Germanium resistance thermometers

Very recently, Bailey and Richards (23) have shown that a high degree of sensitivity for adsorbed species can be achieved by measuring the absorption of infrared radiation on a thin sample cooled to liquid helium temperature. The optical arrangement used in these studies is shown in Figure 10. The modulated beam produced by the interferometer is introduced into the UHV sample chamber and reflected off a thin slice of monocrystalline alumina covered on one side by a 1000 k film of nickel or copper. Radiation absorbed by the sample is detected by a doped germanium resistance thermometer. The minimum absorbed power detected by this device when operated at liquid helium temperature is 5 x 10 14 W for a 1 Hz band width. With this sensitivity absorbtivities of 10"4 could be measured. 

In this section, the design and operation of familiar liquid thermometers, thermocouples, platinum resistance thermometers, thermistors, and optical pyrometers are discussed in detail. Briefer descriptions are also given of a variety of special thermometric devices such as quartz thermometers, germanium resistance thermometers, and sihcon-diode thermometers. 

Germanium. Of the three semiconductors mentioned, germanium has received by far the most attention, and germanium resistance thermometers are available from several commercial sources. The resistance element is usually a small single crystal. Inasmuch as the resistivity is high, the element can be short and thick. It is mounted strain-free in a protective capsule (see Fig. 8.26). Because of this combination of features, the germanium thermometer can be both rugged and reproducible. 

Fig. 8.26. Construction of three commercially available germanium resistance thermometers. (Figure courtesy of Lakeshore Cryotronics, Westerville, OH.)...

Preferences vary as to the exact type, size, and resistance of CRTs. Historically, the important brands have been Allen-Bradley, Ohmite, and Speer (more properly Airco Speer). Manufacture of the Airco Speer grade 1002-1/2 W resistors was discontinued in 1979. The Allen-Bradley radio resistors in sizes from 0.1 to 1 W, and resistance values from 10 to 500 Q, remain the most popular choices above 1 K, and are used occasionally below 1 K. The Speer radio resistors in various sizes and resistances are usually selected for the region below 1 K and may prove useful to as low as 0.02 K. A typical 0.5-W, 220-Q resistor will measure roughly 1 kQ at 1 K, 20 kQ at 0.1 K, and 300 kQ at 0.015 K, the last with about 10 W dissipation. Usually 0.1-1-W resistors with room-temperature resistances of 10-500 Q are used. The temperature dependence of the resistance for three types of carbon resistance thermometers is compared with that of two types of germanium resistance thermometers in Fig. 8.27. 

Carbon thermometers are generally used for low-temperature measurements (T< 80 K) when accuracies of +0.1 K or +1 % of the absolute temperature are needed. Millidegree accuracy is attainable using germanium resistance thermometers at temperatures below 20 K. The primary drawback to germanium thermometers is that no simple analytical representation is available that represents the resistance versus temperature characteristics, even for a given class of doped germanium crystals. A many-point comparison calibration is required if all the inherent stability of the resistor is to be utilized. 

Figure 12.9 Resistance of three representative germanium resistance thermometers versus temperature.

Temperature The level of the temperature measurement (4 K, 20 K, 77 K, or higher) is the first issue to be considered. The second issue is the range needed (e.g., a few degrees around 90 K or 1 to 400 K). If the temperature level is that of air separation or liquefact-ing of natural gas (LNG), then the favorite choice is the platinum resistance thermometer (PRT). Platinum, as with all pure metals, has an electrical resistance that goes to zero as the absolute temperature decreases to zero. Accordingly, the lower useful limit of platinum is about 20 K, or liquid hydrogen temperatures. Below 20 K, semiconductor thermometers (germanium-, carbon-, or silicon-based) are preferred. Semiconductors have just the opposite resistance-temperature dependence of metals—their resistance increases as the temperature is lowered, as fewer valence electrons can be promoted into the conduction band at lower temperatures. Thus, semiconductors are usually chosen for temperatures from about 1 to 20 K. 

NTD (neutron transmutation doped) germanium chips are used as resistive thermometers in low-temperature detectors/calorimeters. 

The bolometer is another type of thermal detector that can offer extreme sensitivity for specialized applications, t This is essentially a resistance thermometer, usually with a platinum, nickel, carbon, or germanium element, although a semiconductor thermistor can also be used. Typically, two elements are used in a bridge circuit with one exposed to radiation and the other kept dark as a reference. The germanium bolometer provides exceptional... 

Another type of resistance thermometer uses metal oxides, instead of metals it is frequently referred to as a thermistor. Electrical resistance of these metal oxides changes rapidly with even rather small temperature changes. Hence, thermistors are often emplyed to measure small temperature changes such as 1°C to 5 °C. The thermistor proper tends to have low purchase prices. Metal oxides, which are semiconductors, include mixtures of the following oxides nickel, manganese, copper, cobalt, tin, germanium, etc. [1]. 

A bolometer is a type of resistance thermometer constructed of strips of metals, such as platinum or nickel, or of a semiconductor. Semiconductor bolometers are often called thermistors. These materials exhibit a relatively large change in resistance as a function of temperature, The responsive element is kept small and blackened to absorb radiant heat. Bolometers are not so extensively used as other infrared transducers for the mid-infrared region, Flowever, a germanium bolometer. operated at 1..3 K, is nearly an ideal transducer for radiation in the. 5 to 400 cm (2000 to 2.3 pm) range. 

There are three main families of temperature sensors thermocouples, resistance thermometers and thermistors. Some old reactors still have bimetalic thermometers (binary or with local readings), but it is suggested that they be replaced in order to allow the operator to follow closer any temperature transient from the control room. Other temperature sensors, like semi-conductor thermometers, consisting of doped germanium sensors, have a complex resistance temperature relationship and are useful only for very low temperature measurements. 

Fig. 9.5. The electrical resistance R(T) for some typical thermometers. A-B denotes Allen-Bradley carbon resistor. Speer is a carbon resistor. CG is carbon-in-glass. CX 1050 is a Cernox and RX 202A is a ruthenium oxide from LakeShore. Ge 100 and Ge 1000 are Cryocal germanium thermometers [45]. 

As was shown in Figure 3.159, cryogenic temperatures can be detected by integrated circuit diodes types K, T, and E thermocouples (TCs) class A and B resistance temperature detectors (RTDs) acoustic and ultrasonic thermometers germanium and carbon resistors and paramagnetic salts. As TCs and RTDs will be discussed in separate subsections, here the focus will be on the other sensors. 

Resistance versus temperature characteristics for the blends show a trend similar to that of a germanium thermometer (GRT) in the temperature range between 0.35 K and 10 K. The data were fitted using Chebychev polynomials [105]. Sensitivity of the blends is high (better than 0.1 mK) at temperatures below about 1 K, because of the higher resistance. In the temperature range between 2 K and 50 K, the sensitivity is about 1.0 mK. Monotonic trend in MR, where... 

Several years ago investigations of carbon and (As-doped) germanium resistors for use as precise secondary thermometers indicated appreciable effects of stray rf fields upon the measured dc resistance. The rf pickup was eliminated by shielding the dc measuring apparatus with a double wall screen cage. Recently, Corruccini [3] has found that the resistance change due to rf pickup is caused by ac joule heating. 

Specific resistivity p and critical temperature Tc were measured by the four-probe method. For the Tc value the point in the middle of the curve of transition to the superconducting state was taken. As a temperature sensor in the cryogenic zone a germanium thermometer of the VG-type was used. The values of the lower Hci and the upper Hc2 of the critical magnetic field at the temperature of 4.2 K were determined from differential curves of magnetization similarly to [8]. 

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