Steady-state Heat Flow Method Research Articles

Influence of Ag Doping on Thermal Conductivity and Magnetic Levitation of Single Grain YBCO Superconductors for High-Temperature Superconducting Maglev

Ag doping in (RE)Ba2Cu3O7−δ [(RE)BCO] bulk high-temperature superconductors has been recognized as an effective way to improve the mechanical and superconducting properties of REBCO bulk superconductors for practical applications such as high-temperature superconducting (HTS) Maglev. Ag2O nanoparticles doped YBCO samples with different doping amounts (x, x varies from 0∼0.5 wt%) were fabricated by Top-Seeded Melt-Growth method (TSMG). The samples’ steady-state thermal conductivity (λ) and the magnetic levitation force (FL) were measured by a testing platform under a similar condition to the HTS Maglev system. The effects of Ag2O doping on the λ and FL of single domain YBCO bulks have been systematically studied. The λ of the c-axis and the ab-plane were calculated through the steady-state heat flow method. We observed that λ increases distinctly when x ≥0.3 wt%. The doping amount of 0.3 wt% has been evidenced to be the optimal value, of which the λ of the c-axis and ab-plane are approximately 52% and 48% higher than the undoped one respectively. According to the results of the FL measurement, we found FL will increase while the relaxation rate of the FL will decrease by adding trace amounts of Ag2O nanoparticles in comparison to the undoped sample. This observation suggests that Ag-doping is an effective way to enhance thermal conductivity as well as the superconducting properties of YBCO superconductors for practical applications.

INSTALLATION FOR DETERMINING THE THERMAL CONDUCTIVITY OF PLATES BY THE STATIONARY METHOD

When measuring the thermal conductivity of thermal insulation materials made by powder metallurgy and building porous materials, complications arise due to the fact that heat flows through the samples are commensurate with heat losses. The steady-state heat flow (SSHF) method is simple, does not require sophisticated equipment, and allows determining the thermal conductivity not in the near-surface layer but in the entire sample volume. Its disadvantage is low accuracy and the need to use reference samples. This work aims to develop an installation for measuring the thermal conductivity of largesized samples of low-conductivity material using the steady-state heat flow method with significantly higher accuracy than existing installations using this method. This is achieved by sandwiching the sample, which is a thin square plate of large dimensions, between the heater chamber and the refrigerator. The heating chamber is made of insulating material and its front wall, which is in contact with the sample, is made of the copper plate (which has good thermal conductivity). In the operating mode, the temperature in the heater chamber is maintained equal to the ambient temperature, which allows us to neglect heat losses and assume that in the steady-state mode, the heater power is equal to the heat flux through the sample. The thickness of the sample is much smaller than the size of its side (the sample should be square). This assumption is necessary to use the condition of isotropic temperature distribution in the cross-section of the sample. The refrigerator is filled with water and ice. The isotropic temperature distribution in the sample is ensured by its contact with the copper walls of the heater and refrigerator chamber. The temperature of the heated surface of the sample is measured using a thermocouple inserted through a hole in the front wall of the heater chamber. The proposed design of the installation and its operating conditions make it possible to significantly improve the accuracy of determining the thermal conductivity coefficient and make the error less than 2%.

Interaction of phonons with spherical nanoparticles embedded densely in simple crystalline matrix. Case of nitrogen-palladium nanocomposite

The results of experimental investigations of thermal conductivity of nanocomposites built of palladium spherical nanoparticles embedded in the structure of crystalline nitrogen are presented in this paper. The investigations were carried out on the samples containing the Pd nanospheres of diameters of 6, 8, 10, 12, 18, and 24 nm at the palladium fraction amounting to 15% of the nanocomposite volume. The measurements were performed with a steady-state heat flow method in the temperature interval of 2–35 K. For the analysis of the experimental results, the relaxation time approximation in the frame of the thermal conductivity Debye model was used. The analysis shows that the nitrogen matrix phonons can effectively interact with spherical nanoparticles in, at least, four different mechanisms. Two of those mechanisms, a diffuse scattering by the boundary of two media (which are the matrix and the material of the nanoparticle) and an oscillating resultant of interaction of nanoparticle phonons with the matrix phonons, result in a decrease of the nanocomposite thermal conductivity. The remaining two mechanisms would be considered nonthermal resistive scattering processes: The first of them is the specular scattering of the matrix phonons by the matrix and the nanoparticle material interface. The second one is a forward scattering, in which the matrix phonon penetrates the nanoparticle and afterward penetrates another one without any resistive interaction with phonons of the crystalline matrix. The analysis shows that the nonresistive processes are significantly more frequent in the investigated nanocomposites than the resistive ones. Such a conclusion is in perfect agreement with the results of the analysis of the phonon mean free path in the nitrogen crystal-palladium nanospheres composite.

