Note: Descriptions are shown in the official language in which they were submitted.
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PRECISION RESISTORS USING AMORPHOUS ALLOYS
Background of the Invention
1. Field of the Invention
The invention is concerned with precision resistors and,
more particularly, with precision resistors utilizing amorphous
metal alloys as resistance elements.
2. Description of the Prior Art
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Resistors for electrical instruments can be divided `
into two arbitrary classifications based on permissible error~
those employed in precision instrumentation in which the overall
error is considerably less than 1% and those employed where less
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precision is needed. This application is devoted to resistors
falliny in the former classification. ~ -
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Precision resistors include resistance elements, which
in turn are composed of materials having many requirements that
must be satisfied for optimum performance. Advantageously, the ;~
resistivity should be high in ordex to minimize the size of the ~ -
resistor. Typical resistance material for use in precision re-
sistors requires a resistivity between 50 and ~30 michrohm-cm.
Resistance material must have a small or negligible thermoelectric
potential against copper because copper is usually the connecting
material. Preferred thermal emf values are about 1 microvolt/ C.
Assuming a wire-wound resistor configuration, coefficients of
expansion of both the resistance element and the insulator on
which it is wound must be considered, because stresses can be
established that will cause changes in both the resistance and ~ ;
the temperature coefficient of resistance. Preferred values of
thermal expansion of the resistance element are approximately
13 microinches/C~ The stability or the maintenance of nominal
resistance within narrow limits over a long period of time is
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perhaps the most important requirement for ultraprecision resistors.
Finally, the temperature coefficient for precision resistors should
be low in order to avoid temperature effects on resistance. The
ideal material for precision resistors should have a zero or near
zero temperature coefficient for the specific working range.
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Investigations have revealed a number of polycrystalline
metal alloys suitable for use as resistance material. NevertheleSs,
new compositions are con-tinually sought in which the foregoing
properties are improved.
The physical properties of amorphous metal alloys have
also been the subject of investigation. Studies have recently ~ ~ ;
disclosed compositions such as IFe,Ni)75P15 C10 and Ni41Pd41B18
having high resistivities; see, e.g., Vol. 42A, PhysLcs Letters,
pp. 407-409 (1973) and Vol. 7, Physical Review B, pp. 3215-3225
(1973). However, these compositions evidence an unacceptably ;
high temperature coefficient of resistivity for useful precision
resistor applications.
Summary of the Invention
In accordance with the invention, precision resistors
utilize certain amorphous metal alloys as resistance elements.
The metal alloys are at least 50~ amorphous, as determined by
X-ray diffraction, and preferably at least 80~ amorphous, and
more preferably, at least 95% amorphous. The amorphous metal
alloys typically consist essentially of about 70 to 87 atom
percent of at least one transition metal element and about 13
to 30 atom percent of at least one of the metalloid elements
of aluminum, antimony, beryllium, boron, germanium, carbon,
indium, phosphorus, silicon and tin. Preferably, the composi- ;
tion can be represenced as MaXb, where M is at least one of the
elements of iron, nickel, cobalt, chromium and vanadium, X is ~ ;
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is at least one of the elements of phosphorus and boron, "a" ranges ~i
from about 75 to 85 atom percent and "b" ranges from about 15 to 25 .
atom
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percent. Used as precision resistance elements, these amorphous ~--
metal alloys evidence generally superior properties, as compared ~ ~
with well-known polycrystalline metal alloys utilized in the prior ;-
art.
Detailed Description of the Invention
A theory has not yet been developed to correlate many
macroscopic physical properties of polycrystalline metal alloys ;~
and of amorphous metal alloys having substantially the same
composition. Many of the physical properties of previously dis-
closed amorphous metal alloys tend to change at elevated tempera-
tures. In contrast to this, however, a class of amorphous metal ~
alloys, whose compositions are given below, exhibit the high resis- ~ -
tivity and, unexpectedly, the low temperature coefficient of resis-
tivity, high stability of resistance and other desirable properties
required for use in precision resistors.
Amorphous metal alloys utilized in the invention consist
essentially of about 70 to 87 atom percent of at least one transi-
tion metal element and about 13 to 30 atom percent of at least
one of the metalloid elements of aluminum, antimony, beryllium, ;
boron, germanium, carbon, indium, phosphorus, silicon and tin.
Transition metal elements are those listed in Groups IB to VIIB
and VIII of the Periodic Table. More specifically, the composi
tion may be represented by the formula MaXb, where M is at least ~`
one of the elements of iron, nickel, cobalt, chromium and vanadium, ;~;
X is at lea~t one of the elements of phosphorus and boron, "a" ~-
ranges from about 75 to 85 atom percent, and "b" ranges from ~;
about 15 to 25 atom percent. Typical compositions include Fe40-
40 14B6~ Fe32Ni36crl4pl2B6, Fe25Ni2sC20CrlOB20~ Fe40Nil5C5M ~
Crl7B18l and FessNigCo5Cr1sB17. The purity of all elements
described is that found in normal commercial practice.
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The amorphous metal alloys are formed by cooling a melt ~
at a rate of about 105 to 106C/ sec~ These amorphous ~lloys axe ~ ~;
usually at least 50% amorphous, as measured by X-ray di~fraction, ~- -
when processed in this manner and may be utilized in some applica-
tions. It is preferred, however, that the amorphous alloy be at
least 80% amorphous, and more preferably, at least 95% amorphous
to realize maximal performance as resistance elements in precision
resistors. ~;
A variety of well-known techniques are available for
10 fabricating splat-quenched foils and rapid-quenched continuous -
ribbon, wire, sheet, etc. of amorphous metal alloys. Typically,
when used in precision resistance applications, these alloys
conveniently take the form of wire or ribbon. The wire and
ribbon are conveniently prepared by casting molten material
; directly onto a chill surface or into a quenching medium of some
sort. Such processing techniquesconsiderably reduce the cost of ~ ;
fabrication, since no intermediate wire-drawing procedures are ~.
required. The alloys may also take the form of vapor deposited
films, such as by electron beam evaporation or vacuum sputtering
and of films formed by plasma spraying.
