[0001] This invention relates to photography. More specifically, this invention is directed
to photographic emulsions containing silver halide grains and to photographic elements
containing these emulsions.
[0002] Silver halide photography has been practiced for more than a century. The radiation
sensitive silver halide compositions initially employed for imaging were termed emulsions,
since it was not originally appreciated that a solid phase was present. The term "photographic
emulsion" has remained in use, although it has long been known that the radiation
sensitive component is present in the form of dispersed microcrystals, typically referred
to as grains.
[0003] Over the years silver halide grains have been the subject of intense investigation.
Although high iodide silver halide grains, those containing at least 90 mole percent
iodide, based on silver, are known and have been suggested for photographic applications,
in practice photographic emulsions almost always contain silver halide grains comprised
of bromide, chloride, or mixtures of chloride and bromide optionally containing minor
amounts of iodide. Up to about 40 mole percent iodide, based on silver, can be accommodated
in a silver bromide crystal structure without observation of a separate silver iodide
phase. However, in practice silver halide emulsions rarely contain more than about
15 mole percent iodide, with iodide well below 10 mole percent being most common.
[0004] All silver halide grains, except high iodide silver halide grains, exhibit cubic
crystal lattice structures. However, grains of cubic crystal lattice structures can
differ markedly in appearance.
[0005] In one form silver halide grains when microscopically observed are cubic in appearance.
A cubic grain 1 is shown in Figure 1. The cubic grain is bounded by six identical
crystal faces. In the photographic literature these crystal faces are usually referred
to as (100) crystal faces, referring to the Miller index employed for designating
crystal faces. While the (100) crystal face designation is most commonly employed
in connection with silver halide grains, these same crystal faces are sometimes also
referred to as {200} crystal faces, the difference in designation resulting from a
difference in the definition of the basic unit of the crystal structure. Although
the cubic crystal shape is readily visually identified in regular grains, in irregular
grains cubic crystal faces are not always square. In grains of more complex shapes
the presence of cubic crystal faces can be verified by a combination of visual inspection
and the 90° angle of intersection formed by adjacent cubic crystal faces.
[0006] The practical importance of the {100} crystal faces is that they present a unique
surface arrangement of silver and halide ions, which in turn influences the grain
surface reactions and adsorptions typically encountered in photographic applications.
This unique surface arrangement of ions as theoretically hypothesized is schematically
illustrated by Figure 2, wherein the smaller spheres 2 represent silver ions while
the larger spheres 3 designate bromine ions. Although on an enlarged scale, the relative
size and position of the silver and bromide ions is accurately represented. When chloride
ions are substituted for bromide ions, the relative arrangement would remain the same,
although the chloride ions are smaller than the bromide ions. It can be seen that
a plurality of parallel rows, indicated by lines 4, are present, each formed by alternating
silver and bromine ions. In Figure 2 a portion of the next tier of ions lying below
the surface tier is shown to illustrate their relationship to the surface tier of
ions.
[0007] In another form silver halide grains when microscopically observed are octahedral
in appearance. An octahedral grain 5 is shown in Figure 3. The octahedral grain is
bounded by eight identical crystal faces. These crystal faces are referred to as (111)
crystal faces. Although the octahedral crystal shape is readily visually identified
in regular grains, in irregular grains octahedral crystal faces are not always triangular.
In grains of more complex shapes the presence of octahedral crystal faces can be verified
by a combination of visual inspection and the 109.5° angle of intersection formed
by adjacent octahedral crystal faces.
[0008] Ignoring possible ion adsorptions, octahedral crystal faces differ from cubic crystal
faces in that the surface tier of ions can be theoretically hypothesized to consist
entirely of silver ions or halide ions. Figure 4 is a schematic illustration of a
(111) crystal face, analogous to Figure 2, wherein the smaller spheres 2 represent
silver ions while the larger spheres 3 designate bromine ions. Although silver ions
are shown at the surface in every available lattice position, it has been suggested
that having silver ions in only every other available lattice position in the surface
tier of atoms would be more compatible with surface charge neutrality. Instead of
a surface tier of silver ions, the surface tier of ions could alternatively be bromide
ions. The tier of ions immediately below the surface silver ions consists of bromide
ions.
[0009] In comparing Figures 1 and 2 with Figures 3 and 4 it is important to bear in mind
that both the cubic and octahedral grains have exactly the same cubic crystal lattice
structure and thus exactly the same internal relationship of silver and halide ions.
The two grains differ only in their surface crystal faces. Note that in the cubic
crystal face of Figure 2 each surface silver ion lies immediately adjacent five halide
ions, whereas in Figure 4 the silver ions at the octahedral crystal faces each lie
immediately adjacent only three halide ions.
[0010] Much less common than either cubic or octahedral silver halide grains are rhombic
dodecahedral silver halide grains. A rhombic dodecahedral grain 7 is shown in Figure
5. The rhombic dodecahedral grain is bounded by twelve identical crystal faces. These
crystal faces are referred to as {110} (or, less commonly in reference to silver halide
grains, {220}) crystal faces. Although the rhombic dodecahedral crystal shape is readily
visually identified in regular grains, in irregular grains rhombic dodecahedral crystal
faces can vary in shape. In grains of more complex shapes the presence of rhombic
dodecahedral crystal faces can be verified by a combination of visual inspection and
measurement of the angle of intersection formed by adjacent crystal faces.
[0011] Rhombic dodecahedral crystal faces can be theoretically hypothesized to consist of
alternate rows of silver ions and halide ions. Figure 6 is a schematic illustration
analogous to Figures 2 and 4, wherein it can be seen that the surface tier of ions
is formed by repeating pairs of silver and bromide ion parallel rows, indicated by
lines 8a and 8b, respectively. In Figure 6 a portion of the next tier of ions lying
below the surface tier is shown to illustrate their relationship to the surface tier
of ions. Note that each surface silver ion lies immediately adjacent four halide ions.
[0012] Although photographic silver halide emulsions containing cubic crystal lattice structure
grains are known which contain only regular cubic grains, such as the grain shown
in Figure 1, regular octahedral grains, such as the grain shown in Figure 3, or, in
rare instances, regular rhombic dodecahedral grains, such as the grain shown in Figure
5, in practice many other varied grain shapes are also observed. For example, silver
halide grains can be cubo-octahedral-that is, formed of a combination of cubic and
octahedral crystal faces. This is illustrated in Figure 7, wherein cubo-octahedral
grains 9 and 10 are shown along with cubic grain 1 and octahedral grain 5. The cubo-octahedral
grains have fourteen crystal faces, six cubic crystal faces and eight octahedral crystal
faces. Analogous combinations of cubic and/or octahedral crystal faces and rhombic
dodecahedral crystal faces are possible, though rarely encountered. Other grain shapes,
such as tabular grains and rods, can be attributed to internal crystal irregularities,
such as twin planes and screw dislocations. In most instances some corner or edge
rounding due to solvent action is observed, and in some instances rounding is so pronounced
that the grains are described as spherical.
[0013] It is known that for cubic crystal lattice structures crystal faces can take any
one of seven possible distinct crystallographic forms. However, for cubic crystal
lattice structure silver halides only grains having {100} (cubic), {111} (octahedral),
or, rarely, {110} (rhombic dodecahedral) crystal faces, individually or in combination,
have been identified.