Thermal Conductivity of Two-Dimensional Benzobisoxazole-Linked Covalent Organic Frameworks with Nanopores: Implications for Thermal Management Applications

Thermal management is essential for maintaining the optimal performance of electronic devices. Although covalent–organic frameworks (COFs) have emerged as a platform for gas and energy storage applications, their thermal transport properties are greatly understudied. Herein, we report the thermal conductivities of three benzobisoxazole (BBO)-linked COFs with nanpores ranging from 1.3 to 2.5 nm over a wide temperature range (80–300 K) using the longitudinal, steady-state heat-flow method. In doing so, thermal conductivity values as high as 0.677 W m–1 K–1 at 300 K were obtained, and no relationship between the thermal conductivity and pore size was observed. These results were supported by density functional theory calculations. The thermal conductivities of the BBO-COFs doped with poly(3-hexylthiophene) were also investigated. The BBO-COFs could be useful as ultralow-k materials for thermal management applications.

Influence of different nanoparticles embedded in crystalline carbon monoxide matrix on heat transfer in the nanocomposite

The preliminary results of investigations of heat transfer in nanocomposites consisting of nanoparticles randomly distributed in solid carbon monoxide matrix are presented. In the experiment the thermal conductivity coefficient dependence on temperature for CO crystal with silica and palladium nanoparticles of different size embedded in the crystal structure was determined over the temperature range 2.2–35 K by steady-state heat flow method. The results of the measurements were analyzed within the frame of relaxation time approximation. The analysis shows that lowering of the thermal conductivity of the nanocomposites relative to pure carbon monoxide crystal observed for both types of the investigated nanoparticles, palladium and silica, is caused mostly by scattering of phonons by boundaries of the nanoparticles. Additionally, the presence of the nanoinclusions promotes higher density of dislocations and influences the matrix lattice dynamics.

Thermal Conductivity of poly-p-phenylene-2,6-benzobisoxazole Film along the In-Plane Axis in the 10–300 K Temperature Range

The thermal conductivities of compression molded thin films of poly-p-phenylene-2,6-benzobisoxazole (PBO) were measured in directions along an in-plane axis in the 10–300 K temperature range by a steady-state heat flow method, with interest in the use of the material for superconductivity applications. The thermal conductivities of the PBO films increased from 0.3 W/mK to 9.0 W/mK with increasing temperature from 10 K to 300 K and these were much higher than those of polyimide films, epoxy resin and glass fiber reinforced plastics at all temperatures. The 9.0 W/mK at 300 K was 60% of that of stainless steel (SUS304). It was 6 W/mK at 150 K, which was half that of SUS304 and was 3.3 W/mK at 77 K, which was 33% of that of SUS304. The thermal conductivities of the PBO films were lower than those of a cloth of high strength ultrahigh molecular weight polyethylene fiber reinforced plastics in the 30 K–180 K temperature range and were almost equivalent to its values in the 180 K–300 K temperature range. The main contribution to the thermal conduction in the PBO films was from thermal phonon conduction along the molecular chains. Although many kinds of high thermal conductivity polymeric materials have been prepared by a uni-directional drawing process or by adding high thermal conductive additives, the PBO film showed high thermal conductivity without a uni-directional drawing process or high thermal conductive additive.

