Superconductor technology: apparatus – material – process – High temperature devices – systems – apparatus – com- ponents,... – Measuring or testing system or device
Reexamination Certificate
1997-11-05
2002-01-08
Utech, Benjamin L. (Department: 1765)
Superconductor technology: apparatus, material, process
High temperature devices, systems, apparatus, com- ponents,...
Measuring or testing system or device
C505S238000
Reexamination Certificate
active
06337991
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Technical Field
The invention relates to resistor materials that are particularly sensitive to changes in temperature and find use in thermistors, bolometers, infrared detectors and the like.
2. Background of the Invention
In many applications, resistors are preferably made from materials whose resistivity is constant with temperature. However, these always exhibit a change in resistance with temperature. Quite often, this change is sensibly linear and is characterized by a sensibly constant temperature coefficient of resistance (TCR) which is numerically equal to the fractional change in resistance, dR, with change in temperature, dT, and given by (1/R) (dR/dT). (TCR is a more useful characteristic than dR/dT because the absolute resistance can be changed by changing the geometry of the resistor.) For resistor applications, some alloys have TCRs in the parts per million per ° C. range. However, other materials have much higher TCRs and can be used as temperature sensors, commonly called thermistors.
Desirable thermistor materials have constant (for ease of calibration) high (for sensitivity) TCRs over a wide operating temperature and reasonable resistivity values. However, some applications preferably have the highest possible TCR at the sacrifice of constancy and operating range. For example, bolometers detect absorbed radiation by producing an electrical signal in response to their increase in temperature. In a typical application, this increase is minute and constancy over a wide temperature range is unimportant. Bolometers may be optimized by maintaining them at a selected constant average ambient temperature and detecting fluctuations from varying radiation.
FIG. 1
is a chart summarizing the TCR and resistivity of some prior art materials and showing one example of the material of this invention in the upper center. A number of materials, thermistor multi-component oxides and GaAs or GaP based semiconductors in bulk form, have very high TCRs but also very high resistivities. These materials rely upon the excitation of electrons across a large thermal barrier for the high TCR. The large thermal barrier generally results in large resistivities which have the potential for non-ohmic contacts, and large 1/f noise levels. Prior art materials with lower resistances all have lower TCRs. Until now, for use in bolometers, for example, the best choice was V0
x
amorphous materials with a typical 0.02/° C. (2%/° C.) TCR.
These problems have received considerable attention for some time, and new materials with a higher TCR are always being sought. Various oxides have been investigated. For example, U.S. Pat. No 4,743,881, issued May 10, 1988 to Howng, discloses La—Cr—O based material, i.e., LaCrO
3
with small amounts of Ti, Si, Mg, and/or Al substituted for Cr. This material is capable of operating in the range of 100 to 600° C. These exhibit a TCR of about 2.5%/° C. However, they are bulk devices made by calcining and sintering the oxides.
Materials with compositions similar to that of this invention are known. La—Mn—O based materials, or more precisely (La
1−x
A
x
)MnO
3
(A=Ca, Sr, Ba, Cd and Pb; 0<x >1), and La—Co—O analogs were synthesized and extensively studied in early 1950s, see, for instance, G. H. Jonker and J. H. Van Santen, Physica, vol. 16 (1950) pp. 337-349 and pp. 599-600 and Physica, vol. 19 (1953) pp. 120-130. Jonker and Santen measured the ferromagnetic properties and conductivity of sintered powders as a function of temperature and composition. Recently, a great deal of interest was rekindled by the discovery of a “giant magnetoresistive effect” in their thin film forms, see, for example, K. Chahara et al. Appl. Phys. Lett., vol 63 (1993) pp. 1990-1993 (reporting on La—Ca—Mn—O ion beam sputtered films on MgO substrates), R. Von Helmolt et al., Phys. Rev. Lett., vol. 71 (1993) pp.2331-2333 (reporting on La—Ca—Mn—O off-axis laser deposited films on SrTiO
3
), and S. Jin et al., Science, vol. 264 (1994) pp.413-415 (reporting on La—Ca—Mn—O films on LaAlO
2
made via laser ablation of powders).
All these powders and films undergo a magnetic phase transition determined by the composition. Below the transition, they behave as metallic conductors but near the transition, the resistance increases sharply and then falls off almost as sharply above the transition temperature where they behave as semiconductors. In the presence of an external magnetic field, the resistance at all temperatures is reduced. For film materials, this decrease can be greater than a thousand-fold on application of a 6T magnetic field. Jin, et al, postulated that such giant magnetoresistance effects should lead to a variety of technical applications, but their results were obtained with transition temperatures of 77° C. U.S. Pat. No. 5,487,356, issued Jan. 30, 1996 to Li (one of the present inventors) et al., incorporated herein by reference, discloses a metal oxide chemical vapor deposition (MOCVD) method of making giant magnetoresistive material of (La
1−x
A
x
)MnO
3
(A=Ca, Sr, Ba, and Mg) with good magnetoresistance effect results at 270 K. However, there are no reports of any investigations of the TCR or use as temperature sensing materials, let alone optimization of the material composition for such use. This may be, in part, because the TCR is highly non-linear and only large within a few degrees of the transition temperature.
Even though it is known that a sharp resistance peak indicates the possibility of a ferromagnetic phase transition, the details are not usually disclosed. An exception is U.S. Pat. No. 5,538,800, issued Jul. 23, 1996 to Li et al., which discloses a polycrystalline material having a very high magnetoresistance ratio of 10,000% in a 6T magnetic field at about 140° K. Although not discussed, the TCR can be deduced from a figure as about 15%/° C., but this is also at about the same low temperature. When making a magnetic field sensor, a high TCR is a disadvantage because the device temperature must be held constant in order to accurately measure the magnetic field effects. A device structure to compensate the TCR was disclosed in U.S. Pat. No. 5,563,331, issued Oct. 8, 1996 to Von Helmolt. Therein, compositions with room temperature TCRs (deduced from figures) in the range of about 2-4%/° C. are illustrated. The invention proposes a method of compensating for the TCR of a magnetoresistive sensor by using two layers of different-composition magnetically sensitive material with a low correlation between their TCRs.
All of these materials are based on a nominal LaMnOz composition with partial substitutions for La and Mn and having a perovskite-like structure. It is known that the perovskite structure is necessary in order to produce a ferromagnetic material. z is nominally 3, but can be in the range of about 2 to 3.5. A decrease from z=3 should occur if, for example, a divalent atom is substituted for the trivalent La. However, there is some uncertainty because some of the Mn atoms, which nominally has a valence of +3, can have a valence of +4 causing an increase in z or a valence of +2 causing a decrease in z. Moreover, there are usually oxygen deficiencies. It is known that in order to exhibit a ferromagnetic effect, there must be some Mn, about 30%, with a +4 valence. For instance, a material with an exact composition LaMnO3 is not ferromagnetic. Since considerable effort is required to determine the exact value of z and the amount of Mn with different valences for each composition, those skilled in the art understand that z has a range of values and often characterize the materials using nominal compositions in conjunction with physical properties.
One application where high a TCR is most useful is in bolometers where high sensitivity is desirable. U.S. Pat. No. 5,450,053, issued Sep. 12, 1995 to Wood, discloses a monolithic integrated focal plane sensitive to both mm-waves (typically 94 GHz) and IR radiation (typically 3-5 and 8-12 micron) constructed on a silicon wafer by selecti
Li Yi-Qun
Zhao Jing
Anderson Matthew
Corning Applied Technologies Corp.
Hamilton, Brook, Smith & Rehnolds, PC
Utech Benjamin L.
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