Low temperature radiative properties of materials used in cryogenics

Radiative heat transfer between two parallel surfaces, a sample surface and a black surface, was measured. One of the surfaces was cooled with liquid helium to about 5 K and the other one was step by step heated to temperatures ranging between 30 and 140 K. As a result, the total hemispherical absorptivity and emissivity of the sample surface were determined in dependence on the temperature of the heat radiation. Aluminium samples were made of Al sheet, Al foil and aluminized mylar. Further measurements were performed on sheets of aluminium alloy, Cu, zinc brass and stainless steel. The influence of different types of sample treatment such as chemical and mechanical surface finishing and material annealing on the radiative properties is presented.     

Effect of different treatments of copper surface on its total hemispherical absorptivity bellow 77 K

Total hemispherical absorptivity of copper surfaces treated with standard industrial methods was measured in dependence on the temperature of thermal radiation, varying from 25 K to 300 K. The sample temperature was typically from 5 K to 40 K and did not exceed 70 K. Usability of chemical and mechanical Cu surface finishing as well as Cu plating with Ni and Au for cryogenic design is discussed. As an example of practical application of our results, the cryogenic design of a LN₂ trap is presented.     

Heat capacity measurements on a thin ribbon sample of Zr0.55Al0.10Ni0.05Cu0.30 glassy alloy and Apiezon N high vacuum grease using a Quantum Design Physical Property Measurement System

We measured the heat capacity of a thin ribbon sample of Zr_0.55Al_0.10Ni_0.05Cu_0.30 glassy alloy using a Heat Capacity option of Quantum Design’s Physical Property Measurement System (PPMS) between 1.9 K and 310 K. The cut ribbon pieces were piled up to form a block, where each ribbon layer was adhered to one another with Apiezon N grease (ANG), and the heat capacity of the block was measured. In order to obtain the heat capacity of the sample, the heat capacity of ANG was subtracted from the block heat capacity. We report how we measured the heat capacity of a thin ribbon sample, and in this connection, we also reviewed the ANG heat capacity previously reported.     

20 K Energy storage unit

A low temperature energy storage unit able to store 36 J between 11 K and 20 K is described. Thanks to an enthalpy reservoir connected to the cold finger of a cryocooler by a heat switch, this device allows measurements in a completely vibration-free environment during a long time after the cryocooler stop. The ESU described in this article uses lead as enthalpy reservoir (9 cm³) and a H₂ gas gap heat switch. Tests varying the initial temperature and the input power were performed. A test showed that a platform can be stabilized at 19 K during more than 1 h using this ESU. The temperature drifts measured after stopping the cryocooler are compared to simulations. A very good agreement is obtained allowing scaling up devices if longer periods are required.     

Thermal characterisation of a tungsten magnetoresistive heat switch

A magnetoresistive heat switch has been developed to improve the performance of our flight-worthy cryogen-free ADR. We have characterised the switch’s thermal conductivity in the temperature range 0.3-4 K under an applied magnetic field of 1.8 T for two tungsten samples of different purity. The results are discussed relating to the key aspects of semi-classical magnetoresistance theory. We show that crystal purity has a strong effect on switch performance and magnetoresistive effect. Our findings are verified by comparison to results obtained by other authors. The measured switching ratio for our best sample is 1.75 × 10⁴ at 1.5 K and 1.51 × 10⁴ at 4.26 K. The lattice conductivity remains dominated by the electronic conductivity in the investigated range of temperatures under an applied magnetic field of 1.8 T. In order for the lattice conductivity to dominate a purity of >99.999% would be required.     

Recommendations for accurate heat capacity measurements using a Quantum Design physical property measurement system

A commercial instrument for determination of heat capacities of solids from ca. 400 K to 0.4 K, the physical property measurement system from Quantum Design, has been used to determine the heat capacities of a standard samples (sapphire [single crystal] and copper). We extend previous tests of the PPMS in three important ways: to temperatures as low as 0.4 K; to samples with poor thermal conductivity; to compare uncertainty with accuracy. We find that the accuracy of heat capacity determinations can be within 1% for 5 K < T < 300 K and 5% for 0.7 K < T < 5 K. Careful attention should be paid to the relative uncertainty for each data point, as determined from multiple measurements. While we have found that it is possible in some circumstances to obtain excellent results by measurement of samples that contribute more than ca. 1/3 to the total heat capacity, there is no “ideal” sample mass and sample geometry also is an important consideration. In fact, our studies of pressed pellets of zirconium tungstate, a poor thermal conductor, show that several samples of different masses should be determined for the highest degree of certainty.     

