[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 tons 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 cuble 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-octahadral-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 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 Crystal- lograph
y, 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. Photog. 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] 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.
[0022] This object is achieved by providing a silver halide photographic emulsion having
radiation sensitive silver halide grains of a cubic crystal lattice structure comprised
of tetrahexahedral crystal faces.
[0023] The invention presents to the art for the first time the opportunity to realize the
unique surface configuration of tetrahexahedral 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
[0024]
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 (210) tetrahexahedron;
Figures 10 and 11 are schematic diagrams of the atomic arrangement at silver bromide
tetrahexahedral crystal surfaces havlng Miller indices of (210) and (410), respectively;
and
Figures 12 through 17 are electron micrographs of tetrahexahedral silver halide grains.
[0025] The present invention relates to silver halide photographic emulsions comprised of
radiation sensitive silver halide grains of a cubic crystal lattice structure comprised
of tetrahexahedral crystal faces and to photographic elements containing these emulsions.
[0026] In one form the silver halide grains can take the form of regular tetrahexahedra.
A regular tetrahexahedron 11 is shown in Figures 8 and 9, which are front and back
views of the same regular tetrahexahedron. A tetrahexahedron has twenty-four identical
faces. Although any grouping of faces is entirely arbitrary, the tetrahexahedron can
be visualized as six separate clusters of crystal faces, each cluster containing four
separate faces. In Figure 8 faces 12a, 12b, 12c, and 12d can be visualized as members
of a first cluster of faces. A second cluster of faces is represented by faces 13a,
13b, and 13c. The fourth face of the cluster, 13d, is shown in Figure 9. Faces 14a
and 14b, shown in Figure 8, and faces 14c and 14d, shown in Figure 9, represent the
four faces of a third cluster of four faces. Similarly, faces 15a and 15b, shown in
Figure 8, and faces 15c and 15d, shown in Figure 9, represent the four faces of a
fourth cluster of four faces. Faces 16a, 16b, and 16c, shown in Figure 8, and face
16d, shown in Figure 9, complete a fifth cluster of faces. Faces 17a, 17b, 17c, and
17d in Figure 9 complete the sixth cluster of faces.
[0027] Looking at the tetrahexahedron it can be seen that there are four intersections of
adjacent faces within each cluster, and there is one face intersection of each cluster
with each of the four clusters adjacent to it for a total of thirty-six face edge
intersections. The relative angles formed by intersecting faces have only two different
values. All intersections of a face from one cluster with a face from another cluster
are identical, forming a first relative angle. Looking at Figure 8, the relative angle
of adjacent faces 12a and 14a, 12b and 13b, 12c and 15a, and 12d and 16d are all at
the identical first relative angle. All adjacent faces within each cluster intersect
at the same relative angle, which Is different from the relative angle of intersection
of faces in different clusters. Looking at one cluster In which all faces are fully
visible, the intersections between faces 12a and 12b, 12b and 12c, 12c and 12d, and
12d and 12a are all at the same relative angle, referred to as a second relative angle.
While the regular tetrahexahedron has a distinctive appearance that can be recognized
by visual inspection, it should be appreciated that measurement of any one of the
two relative angles provides a corroboration of adjacent tetrahexahedral crystal faces.
[0028] In crystallography measurement of relative angles of adjacent crystal faces ie 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 tetrahexahedral
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 on 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 tetrahexahedral
crystal faces can be identified. Relative angles of tetrahexahedral 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 tetrahexahedral
crystal faces.
[0029] 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 (1.10) 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.
[0030] Tetrahexahedral crystal faces include a family of crystal faces that can have differing
Miller index values. Tetrahexahedral crystal faces are generically designated as {hx0}
crystal faces, wherein h and k are different integers each greater than 0, which is
zero and not to be confused with the letter 0. The regular tetrahexahedron 11 shown
In Figures 8 and 9 consists of (210) crystal faces, which corresponds to the lowest
value that h, k, and 0 can each represent. A regular tetrahexahedron having {310},
{320}, (410), {430}, (510), {520},
{530}, or {540} crystal faces would appear similar to the tetrahexahedron 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 and k, tetrahexahedral
crystal faces having a value of h or k of 5 or less are more easily generated. For
this reason, silver halide grains having tetrahexahedral crystal faces of the exemplary
Miller index values identified above are preferred. With practice one tetrahexahedral
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 tetrahexahedral crystal faces present.