Geometrical Effect on Thermal Conductivity of Unidirectional Fiber-Reinforced Polymer Composite along Different In-plane Orientations

This paper focuses on the anisotropic characteristics of the in-plane thermal conductivity of fiber-reinforced polymer composite based on experiment and simulation. Thermal conductivity along different in-plane orientations was measured by laser flash analysis (LFA) and steady-state heat flow method. Their heat transfer processes were simulated to reveal the geometrical effect on thermal conduction. The results show that the in-plane thermal conduction of unidirectional carbon-fiber-reinforced polymer composite is greatly influenced by the sample geometry at an in-plane orientation angle between 0° to 90°. By defining radius-to-thickness as a dimensionless shape factor for the LFA sample, the apparent thermal conductivity shows a dramatic change when the shape factor is close to the tangent of the orientation angle (tanθ). Based on finite element analysis, this phenomenon was revealed to correlate with the change of the heat transfer process. When the shape factor is larger than tanθ, the apparent thermal conductivity is consistent with the estimated value according to the theoretical model. For a sample with a shape factor smaller than tanθ, the apparent thermal conductivity shows a slow growth around a low value, which seriously deviates from the theory estimation. This phenomenon was revealed to correlate with the change of the heat transfer process from a continuous path to a zigzag path. These results will be helpful in optimizing the ply scheme of composite laminates for thermal management applications.

The influence of silica nano-inclusions on the structure of methane crystal observed in thermal conductivity experiment

The preliminary results of experimental investigations of the thermal conductivity of nanocomposites built of amorphous silica nanoparticles embedded in the structure of crystalline methane are presented. The investigations were carried out on the samples containing the particles of linear dimensions of 5, 10–20 and 42nm. The measurements were performed with steady-state heat flow method in the temperature interval 2–35K.A very simple thermal conductivity model shows that the dependence of the thermal conductivity coefficient of the investigated nanocomposites on temperature can be successfully described by taking into account only two mechanisms of the heat carrier dissipation: scattering of phonons by methane crystal grain boundaries and phonon-phonon interaction in U-processes. It was found that the low-temperature phonon mean free path is inversely proportional to the linear dimension of the nanopowder particles embedded in the methane crystal structure.

Heat transfer in Ar and N2 doped solid CO

The measurements of thermal conductivity coefficient of a solid carbon monoxide crystal containing argon and nitrogen admixtures at different concentrations (1.5, 3, 6, 12.5, 25% for N2 and 0.5, 1, 1.25, 2, 4% for Ar) were performed in the temperature range from 1.5 to 40 K by steady-state heat flow method. For analysis of the experimental data the Callaway method in the framework of the Debye model was used. The contribution of various mechanisms of phonon scattering, including scattering by disordered dipoles of the CO matrix, to the thermal conductivity of CO–N2 and CO–Ar solid solutions were taken into account.

Measurement of the effective thermal conductivity of particulate materials by the steady-state heat flow method in a cuvette

To measure the thermal conductivity of particle beds, a specially designed cuvette is inserted into the chamber of an ITP-MG4 device fitted with a vertical heat flux sensor. The cuvette with a transparent wall makes it possible to reduce the amount of test material to 25 cm3, to monitor visually the uniformity of a charge, to determine the bulk density of the particle bed (and to increase it if necessary using vibrocompaction) and to apply external pressure to the bed from 2.5 to 30 kPa. Using various continuous-solid and particulate materials as references, a calibration equation is obtained for thermal conductivity in the range of 0.03–1.1 W (m K)−1. To eliminate thermal contact resistance when measuring references, the end faces of glass specimens with a departure from flatness of up to 50 μm are wetted with water. To model the calibration, a calculation is carried out by the electrical circuit analogy. The calculated curve is close to the experimental points if a value for the contact thermal resistances r# = 2 × 10−3 m2 K W−1 is taken. Values of r# calculated by the Yovanovich model, based on the known roughnesses of the contact surfaces of the cuvette and the solid specimens, are an order of magnitude lower due to the decisive influence of nonflatness and not surface roughness at the low pressures used. The conditions under which our measurements were made are compared with the instructions of Russian, American and international standards for the measurement of thermal conductivity by the steady-state heat flow method (specimen size, flatness of working surfaces, etc). The sources of measurement inaccuracy and ways to improve the technique are examined.

