Changhu Xing

Analysis of the electrothermal technique for thermal property characterization of thin fibers

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).

Parametric Study of the Frequency-Domain Thermoreflectance Technique

Without requiring regression for parameter determination, one-dimensional (1D) analytical models are used by many research groups to extract the thermal properties in frequency-domain thermoreflectance measurements. Experimentally, this approach involves heating the sample with a pump laser and probing the temperature response with spatially coincident probe laser. Micron order lateral resolution can be obtained by tightly focusing the pump and probe lasers. However, small laser beam spot sizes necessarily bring into question the assumptions associated with 1D analytical models. In this study, we analyzed the applicability of 1D analytical models by comparing to 2D analytical and fully numerical models. Specifically, we considered a generic n-layer two-dimensional (2D), axisymmetric analytical model including effects of volumetric heat absorption, contact resistance, and anisotropic properties. In addition, a finite element numerical model was employed to consider nonlinear effects caused by temperature d...

Thermophysical Property Measurement of Electrically Nonconductive Fibers by the Electrothermal Technique

The transient electrothermal technique is a powerful tool to obtain thermal properties of fine fibers. However, the technique suffers from several inherent pitfalls, which affect measurement accuracy, especially with application to coated, nonconductive samples. In this paper, measurement challenges are described and quantified for several associated parameters and physics including: sample length, time of Joule-heating initiation, sample resistance including measurement uncertainty as well as evolving resistance for coated samples, coating influence, lateral surface heat losses, vacuum level, and variable heat generation. Several methods to overcome these challenges to ensure good measurement accuracy are provided. These methods are applied to the measurement of thermal conductivity and thermal diffusivity of gold-coated glass fibers (nonconductor). The resulting measured thermal conductivity of 1.35 Wm−1 K−1, and thermal diffusivity of 7.6 × 10−7 m2 s−1 compare well to literature values. Additionally, an analytic formula is developed along with limiting conditions for simplified application, which accounts for neglected heat losses. The result is a factor that can be applied to correct a more straightforward heat model of the sample, which neglects heat losses. To further validate the method and quantify measurement variability, a detailed uncertainty analysis is performed using methods based on the Taylor series method for propagation of uncertainty and Monte Carlo simulation. The resulting measurement uncertainty is found to be ~7% for thermal conductivity and ~4% for thermal diffusivity.

Investigation of Synthetic Spider Silk Crystallinity and Alignment via Electrothermal, Pyroelectric, Literature XRD, and Tensile Techniques

The processes used to create synthetic spider silk greatly affect the properties of the produced fibers. This paper investigates the effect of process variations during artificial spinning on the thermal and mechanical properties of the produced silk. Property values are also compared to the ones of the natural dragline silk of the N. clavipes spider, and to unprocessed (as-spun) synthetic silk. Structural characterization by scanning pyroelectric microscopy is employed to provide insight into the axial orientation of the crystalline regions of the fiber and is supported by XRD data. The results show that stretching and passage through liquid baths induce crystal formation and axial alignment in synthetic fibers, but with different structural organization than natural silks. Furthermore, an increase in thermal diffusivity and elastic modulus is observed with decreasing fiber diameter, trending towards properties of natural fiber. This effect seems to be related to silk fibers being subjected to a radial gradient during production.

A Thermal Conductivity Measurement System for Fuel Compacts

A measurement system has been designed and built for the specific application of measuring thermal conductivity of a composite nuclear fuel over a temperature range of 400 K to 1100 K. Because the composite nature of the sample requires measurement of the whole compact, there is no existing method available for obtaining its thermal conductivity in a non-destructive manner. The designed apparatus is an adaptation of the guarded-comparative-longitudinal heat flow technique. Initial testing has been performed on stainless steel 304. The determinate uncertainty is presented and found to be 2.5% excluding the error associated with the reference samples. The prototype system is expected to be further tested and modified for the measurement of thermal conductivity of fresh fuel compacts at elevated temperatures in a radioactive environment.Copyright

A correction scheme for thermal conductivity measurement using the comparative cut-bar technique based on 3D numerical simulation

As an important factor affecting the accuracy of thermal conductivity measurement, systematic (bias) error in the guarded comparative axial heat flow (cut-bar) method was mostly neglected by previous researches. This bias is primarily due to the thermal conductivity mismatch between sample and meter bars (reference), which is common for a sample of unknown thermal conductivity. A correction scheme, based on finite element simulation of the measurement system, was proposed to reduce the magnitude of the overall measurement uncertainty. This scheme was experimentally validated by applying corrections on four types of sample measurements in which the specimen thermal conductivity is much smaller, slightly smaller, equal and much larger than that of the meter bar. As an alternative to the optimum guarding technique proposed before, the correction scheme can be used to minimize the uncertainty contribution from the measurement system with non-optimal guarding conditions. It is especially necessary for large thermal conductivity mismatches between sample and meter bars.