[0014] It is thus apparent that the photographic art has been limited in the crystal faces
presented by silver halide grains of cubic crystal lattice structure. As a result
the art has been limited in modifying photographic properties to the choice of surface
sensitizers and adsorbed addenda that are workable with available crystal faces, in
most instances cubic and octahedral crystal faces. This has placed restrictions on
the combinations of materials that can be employed for optimum photographic performance
or dictated accepting less than optimum performance.
[0015] F. C. Phillips, An Introduction to Crystallography, 4th Ed., John Wiley & Sons, 1971,
is relied upon as authority for the basic precepts and terminology of crystallography
herein presented.
[0016] James, The Theory of the Photographic Process, 4th Ed., Macmillan, New York, 1977,
pp. 98 through 100, is corroborative of the background of the invention described
above. In addition, James at page 98 in reference to silver halide grains states that
high Miller index faces are not found.
[0017] Berry, "Surface Structure and Reactivity of AgBr Dodecahedra", Photographic Science
and Engineering, Vol. 19, No. 3, May/June 1975, pp. 171 and 172, illustrates silver
bromide emulsions containing (110) crystal faces.
[0018] Klein et al, "Formation of Twins of AgBr and AgCl Crystals in Photographic Emulsions",
Photo- graphische Korrespondenz, Vol. 99, No. 7, pp. 99-102 (1963) describes a variety
of singly and doubly twinned silver halide crystals having {100} (cubic) and {111}
(octahedral) crystal faces. Klein et al is of interest in illustrating the variety
of shapes which twinned silver halide grains can assume while still exhibiting only
{111} or {100} crystal faces.
[0019] A. P. H. Trivelli and S. E. Sheppard, The Silver Bromide Grain of Photographic Emulsions,
Van Nostrand, Chapters VI and VIII, 1921, is cited for historical interest. Magnifications
of 2500X and lower temper the value of these observations. Much higher resolutions
of grain features are obtainable with modern electron microscopy.
[0020] W. Reinders, "Studies of Photohalide Crystals", Kolloid-Zeitschrift, Vol. 9, pp.
10-14 (1911); W. Reinders, "Study of Photohalides III Absorption of Dyes, Proteins
and Other Organic Compounds in Crystalline Silver Chloride", Zeitschrift fur Ph
ysikalische Chemie, Vol. 77, pp. 677-699 (1911); Hirata et al, "Crystal Habit of Photographic
Emulsion Grains", J. Photon. Soc. of Japan, Vol. 36, pp. 359-363 (1973); Locker U.S.
Patent 4,183,756; and Locker et al U.S. Patent 4,225,666 illustrate teachings of modifying
silver halide grain shapes through the presence of various materials present during
silver halide grain formation.
[0021] Wulff et al U.S. Patent 1,696,830 and Heki et al Japanese Kokai 58[1983]-54333 describe
the precipitation of silver halide in the presence of benzimidazole compounds.
[0022] Halwig U.S. Patent 3,519,426 and Oppenheimer et al, "Role of Cationic Surfactants
in Recrystallization of Aqueous Silver Bromide Dispersions", Smith Particle Growth
and Suspension, Academic Press, London, 1973, pp. 159-178, disclose additions of 4-hydroxy-6-methyl-1,3,3a,7-tetraazaindene
to silver chloride and silver bromide emulsions, respectively.
[0023] It is an object of this invention to provide a silver halide photographic emulsion
comprised of radiation sensitive silver halide grains of a cubic crystal lattice structure
presenting crystal faces offering enhanced adsorption sites.
[0024] This object is achieved by providing a silver halide photographic emulsion comprised
of radiation sensitive silver halide grains of a cubic crystal lattice structure comprised
of hexoctahedral crystal faces.
[0025] The invention presents to the art for the first time the opportunity to realize the
unique surface configuration of hexoctahedral crystal faces in photographic silver
halide emulsions. The invention thereby renders accessible for the first time a new
choice of crystal faces for modifying photographic characteristics and improving interactions
with sensitizers and adsorbed photographic addenda.
Description of the Drawings
[0026]
Figure 1 is an isometric view of a regular cubic silver halide grain;
Figure 2 is a schematic diagram of the atomic arrangement at a silver bromide cubic
crystal surface;
Figure 3 is an isometric view of a regular octahedral silver halide grain;
Figure 4 is a schematic diagram of the atomic arrangement at a silver bromide octahedral
crystal surface;
Figure 5 is an isometric view of a regular rhombic dodecahedron;
Figure 6 is a schematic diagram of the atomic arrangement at a silver bromide rhombic
dodecahedral crystal surface;
Figure 7 is an isometric view of a regular cubic silver halide grain, a regular octahedral
silver halide grain, and intermediate cubo-octahedral silver halide grains.
Figures 8 and 9 are front and rear isometric views of a regular {321} hexoctahedron;
Figure 10 is a schematic diagram of the atomic arrangement at a silver bromide {321}
hexoctahedral crystal surface; and
Figures 11 through 15 are electron micrographs of hexoctahedral silver halide grains.
[0027] The present invention relates to silver halide photographic emulsions comprised of
radiation sensitive silver halide grains of a cubic crystal lattice structure comprised
of hexoctahedral crystal faces and to photographic elements including the emulsions.
[0028] In one form the silver halide grains can take the form of regular hexoctahedra. A
regular hexoctahedron 11 is shown in Figures 8 and 9. A hexoctahedron has forty-eight
identical faces. Although any grouping of faces is entirely arbitrary, the hexoctahedron
can be visualized as six separate clusters of crystal faces, each cluster containing
eight separate faces. In Figure 8 faces 12a, 12b, 12c, 12d, 12e, 12f, 12g, and 12h
can be visualized as members of a first cluster of faces. A second cluster of faces
is represented by faces 13a, 13b, 13c, 13d, 13e, 13f, and 13g. The eighth face of
the cluster, 13h, is shown substantially normal to the field of view. Faces 14a, 14b,
14c, and 14d represent four visible faces of a third cluster of eight faces, and faces
15a and 15b represent two visible faces of a fourth cluster of eight faces. Two remaining
clusters of eight faces each are entirely hidden from view on the opposite side of
the hexoctahedron.
[0029] Figure 9 shows a back view of the hexoctahedron 11 obtained by 180° rotation of the
hexoctahedron about a vertical axis. Faces 14e, 14f, 14g, and 14h of the third cluster
are shown. Faces 15c, 15d, 15e, 15f, 15g, and 15h of the fourth cluster are shown.
Faces 16a, 16b, 16c, 16d, 16e, 16f, 16g, and 16h forming a fifth cluster are shown.
Faces 17a, 17b, 17c, 17d, 17e, 17f, 17g, and 17h complete the sixth cluster.
[0030] Looking at the hexoctahedron it can be seen that there are eight intersections of
adjacent faces within each cluster, and there are two face intersections of each cluster
with each of the four clusters adjacent to it for a total of seventy-two face edge
intersections. The relative angles formed by intersecting faces have only three different
values. All intersections of a face from one cluster with a face from another cluster
are identical, forming a first relative angle. All adjacent faces within each cluster
intersect at one of two different relative angles. Looking at one cluster in which
all faces are fully visible, the intersections between faces 12a and 12b, 12c and
12d, 12e and 12f, and 12g and 12h are all at the same relative angle, referred to
as a second relative angle. The intersections between faces 12b and 12c, 12d and 12e,
12f and 12g, and 12h and 12a are all at the same relative angle, referred to as a
third relative angle, since it is of a different value than both the first and second
relative angles. While the regular hexoctahedron has a distinctive appearance that
can be recognized by visual inspection, it should be appreciated that measurement
of any one of the three relative angles provides a corroboration of adjacent hexoctahedral
crystal faces.