S111054 高配向カーボンナノチューブ膜の接触熱抵抗に及ぼす接触条件の影響([S111-05]第19回卒業研究コンテスト(5))

Carbon nanotubes have been reported to have high thermal conductivity. A vertically aligned carbon nanotube (VA-CNT) film is composed of densely packed and highly-aligned CNTs on a SiC substrate. This VA-CNT film is expected to have superior heat conductivity than a conventional thermal-grease heat transfer method. However, it is not clear that how much contact pressure and contact roughness affect the contact thermal conductivity of the VA-CNT films. We developed an apparatus that measures the contact thermal resistance by an one-directional steady-state heat flow method. We found out that there was suitable surface roughness for the minimum contact thermal resistance of the VA-CNT film.

Thermal conductivity of geomaterial foams based on silica fume

Porous materials have been prepared from a solution containing sodium silicate and sodium hydroxide with the addition of silica fume. Kaolin and diatomite were also tested as additives to the initial formulation. This method yields consolidated geomaterial foams without requiring thermal treatment above $$50\,{}^{\circ}\hbox{C}$$ . The influence of chemical composition on the thermal conductivity was studied. The choice of raw materials was found to play an important role. The accuracy of thermal conductivity measurements was evaluated by comparing the steady state heat flow method with the laser flash technique for five different reference materials giving values within 6%. Using the steady state heat flow method, a value of $$0.12 \pm 0.01\,{\rm W}\,{\rm m}^{-1}\,{\rm K}^{-1}$$ was then obtained for consolidated foams, made with kaolin as the precursor, containing approximately 70% of porosity.

Bi2212 superconducting tubular conductors prepared by the diffusion process for current lead

Bi2212 HTS conical tubular conductors have been prepared by the diffusion process for current lead application. The Bi2212 HTS layers are synthesized through the diffusion reaction between a Sr-Ca-Cu oxide substrate and a Bi-Cu oxide coating with Ag addition. Highly oriented HTS diffusion layers about 150μm in thickness are formed around both outside and inside of the conical tube 34/29mm in outside/inside diameter at larger end, 24/19mm in outside/inside diameter at smaller end, and 100mm in length. The transport current has been measured using two cryocoolers between 12 K at cold end and 40–70 K at warm end under 0.5 T. The transport current of 1,000 A was stably passed for 10 minutes at 50 K. The joint voltage of cold end was as low as 30μV at 50 K during 1,000 A running. The low voltage results from the low contact resistance between Bi2212 grains and Ag precipitated on the surface. Heat leakage conducted through the specimen has been newly evaluated using heat flux meter by steady-state heat flow method.

Thermal contact resistance of different materials between 0.3 K and 4.5 K

The choice of a good bonding agent often depends on the temperature range of use and on its corresponding thermal contact resistance. Measurements obtained between 0.3 K and 4.5 K were made applying the steady state heat flow method across the interfaces of small crystals of quartz (SiO 2) sandwiched by two copper blocks and attached with various types of contact materials. Below 1 K, we found that for one crystal attached with indium solder its thermal contact resistance was R B - 1 ≈ 0.68 T 3 W cm 2 K . This value in comparison with those attached with a conductive and an insulating epoxies, turns to be the lowest.

Thermal conductivity of a metal-organic framework (MOF-5): Part II. Measurement

The thermal conductivity of MOF-5 single crystals is measured over a wide temperature range between 6 K and 300 K, using the longitudinal, steady-state heat flow method. Between 6 K and 20 K, the thermal conductivity increases with the increase in temperature and exhibits a peak near 20 K. This peak results from the crossover between the decreasing mean free path and the increasing phonon specific heat with the increasing temperature. From 20 K to 100 K, the thermal conductivity decreases rapidly with increasing temperature. Above 100 K, the thermal conductivity is nearly temperature independent, and its value at 300 K is 0.32 W/m K, a rather low value for crystals. The mean free path analysis shows that at high temperature, the phonon mean free path is minimized to the cage size due to the porous, flexible structure of MOF-5.

