Combinatorial approaches as effective tools in the study of phase diagrams and composition-structure-property relationships

This article will provide an overview of state-of-the-art combinatorial/high-throughput methodologies and tools for accelerated materials research and discovery. Combinatorial thin films with discrete composition libraries or continuous composition gradients (spreads) have been widely used to study composition-structure-property relationships and to discover new functional materials. A diffusion-multiple approach—the creation of composition gradients and intermetallic phases by long-term annealing of junctions of three or more phases/alloys—enables effective studies of phase diagrams, kinetics, and composition-structure-property relationships of bulk alloys. Such studies are made possible by localized property measurements using micro-scale probes/measurement tools. Micro-scale probes for several properties such as elastic modulus, hardness, thermal conductivity, dielectric properties, optical properties, and crystal structures are relatively well developed and will be discussed in detail. The probes for electrical conductivity, magnetic properties, and compressive yield strength need further improvement or more benchmark studies. All these micro-scale probes are very useful for materials research. For instance, a micro-scale thermal conductivity probe can be used to study order-disorder transformation, site preference in intermetallic compounds, solid-solution effects on conductivity, and compositional point defect propensity. Several probes can be combined to accelerate the development of structural materials to obtain phase diagrams, diffusion coefficients, precipitation kinetics, solution-strengthening effects, and precipitation-strengthening effects. The probes yet to be developed that would have a significant impact on materials research include ones for lattice parameter measurements at micron-scale resolution, localized melting point measurements, ductility, thermal expansion coefficients, and thermodynamic properties. The impact of the development of the micro-scale probes goes beyond combinatorial materials research since most of them can be applied to non-combinatorial metallographic or thin film samples as well. Examples will show that in addition to improved efficiency, the systematic nature of the combinatorial approaches can reveal complex phenomena and interactions that otherwise would be difficult to be aware of or find using conventional one-composition-at-a-time practice, especially when measurements of several properties are made using multiple probes.     

NPL Initiatives in the Development of Thermal Properties Reference Materials

The present paper describes the status of a number of studies involving NPL which are directed towards development of internationally accepted reference materials to serve different requirements. These include: (1) thermal diffusivity and thermal conductivity measurements on candidate molten metal reference materials; (2) co-operative flash diffusivity measurements on glassy carbon, a dense fine grain alumina, and other ceramics; (3) inter-European studies, one using different steady-state and transient methods to evaluate the properties of a high temperature ceramic and the other using the guarded hot plate method to produce a high temperature thermal insulation reference; (4) a six-laboratory European program to certify a replacement European thermal insulation reference material for use at room temperature; and (5) an intercomparison between standards organizations in five countries on the current reference materials supplied in each country or geographical area. Brief mention is made concerning other on-going and planned work together with recommendations for materials to address additional needs.     

Photothermal Radiometry Characterization of Limestone Rocks from the Península of Yucatán

Limestone is a sedimentary rock composed of calcium carbonate with minor amounts of silica, iron oxide, clay, dolomite, and organic material. These types of stones have been used extensively as building materials. Due to this, determination of their thermal properties is of the utmost importance. These properties depend on the microstructure and composition of each type of rock. In this study, the effect of the thermal treatment of three different limestone rocks from the Peninsula of Yucatán were studied, in the range from 100?°C up to 600?°C, using photothermal radiometry. These studies were complemented by the characterization of the crystalline phases using X-ray diffraction and effective porosity measurements performed by the saturation technique. It is shown that the thermal diffusivity, thermal conductivity, and specific heat of the limestone decrease as the temperature increases. This behavior can be related to increases in microcracks and effective porosity due to thermal treatments.     