[0031] In crystallography measurement of relative angles of adjacent crystal faces is employed
for positive crystal face identification. Such techniques are described, for example,
by Phillips, cited above. These techniques can be combined with techniques for the
microscopic examination of silver halide grains to identify positively the hexoctahedral
crystal faces of silver halide grains. Techniques for preparing electron micrographs
of silver halide grains are generally well known in the art, as illustrated by B.M.
Spinell and C.F. Oster, "Photographic Materials", The Encyclopedia of Microscopy and
Microtechnique, P. Gray, ed., Van Nostrand, N.Y., 1973, pp.427-434, note particularly
the section dealing with carbon replica electron microscopy at pages 429 and 430.
Employing techniques well known in electron microscopy, carbon replicas of silver
halide grains are first prepared. The carbon replicas reproduce the grain shape while
avoiding shape altering silver print-out that is known to result from employing the
silver halide grains without carbon shells. An electron scanning beam rather than
light is employed for imaging to permit higher ranges of magnification to be realized
than when light is employed. When the grains are sufficiently spread apart that adjacent
grains are not impinging, the grains lie flat on one crystal face rather than on a
coign (i.e., a point). By tilting the sample being viewed relative to the electron
beam a selected grain can be oriented so that the line of sight is substantially parallel
to both the line of intersection of two adjacent crystal faces, seen as a point, and
each of the two intersecting crystal faces, seen as edges. When the grain faces are
parallel to the imaging electron beam, the two corresponding edges of the grain which
they define will appear sharper than when the faces are merely close to being parallel.
Once the desired grain orientation with two intersecting crystal faces presenting
a parallel edge to the electron beam is obtained, the angle of intersection can be
measured from an electron micrograph of the oriented grain. In this way adjacent hexoctahedral
crystal faces can be identified. Relative angles of hexoctahedral and adjacent crystal
faces of other Miller indices can also be determined in the same way. Again, the unique
relative angle allows a positive identification of the crystal faces. While relative
angle measurements can be definitive, in many, if not most, instances visual inspection
of grains by electron microscopy allows immediate identification of hexoctahedral
crystal faces.
[0032] Referring to the mutually perpendicular x, y, and z axes of a cubic crystal lattice,
it is well recognized in the art that cubic crystal faces are parallel to two of the
axes and intersect the third, thus the {100} Miller index assignment; octahedral crystal
faces intersect each of the three axes at an equal interval, thus the (111) Miller
index assignment; and rhombic dodecahedral crystal faces intersect two of the three
axes at an equal interval and are parallel to the third axis, thus the {110} Miller
index assignment. For a given definition of the basic crystal unit, there is one and
only one Miller index assignment for each of cubic, octahedral, and rhombic dodecahedral
crystal faces.
[0033] Hexoctahedral crystal faces include a family of crystal faces that can have differing
Miller index values. Hexoctahedral crystal faces are generically designated as {hkℓ}
crystal faces, wherein h, k, and ℓ are each integers greater than 0; h is greater
than k; and k is greater than 1. The regular hexoctahedron 11 shown in Figures 8 and
9 consists of {321} crystal faces, which corresponds to the lowest value that h, k,
and ℓ can each represent. A regular hexoctahedron having {421}, {431}, {432}, {521},
{531}, {532}, {541}, {542}, or (543) crystal faces would appear similar to the hexoctahedron
11, but the higher Miller indices would result in changes in the angles of intersection.
Although there is no theoretical limit on the maximum values of the integers h, k,
and 1, hexoctahedral crystal faces having a value of h of 5 or less are more easily
generated. For this reason, silver halide grains having hexoctahedral crystal faces
of the exemplary Miller index values identified above are preferred. With practice
one hexoctahedral crystal face can often be distinguished visually from another of
a different Miller index value. Measurement of relative angles permits positive corroboration
of the specific Miller index value hexoctahedral crystal faces present.
[0034] In one form the emulsions of this invention contain silver halide grains which are
bounded entirely by hexoctahedral crystal faces, thereby forming basically regular
hexoctahedra. In practice although some edge rounding of the grains is usually present,
the unrounded residual flat hexoctahedral faces permit positive identification, since
a sharp intersecting edge is unnecessary to establishing the relative angle of adjacent
hexoctahedral crystal faces. Sighting to orient the grains is still possible employing
the residual flat crystal face portions.
[0035] The radiation sensitive silver halide grains present in the emulsions of this invention
are not confined to those in which the hexoctahedral crystal faces are the only flat
crystal faces present. Just as cubo-octahedral silver halide grains, such as 9 and
10, exhibit both cubic and octahedral crystal faces and Berry, cited above, reports
grains having cubic, octahedral, and rhombic dodecahedral crystal faces in a single
grain, the radiation sensitive grains herein contemplated can be formed by hexoctahedral
crystal faces in combination with any one or combination of the other types of crystal
faces possible with a silver halide cubic crystal lattice structure. For example,
if conventional silver halide grains having cubic, octahedral, and/or rhombic dodecahedral
crystal faces are employed as host grains for the preparation of silver halide grains
having hexoctahedral crystal faces, stopping silver halide deposition onto the host
grains before the original crystal faces have been entirely overgrown by silver halide
under conditions favoring hexoctahedral crystal face formation results in both hexoctahedral
crystal faces and residual crystal faces corresponding to those of the original host
grain being present. Starting with cubic host grains, the preparation of cubo-hexoctahedral
grains is illustrated in the examples.
[0036] In another variant form deposition of silver halide onto host grains under conditions
which favor hexoctahedral crystal faces can initially result in ruffling of the grain
surfaces. Under close examination it has been observed that the ruffles are provided
by protrusions from the host grain surface. Protrusions in the form of ridges have
been observed, but protrusions, when present, are more typically in the form of pyramids.
Pyramids presenting hexoctahedral crystal faces on host grains initially presenting
{100} crystal faces have eight surface faces. These correspond to the eight faces
of any one of the 12, 13, 14, 15, 16, or 17 series clusters described above in connection
with the hexoctahedron 11. When the host grains initially present (111) crystal faces,
pyramids bounded by six surface faces are formed. Turning to Figure 8, the apex of
the pyramid corresponds to the coign formed faces 12a, 12h, 13d, 13c, 14b, and 14c.
If the host grains initially present {110} crystal faces, pyramids bounded by four
surface faces are formed. Turning to Figure 8, the apex of the pyramid corresponds
to the coign formed faces 12a, 12b, 13c, and 13d. The protrusions, whether in the
form of ridges or pyramids, can within a short time of initiating precipitation onto
the host grains substantially cover the original host grain surface. If silver halide
deposition is continued after the entire grain surface is bounded by hexoctahedral
crystal faces, the protrusions become progressively larger and eventually the grains
lose their ruffled appearance as they present larger and larger hexoctahedral crystal
faces. It is possible to grow a regular hexoctahedron from a ruffled grain by continuing
silver halide deposition.
[0037] Even when the grains are not ruffled and bounded entirely by hexoctahedral crystal
faces, the grains can take overall shapes differing from regular hexoctahedrons. This
can result, for example, from irregularities, such as twin planes, present in the
host grains prior to growth of the hexoctahedral crystal faces or introduced during
growth of the hexoctahedral crystal faces.
