Radiation Thermocouples Research Articles

Responsivity and detectivity modelling of thermal radiation sensors based on a biased thermocouple

Thermal radiation sensors are based on two signal transduction stages: radiation to thermal and thermal to electrical. The most common of these sensors are the radiation thermocouple using the Seebeck effect and the bolometer applying the thermoresistive effect. While the bolometer requires a bias current for signal generation the thermocouple is generally operated unbiased. The paper theoretically investigates a biased thermocouple instead, which can be thought of as a combination of both thermal radiation sensor types. Its responsivity and detectivity are calculated based on previous theories of the performance of bolometers and radiation thermocouples, respectively, thereby including the Peltier effect. The electrical resistance and thermal conductance of the thermocouple as input parameters for these calculations are modelled using a simple strip geometry to facilitate one-dimensional analytical electrothermal modelling.

Fluctuation phenomena in instrument science

Definitions are presented of instrument science, and of fundamental and semi-fundamental fluctuations in physical quantities. Reference is made to the significance of instrument science both in pure science and in practical applications. Sources of fundamental fluctuations are reviewed, together with the corresponding formulae for the amount of fluctuation. The significance of energy stores, and of means of dissipation, in thermal fluctuations are contrasted. The theory is extended to thermal and radiation fluctuations, using the concept of transduction impedance in passive transducers. Particular reference is made to the application of the theory to radiation thermocouples and to seismometers.

Radiation Thermocouples with (Bi, Sb)2(Te, Se)3

New semiconducting thermoelectric materials were developed to improve the performance of radiation thermocouples. They were Bi1.8Sb0.2Te2.7Se0.3+0.08 wt.% CuBr for n-type and Bi1.4Sb0.6Te3 for p-type. Radiation thermocouples were made with combinations of two types of the new materials as well as those already known by welding them on receivers having an area of 2 mm×0.2 mm. The radiation thermocouple with Bi1.8Sb0.2Te2.7Se0.3+0.08 wt.% CuBr and Bi1.4Sb0.6Te3 showed the best characteristics with NEP (Noise Equivalnet power) of 2×10-11 Watt, responsivity of 30 µV/µW, internal resistance of 8 Ω and time constant of 20 milliseconds. The NEP decreased as the figure of merit of the thermocouple increased. It was found that contact noise was generated at the welded part of the receiver. This noise was noticeable when the area of the welded part became smaller than about 10-6 cm2, and set the lower limit of NEP.

Solid-Backed Evaporated Thermopile Radiation Detectors

Modern space-borne infrared systems require uncooled detectors sensitive to long wavelength radiation from 8 μ to 40 μ. The thermal detectors, such as thermistor and metal film bolometers, and radiation thermocouples are most practical. It is shown that the thermocouple detector has certain characteristics which make it particularly attractive for space-borne use. Chief among these is the fact that it has inherent dc stability and therefore does not require optical modulation. This permits systems to be designed with no moving parts. A technique is described for the construction of rugged thermocouple and thermopile detectors by vacuum deposition of evaporated materials through masks onto a solid substrate. This technique permits quantity production of complex detector element arrays that can survive and operate in severe military and space environments. A brief theoretical analysis of the performance to be expected from the new design is given and the properties of several configurations which have been fabricated are described. A discussion is included of some application considerations for use of these detectors.

The construction of radiation thermocouples using semi-conducting thermoelectric materials

An investigation into the methods of construction of radiation thermocouples using semiconducting thermoelectric materials is described. This work has been based on the description given in patent applications by E. Schwarz. Performance data for these thermocouples is included together with that obtained for certain commercial couples made by Schwarz and measured by the authors.

Dynamic Impedance and Sensitivity of Radiation Thermocouples

The concept of dynamic impedance is applied to radiation thermocouples. An equivalent circuit is derived, in terms of which the working of a thermocouple and the factors leading to high sensitivity can be visualized. Methods are given for measuring the dynamic impedance and it is shown that these measurements lead to values for thermo-electric power, heat loss from the receiver, and time constant of the thermocouple. Expressions are given, in terms of the dynamic impedance, for the important properties of radiation thermocouples, namely the signal-to-noise sensitivity, ultimate sensitivity, noise factor and power efficiency. These properties are thus obtained in terms of parameters that can be predicted from the design, or measured in the finished instrument. The sensitivity is found without recourse to micro-measurements and therefore independent of limitations set by the thermocouple amplifier. The expression for sensitivity differs from those previously published, and the discrepancy is discussed. The way in which the ultimate sensitivity might be approached in practice is indicated. The efficiency is shown to depend on the amount of radiation falling on the receiver of the thermocouple, but to be a constant fraction of the thermodynamic limit ΔT/T. For an ideal thermocouple the fraction is one-half, for an actual thermocouple it is less by a factor which we call the relative efficiency.

Effect of Temperature on the Steady-State Sensitivity of Vacuum Radiation Detectors

The steady-state sensitivities of two vacuum radiation thermocouples of very different design and a vacuum compensated bolometer have been measured at 298°, 205°, and 96°K, and the effect of temperature has been found to vary widely for the three detectors. The observed results have been accounted for quantitatively in terms of theoretical sensitivity relations which are based on exact heat flow theory and take into account the specific structural characteristics of the detectors. The treatment is sufficiently general to permit the prediction of steady-state sensitivities as a function of temperature for any vacuum radiation thermocouple or bolometer whose structural characteristics are known.

TELLURIUM-BISMUTH VACUUM RADIATION THERMOCOUPLE

A simple method of constructing delicate radiation thermocouples with gold leaf receivers is described in detail. A tellurium wire ¼ mm long could be easily welded to the receiver by using a condenser discharge. The thermoelectric power of tellurium in its β-modification against bismuth is about 600 μv/°C. The thermocouples are about three times more sensitive than those made with bismuth alloy wires. Some methods for evacuating and permanently evacuating the thermocouples are given.