No abstract

temperature, has become a promisingly alternative technology to the existing vapor-compression refrigeration due to its energy efficiency and environment friendly. [4][5][6] As the core part of magnetic refrigeration, the comprehensive performance including MCE, thermal conductivity (λ), preparation and fabrication, corrosion resistance, and thermal and magnetic field cycling stabilities for magnetocaloric materials greatly affect the performance of the magnetic refrigerator. [7,8] Pure Gd, as the benchmark magnetic refrigerant, is commonly used in the active magnetic regenerator prototypes. [9,10] In particular, Gd metal could be produced into different shapes due to its good mechanical properties, such as thin plate, sphere, and microwire, and it has been demonstrated that microwire is more desirable for realizing a higher cooling performance. [11][12][13][14][15][16] However, Gd metal also has its disadvantages such as relatively low magnetic entropy change (ΔS M ), limited availability, and expensive price that hinder its large-scale commercialization. Therefore, a great number of magnetic materials with large MCE around room temperature have been extensively explored in the past decades. [2,6,[17][18][19][20] Unfortunately, although these new materials exhibit much better MCE than that of Gd, they still have severe shortcomings such as hard preparation Magnetic refrigeration based on magnetocaloric effect (MCE) has become a promisingly alternative technology to the conventional vapor-compression refrigeration. A great number of magnetic materials have been reported to exhibit larger MCE than that of the benchmark magnetic refrigerant Gd.However, these materials still have severe shortcomings on the "Non-MCE" properties, such as hard preparation and fabrication, low thermal conductivity λ, and poor corrosion resistance and cycling stabilities, which hinder the practical application of these materials. In this paper, a novel La(Fe, Si) 13 H y / In composite which is prepared by a readily available hot pressing method is demonstrated to exhibit an outstanding comprehensive performance with durable service life in various aspects. Noteworthily, the ΔS M does not decrease but increases with increasing nonmagnetic In metal. This advantageous anomaly is related to the strengthening of the first-order itinerant electron metamagnetic transition induced by residual compression stress and surrounded constraints in the composite. The present results make La(Fe, Si) 13 H y /In composite the most attractive alternative to Gd for magnetic refrigeration. Moreover, this work also provides a feasible way to solve the serious issues toward applications for La(Fe, Si) 13 -based materials and other brittle magnetocaloric materials.

A measurement system has been designed and built for the specific application of measuring the effective thermal conductivity of a composite, nuclear-fuel compact (small cylinder) over a temperature range of 100 • C to 800 • C. Because of the composite nature of the sample as well as the need to measure samples pre-and postirradiation, measurement must be performed on the whole compact non-destructively. No existing measurement system is capable of obtaining its thermal conductivity in a non-destructive manner. The designed apparatus is an adaptation of the guardedcomparative-longitudinal heat flow technique. The system uniquely demonstrates the use of a radiative heat sink to provide cooling which greatly simplifies the design and setup of such high-temperature systems. The design was aimed to measure thermalconductivity values covering the expected range of effective thermal conductivity of the composite nuclear fuel from 10 W · m −1 · K −1 to 70 W · m −1 · K −1 . Several materials having thermal conductivities covering this expected range have been measured for system validation, and results are presented. A comparison of the results has been made to data from existing literature. Additionally, an uncertainty analysis is presented finding an overall uncertainty in sample thermal conductivity to be 6 %, matching well with the results of the validation samples.

h i g h l i g h t sMeasured the ETC of surrogate TRISO fuel compacts experimentally and numerically. The ETC of the surrogate fuel compacts varies between 50 and 30 W m À1 K À1 . A new model/approach to analyze the effect of constituent materials on ETC is proposed. a b s t r a c t Accurate modeling capability of thermal conductivity of tristructural-isotropic (TRISO) fuel compacts is important to fuel performance modeling and safety of Generation IV reactors. To date, the effective thermal conductivity (ETC) of TRISO fuel compacts has not been measured directly. The composite fuel is a complicated structure comprised of layered particles in a graphite matrix. In this work, finite element modeling is used to validate an analytic ETC model for application to the composite fuel material for particle-volume fractions up to 40%. The effect of each individual layer of a TRISO particle is analyzed showing that the overall ETC of the compact is most sensitive to the outer layer constituent. In conjunction with the modeling results, the thermal conductivity of matrix-graphite compacts and the ETC of surrogate TRISO fuel compacts have been successfully measured using a previously developed measurement system. The ETC of the surrogate fuel compacts varies between 50 and 30 W m À1 K À1 over a temperature range of 50-600°C. As a result of the numerical modeling and experimental measurements of the fuel compacts, a new model and approach for analyzing the effect of compact constituent materials on ETC is proposed that can estimate the fuel compact ETC with approximately 15-20% more accuracy than the old method. Using the ETC model with measured thermal conductivity of the graphite matrix-only material indicate that, in the composite form, the matrix material has a much greater thermal conductivity, which is attributed to the high anisotropy of graphite thermal conductivity. Therefore, simpler measurements of individual TRISO compact constituents combined with an analytic ETC model, will not provide accurate predictions of overall ETC of the compacts emphasizing the need for measurements of composite, surrogate compacts.

The transient electrothermal technique has been developed to measure the thermal conductivity and thermal diffusivity of electrically conductive or non-conductive nano-to-microscale fibers. In this work, a full theoretical model is developed in detail including the effects of radiation heat loss and non-constant heating as a result of sample temperature rise during measurement, and is compared to the more commonly used reduced model, which neglects these effects. Non-dimensional parameters are derived representing radiation heat loss and non-constant heating to identify the true parameter dependences on these effects. A numerical model is used to perform parametric analyses on the experimental setup providing results that were fitted with the full and reduced models to find thermal conductivity and thermal diffusivity. Additionally, the numerical model was used to investigate nonlinear radiation heat losses and spatially non-uniform heating effects resulting from uneven coating of the conductive layer on electrically non-conductive samples. As a result, these influences are shown to require careful consideration in the application of this technique. A clear linear relationship was found between the non-dimensional parameters and measurement error, which provides a measure for the proper estimation of systematic error induced by these effects. Using the reduced model for data reduction results in measurement percentage error equal to ten times the radiation and non-constant heating dimensionless parameters under the assumption of linear radiation heat losses (small sample temperature rise compared to ambient temperature).