[0038] The important feature to note is that if any crystal face of a silver halide grain
is a hexoctahedral crystal face, the resulting grain presents a unique arrangement
of surface silver and halide ions that differs from that presented by all other possible
crystal faces for cubic crystal lattice structure silver halides. This unique surface
arrangement of ions as theoretically hypothesized is schematically illustrated by
Figure 10, wherein a (321) hexoctahedral crystal face is shown formed by silver ions
2 and bromide ions 3. Comparing Figure 10 with Figures 2, 4, and 6, it is apparent
that the surface positioning of silver and bromide ions in each figure is distinctive.
The (321) hexoctahedral crystal face presents an ordered, but more varied arrangement
of surface silver and bromide ions than is presented at the cubic, octahedral, or
rhombic dodecahedral silver bromide crystal faces. This is a result of the oblique
tiering that occurs at the (321) hexoctahedral crystal face. Hexoctahedral crystal
faces with differing Miller indices also exhibit oblique tiering. The differing Miller
indices result in analogous, but nevertheless unique surface arrangements of silver
and halide ions.
[0039] While Figures 2, 4, 6, and 10 all contain bromide ions as the sole halide ions, it
is appreciated that the same observations as to differences in the crystal faces obtain
when each wholly or partially contains chloride ions instead. Although chloride ions
are substantially smaller in effective diameter than bromide ions, a [321] hexoctahedral
crystal surface presented by silver chloride would appear similar to the surface shown
in Figure 10
[0040] The cubic crystal lattice structure silver halide grains containing hexoctahedral
crystal faces can contain minor amounts of iodide ions, similarly as conventional
silver halide grains. Iodide ions have an effective diameter substantially larger
than that of bromide ions. As is well known in silver halide crystallography, this
has a somewhat disruptive effect on the order of the crystal structure, which can
be accommodated and actually employed photographically to advantage, provided the
iodide ions are limited in concentration. Preferably iodide ion concentrations below
15 mole percent and optimally below 10 mole percent, based on silver, are employed
in the practice of this invention. Iodide ion concentrations of up to 40 mole percent,
based on silver, can be present in silver bromide crystals. Since iodide ions as the
sole halide ions in silver halide do not form a cubic crystal lattice structure, their
use alone has no applicability to this invention.
[0041] It is appreciated that the larger the proportion of the total silver halide grain
surface area accounted for by hexoctahedral crystal faces the more distinctive the
silver halide grains become. In most instances the hexoctahedral crystal faces account
for at least 50 percent of the total surface area of the silver halide grains. Where
the grains are regular, the hexoctahedral crystal faces can account for all of the
flat crystal faces observable, the only remaining grain surfaces being attributable
to edge rounding. In other words, silver halide grains having hexoctahedral crystal
faces accounting for at least 90 percent of the total grain surface area are contemplated.
[0042] It is, however, appreciated that distinctive photographic effects may be realized
even when the hexoctahedral crystal faces are limited in areal extent. For example,
where in an emulsion containing the silver halide grains a photographic addendum is
present that shows a marked adsorption preference for a hexoctahedral crystal face,
only a limited percentage of the total grain surface may be required to produce a
distinctive photographic effect. Generally, if any hexoctahedral crystal face is observable
on a silver halide grain, it accounts for a sufficient proportion of the total surface
area of the silver halide grain to be capable of influencing photographic performance.
Stated another way, by the time a hexoctahedral crystal face becomes large enough
to be identified by its relative angle to adjacent crystal faces, it is already large
enough to be capable of influencing photographic performance. Thus, the minimum proportion
of total grain surface area accounted for by hexoctahedral crystal faces is limited
only by the observer's ability to detect the presence of hexoctahedral crystal faces.
[0043] The successful formation of hexoctahedral crystal faces on silver halide grains of
a cubic crystal lattice structure depends on identifying silver halide grain growth
conditions that retard the surface growth rate on hexoctahedral crystal planes. It
is generally recognized in silver halide crystallography that the predominant crystal
faces of a silver halide grain are determined by choosing grain growth conditions
that are least favorable for the growth of that crystal face. For example, regular
cubic silver halide grains, such as grain 1, are produced under grain growth conditions
that favor more rapid deposition of silver and halide ions on all other available
crystal faces than on the cubic crystal faces. Referring to Figure 7, if an octahedral
grain, such as regular octahedral grain 5 is subjected to growth under conditions
that least favor deposition of silver and halide ions onto cubic crystal faces, grain
5 during continued silver halide precipitation will progress through the intermediate
cubo-octahedral grain forms 9 and 10 before reaching the final cubic grain configuration
1. Once only cubic crystal faces remain, then silver and halide ions deposit isotropically
on these surfaces. In other words, the grain shape remains cubic, and the cubic grains
merely grow larger as additional silver and halide ions are precipitated.
[0044] By analogy, grains having hexoctahedral crystal faces have been prepared by introducing
into a silver halide precipitation reaction vessel host grains of conventional crystal
faces, such as cubic grains, while maintaining growth conditions to favor retarding
silver halide deposition along hexoctahedral crystal faces. As silver halide precipitation
continues hexoctahedral crystal faces first become identifiable and then expand in
area until eventually, if precipitation is continued, they account for all of the
crystal faces of the silver halide grains being grown. Since hexoctahedral crystal
faces accept additional silver halide deposition at a slow rate, renucleation can
occur, creating a second grain population. Precipitation conditions can be adjusted
by techniques generally known in the art to favor either continued grain growth or
renucleation.
[0045] Failure of the art to observe hexoctahedral crystal faces for silver halide grains
over decades of intense investigation as evidenced by published silver halide crystallographic
studies suggests that there is not an extensive range of conditions that favor the
selective retarding of silver halide deposition along hexoctahedral crystal faces.
It has been discovered that growth modifiers can be employed to retard silver halide
deposition selectively at hexoctahedral crystal faces, thereby producing these hexoctahedral
crystal faces as the external surfaces of the silver halide grains being formed. The
growth modifiers which have been identified are organic compounds. They are believed
to be effective by reason of showing an adsorption preference for a hexoctahedral
crystal face by reason of its unique arrangement of silver and halide ions. Growth
modifiers that have been empirically proven to be effective in producing hexoctahedral
crystal faces are described in the examples, below.
[0046] These growth modifiers are effective under the conditions of their use in the examples.
From empirical screening of a variety of candidate growth modifiers under differing
conditions of silver halide precipitation it has been concluded that multiple parameters
must be satisfied to achieve hexoctahedral crystal faces, including not only the proper
choice of a growth modifier, but also proper choice of other precipitation parameters
identified in the examples. Failures to achieve hexoctahedral crystal faces with compounds
shown to be effective as growth modifiers for producing hexoctahedral crystal faces
have been observed when accompanying conditions for silver halide precipitation have
been varied. However, it is appreciated that having demonstrated success in the preparations
of silver halide emulsions containing grains with hexoctahedral crystal faces, routine
empirical studies systematically varying parameters are likely to lead to additional
useful preparation techniques.
[0047] Once silver halide grain growth conditions are satisfied that selectively retard
silver halide deposition at hexoctahedral crystal faces, continued grain growth usually
results in hexoctahedral crystal faces appearing on all the grains present in the
silver halide precipitation reaction vessel. It does not follow, however, that all
of the radiation sensitive silver halide grains in the emulsions of the present invention
must have hexoctahedral crystal faces. For example, silver halide grains having hexoctahedral
crystal faces can be blended with any other conventional silver halide grain population
to produce the final emulsion. While silver halide emulsions containing any identifiable
hexoctahedral crystal face grain surface are considered within the scope of this invention,
in most applications the grains having at least one identifiable hexoctahedral crystal
face account for at least 10 percent of the total grain population and usually these
grains will account for greater than 50 percent of the total grain population.
