Optimization of a Graphite Tube Blackbody Heater for a Thermogage Furnace

Design modifications are presented for a 289-mm long, 25.4-mm inner diameter blackbody heater element of a 48 kW Thermogage blackbody furnace, based on (i) cutting a small “heater zone” into the ends of the tube and (ii) using a mixture of He and Ar or N₂ to “tune” the heat losses and, hence, gradients in the furnace. A simple numerical model for the heater tube is used to model and optimize these design changes, and experimental measurements of the modified temperature profile are presented. The convenience of the Thermogage graphite-tube furnace, commonly used in many NMIs as a blackbody source for radiation–thermometer calibration and as a spectral irradiance standard, is limited by its effective emissivity, typically between 99.5% and 99.9%. The design simplicity of the furnace is that the blackbody cavity, heater, and electrical and mechanical connections are achieved through a single piece of machined graphite. As the heater also performs a mechanical function, the required material thickness leads to significant axial heat flux and resulting temperature gradients. For operation at a single temperature, changes to the tube profile could be used to optimize the gradient. However, it is desired to use the furnace over a wide temperature range (1,000–2,900°C), and the temperature-dependence of the electrical conductivity and thermal conductivity, and that of the insulation, makes this approach much more complex; for example, insulation losses are proportional to T ⁴, whereas conduction losses are proportional to T. In the results presented here, a slightly thinner graphite region near each end of the tube was used to “inject heat” to compensate for the axial conduction losses, and the depth, width, and position of this region was adjusted to achieve a compromise in performance over a wide temperature range. To assist with this optimization, the insulation purging gas was changed from N₂ to He at the lower temperatures to change the thermal conductivity of the felt insulation, and the effectiveness of this approach has been experimentally confirmed.     

Heat generation properties of Ga doped ZnO thin films prepared by rf-magnetron sputtering for transparent heaters

Ga doped ZnO (GZO) thin films were prepared by rf-magnetron sputtering on glass substrate for window heater applications. Electrical and optical properties of these films were analyzed in order to investigate on substrate temperature and rf power dependencies. High quality GZO films with a resistivity of 1.30 × 10^− 4 Ω cm and a transparency above 90% in the visible range were able to be formed. GZO films have been patterned on glass substrate as a line heater. This GZO line heater showed the rapid heat radiation property from room temperature to 90 °C for 22 s at the applied voltage of 42 V. These results could provide a possibility to use GZO as effective transparent heaters.     

Nanokelvin thermometry at temperatures near 2 K

We present a method of temperature measurement based on a liquid³He vapor pressure thermometer with a resolution of one part in 10⁹ of the absolute temperature over the range from 1.6 to 2.2 K. The thermometer, as well as apparatus suitable for the assessment of the resolution and stability of the device, are described in detail. A method for the determination of a fixed point on the temperature scale with a resolution of 2×10^?9 is presented. Two different procedures for monitoring the long-term stability of the thermometer are discussed. The present resolution and stability of the thermometer are an improvement by two orders of magnitude over conventional germanium resistance thermometry. Although this performance level is adequate for presently planned phase transition experiments using liquid⁴He, future improvements by yet another order of magnitude seem feasible and will bring the device within an order of magnitude of the thermal noise limit.     

Realization of a 3He�C4He Vapor-Pressure Thermometer for Temperatures Between 0.65?K and 5?K at LNE-CNAM

In the temperature range between 0.65 K and 5 K, the International Temperature Scale of 1990 (ITS-90) is based on ³He and ⁴He vapor-pressure thermometers. Between 0.65 K and 1 K, the ITS-90 overlaps with the Provisional Low Temperature Scale of 2000 (PLTS-2000), defined in term of the melting pressure of ³He. Some differences, up to more than 1 mK, exist between the two scales in the overlapping interval. The LNE-CNAM has recently started the construction of a ³He?C⁴He vapor-pressure thermometer to realize the ITS-90 in its lowest subrange at the highest degree of accuracy. The device is provided with two separate vapor-pressure chambers, one for ³He and the other for ⁴He, built in a single copper block, and is installed in the experimental space of a dilution refrigerator. The vapor-pressure thermometer is designed to accommodate on the same copper block several transfer standards, an acoustic thermometer, and the ³He melting-pressure thermometer. This configuration is intended for realizing calibrations of transfer standards down to 0.65 K, for investigating the possibility to extend the acoustic thermometer below 4 K, and to perform a direct comparison between the ITS-90 and the PLTS-2000 in the overlapping temperature range, in order to study their differences. The realization of the system has been recently accomplished, and this report illustrates the characteristics of such an experimental device.     