A vibration free cryostat using pulse tube cryocooler

This paper introduces a new vibration free cryostat cooled by liquid helium and a 4 K pulse tube cryocooler. The cryogenic device mounts on the sample cooling station which is cooled by liquid helium. The boil off helium is recondensed by the pulse tube cryocooler, thus the cryostat maintains zero boil off. There is no mechanical contact between the cryogenic part of the cryocooler and the sample cooling station. A bellows is used to isolate the vibration which could transfer from the cryocooler flange to the cryostat flange at the room temperature. Any vibrations generated by the operation of the cryocooler are almost entirely isolated from the cryogenic device. The cryostat provides a cooling capacity of 0.65 W at 4.21 K on the sample cooling station while maintaining a vapor pressure of 102 kPa. The sample cooling station has a very stable temperature with oscillations of less than ±3 mK during all the operations. A cryogenic microwave oscillator has been successfully cooled and operated with the cryostat.     

Specific heat, electrical resistivity and thermoelectric power of YbNi4Si

The studies of the specific heat, electrical resistivity and thermoelectric power of YbNi₄Si are reported. These studies are supported by magnetic susceptibility and X-ray photoemission spectroscopy (XPS) measurements. YbNi₄Si does not order magnetically down to 4 K. Nearly in the whole temperature range studied the magnetic susceptibility follows a Curie law with μ_eff = 4.15 μ_B/f.u. This effective magnetic moment is close to the value expected for the 4f¹³ configuration (4.54 μ_B). The Yb²⁺ and Yb³⁺ peaks observed by XPS in the valence band region confirm the domination of the Yb³⁺ valence state. Based on the specific heat measurements, the electronic specific heat coefficient γ = 25 mJ/mol/K² and the Debye temperature θ_D = 320 K were derived. A quadratic dependence of electrical resistivity at low temperatures has been observed. The Kadowaki-Woods ratio has been discussed. The thermoelectric power has been analyzed in the framework of the two band model.     

Thermochemical studies on SrFe12O19(s)

The citrate-nitrate gel combustion route was used to prepare SrFe₁₂O₁₉(s) powder sample and the compound was characterized by X-ray diffraction analysis. A solid-state electrochemical cell of the type: (−)Pt, O₂(g)/{CaO(s) + CaF₂(s)}//CaF₂(s)//{SrFe₁₂O₁₉(s) + SrF₂(s) + Fe₂O₃(s)}/O₂(g), Pt(+) was used for the measurement of emf as a function of temperature from 984 to 1151 K. The standard molar Gibbs energy of formation of SrFe₁₂O₁₉(s) was calculated as a function of temperature from the emf data and is given by: (SrFe₁₂O₁₉, s, T)/kJ mol⁻¹ (±1.3) = −5453.5 + 1.5267 × (T/K). Standard molar heat capacity of SrFe₁₂O₁₉(s) was determined in two different temperature ranges 130-325 K and 310-820 K using a heat flux type differential scanning calorimeter (DSC). A heat capacity anomaly was observed at 732 K, which has been attributed to the magnetic order-disorder transition from ferrimagnetic state to paramagnetic state. The standard molar enthalpy of formation, (298.15 K) and the standard molar entropy, (298.15 K) of SrFe₁₂O₁₉(s) were calculated by second law method and the values are −5545.2 kJ mol⁻¹ and 633.1 J K⁻¹ mol⁻¹, respectively.     

Thermal conductivity measurements of pitch-bonded graphites at millikelvin temperatures: Finding a replacement for AGOT graphite

Pitch-bonded graphites are among the best known thermal insulators at sub-kelvin temperatures, but are very good conductors at higher temperatures. This makes them ideal for mechanical supports which must provide good thermal isolation at an operating temperature below 1 K, but must have good conductance at higher temperatures to aid in initially cooling down an instrument (a “passive heat switch”). One type of graphite, AGOT, has been known as having the lowest thermal conductivity below 1 K not only among graphites, but also compared with any other material. It is, however, no longer available. We have carried out thermal conductivity measurements at temperatures between 60 mK and 4 K on a proposed replacement, POCO AXM-5Q graphite, as well as a sample of AGOT graphite. Our measurements show that both graphites have a difference of about six orders of magnitude in conductivity between room temperature and 100 mK, but that AGOT graphite is not as good an insulator as previously believed. We conclude that AXM-5Q graphite is not only a suitable replacement for AGOT, but in fact is somewhat superior.     