[0048] The emulsions of this invention can be substituted for conventional emulsions to
satisfy known photographic applications. In addition, the emulsions of this invention
can lead to unexpected photographic advantages.
[0049] For example, when a growth modifier is present adsorbed to the hexoctahedral crystal
faces of the grains and has a known photographic utility that is enhanced by adsorption
to a grain surface, either because of the more intimate association with the grain
surface or because of the reduced mobility of the growth modifier, improved photographic
performance can be expected. The reason for this is that for the growth modifier to
produce a hexoctahedral crystal face it must exhibit an adsorption preference for
the hexoctahedral crystal face that is greater than that exhibited for any other possible
crystal face. This can be appreciated by considering growth in the presence of an
adsorbed growth modifier of a silver halide grain having both cubic and hexoctahedral
crystal faces. If the growth modifier shows an adsorption preference for the hexoctahedral
crystal faces over the cubic crystal faces, deposition of silver and halide ions onto
the hexoctahedral crystal faces is retarded to a greater extent than along the cubic
crystal faces, and grain growth results in the elimination of the cubic crystal faces
in favor of hexoctahedral crystal faces. From the foregoing it is apparent that growth
modifiers which produce hexoctahedral crystal faces are more tightly adsorbed to these
grain surfaces than to other silver halide grain surfaces during grain growth, and
this enhanced adsorption carries over to the completed emulsion.
[0050] To provide an exemplary photographic application, Locker U.S. Patent 3,989,527 describes
improving the speed of a photographic element by employing an emulsion containing
radiation sensitive silver halide grains having a spectral sensitizing dye adsorbed
to the grain surfaces in combination with silver halide grains free of spectral sensitizing
dye having an average diameter chosen to maximize light scattering, typically in the
0.15 to 0.8
lim range. Upon imagewise exposure radiation striking the undyed grains is scattered
rather than being absorbed. This results in an increased amount of exposing radiation
striking the radiation sensitive imaging grains having a spectral sensitizing dye
adsorbed to their surfaces.
[0051] A disadvantage encountered with this approach has been that spectral sensitizing
dyes can migrate in the emulsion, so that to some extent the initially undyed grains
adsorb spectral sensitizing dye which has migrated from the initially spectrally sensitized
grains. To the extent that the initially spectrally sensitized grains were optimally
sensitized, dye migration away from their surfaces reduces sensitization. At the same
time, adsorption of dye on the grains intended to scatter imaging radiation reduces
their scattering efficiency.
[0052] In the examples below it is to be noted that a specific spectral sensitizing dye
has been identified as a growth modifier useful in forming silver halide grains having
hexoctahedral crystal faces. When radiation sensitive silver halide grains having
hexoctahedral crystal faces and a growth modifier spectral sensitizing dye adsorbed
to the hexoctahedral crystal faces are substituted for the spectrally sensitized silver
halide grains employed by Locker, the disadvantageous migration of dye from the hexoctahedral
crystal faces to the silver halide grains intended to scatter light is reduced or
eliminated. Thus, an improvement in photographic efficiency can be realized.
[0053] To illustrate another advantageous photographic application, the layer structure
of a multicolor photographic element which introduces dye image providing materials,
such as couplers, during processing can be simplified. An emulsion intended to record
green exposures can be prepared using a growth modifier that is a green spectral sensitizing
dye while an emulsion intended to record red exposures can be prepared using a growth
modifier that is a red spectral sensitizing dye. Since the growth modifiers are tightly
adsorbed to the grains and non-wandering, instead of coating the green and red emulsions
in separate color forming layer units, as is conventional practice, the two emulsions
can be blended and coated as a single color forming layer unit. The blue recording
layer can take any conventional form, and a conventional yellow filter layer can be
employed to protect the blended green and red recording emulsions from blue light
exposure. Except for blending the green and red recording emulsions in a single layer
or group of layers differing in speed in a single color forming layer unit, the structure
and processing of the photographic element is unaltered. If silver chloride emulsions
are employed, the approach described above can be extended to blending in a single
color forming layer unit blue, green, and red recording emulsions, and the yellow
filter layer can be eliminated. The advantage in either case is a reduction in the
number of emulsion layers required as compared to a corresponding conventional multicolor
photographic element.
[0054] In more general applications, the substitution of an emulsion according to the invention
containing a growth modifier spectral sensitizing dye should produce a more invariant
emulsion in terms of spectral properties than a corresponding emulsion containing
silver halide grains lacking hexoctahedral crystal faces. Where the growth modifier
is capable of inhibiting fog, such as nitrobenzimidazole or 5-carboxy-4-hydroxy-1,3,3a,7-tetraazaindene,
shown to be effective growth modifiers in the examples, more effective fog inhibition
at lower concentrations may be expected. It is recognized that a variety of photographic
effects, such as photographic sensitivity, minimum background density levels, latent
image stability, nucleation, developability, image tone, absorption, and reflectivity,
are influenced by grain surface interactions with other components. By employing components,
such as peptizers, silver halide solvents, sensitizers or desensitizers, supersensitizers,
halogen acceptors, dyes, antifoggants, stabilizers, latent image keeping agents, nucleating
agents, tone modifiers, development accelerators or inhibitors, development restrainers,
developing agents, and other addenda that are uniquely matched to the hexoctahedral
crystal surface, distinct advantages in photographic performance over that which can
be realized with silver halide grains of differing crystal faces are possible.
[0055] The silver halide grains having hexoctahedral crystal faces can be varied in their
properties to satisfy varied known photographic applications as desired. Generally
the techniques for producing surface latent image forming grains, internal latent
image forming grains, internally fogged grains, surface fogged grains, and blends
of differing grains described in Research Disclosure, Vol. 176, December 1978, Item
17643, Section I, can be applied to the preparation of emulsions according to this
invention. Research Disclosure is published by Kenneth Mason Publications, Ltd., Emsworth,
Hampshire P010 7DD, England. The silver halide grains having hexoctahedral crystal
faces can have silver salt deposits on their surfaces, if desired. Selective site
silver salt deposits on host silver halide grains are taught by Maskasky U.S. Patents
4,463,087 and 4,471,050, here incorporated by reference. The silver halide grains
having hexoctahedral crystal faces can have silver salt deposits on their surfaces,
if desired. Selective site silver salt deposits on host silver halide grains are taught
by Maskasky U.S. Patents 4,463,087 and 4,471,050, here incorporated by reference.
[0056] The growth modifier used to form the hexoctahedral crystal faces of the silver halide
grains can be retained in the emulsion, adsorbed to the grain faces or displaced from
the grain faces. For example, where, as noted above, the growth modifier is also capable
of acting as a spectral sensitizing dye or performing some other useful function,
it is advantageous to retain the growth modifier in the emulsion. Where the growth
modifier is not relied upon to perform an additional useful photographic function,
its presence in the emulsion can be reduced or eliminated, if desired, once its intended
function is performed. This approach is advantageous where the growth modifier is
at all disadvantageous in the environment of use. The growth modifier can itself be
modified by chemical interactions, such as oxidation, hydrolysis, or addition reactions,
accomplished with reagents such as bromine water, base, or acid-e.g., nitric, hydrochloric,
or sulfuric acid.