The fluctuation-imposed limit for temperature measurement

In experimental sciences, random processes often place a fundamental limit on the achievable measurement resolution. A well known example is the Johnson noise voltage across a resistor. In this paper, we describe observations of the spontaneous transfer of heat in two equilibrium systems: one consisting of a thermometer linked to a reservoir, the other consisting of two thermometers connected to each other and linked to a reservoir for the purpose of temperature stabilization. In the second system, we find anti-correlations between the temperature fluctuations of the two thermometers at intermediate frequencies. We also find that the low frequency temperature noise density of the thermometers, in units of , is given by , whereR is the thermal resistance of the link between the thermometer and the reservoir. This implies that for noise reduction purposes,R is the only available engineering parameter to adjust. In a recent thermometer design, we have reduced R to achieve a low frequency temperature noise density of 5×10⁻¹¹ at 2.18 K.     

Some characteristics of the CROG thermometer

Summary The results imply the following conclusions on the practical utilization of the thermometer, and on the characteristics of the thermometer: Temperatures measured by the CROG thermometer lie within the range from 3.7° to 3.1°K when the supply current ranges from 120 to 990 mA; no changes were observed in the sensitivity of the CROG thermometer over a year's time; the sensitivity of the CROG thermometer over the 3.7° to 3.1°K temperature range is respectively 6·10³ to 5·10² Hz · °K^–1; the error in measurements of the absolute values of the temperature on the T₅₈ scale may be small, and can attain values of 10^–4 to 10^–5 °K.Translated from Izmeritel'naya Tekhnika, No. 9, pp. 30–31, September, 1972.     

A High-Resolution Thermometer for the Range 1.6 to 5 K

This paper presents a detailed design, a theoretical analysis, and experimental tests of a high-resolution thermometer for use in the temperature range from 1.6 to 5 K. The device uses a dc-SQUID magnetometer to determine the change in magnetization with temperature of a paramagnetic salt in a magnetic field. The field is provided by a small permanent magnet attached to the thermometer. Measurements of the sensitivity of the device agree well with the theoretical analysis. Near 2.17 K (the superfluid transition of ⁴ He at saturated vapor pressure) the thermometer has a specific sensitivity of 4000 ₀ /K Gauss. There it achieves a temperature resolution better than 10 ^–9 K when it is charged with a field of about 300 Gauss. At 4.2 K, the specific sensitivity is smaller by a factor of 50, but should still allow temperature measurements with a resolution better than 10 ^–7 K. Near 2.17 K, drifts of the device are below the level of 10 ^–13 K/s. The thermometer has a small mass of about 7 g (excluding the magnet), and thus the advantage of relatively small cosmic radiation heating during microgravity experiments in Earth orbit.     

Measurement of Thermal Conductivity of Anisotropic SiC Crystal

Silicon carbide (SiC) crystals with excellent heat conduction and thermal stability can be widely used in microelectronic devices and integrated circuits. It is important for the study of a functional type SiC material to have accurate thermal-conductivity and thermal-diffusivity values of SiC crystal. A 3ω technique is employed to determine the anisotropic thermal conductivity of SiC crystal. Three micrometal probes with different widths are deposited by chemical-vapor deposition on the surface of SiC crystal. Each micrometal probe is used as a heater, and also as a thermometer. The temperature fluctuation signals of a micrometal probe represent heat conduction in different directions in the specimen. Thermal conductivities both in the cross-plane and in-plane directions of SiC crystal are achieved through fitted values. The results indicate that thermal conductivities in three different directions of SiC crystal can be characterized using the metal heater construction.     

The Bayreuth nuclear demagnetization refrigerator

The design, construction, and performance of a Cu nuclear refrigerator are reported. The first nuclear stage (total 275 moles Cu with 104 moles Cu in 8 T) has refrigerated a¹⁹⁵Pt NMR thermometer in the low-field experimental region to 15 µK; it can keep experiments below 20 µK for more than 1 week. It can alternatively precool a second nuclear stage (2 moles Cu in 9 T). Demagnetizing this stage has resulted in a temperature of at most 12 µK as measured by another¹⁹⁵Pt NMR thermometer attached to the stage. The details of the thermometry are described and possible origins of the observed internal heat leaks as well as unexpected contributions to the specific heat of the nuclear stages are discussed.     

