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(11) | EP 0 046 053 A2 |
| (12) | EUROPEAN PATENT APPLICATION |
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| (54) | A resistive element composition |
| (57) A resistive element for use in electrical and electronic circuits features a composition
having at least one precious metal-containing pyrochlore of the formula:
wherein A is Pb. Bi or a mixture thereof; B is Ru, Ir or a mixture thereof; wherein x is greater than zero and is approximately equal to or less than 1.0; and wherein y is equal to or greater than zero and is approximately equal to or less than one. |
[0001] The invention relates to electrical resistive elements of new composition which exhibit improved properties.
[0002] The following patents concern pyrochlore materials which can be used in this invention, and their synthesis. These patents are included herein as a background for the present invention:
U.S. Patent Nos. 4,124,539; 4,203,871; 4,129,525; 4,163,706 4,192, 780 and 4,146,458.
[0003] The polynary oxide compounds utilized in this invention display the pyrochlore crystal structure and have a composition that is best described by the following general formula
wnerein A ana B are metal cations. A detailed description of this structure is given-by A. F. Wells, Structural Inorganic Chemistry, Fourth Edition, Clarenden Press, Oxford, (1975), p. 209. Briefly, oxides of this type display a face-centered cubic structure having a unit cell dimension of about .10A. The B cations are octahedrally coordinated by oxygen anions (C). The structural framework is formed by a three-dimensional array of these corner shared octahedra, each sharing corners with six others. This framework has the composition B2O6. As-Wells describes, this framework of octahedra is "based on the diamond net, having large holes which contain the 0' and two A atoms which themselves form a cuprite-like net A2O' interpenetrating the octahedral framework". The octahedra are actually arranged in tetrahedral clusters. These clusters of octahedra are then tetrahedrally arranged so as to form the large holes in the structure described by Wells. Each of these large holes may also be defined by four tetrahedrally arranged puckered, hexagonal rings which are formed by the corner shared octahedra. The A cations reside in the center of these puckered hexagonal rings and are coordinated by the six 0 anions which define the rings plus two more 0' cations at a slightly different distance. These 0' anions reside at the center of the large holes in the octahedral framework. It is the 0' anions which may be partially or totally absent, leading to the general pyrochlore oxide formula A2B2O7-y where 0 ≤ y ≤ 1.
[0004] Some precious metal-containing pyrochlores are known to be useful in resistor element compositions; see for example U.S. Patents 3,583,931; 3,581,262; 3,682,840; 3,896,055; 3,974,107; 3,960,778; and 3,951,672.
[0005] For certain precious metal-containing pyrochlores, it has rather unexpectedly been found that substitution of the cations on the A site such as Pb or Bi for the cations on the B site such as Ru or Ir provides new compositions that are useful as resistive elements-More surprisingly these lead-rich and bismuth-rich pyrochlores have temperature coefficients of resistivity which vary uniformly as a function of the lead or bismuth content and therefore provide an unparalleled method to control the temperature coefficient of resistivity (TCR). Especially important has been the discovery that certain ones of these lead-rich and bismuth-rich pyrochlore compositions have been discovered to further provide a substantially constant resistivity over a wide temperature range. Resistor elements comprising these compositions can function in temperature compensated circuitry.
[0006] In addition, the lead-rich and bismuth-rich pyrochlore compositions of this invention have reduced amounts of precious metal which provides an important economic advantage. A weight reduction of more than 50% for the precious metal content is possible for the resistive compositions of this invention. The particular ease of preparing the inventive compositions as taught in the following U.S. Patents: 4,129,525; 4,146,458; 4,163,706; 4,192,780; and 4,203,871 provide further reliabilities and economics over the prior art. The physical form of the product from this synthesis approach produces a resistive composition in a form which is ideally suited for its dispersion in a non-conductive organic or inorganic binder, which can be essential in the fabrication of resistive elements. Consequently, resistive elements comprising one or more of the lead-rich or bismuth-rich pyrochlore compounds, as set forth by this invention, provide significant advantages over the prior art compositions.
[0007] The invention pertains to a resistive element for use in electrical and electronic circuits. The resistive element comprises a composition having at least one precious metal-containing pyrochlore of the formula:
wherein A is Pb, Bi or amixture thereof; B is
[0008] Ru, Ir or a mixture thereof; wherein x is greater than zero and is approximately equal to or less than 1.0; and wherein y is equal to or greater than zero and is approximately equal to or less than one.