[0057] Apart from the novel grain structures identified above, the radiation sensitive silver
halide emulsions and the photographic elements in which they are incorporated of this
invention can take any convenient conventional form. The emulsions can be washed as
described in Research Disclosure, Item 17643, cited above, Section II.
[0058] The radiation sensitive silver halide grains of the emulsions can be surface chemically
sensitized. Noble metal (e.g., gold), middle chalcogen (e.g., sulfur, selenium, or
tellurium), and reduction sensitizers, employed individually or in combination are
specifically contemplated. Typical chemical sensitizers are listed in Research Disclosure,
Item 17643, cited above, Section III. From comparisons of surface halide and silver
ion arrangements in general the chemical sensitization response of silver halide grains
having hexoctahedral crystal faces should be analogous, but not identical, to that
of cubic and octahedral silver halide grains. That observation can be extended to
emulsion addenda generally which adsorb to grain surfaces.
[0059] The silver halide emulsions can be spectrally sensitized with dyes from a variety
of classes, including the polymethine dye class, which includes the cyanines, merocyanines,
complex cyanines and merocyanines (i.e., tri-, tetra-, and polynuclear cyanines and
merocyanines), oxonols, hemioxonols, styryls, merostyryls, and streptocyanines. Illustrative
spectral sensitizing dyes are disclosed in Research Disclosure, Item 17643, cited
above, Section IV.
[0060] The silver halide emulsions as well as other layers of the photographic elements
of this invention can contain as vehicles hydrophilic colloids, employed alone or
in combination with other polymeric materials (e.g., latices). Suitable hydrophilic
materials include both naturally occurring substances such as proteins, protein derivatives,
cellulose derivatives-e.g., cellulose esters, gelatin-e.g., alkali treated gelatin
(cattle, bone, or hide gelatin) or acid treated gelatin (pigskin gelatin), gelatin
derivatives-e.g., acetylated gelatin, phthalated gelatin, and the like, polysaccharides
such as dextran, gum arabic, zein, casein, pectin, collagen derlvatives, collodion,
agar-agar, arrowroot, and albumin. It is specifically contemplated to employ hydrophilic
colloids which contain a low proportion of divalent sulfur atoms. The proportion of
divalent sulfur atoms can be reduced by treating the hydrophilic colloid with a strong
oxidizing agent, such as hydrogen peroxide. Among preferred hydrophilic colloids for
use as peptizers for the emulsions of this invention are gelatino-peptizers which
contain less than 30 micromoles of methionine per gram. The vehicles can be hardened
by conventional procedures. Further details of the vehicles and hardeners are provided
in Research Disclosure, Item 17643, cited above, Sections IX and X.
[0061] The silver halide photographic elements of this invention can contain other addenda
conventional in the photographic art. Useful addenda are described, for example, in
Research Disclosure, Item 17643, cited above. Other conventional useful addenda include
antifoggants and stabilizers, couplers (such as dye forming couplers, masking couplers
and DIR couplers) DIR compounds, anti-stain agents, image dye stabilizers, absorbing
materials such as filter dyes and UV absorbers, light scattering materials, antistatic
agents, coating aids, and plasticizers and lubricants.
[0062] The photographic elements of the present invention can be simple black-and-white
or monochrome elements comprising a support bearing a layer of the silver halide emulsion,
or they can be multilayer and/or multicolor elements. The photographic elements produce
images ranging from low contrast to very high contrast, such as those employed for
producing half tone images in graphic arts. They can be designed for processing with
separate solutions or for in-camera processing. In the latter instance the photographic
elements can include conventional image transfer features, such as those illustrated
by Research Disclosure, Item 17643, cited above, Section XXIII. Multicolor elements
contain dye image forming units sensitive to each of the three primary regions of
the spectrum. Each unit can be comprised of.a single emulsion layer or of multiple
emulsion layers sensitive to a given region of the spectrum. The layers of the element,
including the layers of the image forming units, can be arranged in various orders
as known In the art. In an alternative format, the emulsion or emulsions can be disposed
as one or more segmented layers, e.g., as by the use of microvessels or microcells,
as described in Whitmore U.S. Patent 4,387,154.
[0063] A preferred multicolor photographic element according to this invention containing
incorporated dye image providing materials comprises a support bearing at least one
blue sensitive silver halide emulsion layer having associated therewith a yellow dye
forming coupler, at least one green sensitive silver halide emulsion layer having
associated therewith a magenta dye forming coupler, and at least one red sensitive
silver halide emulsion layer having associated therewith a cyan dye forming coupler,
at least one of the silver halide emulsion layers containing grains having hexoctahedral
crystal faces as previously described.
[0064] The elements of the present invention can contain additional layers conventional
in photographic elements, such as overcoat layers, spacer layers, filter layers, antihalation
layers, and scavenger layers. The support can be any suitable support used with photographic
elements. Typical supports include polymeric films, paper (including polymer-coated
paper), glass, and metal supports. Details regarding supports and other layers of
the photographic elements of this invention are contained in Research Disclosure,
Item 17643, cited above, Section XVII.
[0065] The photographic elements can be imagewise exposed with various forms of energy,
which encompass the ultraviolet, visible, and infrared regions_of the electromagnetic
spectrum as well as electron beam and beta radiation, gamma ray, X ray, alpha particle,
neutron radiation, and other forms of corpuscular and wave-like radiant energy in
either noncoherent (random phase) forms or coherent (in phase) forms, as produced
by lasers. When the photographic elements are intended to be exposed by X rays, they
can include features found in conventional radiographic elements, such as those illustrated
by Research Disclosure, Vol. 184, August 1979, Item 18431.
[0066] Processing of the imagewise exposed photographic elements can be accomplished in
any convenient conventional manner. Processing procedures, developing agents, and
development modifiers are illustrated by Research Disclosure, Item 17643, cited above,
Sections XIX, XX, and XXI, respectively.
Examples
[0067] The invention can be better appreciated by reference to the following specific examples.
In each of the examples the term "percent" means percent by weight, unless otherwise
indicated, and all solutions, unless otherwise indicated, are aqueous solutions. Dilute
nitric acid or dilute sodium hydroxide was employed for pH adjustment, as required.
Example 1
[0068] This example illustrates the preparation of a hexoctahedral silver bromide emulsion
having the Miller index [321], beginning with a cubic host emulsion.
[0069] To a reaction vessel supplied with a stirrer was added 0.5 g of bone gelatin dissolved
in 28.5 g of water. To this was added 0.05 mole of a cubic silver bromide emulsion
of mean grain size 0.8µm, containing about 10 g/Ag mole gelatin, and having a total
weight of 21.6 g. The emulsion was heated to 40°C, and 0.3 millimole/Ag mole of 6-nitrobenzimidazole
dissolved in 2 mL. methanol was added. The mixture was held for 15 min. at 40°C. The
pH was adjusted to 6.0 at 40°C. The emulsion was then heated to 60°C, and the pAg
adjusted to 8.5 at 60°C with KBr, and maintained at that value during the precipitation.