[0031] In one form the emulsions of this invention contain silver halide grains which are
bounded entirely by tetrahexahedral crystal faces, thereby forming basically regular
tetrahexahedra. In practice although some edge rounding of the grains is usually present,
the unrounded residual flat tetrahexahedral faces permit positive identification,
since a sharp intersecting edge is unnecessary to establishing the relative angle
of adjacent tetrahexahedral crystal faces. Sighting to orient the grains is still
possible employing the residual flat crystal face portions.
[0032] The radiation sensitive silver halide grains present in the emulsions of this invention
are not confined to those in which the tetrahexahedral 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
tetrahexahedral 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 tetrahexahedral 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 tetrahexahedral crystal face
formation results in both tetrahexahedral crystal faces and residual crystal faces
corresponding to those of the original host grain being present.
[0033] In another variant form deposition of silver halide onto host grains under conditions
which favor tetrahexahedral 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 tetrahexahedral crystal faces on host grains initially
presenting {100} crystal faces have four surface faces. These correspond to the four
faces of any one of the 12, 13, 14, 15, 16, or 17 series clusters described above
in connection with the tetrahexahedron 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, 12b,
13a, 13b, 14a, and 14b. 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 tetrahexahedral crystal faces, the protrusions
become progressively larger and eventually the grains lose their ruffled appearance
as they present larger and larger tetrahexahedral crystal faces. It is possible to
grow a regular tetrahexahedron from a ruffled grain by continuing silver halide deposition.
[0034] Even when the grains are not ruffled and bounded entirely by tetrahexahedral crystal
faces, the grains can take overall shapes differing from regular tetrahexahedrons.
This can result, for example, from Irregularities, such as twin planes, present in
the host grains prior to growth of the tetrahexahedral crystal faces or introduced
during growth of the hexoctahedral crystal faces.
[0035] The important feature to note is that if any crystal face of a silver halide grain
is a tetrahexahedral 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 (210) tetrahexahedral 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 (210) tetrahexahedral 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 the result of the tiering
that occurs at the (210) tetrahexahedral crystal face. Tetrahexahedral crystal faces
with differing Miller indices also exhibit tiering. The differing Miller indices result
in analogous, but nevertheless unique surface arrangements of silver and halide ions.
The difference between tetrahexahedral crystal faces of differing Miller indices is
illustrated by comparing Figure 10, which is a hypothetical schematic diagram of a
(210) crystal face, and Figure 11, which is a corresponding diagram of a (410) crystal
face.
[0036] While Figures 2, 4, 6, 10, and 11 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 tetrahexahedral
crystal surface presented by silver chloride Ions would be similar to the corresponding
silver and bromide ion surfaces.
[0037] The cubic crystal lattice structure silver halide grains containing tetrahexahedral
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.
[0038] It is appreciated that the larger the proportion of the total silver halide grain
surface area accounted for by tetrahexahedral crystal faces the more distinctive the
silver halide grains become. In most instances the tetrahexahedral crystal faces account
for at least 50 percent of the total surface area of the silver halide grains. Where
the grains are regular, the tetrahexahedral 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 tetrahexahedral crystal
faces accounting for at least 90 percent of the total grain surface area are contemplated.
[0039] It is, however, appreciated that distinctive photographic effects may be realized
even when the tetrahexahedral 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 tetrahexahedral crystal face,
only a limited percentage of the total grain surface may be required to produce a
distinctive photographic effect. Generally, if any tetrahexahedral 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 tetrahexahedral 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 tetrahexahedral
crystal faces ts limited only by the observer's ability to detect the presence of
tetrahexahedral crystal faces.
[0040] The successful formation of tetrahexahedral 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 tetrahexahedral 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-octahedr.al 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.
[0041] By analogy, grains having tetrahexahedral 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 tetrahexahedral crystal faces. As silver halide precipitation
continues tetrahexahedral 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 tetrahexahedral 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.
[0042] Failure of the art to observe tetrahexahedral 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 tetrahexahedral crystal faces.