Kapton Capacitance Thermometry at Low Temperatures and in High Magnetic Fields

A sensitive Kapton foil capacitance sensor, with size of 9.5 mm×4.5 mm, has been developed and used as a thermometer at ultra-low temperatures down to 1.2 mK and in high magnetic fields. There is no visible magnetic field dependence up to 15 T. The sensor was calibrated with ³He melting pressure thermometer (MPT) and vibrating wire (VW) viscometer. With the silver powder sintered heat exchanger sandwich-like design, the thermal relaxation time is as short as a few minutes at the base temperature. The low temperature (below 1.2 K) reproducibility has been tested and is satisfied within experimental errors.     

A planar Bi-Sb multijunction thermal converter with small ac-dctransfer differences

A planar Bi-Sb multijunction thermal converter was fabricated on the LPCVD Si₃N₄/SiO₂/Si₃N ₄-diaphragm, prepared by silicon bulk micromachining, which thermally isolated a bifilar Pt- or NiCr-heater and the hot junctions of a Bi-Sb thermopile from the silicon substrate. The voltage responsivity, the ac-dc transfer difference, and the fluctuations of the output thermoelectric voltage and heater resistance were discussed to investigate the design factors of a thermal converter. The respective voltage responsivities in air and in a vacuum of the thermal converter with a built-in NiCr-heater were about 14.0 mV/mW and 54.0 mV/mW. The ac-dc voltage and the current transfer differences in air were about ±0.60 ppm and ±0.11 ppm in the dc reversing frequency range from 10 Hz to 10 kHz. They are sufficiently small to be used as practical ac standards. Compared to the thermal converter with a built-in Pt-heater, the thermal converter with a built-in NiCr-heater demonstrated a higher voltage responsivity and smaller ac-dc transfer differences, while exhibiting slightly larger fluctuations in output thermoelectric voltage and in heater resistance     

Construction and use of a pulsed copper nuclear magnetic resonance thermometer

A pulsed nuclear magnetic resonance thermometer, constructed for temperature measurements below 0.5 K, is described. The nuclear free precession signal of copper nuclei is recorded with a gated low noise amplifier and with a phase sensitive detector gated to integrate the signal over an adjustable number of periods. The physical significance of the signal is discussed. A comparison of the resonance thermometer against a slurry type cerium magnesium nitrate (CMN) thermometer for the temperature region 10 mK to 100 mK is presented. The deviation Δ = T_NMR − T_CMN, representing the departure of the CMN thermometer from a Curie law behaviour, was measured as (0.5 ± 0.2) mK. In experiments with a nuclear refrigeration cryostat the resonance thermometer was calibrated against a nuclear orientation thermometer which provided an independent absolute temperature standard. At the low temperature end, from 1 mK to 10 mK, the linearity of the thermometer in T⁻¹ was confirmed by measurements of the nuclear spin-lattice relaxation time.     

An intercomparison of a Cu nuclear susceptibility thermometer and an Sn99 mösbauer effect thermometer between 6 mK and 1 K

The cold-absorber Sn¹¹⁹ Mössbauer effect thermometer discussed by Parshin et al, has been compared with the steady-field Cu nuclear magnetic susceptibility thermometer. The Curie constant of the Cu thermometer was also determined from simultaneous measurements using a ballistic CMN thermometer. The sensors were installed in the mixing chamber of a dilution refrigerator capable of producing temperatures below 6 mK in the single cycle mode. The absorber never cooled below 7.8 mK due to heat leaks. Good consistency and reproducibility were established between 10 and 100 mK.     

3He constant-volume gas thermometry: Calculations for a temperature scale between 0.8 and 25 K

A discussion is presented on the possibilities of a³He gas thermometer for defining a temperature scale below 30 K, based on recent new measurements of the virial coefficient. The influence of all corrections of interest is given in comparison with⁴He gas thermometry and with⁴He and³He vapor pressure thermometry. It is shown that a³He gas thermometer can be operated down to temperatures <1 K, with an estimated inaccuracy of less than ±0.5 mK, thereby obviating the explicit need of the³He and⁴He vapor pressure scales below 5 K, and directly joining a possible scale based on the³He melting curve.     

3He melting pressure thermometry

High-precision measurements of the³He melting pressure versus temperature have been made from 500 K to 25 mK using a⁶⁰Co nuclear orientation primary thermometer and a Pt NMR susceptibility secondary thermometer. Temperatures for the fixed points on the melting curve are: the superfluid A transition T_A = 2.505 mK, the A-B transition T_AB = 1.948 mK, and the solid ordering temperature t_n = 0.934 mK. These fixed points and a functional form for P(T) constitute a convenient temperature scale, based on a primary thermometer, usable to well below 1 mK.     