Physicochemical properties and theoretical explanation of ZnCd(SCN)4 crystal

The crystal density and Mohs hardness of zinc cadmium thiocyanate (ZCTC), ZnCd(SCN)₄ have been measured at room temperature. The specific heat of the crystal is 699.5 J mol⁻¹ K⁻¹ at 300 K. The thermal expansion coefficient (TEC) along the a and c axes, respectively, is interpreted on the basis of crystal structure. The thermal decomposition process is characterized by thermogravimetry analysis and differential scanning calorimetry (TGA/DSC). The intermediates and final products of the thermal decomposition were identified by X-ray powder diffraction at room temperature. The high-temperature effect in air on the optical transmission of the crystal was also studied.     

Growth and properties of UV nonlinear optical crystal ZnCd(SCN)4

A second-order nonlinear optical coordination crystal, zinc cadmium thiocyanate, ZnCd(SCN)₄ (ZCTC) was grown as a frequency doubler, emitting UV light. A large typical single crystal with dimensions up to 15×7×7 mm³ has been obtained by slow solvent-evaporation method for the first time. The infrared (IR) spectroscopy and X-ray powder diffraction (XRPD) of single crystals were performed at room temperature. The specific heat of the crystal has been measured to be 367.9 J/mol K at 300 K. The thermal expansion coefficients a- and c-oriented, have been measured to be −1.69×10⁻⁵ and 1.95×10⁻⁴ K⁻¹, respectively. The second harmonic generation (SHG) efficiency of ZCTC crystal is 51.6 times as high as that of urea reference, and the measured transmittance spectra from 190 to 3200 nm showed that the UV transparency cutoff occurs at 290 nm and the transmission is 73.22% at 380 nm. UV laser light of wavelength 380 nm has been achieved by the frequency doubling of a 760 nm laser diode at room temperature.     

6 K solid state Energy Storage Unit

A cryogen-free cold source for temperature below 6 K without mechanical, thermal and electromagnetic perturbations would be welcome in many sensitive applications. This article describes such a device (Energy Storage Unit-ESU) built to store 36 J between 3 K and 6 K. This ESU consists of a solid state enthalpy reservoir connected to a cryocooler by a heat switch. Its different parts as well as the experimental results are presented. The choice of Gd₂O₂S (GOS) as high specific heat solid material for the enthalpy reservoir is discussed. Tests in different conditions were performed. A very good agreement was found between the experimental data and those predicted using the heat switch characteristics and the specific heat measurements of the GOS used in this experimental set-up. A stable 6 K temperature was maintained during more than 50 min in a completely silent environment. A semi-continuous operation for this cold source was successfully tested during 2.5 h.     

High temperature thermoelectric properties of (Ca, Ce)4Mn3O10

Thermoelectric (TE) properties such as resistivity (ρ), Seebeck coefficient (S), and thermal conductivity (κ) of Ca_4−3xCe_3xMn₃O₁₀ (0<x≤0.03) polycrystalline samples were measured from room temperature to 1000 K. ρ shows an obvious decrease with the increment of Ce content. The hopping conduction mechanism is used to explain the conduction behavior of these samples. The negative S values indicate that these materials are n-type. The sample of x=0.03 has the largest power factor, 0.52×10⁻⁴ Wm⁻¹ K⁻² at 1000 K. The value of κ and the dimensionless figure of merit of this sample is 1.51 Wm⁻¹ K⁻¹ and 0.034 at 1000 K, respectively.     

Growth and properties of Ca0.28Ba0.72Nb2O6 single crystals

Calcium barium niobate Ca_0.28Ba_0.72Nb₂O₆ (CBN-28) crystals were successfully grown by the Czochralski method. X-ray powder diffraction experiments indicated that CBN single crystals are tetragonal with a = 12.432(±0.002) Å and c = 3.957(±0.001) Å, which have almost the same structure as the Sr_0.50Ba_0.50Nb₂O₆ (SBN-50) crystal. The thermal expansion coefficient perpendicular to Z-direction had been measured to be 1.25 × 10⁻⁵ K⁻¹ between 293.15 and 572.15 K, and along Z-axis was negative between 298.15 and 543.15 K. The specific heat of the crystal had been measured by the differential scanning calorimetric experiments. The transmittance spectra from 200 to 3200 nm were also measured. The measured temperature dependence of dielectric constants showed that the Curie temperature of the CBN-28 crystals is 260 °C, which is about 200 °C higher than that of the (SBN) crystal.     