Thermoelectric properties of Bi‐Sb samples grown by mechanical alloy

Seebeck coefficient S(T), electric resistivity ϱ(T) and thermal conductivity κ(T) measurements are reported on polycrystalline Bi0.88Sb0.12 samples grown by mechanical alloy (MA) by using different milling time. The electrical and thermal characterization was carried out in the temperature range between 77 K and 300 K. The Seebeck coefficient is negative in whole measurement range and its magnitude increases with the temperature, S(T) exhibit an enhancement with the milling time up to reach values close to –300 µV/K in the samples milled through 45 hours. The electrical resistivity shows a weak semiconductor behavior with the temperature, ϱ(T) was increasing with the processing time, but in all cases it shown values close to 10–3 Ω cm. The thermal conductivity measurements were carried out by using a steady-state heat-flow method. κ(T) show a linear behavior with the temperature and its magnitude decreases with processing time up to values smaller than 1 W/mK. By using SEM analysis the behavior of the granularity of the samples was studied as a function of the milling time. The dimensionless thermoelectric figure of merit ZT, reaches values close to 0.3, which are characteristic values for single crystalline alloys of these kind compounds. These results show that the mechanical alloy could be a useful technique in the production of thermoelectric materials. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Thermal contact resistance between high- Tc superconductor and copper

The thermal contact resistivity ( W c) through the connection between a high- T c superconductor and a copper block was measured from 10 to 200 K for various connecting methods using In solder, silver paste, epoxy resin and varnish as adhesives. W c was measured by a steady state heat flow method and with a three-terminal non-steady state method by which the thermal diffusivity α was also determined simultaneously. It was found that W c seriously depended on the connecting method and increased with decrease in temperature. The measured W c values by both steady and non-steady methods were roughly consistent with each other. The proposed three-terminal non-steady state method was proved to be a useful method to determine both α and W c.

Thermal conductivity of Gd2(MoO4)3 crystal from 0.5 to 80 K

Thermal conductivity of the improper ferroelastic Gd2(MoO4)3 crystal has been studied in the temperature range from 0.5 to 80 K by the steady-state heat-flow method. The ferroelastic domain walls were found to be strong centers of phonon scattering. At temperatures lower than 40 K, the thermal conductivity of the singledomain samples was by almost one order of magnitude greater than that of the multidomain ones.

スタイキャスト補強Ｂｉ系（２２２３）超伝導材料の熱伝導率

The thermal conductivity of STYCAST reinforced Bi(2223) superconducting materials has been studied between 10 and 150K using a steady-state heat flow method. The value of heat leakage per pair of bulk leads from 4.2K to 77.3K for the composite materials has been estimated from the thermal conductivity data. The measured thermal conductivity of the composite materials below 100K is in rough agreement with the value obtained from the mixtures rule of Bi(2223) and STYCAST. It is found that the heat intrusion of the reinforced materials is one-fourth as small as that of 800 A-class conventional gas-cooled current leads. The thermal conductivity of Bi(2223) superconducting material has been studied from 30K to 130K in constant magnetic fields up to 13T. The thermal conductivity value of the Bi(2223) material is strongly suppressed as the applied field is increased along the c-axis of the sample. The contribution of the magnetic field to the value of heat leakage for the current leads of the high-Tc materials is negligible even in high fields. Therefore, STYCAST reinforced Bi(2223) superconducting materials are suitable for current leads to design the high-field and/or large-scale superconducting magnet with a cryocooler-cooled type using no liquid helium.

低温における熱拡散率と熱伝導率の同一セッティング測定

A thermal diffusivity measuring system employing a closed cycle helium refrigerator was developed, which also enabled the thermal conductivity measurement under an identical experimental setup. The diffusivity was measured by a discretional heating method and the conductivity was measured by a steady-state heat flow method. The errors and the accuracy in measurements were evaluated by measuring the diffusivities of an austenitic stainless steel standard sample (SRM 1460), a Pyrex glass (Corning #7740) and a high purity copper between 12 to 200K. The diffusivity ranging from 10-3 to 2cm2/s could be determined with an uncertainty of 3%. The results agreed with the reported values within 10%.