A 2.5M solution of AgN0
3 and a 2.5M solution of KBr were then introduced with a constant silver addition rate
over a period of 50 min., consuming 0.025 mole Ag. The precipitation was then stopped,
and an additional 6.0 millimoles/- original Ag mole of 6-nitrobenzimidazole dissolved
in 2 ml of methanol were added. The precipitation was then continued at the same rate
as before for 10 minutes, consuming an additional 0.005 mole Ag. At this stage a sample
(Emulsion 1A) was removed. The precipitation was continued for a further 65 min.,
during which an additional 0.0325 mole Ag was consumed, to produce Emulsion IB.
[0070] A carbon replica electron micrograph (Figure 11) shows Emulsion 1A to have a combination
of cubic and hexoctahedral faces. Emulsion 1B (Figure 12) has hexoctahedral faces
only. The Miller index of the hexoctahedral faces was determined by measurement of
the relative angle between two adjacent hexoctahedral crystal faces. From this angle,
the supplement of the relative angle, which is the angle between their respective
crystallographic vectors, Φ, could be obtained, and the Miller index of the adjacent
hexoctahedral crystal faces was identified by comparison of this angle Φ with the
theoretical intersecting angle θ between [h
1k
1ℓ
1]] and [h
2k
2ℓ
2] vectors. The angle 6 was calculated as described by Phillips, cited above, at pages
218 and 219.
[0071] To obtain the angle Φ, a carbon replica of the crystal sample was rotated on the
stage of an electron microscope until, for a chosen crystal, the angle of observation
was directly along the line of intersection of the two adjacent crystal faces of interest.
An electron micrograph was then made, and the relative angle was measured on the micrograph
with a protractor. The supplement of the measured relative angle was the angle Φ between
vectors. The results for Emulsions 1A and 1B for each of the vector angles corresponding
to the three different relative angles measured are given below. The number of measurements
made is given in parentheses. Theoretical Miller indices as high as {543} were considered.
[0072] Angle Between Vectors Theoretical {321} 31.0° 21.8° 44.4° Measured, Emulsion 1A 30.5±1.0°(4)
21°(1) 45°(1) Emulsion 1B 32.0±1.9°(4) 21°(1) - The emulsions of this example therefore
show {321} hexoctahedral faces, with Emulsion 1B, which is composed of regular hexoctahedra,
showing only {321} crystal faces.
Example 2
[0073] This example illustrates the preparation of a hexoctahedral silver bromide emulsion
having the Miller index {321} beginning with an octahedral host emulsion.
[0074] To a reaction vessel supplied with a stirrer was added 0.10 mole of an octahedral
AgBr emulsion, containing 40 g/Ag mole gelatin, of mean grain size 1.3µm, diluted
to 55 mL. with water. The emulsion was heated to 40°C, and 4.0 millimole/mole startup
Ag of 6-nitrobenzimidazole dissolved in 3 mL. of methanol was added. The mixture was
held 15 min. at 40°C. The temperature was then raised to 60°C. The p
Ag was adjusted to 8.5 at 60° with KBr and maintained at that value during the precipitation.
The pH was adjusted to 6.0 at 60°C and maintained at that value. A 2.0 M solution
of AgN0
3 and a 2.0 M solution of KBr were simultaneously added over a period of 400 min.,
with a constant silver addition rate consuming 0.08 mole Ag.
[0075] Figure 13 is an electron micrograph showing the hexoctahedral habit of the emulsion
prepared. The Miller index was observed to be {321}.
Example 3
[0076] This example illustrates the preparation of a hexoctahedral silver bromide emulsion
having the Miller index {521} beginning with a cubic host emulsion.
[0077] To a reaction vessel supplied with a stirrer was added 0.05 mole of a cubic silver
bromide emulsion of mean grain size 0.8pm, containing about 10 g/Ag mole of gelatin.
Water was added to make the total weight 50 g. To the emulsion at 40°C was added 3.0
millimole/Ag mole of the growth modifier spectral sensitizing dye 3-carboxymethyl-5-{[3-(3-sulfopropyl)-2-thiazolidinylidene]ethylidene)rhodanine,
sodium salt (structure shown below), hereinafter referred to as Dye I, dissolved in
3 mL. of methanol, 2 mL. water, and 3 drops of triethylamine.

[0078] The emulsion was then held for 15 min. at 40°C. The pH was adjusted to 6.0 at 40°C.
The temperature was raised to 60°C, and the pAg adjusted to 8.5 at 60°C with KBr and
maintained at that value during the precipitation. A 2.5 M solution of A
GNO
3 was introduced at a constant rate over a period of 125 min. while a 2.5 M solution
of KBr was added as needed to hold the pAg constant. A total of 0.0625 mole Ag was
added. An electron micrograph of the resulting hexoctahedral emulsion grains is shown
in Figure 14.
[0079] The Miller index of the hexoctahedra of the prepared emulsion was determined to be
{521} by the method described for Example 1.
[0080] Angle Between Vectors Theoretical {521} 21.0° 45.6° Measured 22.9±1.4°(10) 45.6±3.2°(15)
Example 4
[0081] This example illustrates the preparation of a hexoctahedral silver chloride emulsion
having the Miller index {521}.
[0082] To a reaction vessel supplied with a stirrer was added 0.05 mole of a cubic silver
chloride emulsion of mean grain size 0.65 µm and containing 40 g/Ag mole gelatin.
Water was added to make the total weight 48 g. To the emulsion at 40°C was added 2.0
millimole/Ag mole of Dye I dissolved in 3 mL. of methanol, 1.5 mL. water, and 2 drops
of triethylamine. The emulsion was then held for 15 min. at 40°C. The temperature
was then raised to 50°C. The pH was adjusted to 5.92 at 50°C, and maintained at about
this value during the precipitation by NaOH addition. The pAg was adjusted to 7.7
at 50°C with NaC1 solution and maintained during the precipitation. A 2.0 M solution
of AgN0
3 was introduced at a constant rate over a period of 200 min., while a 2.2 M solution
of NaCl was added as needed to hold the pAg constant. A total of 0.04 mole Ag was
added. An electron micrograph of the resulting hexoctahedral emulsion grains is shown
in Figure 15. The Miller index of the grains was observed to be {521}.
Example 5
[0083] This example illustrates additional growth modifiers capable of producing hexoctahedral
crystal faces and lists potential growth modifiers investigated, but not observed
to produce hexoctahedral crystal faces.
[0084] The grain growth procedures employed were of two different types:
[0085] A. The first grain growth procedure was as follows: To a reaction vessel supplied
with a stirrer was added 0.5 g of bone gelatin dissolved in 28.5 g of water. To this
was added 0.05 mole of silver bromide host grain emulsion of mean grain size 0.8µm,
containing about lOg/Ag mole gelatin, and having a total weight of 21.6 g. The emulsion
was heated to 40°C, and 6.0 millimoles/Ag mole of dissolved growth modifier were added.
The mixture was held for 15 min. at 40°C. The pH was adjusted to 6.0 at 40°C. The
emulsion was then heated to 60°C, and the pAg was adjusted to 8.5 at 60°C with KBr
and maintained at that value during the precipitation. The pH, which shifted to 5.92
at 60°C, was held at that value thereafter. A 2.5M solution of A
GNO
3 and a 2.5M solution of KBr were then introduced with a constant silver addition rate
over a period of 125 min., consuming 0.0625 mole Ag.
[0086] Cubic or octahedral host grains were employed as noted in Table I. Small samples
of emulsion were withdrawn at intervals during the precipitation for electron microscope
examination, any hexoctahedral crystal faces revealed in such samples are reported
in Table I.