Hygroscopicity and bulk thermal expansion in Y2W3O12

Negative thermal expansion material, Y₂W₃O₁₂ has been synthesized by the solid-state method and bulk thermal expansion of the material has been investigated from 300 to 1100 K. The material reversibly forms a trihydrate composition whose X-ray diffraction pattern can be indexed to an orthorhombic unit cell with a = 10.098(1) Å, b = 13.315(3) Å, c = 9.691(4) Å. The cell volume of the hydrated pattern is 7% smaller than the unhydrated cell volume. According to the dilatometric studies, the material shows a 3-6% increase in the linear strain at about 400 K, which can be attributed to the removal of water. Sintering the material at 1473 K leads to large grain size of >100 μm, which results in a large hysteresis in the bulk thermal expansion behavior. Hot pressing at 1273 K under a uniaxial pressure of 25 MPa results in a fine-grained (2-5 μm) ceramic. Glazing the ceramic prevents moisture pick up and a linear thermal expansion over the entire temperature range 1100-300 K and an average linear thermal expansion co-efficient of −9.65 × 10⁻⁶/K is observed. The effect of water on the thermal expansion behavior of this system is discussed.     

Thermalization of a manganin band-cable at 1.4 K

The thermalization of wires for electrical connections was measured at temperatures of ∼1.4 K in a pumped ⁴He cryostat, using a simple but efficient device composed of a band-cable of 50 manganin wires clamped into a heat sink. We applied a varied input power from 1 up to to the wires and we observed that the power dissipation can be reduced down to very low levels.     

Thermoelectric properties of Zn4Sb3 prepared by hot pressing

Polycrystalline specimens of the thermoelectric material Zn₄Sb₃ were prepared by the hot-pressing method at various temperatures and pressures and their thermoelectric properties were evaluated in a temperature range from 298 K to 673 K. A single phase of Zn₄Sb₃ was obtained in the samples prepared at 673 K with a pressure above 150 MPa, whereas ZnSb was placed in the Zn₄Sb₃ matrix for the samples prepared at 100 MPa. The electrical transport properties of the single phase compound showed p-type conduction and metallic transport behavior based on the temperature dependence. The sample produced at 673 K under a pressure of 200 MPa exhibited the highest ZT value of 1.36 at 673 K. This study suggests that the dense and single-phase Zn₄Sb₃ compound is a route to achieve a high thermoelectric performance.     

Synthesis of nanocrystalline zirconia by amorphous citrate route: structural and thermal (HTXRD) studies

Nanocrystalline zirconia powder with a fairly narrow particle size distribution has been synthesized by the amorphous citrate route. The sample obtained has a high BET surface area of 89 m² g⁻¹. Rietveld refinement of the powder X-ray diffraction (XRD) profile of the zirconia sample confirms stabilization of zirconia in the tetragonal phase with around 8% monoclinic impurity. The data show the presence of both anionic as well as cationic vacancies in the lattice. Crystallite size determined from XRD is 8 nm and is in close agreement with the particle size determined by TEM. The in situ high temperature-X-ray diffraction (HTXRD) study revealed high thermal stability of the mixture till around 1023 K after which the transformation of tetragonal phase into the monoclinic phase has been seen as a function of temperature till 1473 K. This transformation is accompanied by an increase in the crystallite size of the sample from 8 to 55 nm. The thermal expansion coefficients are 9.14 × 10⁻⁶ K⁻¹ along ‘a’- and 15.8 × 10⁻⁶ K⁻¹ along ‘c’-axis. The lattice thermal expansion coefficient in the temperature range 298-1623 K is 34.6 × 10⁻⁶ K⁻¹.     

Crystal structure and spectroscopic investigations of an organic monophosphate

Single crystals of (p-ClC₆H₄NH₃)H₂PO₄ are synthesized in water by interaction of H₃PO₄ and (p-ClC₆H₄NH₂). This compound crystallizes in the orthorhombic system with the Pbca space group. Its unit-cell parameters are a = 9.724(3), b = 7.861(1), c = 25.078(6) Å, V = 1917.1(6) Å³ and Z = 8. The crystal structure has been solved and refined to R = 0.039, using 4298 independent reflections. The atomic arrangement can be described by inorganic layers parallel to ab plane, between which the organic cations are located. This compound exhibits a reversible phase transition at 403 K. The electrical conductivity measurements show that the (p-ClC₆H₄NH₃)H₂PO₄ has a conductivity value which goes from σ = 0.88 × 10⁻⁶ Ω⁻¹ cm⁻¹ at room temperature (293 K) to 3.31 × 10⁻⁴ Ω⁻¹ cm⁻¹ at 433 K. Its characterisation by TA, NMR and IR is reported too.