In the prior art, stress relief processing is required
in order to fabricate resistance elements. In constrast, further
processing steps for stress relief are not required to fabricate
precision resistors utilizing amorphous metal alloy materials.
These amorphous metal alloys also evidence high mechani~
cal strength. The tensile strength typically is about 30n,000 to
600,000 psi, as compared with polycrystalline alloys, which usually
range from about 30,000 to 200,000 psi. For example, the poly-
crystalline composition Ni76Crl7Si4Mn3 has a tensile strength
30 ranging from 175,000 to 200,000 psi.
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The resistivity of -the amorphous metal alloys is on
~he order of about 170 microhm-cm, as compared with the best
available material having a resistivity of 136 microhm-cm
(Fe72cr23Alscoo.s)~ This means that smaller dimensions can be
utilized to achieve the same resistance value where amorphous
metal alloy composltions are involved.
A typical temperature coefficient of resistance for
precision resistors is about ~10 ppm/C over the range of 20 to ;
45C (Cu85Mnl0Ni4). Over the temperature range of 20 to 100C,
the temperature coefficient of a good precision resistor is
+150 ppm/C. In contrast, the same value for some amorphous metal
alloy compositions is about 2 ppm/C over the range of 25 to 200C.
Perhaps the greatest advantage that the amorphous metal alloys
offers is the temperature range over which the change in the
temperature coefficient of resistance is small.
The thermal emf against copper is measured to be about
+3.0 microvolts/C over the range of 25C to 200C. This compares
favorably with prior art materials such as Ni60Crl6Fe24 (~0.8 micro-
volts/C over the range of 0 to 75C) and Ni35Cr20Fe45 (-3 micro-
volts/C over the range of 0 to 100C). Likewise, the thermal
expansion of amorphous metal alloy compositions is about 13 micro- ~;~
inches/C, again comparable to the best prior art alloys.
Precision resistors employing amorphous metal alloys
may advantageously take the form of either fixed or variable
resistors.
Examples `
1. Resistance measurements were made on several
amorphous specimens, as indicated below, employing the four-point ;~
probe method as described by G. T. Meaden, in Electrical Resistance
30 of Metals, Plenum Press (N.Y.), 1965, pp. 147 et seq. In each
measurement, the two ends of a ribbon of material about 1 meter
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in length were clamped with copper leads and connected in series
to an ammeter, a current source, and a variable resistor of about
2 x 103 ohms. A few inches in from each end, brass springs were
clamped and a connection was made to a potentiometer. The resis-
tance of the ribbons was about 50 ohms. By using this technique,
contact and lead resistances have no effect on the measurement.
Resistivity (p) was calculated from the relationship
p = AR/Q
where R is the measured resistance, A is the cross-sectional area
1~ of the ribbon and "Q" is its length.
Measurements of temperature coefficient of resistance ~,
(TC) were made between 25C and 200C, employing the relationship ~ ;
TC = (R-Ro)/~O(t-to)~
where R is the resistance at tC and Ro is the resistance at the
reference temperature llto~ (here, 25C). A four-point probe
employing a digital ammeter was used.
A determination of isothermal aging effects on electri- ;
cal resistance was made at 150C, using a two-point probe and a
digital ohmeter. This measurement indicates thermal stability.
The results for three examples of amorphous metal alloys
are tabulated in the Table below. Sample 1 had a composition of
Fe40Mi40P14B6 (the subscripts are in atom percent). Measurements
were made on ribbon of Sample 1 having dimensions 0.0015 inch
thick by 0.061 inch~wide. Sample 2 had a composition Fe32Ni36Cr
P12B6. Measurements were made on ribbon of Sample 2 having
dimensions 0.0016 inch thick by 0.0675 inch wide. Sample 3 had
p on Fe25Ni25C20CrlOB20- MeaSurements were made on
ribbon of Sample 3 having dimensions 0.0018 inch thick by 0.039
inch wide.
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TABLE
Physical Properties Measured for Amorphous
Metal Alloys in Precision Resistor Applicat~ons
Temperature
Resistivity Temperature Coefficient Stability at
_mple p, microhm-cm of Resistance, ppm/C 150C,%/1000 hr. ;
1 183 -32 0,3
2 176 +145 (25-125C) 0.3
3 170 ~2.0 0.4
The thermal emf of Sample 3 was measured against copper
and was found to be 3.0 microvolts/C over the range of 25 to
200C and 2.7 microvolts/C over the range of 25 to 100C. The
thermal expansion of this material was found to be 12.6 micro-
inch/C.
2. A variable resistor was constructed, using D-wire
of compositlon Fe40Ni40P14B6 as the resistance element and a
conventional graphite slider contact. The D-wire, which in
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cross-section is half an ellipse, had dimensions as follows: the
20- major axis was 0.033 inch and one-half the minor axis was 0.010
lnch. The D-wire was wound on a polymethyl methacrylate core
having dimensions of 2.5 inch diameter and 10.0 inch length.
Copper leads were clamped to both ends of the D-wire. The active
length of the resistance element was 301.6 inch. The measured
resistance was 400.42 ohms. This value remained constant over
100 operations of the slider contact. The value remained stable
after nearly 4000 hours. There was no visible sign of wear of the
resistance element.
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