[0087] B. The second grain growth procedure employed 7.5 millimoles of a freshly prepared
very fine grain (approximately 0.02 um) AgBr emulsion to which was added 0.09 millimole
of growth modifier. In this process these very fine AgBr grains were dissolved and
reprecipitated onto the host grains. The host grain emulsion contained 0.8 µm AgBr
grains. A 7.5 millimole portion of the host grain emulsion was added to the very fine
grain emulsion. A pH of 6.0 and pAg of 9.3 at 40° C was employed. The mixture was
stirred at 60° C for about 19 hours.
[0088] The crystal faces presented by the host grains are as noted in Table I. Where both
octahedral and cubic host grains are noted using the same growth modifier, a mixture
of 5.0 millimoles cubic grains of 0.8 µm and 2.5 millimoles of octahedral grains of
0.8 µm was employed giving approximately the same number of cubic and octahedral host
grains. In looking at the grains produced by ripening, those produced by ripening
onto the cubic grains were readlly visually distinguished, since they were larger.
Thus, It was possible in one ripening process to determine the crystal faces produced
using both cubic and octahedral host grains.
Comparative Example 6
[0090] The purpose of this comparative example is to report the result of adding 6-nitrobenzimidazole
to a reaction vessel prior to the precipitation of silver bromide, as suggested by
Wulff et al U.S. Patent 1,696,830.
[0091] A reaction vessel. equipped with a stirrer was charged with 0.75 g of deionized bone
gelatin made up to 50 g with water. 6-Nitrobenzimidazole, 16.2 mg (0.3 weight % based
on the Ag used), dissolved in 1mL of methanol, was added, followed by 0.055 mole of
KBr. At 70°C 0.05 mole of a 2M solution of AgN0
3 was added at a uniform rate over a period of 25 min. The grains formed were relatively
thick tablets showing {111} crystal faces. There was no indication of the novel hexoctahedral
crystal faces of the invention.
Comparative Example 7
[0092] The purpose of this comparative example is to report the result of employing 4-hydroxy-6-methyl-1,3,3a,1-tetraazaindene,
sodium salt during grain precipitation, as suggested by Smith Particle Growth and
Suspension, cited above.
[0093] To 100 mL of a 3% bone gelatin solution were added simultaneously 10 mL of 1.96 M
AgNO
3 and 10mL of 1.96 M KBr at 50°C with stirring over a period of about 20 sec. The AgBr
dispersion was aged for 1 min at 50°C, then diluted to 500 mL. The dispersion was
adjusted to pBr 3 with KBr.
Samples 7a. 7b.
[0094] To 80mL of 1X10
-3 M KBr containing 0.4 mmole/ℓ of 4-hydroxy-6-methyl-1,3,3a,7-tetraaza- indene, sodium
salt and 0.6 mmole/ℓ of l-dodecylquinolinium bromide was added 20 mL of the above
dispersion, which was then stirred at 23°C. Samples were removed after 15 min (Sample
7a) and 60 min (Sample 7b).
Samples 7c, 7d
[0095] Samples 7c and 7d were prepared similarly as Samples 7a and 7b, respectively, except
that 0.8 mmole/ℓ of 4-hydroxy-6-methyl-1,3,3a,7-tetraaza- indene and 0.6 mmole/ℓ of
1-dodecylquinolinium bromide were used.
[0096] Examination of the grains of each of the samples revealed rounded cubic grains. No
hexoctahedral crystal faces were observed.
Example 8
[0097] This example illustrates that a hexoctahedral emulsion exhibits an increase in photographic
speed at a given fog level as compared to an octahedral emulsion of the same halide
composition and grain volume.
Example Hexoctahedral Emulsion (A)
[0098] To a reaction vessel supplied with a stirrer was added 0.4 moles of an 0.7µm AgIBr
(6 mole percent I) octahedral emulsion containing =8g bone gelatin/Ag mole. The contents
of the kettle weighed 400g. The emulsion was heated to 40°C, and 6.0 mmoles/Ag mole
of 6-nitrobenzimidazole dissolved in 24 mL methanol was added. The mixture was held
for 15 min at 40°C. The pH was adjusted to 6.0 at 60°C and the pAg adjusted to 8.5
at 60°C with NaBr solution, and maintained at these values during the precipitation.
A 2.5M solution of AgN0
3 and a solution 2.48M in NaBr and 0.5M in NaI were then introduced with a constant
silver addition rate over a period of 145 min, consuming 0.4 moles of Ag. The resulting
emulsion was centrifuged and the solid silver halide phase was resuspended in 250mL
of 3% bone gelatin solution. Electron micrographs of this emulsion showed grains with
distinct hexoctahedral crystal faces had been formed.
Control Emulsion (B)
[0099] This control emulsion was precipitated identically to the above hexoctahedral emulsion,
except the 6-nitro-benzimidazole was added after the precipitation was complete, but
before the centrifugation step. After this compound had been added, the emulsion was
stirred for 15 min at 40°C, then centrifuged. The resulting grains were octahedral
in shape.
Sensitization
[0100] Emulsions A and B were chemically sensitized, as listed below, and then coated on
acetate support at 1.08g Ag/m
2, 4.31g bone gelatin/m
2, 0.81g of a dispersion of the coupler 2-benzamido-5-[2-(4-butanesulfonylamidophenoxy)tetradecanamido]-4-chlorophenol/m
2, 0.14g saponln/m
2 as spreading agent, and 18mg bis(vinylsulfonylmethyl) ether/g gelatin as hardener.
Coating Emulsion
[0101]
1 B heated 10 min at 70°C with 2.4mg/Ag mole sodium thiosulfate & 0.8mg/Ag mole potassium
chloroaurate
2 B heated 10 min at 70°C with 4.8mg/Ag mole sodium thiosulfate & 1.6mg/Ag mole potasium
chloroaurate
3 A heated 10 min at /0°C with 2.4mg/Ag mole sodium thiosulfate & 0.8mg/Ag mole potassium
chloroaurate
[0102] These coatings were exposed for 0.1 s to a 2850°K tungsten light source through a
variable density tablet. These coatings were then processed for 1 min, 2 min, 3 min,
4 min, 5 min, 8 min in Kodak C-41 Color Negative developer at 38°C. The results are
summarized below in Table II.

[0103] From Table II it is apparent that the example emulsion satisfying the requirements
of this invention exhibits higher photographic speeds than the control octahedral
emulsion. Further, this increased speed is realized even when the chemical sensitizers
are doubled in concentration in the control emulsion. Whether compared at the same
development times or at the same fog levels, the example emulsion of the invention
is in all instances superior in photographic performance.
Example 9
[0104] This example illustrates the selective site epitaxial deposition of a silver salt
onto hexoctahedral grains of an emulsion satisfying the requirements of this invention.
[0105] To a reaction vessel supplied with a stirrer was added 0.05 moles of Emulsion A of
Example 8. Distilled water was added to make a total contents weight of 50g. The contents
were heated to 40°C and 0.92 mmole of NaC1 was added. A 0.50M solution of AgNO
3 and a 0.52M solution of NaCl were then introduced with a constant silver addition
rate over a period of 5 min, consuming 1.25 mmoles of silver. During the precipitation,
the pAg was held constant at 7.5 and the temperature held constant at 40°C.
[0106] A 20,000X carbon replica electron micrograph of the resulting emulsion showed discrete
epitaxial growths on the surfaces of the hexoctahedral host emulsion grains The host
grains showed some edge rounding after epitaxy.