It has been discovered that growth modifiers can be employed to retard silver halide
deposition selectively at tetrahexahedral crystal faces, thereby producing these tetrahexahedral
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 tetrahexahedral
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 tetrahexahedral
crystal faces are described in the examples, below.
[0043] 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 tetrahexahedral 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 tetrahexahedral crystal
faces with compounds shown to be effective as growth modifiers for producing tetrahexahedral
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 tetrahexahedral crystal
faces, routine empirical studies systematically varying parameters are likely to lead
to additional useful preparation techniques.
[0044] Once silver halide grain growth conditions are satisfied that selectively retard
silver halide deposition at tetrahexahedral crystal faces, continued grain growth
usually results in tetrahexahedral 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 tetrahexahedral crystal faces. For example, silver halide grains
having tetrahexahedral 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 tetrahexahedral crystal face grain surface are considered
within the scope of this invention, in most applications the grains having at least
one identifiable tetrahexahedral 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.
[0045] 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.
[0046] For example, when a growth modifier is present adsorbed to the tetrahexahedral 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 tetrahexahedral crystal face it must exhibit an adsorption preference for
the tetrahexahedral 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 tetrahexahedral
crystal faces. If the growth modifier shows an adsorption preference for the tetrahexahedral
crystal faces over the cubic crystal faces, deposition of silver and halide ions onto
the tetrahexahedral 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 tetrahexahedral crystal faces. From the foregoing it is apparent that
growth modifiers which produce tetrahexahedral 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.
[0047] 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 vm 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.
[0048] 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 sensitizatlon. At the same
time, adsorption of dye on the grains intended to scatter imaging radiation reduces
their scattering efficiency.
[0049] 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
tetrahexahedral crystal faces. When radiation sensitive silver halide grains having
tetrahexahedral crystal faces and a growth modifier spectral sensitizing dye adsorbed
to the tetrahexahedral crystal faces are substituted for the spectrally sensitized
silver halide grains employed by Locker, the disadvantageous migration of dye from
the tetrahexahedral crystal faces to the silver halide grains intended to scatter
light is reduced or eliminated. Thus, an improvement in photographic efficiency can
be realized.
[0050] 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.
[0051] 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 tetrahexahedral crystal faces. Where the growth modifier
is capable of spectral sensitlzation, such as the dyes shown to be effective growth
modifiers in the examples, more effective spectral sensitization 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 unlquely matched to the tetrahexahedral
crystal surface, distinct advantages in photographic performance over that which can
be realized with silver halide grains of differing crystal faces are possible.
[0052] The silver halide grains having tetrahexahedral 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 1918, 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 tetrahexahedral 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.
[0053] The growth modifier used to form the tetrahexahedral crystal faces of the silver
halide grains can be retained in the emulsion, adsorbed to the grain faces, displaced
from the grain faces or destroyed entirely. 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., nttric, hydrochloric, or sulfuric acid.
[0054] 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 11643, cited above, Section II.
[0055] 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 tetrahexahedral 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.
[0056] 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 176.43, cited
above, Section IV.
[0057] 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 derivatives, collodion,
agar-agar, arrowroot, and albumin. It is specifically contemplated to employ hydrophilic
colloids which contain a low proportion 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.
[0058] 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.
[0059] 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 mlcrovessels or microcells,
as described in Whitmore U.S. Patent 4,387,154.
[0060] 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 tetrahexahedral
crystal faces as previously described.
[0061] 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.
[0062] 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.
[0063] 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
[0064] 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
[0065] This example illustrates the preparation in the presence of ammonia of a tetrahexahedral
silver bromide emulsion having the Miller index (210), beginning with a cubic host
emulsion.
[0066] To a reaction vessel supplied with a stirrer was added 0.5 g. of deionized 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 of gelatin and having
a total weight of 21.6 g. The emulsion was heated to 40°C, and 3.0 millimoles/Ag mole
of Dye I dissolved in 2.5 mL. water was added. The mixture was held at 40°C for 15
min.