VALIDATION OF A THERMAL CONDUCTIVITY MEASUREMENT SYSTEM FOR FUEL COMPACTS

A high temperature guarded-comparative-longitudinal heat flow measurement system has been built to measure the thermal conductivity of a composite nuclear fuel compact. It is a steady-state measurement device designed to operate over a temperature range of 300 K to 1200 K. No existing apparatus is currently available for obtaining the thermal conductivity of the composite fuel in a non-destructive manner due to the compact’s unique geometry and composite nature. The current system design has been adapted from ASTM E 1225. As a way to simplify the design and operation of the system, it uses a unique radiative heat sink to conduct heat away from the sample column. A finite element analysis was performed on the measurement system to analyze the associated error for various operating conditions. Optimal operational conditions have been discovered through this analysis and results are presented. Several materials have been measured by the system and results are presented for stainless steel 304, inconel 625, and 99.95% pure iron covering a range of thermal conductivities of 10 W/m*K to 70 W/m*K. A comparison of the results has been made to data from existing literature.

An Electromotive Force Measurement System for Alloy Fuels

The development of advanced nuclear fuels requires a better understanding of the transmutation and micro-structural evolution of the materials. Alloy fuels have the advantage of high thermal conductivity and improved characteristics in fuel-cladding chemical reaction. However, information on thermodynamic and thermophysical properties is limited. The objective of this project is to design and build an experimental system to measure the thermodynamic properties of solid materials from which the understanding of their phase change can be determined. The apparatus was used to measure the electromotive force (EMF) of several materials in order to calibrate and test the system. The EMF of chromel was measured from 100°C to 800°C and compared with theoretical values. Additionally, the EMF measurement of Ni-Fe alloy was performed and compared with the Ni-Fe phase diagram. The prototype system is to be modified eventually and used in a radioactive hot-cell in the future.Copyright

Thermal Property Measurement of Thin Fibers: A Direct Approach

Fiber thermal characterization is often accomplished by indirect means, such as embedding the fiber in a matrix, measuring the thermal response of the composite, and relating for the contributions of the fiber and matrix to the overall behavior or measuring bundles of fibers. To improve the accuracy of the composite-based or bundle-based techniques, several different contact (hot wire and dc thermal bridge) and non-contact (Raman shift and IR thermography) methods have been developed to directly measure the thermal properties of individual fibers. To improve on the shortcomings of these methods, this paper presents the experimental results of an improved transient electrothermal (TET) method, as well as a 3ω-based method that better accounts for all sources of heat transfer, particularly heat loss by radiation. The incorporation of radial radiation heat loss becomes a significant factor as the size of the fibers decrease. This work describes practical applications of the methods to measure the properties of the fibers, including sample preparation for electrically conductive and non-conductive samples, data acquisition and calibration, data analysis, and sample property determination.Results include validation of the methods with electrically conductive (platinum) and non-conductive (glass) fibers to improve upon the initial validation of the generalized electrothermal method which focused only on short, conductive fibers. The axial thermal conductivity and diffusivity of several high performance fibers are presented. The novelty of this paper is that it serves as both a compilation of previous research on the transient electrothermal and 3ω methods [1–6], measurements of new silk fibers, and practical information associated with the methods that improve the accuracy of the measured thermal property, as well as presenting thermal properties of additional fibers (carbon fiber and natural and synthetic spider silks).To improve upon the long sample preparation time required for the TET and 3ω methods, future work focused on the development of a quantum dot-based photothermal fluorescence method is presented.Copyright

Description of Test Setup and Approach to Measure Thermal Properties of Natural and Synthetic Spider Silks at Cryogenic Temperatures

Spider silk is well-known for its exceptional mechanical properties, such as strength, elasticity and flexibility. Recently, it has been reported that dragline silk from a Nephila clavipes also has an exceptionally high thermal conductivity, comparable to copper when the fiber is stretched. Synthetic spider silks have been spun from spider silk proteins produced in transgenic sources, and their production process has the optimization potential to have properties similar to or better than the natural spider silk. There is interest to measure the thermal properties of natural and synthetic silk at cryogenic temperatures for use of spider silk fibers as heat conduits in systems where component weight is an issue, such as in spacecraft. This low temperature measurement is also of particular interest because of the conformational changes in protein structures, which affect material properties, that occurs at lower temperatures for some proteins. A measurement system has been designed and is being tested to characterize the thermal properties of natural and synthetic spider silks by means of a transient electrothermal method.Copyright