[0009] Of those compositions which are preferred, A is Pb, B is Ru, and x is chosen to have a range of approximately 0.15 to 0.65. More particularly, x is chosen as approximately 0.40 for applications requiring the lowest possible TCR.
[0010] This invention is designed to provide an improved resistive element for use in electrical and electronic circuits; to provide lead-rich or bismuth-rich pyrochlore compositions for electrical and electronic resistive elements, which are more economical than other similar resistive elements; and to provide lead-rich or bismuth-rich pyrochlore compositions which can be easily adjusted to precisely tailor the temperature coefficient of resistivity for use in electrical and electronic circuits.
[0011] The invention will become more apparent and will be better understood with respect to the accompanying drawings in which
Figure 1 is a graph illustrating the resistivity of various lead-rich pyrochlore compositions with respect to temperature;
Figure 2 is a similar graph of resistivity as that shown in Figure 1, for other and similar lead-rich pyrochlore compositions extended over a wider temperature range; and
Figure 3 is a graph depicting the resistivity of various bismuth-rich pyrochlore compositions with respect to temperature.
[0012] Generally-speaking, this invention pertains to lead-rich and bismuth-rich pyrochlore compositions for use in resistive elements of electrical or electronic circuits. As defined herein, "resistive element" can refer to resistive deposits of only a few microns, which can be deposited upon integrated circuit chips or devices, or to a solid bar of resistive material which can be hardwired into a circuit or electrical device.
[0013] The resistive element of the invention comprises a composition of at least one metal-containing pyrochlore of the formula:
wherein A is Pb, Bi or a mixture thereof; B is
[0014] Ru, Ir or a mixture thereof; wherein x is greater than zero, and is approximately equal to or less than 1.0; and wherein y is equal to or greater than zero, and is approximately equal to or less than one.
[0015] Such composition can be made by the low temperature alkaline medium synthesis technique described in U.S. Patent No. 4,129,525.
[0016] The electrical resistivity versus temperature behavior for "expanded" lead ruthenate, Pb2[Ru2-xpbx] 07-y, in a range 0< x < 0.98 is plotted in Figure 1. This data represents the results of four probe measurements on pressed powder samples in which a constant load was maintained on the sample over the entire temperature range. The resistivity measurements were extended to higher temperatures for several compositions in the range 0 < x < 1.06; and the results are shown in Figure 2. Comparison of the results shown in Figures 1 and 2 indicate good reproducibility for samples of the same or similar composition.
[0017] It is evident from these results that the expanded lead ruthenates are high conductivity materials (10-3 ohm-cm < ρ < 10-1 ohm-cm) which exhibit a smooth variation from a positive to a negative TCR as a function of increasing lead contents It is easy to "tailor" the temperature coefficient of resistivity of resistor compositions based on Pb2[Ru2-xPbx]07-y by proper adjustment of Pb content. Moreover a composition with x=0.4 shows a variation of resistivity with temperature which is essentially zero ( Δ ρ/ρ =±1.4% for 80 K<T<400K). It is evident from Figure 3 that the expanded bismuth ruthenates are also high conductivity materials and that they exhibit a gradual increase in a small positive TCR as a function of increasing bismuth content. The capability of simply preparing resistive compositions with a positive, negative or zero TCR by adjustment of the A/B ratio is very advantageous in.the manufacture of temperature compensated electronic circuits. Current practice provides for only limited adjustment of electrical properties of resistor compositions based on precious metal-containing pyrochlores of the type illustrated in U.S. Patent Nos.3,583,931; 3,682,840; and 3,951,672, through chemical substitution of elements which themselves are often precious metals.
1. A resistive element for use in electrical and electronic circuits, comprising a
composition having at least one precious metal-containing pyrochlore of the-formula:
wherein A is Pb, Bi or a mixture thereof; B is Ru, Ir or a mixture thereof; wherein x is greater than zero and is approximately equal to or less than 1.0; and wherein y is equal to or greater than zero and is approximately equal to or less than one.
wherein A is Pb, Bi or a mixture thereof; B is Ru, Ir or a mixture thereof; wherein x is greater than zero and is approximately equal to or less than 1.0; and wherein y is equal to or greater than zero and is approximately equal to or less than one.
2. A resistive element according to claim 1, wherein A is Pb; B is Ru; and x is in
an approximate range of from 0.15 to 0.65.
3. A resistive element according to claim 2, wherein x is approximately 0.4.
4. A resistive element according to claim 1, wherein A is Bi, B is Ru; and x is in
an approximate range of from 0.05 to 0.4.