[0067] Just prior to beginning the precipitation, 3.4 millimoles of an aqueous (NH
4)
2SO
4 solution (1.0 mL) was added, followed by 25.9 millimoles of ammonium hydroxide (1.75
mL) and 0.25 millimoles of KBr solution (0.5 mL). The pAg was measured as 9.3 at 40°C,
and was maintained through the precipitation. At 40°C a 2.5M solution of AgNO
3 was added at a constant flow rate along with a 2.5M solution of KBr, which was added
as necessary to maintain the pAg. The precipitation consumed 0.0175 mole Ag in 35
min. The pH was then slowly adjusted to 5.5.
[0068] A carbon replica electron micrograph (Figure 12) shows Emulsion 1 to have tetrahexahedral
faces. The Miller index of the tetrahexahedral faces was determined by measurement
of the relative angle between two adjacent tetrahexahedral crystal faces. From thls
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
tetrahexahedral crystal faces was identified by comparison of this angle φ with the
theoretical intersecting angle 6 between [h
1k
10
1] and [h
2k
20
2] vectors. The angle 6 was calculated as described by Phillips, cited above, at pages
218 and 219.
[0069] 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. Comparison of φ with 6 enabled the crystal faces to be assigned. If the experimentally
determined angle was nearly mld-way between two theoretical angles, the one associated
with the lower Miller index was used for the assignment. The results for Emulsion
1 were as follows. The number of measurements made Is given in parentheses. Theoretical
values for vectors up to (540) were considered.
[0070] Emulsion 1 is thus composed of regular tetrahexahedra showing (210) faces.
Example 2
[0071] This example illustrates the preparation under non-ammoniacal conditions of a silver
bromide emulsion having grains with Miller index (410) crystal faces beginning with
a cubic host emulsion.
[0072] To a reaction vessel supplied with a stirrer was added 0.05 mole of the same host
emulsion as used in Example 1 (about 10 g/Ag mole gelatin) made up to 50 g. with water.
To this was added 2 millimoles/Ag mole of Dye II dissolved in 2 ml. of N,N-dimethylformamide.
[0073] The mixture was held at 40°C for 15 min. The pH was adjusted to 6.0 at 40°C, and
the emulsion was heated to 60°C. The pAg was adjusted to 8.5 at 60°C with KBr and
maintained at that value during the precipitation. A 2.0 M solution of AgNO
3 and a 2.0 M solution of KBr were then simultaneously added over a period of 50 min.
The AgN0
3 solution was added at a constant rate and 0.01 moles Ag were added.
[0074] Figure 13 is an electron micrograph of the resulting emulsion, showing the crystals
to have a regular tetrahexahedral habit. The Miller index, determined as described
for Example 1, was found to be (410).
Example 3
[0075] This example illustrates the preparation of another tetrahexahedral emulsion having
a {410} Miller index, but using a different growth modifier. The emulsion was prepared
as described for Example 2, but for Dye II was substituted Dye III, 4 millimoles/Ag
mole, dissolved in 3 mL. water. The precipitation was carried out for 50 min. at a
rate consuming 0.02 mole Ag.
[0076] Figure 14 is an electron micrograph of Emulsion 3 showing the crystals to have a
regular tetrahexahedral habit. The Miller index was determined as described in Example
1 and found to be {410}.
Example 4
[0077] Example 4 illustrates the preparation of a silver bromide tetrahexahedral emulsion
having a (410) Miller index by the Ostwald ripening of a Lippmann emulsion onto a
mixture of cubic and octahedral host grains in the presence of a growth modifier.
[0078] To a reaction vessel was added 32.5 g. (7.5 millimole) of a freshly prepared AgBr
Lippmann emulsion of mean grain size 0.02µm and containing 167 g/Ag mole of gelatin.
At 35°C, 0.09 millimole of Dye IV Ln 2 mL of methanol containing 2 drops of triethylamine
was added.
Then 3.0 ml, 7.5 millimoles of AgBr consisting of a mixture of two emulsions containing
approximately equal numbers of cubes (0.8µm mean size; 10 g/Ag mole gelatin) and octahedra
(0.8µm mean size; 10 g/Ag mole gelatin) was added. The pH was adjusted to 6.0 at 40°C,
and the pAg to 9.3 with KBr solution. The mixture was then heated to 60°C and stirred
at that temperature for 19 hrs.
[0079] Figure 15 is an electron micrograph of the resulting emulsion, showing the crystals
to have a regular tetrahexahedral habit. The Miller index was determined to be (410).