Tin,a candidate for low temperature NMR thermometer?

The nuclear spin-lattice relaxation times for the tin isotopes Sn¹¹⁷ and Sn¹¹⁹ have been measured in the temperature range between 20 and 50 mK with a SQUID magnetometer by watching the longitudinal magnetization of the nuclear spins after the application of a rf pulse. The temperature was determined using a SQUID noise thermometer. The Korringa constants were estimated from the τ₁ values. The experimental device and the principal methods are described and the results and thermometer applications discussed.     

Capabilities of a New Pressure Controller for Gas-Controlled Heat Pipes

Pressure control is used in many metrological applications and for the control of thermodynamic quantities. At the Italian National Research Institute of Metrology (INRiM), a new pressure controller has been designed and assembled, operating in the pressure range between 4 kPa and 400 kPa. This innovative instrument uses a commercial pressure transducer with a sensitivity of 10⁻⁴ and several electro-valves interposed among calibrated volumes of different dimensions in order to realize known ratios for very fine pressure changes. The device is provided with several circuits to drive the electro-valve actions, for signal processing and transmission, and for both manual and automatic control. Input/output peripherals, such as a 4 × 20 dot matrix display and a 4 × 4 keyboard, allow setting of the parameters and data visualization, while a remote control port allows interfacing with a computer. The operating principle of this pressure controller has been recently applied, with excellent results, to control the pressure in gas-controlled heat pipes by using a standard platinum resistance thermometer as a temperature/pressure sensor, achieving in this case a relative sensitivity better than 10⁻⁶ in pressure. Several tests were also made to control the pressure by means of a commercial sensor. The device, its main components, and its capabilities are here reported, together with application tests and results.     

A simplified time-domain design and implementation of cascaded PI-sliding mode controller for dc–dc converters used in off-grid photovoltaic applications with field test results

A time-domain design methodology for voltage regulation control of dc–dc boost and buck-boost converters based on a multi-loop controller with PI regulator for the outer loop and an inner loop with sliding mode current controller has been developed for renewable energy applications such as photovoltaic (PV)-fed dc–dc converters. This paper proposes a new method for the design of PI regulators in such multi-loop control scheme. The proposed design presents a simple analytical method for selecting controller gains and has been validated by simulation as well as hardware implementation. Also, this paper presents an illustrative example based on the proposed design for the voltage regulation control of PV-fed boost converters for off-grid applications. The simulation results for varying irradiation, temperature and load along with stability analysis have been presented in this paper. The proposed controller is implemented in hardware for a 1.1 kW PV-array-fed boost converter. Performance analysis based on field test results using real-time weather data validates the proposed design. Therefore the proposed controller could be considered as an attractive solution for off-grid renewable energy applications like PV- or fuel-cell-fed dc–dc converter, where the variations are stochastic in nature.     

Measurements of absolute temperature below 0.75 K using a Josephson-junction noise thermometer

In order to extend the international temperature scale of 1990, ITS-90, below its lower limit of 0.65 K, we have developed a temperature scale ranging from 6 to 750 mK. Values of absolute temperature are defined on this scale by an R-SQUID noise thermometer. A review is given here of the decade of our experience in the operation of this thermometer and in modeling its systematic errors. The reproducibility of noise temperature values was assessed using superconductive fixed points and the melting curve of³He. To assess the accuracy of the noise thermometer, it was compared with another absolute thermometer (based on nuclear orientation) at the lowest temperatures and with an internationally recognized scale above 0.5 K. The noise thermometer temperatures were used to calibrate two paramagnetic salt thermometers, the result being the construction of a temperature scale that is smooth to approximately 0.01%. On the basis of these comparisons, temperature values defined by the R-SQUID are deemed to be reproducible to within 0.1% and accurate to within 0.3%.Retired.     

3He melting pressure temperature scale below 25 mK

Using⁶⁰Co -ray anisotropy radiation as a primary thermometer, with a Pt NMR susceptibility secondary thermometer, we have made high-precision measurements of the³He melting pressure versus temperature from 500 K to 25 mK. Temperatures obtained for the fixed points on the melting curve are: the superfluid A transition T_A = 2.505mK, the A-B transition T_AB = 1.948 mK, and the solid ordering temperature T_N = 0.934 mK. We provide a functional form for P(T), which, with the fixed points, constitutes a convenient temperature scale, based on a primary thermometer, usable to well below 1 mK.