Example 5
[0080] Example 5 illustrates the preparation of a silver bromide tetrahexahedral emulsion
by Ostwald ripening, but using Dye III instead of Dye IV as growth modifier.
[0081] The emulsion of Example 5 was prepared as described for Example 4, but using as growth
modifier 0.09 millimole of Dye III dissolved in 3 mL of methanol, 1 mL of N,N-dimethylformamide,
and 2 drops of triethylamine. An electron micrograph of the resulting emulsion is
shown in Figure 16. The habit is a regular tetrahexahedron, with (410) faces.
Example 6
[0082] This example illustrates the preparation of a tetrahexadral silver chloride emulsion
having the Miller index {410}.
[0083] 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
milllmole/Ag mole of Dye III dissolved in 2 mL of water. 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 this value during the precipitation by NaOH addition.
The pAg was adjusted to 7.9 at 50°C with NaCl solution and maintained during the precipitation.
A 1.5M solution of AgNO
3 was introduced at a constant rate over a period of 500 min., while a 2.7M solution
of NaCl was added as needed to hold the pAg constant. A total of 0.075 mole Ag was
added.
[0084] An electron micrograph of the resulting tetrahexahedral emulsion grains is shown
in Figure 17. The habit was a regular tetrahexahedron, and the Miller index was determined
to be {410}.
Example 7
[0085] This example illustrates additional growth modifiers capable of producing tetrahexahedral
crystal faces and lists potential growth modifiers investigated, but not observed
to produce tetrahexahedral crystal faces.
[0086] The grain growth procedures employed were of three different types:
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 AgNO3 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.
B. The second grain growth procedure was as follows: To a reaction vessel supplied
with a stirrer was added 27.5 mL of water. To this was added 0.05 mole of a silver
bromide host grain emulsion of mean grain size 0.8 µm, containing about 10 g/Ag mole
of gelatin and having a total weight of 21.6 g. The emulsion was heated to 40"C, and
3.0 millimole/initial Ag mole of dissolved growth modifier was added. The mixture
was held at 40°C for 15 min. Just prior to beginning the precipitation 3.4 millimoles
of an aqueous (NH4)2SO4 solution (1.0 mL), containing also 0.25 millimole of KBr, was added, followed by
25.9 millimoles of ammonium hydroxide (2.0 mL). The pAg was measured as 9.3 at 40°C
and was maintained at that level throughout the precipitation. At 40°C a 2.5M solution
of AgN03 was added at a constant flow rate along with a 2.5M solution of KBr, the latter being
added at the rate necessary to maintain the pAg. The precipitation consumed 0.05 mole
Ag over a period of 100 min. The pH was then slowly adjusted to 5.5.
[0087] In the first and second procedures 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 tetrahexahedral crystal
faces revealed in such samples are reported in Table I.
C. The third 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 readily 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.
Example 8
[0090] This example illustrates the modification of a growth modifier used to prepare an
emulsion according to the invention containing grains with tetrahexahedral crystal
faces.
[0091] Two emulsions according to the invention containing grains with tetrahexahedral crystal
faces were prepared using preparation procedures similar to that described in Example
1. Emulsion A consisted of pure silver bromide tetrahexahedral grains while Emulsion
B consisted of silver bromoiodide (2.5 mole percent iodide) tetrahexahedral grains.
[0092] Both Emulsions A and B were pink in color, the color being attributable to Dye I
(see Example 1) employed as a growth modifier during their preparation. To each emulsion
bromine water was added with stirring. With the addition of the bromine water the
pink color completely disappeared, leaving only a yellow color expected for the emulsions
absent the presence of a spectral sensitizing dye.
[0093] Beyond illustrating how a growth modifier can be effectively destroyed within an
emulsion according to the invention after its preparation, the example more specifically
illustrates that spectral sensitizing dye employed as a growth modifier can be destroyed
after emulsion preparation, if desired. By destuction of the spectral sensitizer,
the emulsion is placed in a form in which it retains only its native spectral sensitivity,
as is desirable for many known photographic applications. Alternatively, once the
spectral sensitizing dye employed as a growth modifier has been effectively destroyed,
another spectral sensitizing dye can be adsorbed to the grain surfaces.