Experimental investigation of the effect of moisture on thermal conductivity and specific heat of porous ceramic materials

Thermal conductivity and specific heat of porous ceramic materials display a unique behaviour when moisture is present in their structure. When compared with the corresponding dried materials, these properties are drastically altered, and the magnitude of these changes depends on the moisture content. In this work it was experimentally investigated the effect of adsorbed water on the thermal conductivity and specific heat of the most commonly structural material: castables and concrete of Portland cement. The experimental technique employed was the hot wire parallel technique, and measurements were carried out from room temperature up to 300°C during the heating and cooling cycle. The thermal conductivity and the specific heat were simultaneously determined from the same experimental thermal transient. Experimental results show a drastic influence of the adsorbed water on the thermal conductivity and specific heat of green castables. It was also observed that the addition of glassy phase on sample composition decreases the thermal conductivity and promotes the inversion of the slope of the curve thermal conductivity versus temperature for the dried material.     

Zum Zusammenhang zwischen Eigenschaften und Gefügestruktur mehrphasiger Werkstoffe. Teil IV: Gefügestruktur und thermischer Ausdehnungskoeffizient

On the relationship between the properties and the microstructure of multiphase materials. Part IV: Microstructure and thermal expansion coefficient. The properties of multiphase materials depend on the properties as well as on the geometry and geometrical arrangement (microstructure) of their phases. In many cases, where the material exists under extreme conditions (e. g. high temperatures, radioactive irradiation) not admitting direct property measurements or making them difficult and less accurate two dimensional pictures (micrographs) are available as the only reliable source of information. A stereologic image analysis then provides the tool to characterize the microstructure of multiphase materials quantitatively from their micrographs. Knowing equations describing the dependence of the properties on the microstructure characterized by appropriate parameters direct property measurements would become substitutable by stereologic analyses of the microstructure. Due to the fact, that this holds not only under extreme conditions the stereological microstructure analysis could also be used in conventional quality control to obtain properties without measuring them directly. Furthermore quantitative relationships between properties and microstructural parameters provide a better insight into the property-microstructuredependence thus enabling theoretically the pre-calculation of property improvements by optimalisation of the microstructure (“taylormade- materials”). This paper – subdivided in four articles – considers the derivation and the experimental proof of quantitative relationships between the microstructure and the conductivity, elastic modulus and thermal expansion coefficient of two phase matrix materials. In the present fourth part of this paper the derivation of the equations is considered which describe the dependence of the linear thermal expansion coefficient on the microstructure of two phase matrix materials quantitatively, the phases of which show linear elastic and isotropic behaviour. Calculated and measured thermal expansion coefficients of two phase materials from about 15 binary systems are compared agreeing sufficiently well. The general equation describing mathematically the relationship between the microstructure and the linear thermal expansion coefficient of two phase materials confirms theoretically, that closed pores do not affect the thermal expansion coefficient of porous materials. – At the end of the present article an engineering statement is made summarizing and judging the results obtained in the alltogether four parts of this publication.     

Nano-thermal transport array: An instrument for combinatorial measurements of heat transfer in nanoscale films

The nano-Thermal Transport Array is a silicon-based micromachined device for measuring the thermal properties of nanoscale materials in a high-throughput methodology. The device contains an array of thermal sensors, each one of which consists of a silicon nitride membrane and a tungsten heating element that also serves as a temperature gauge. The thermal behavior of the sensors is described with an analytical model. The assumptions underlying this model and its accuracy are checked using the finite element method. The analytical model is used in a data reduction scheme that relates experimental quantities to materials properties. Measured properties include thermal effusivity, thermal conductivity, and heat capacity. While the array is specifically designed for combinatorial analysis, here we demonstrate the capabilities of the device with a high-throughput study of copper multi-layer films as a function of film thickness, ranging from 15 to 470 nm. Thermal conductivity results show good agreement with earlier models predicting the conductivity based on electron scattering at interfaces.     

Thermal Conductivity Measurement of Submicron-Thick Films Deposited on Substrates by Modified ac Calorimetry (Laser-Heating Ångstrom Method)

Technological development, especially in microelectronics, necessitates the development of new and improved methods for measuring the thermal properties of materials, especially in the form of ultrathin films. Previously, modified ac calorimetry (laser-heating Ångstrom method) using a scanning laser as the energy source was developed and shown to provide accurate values of thermal diffusivity and derived thermal conductivity for a broad range of materials in the form of free-standing thin sheets or films, wires including fiber bundles, and some films on substrates. This paper describes further applications of the modified ac-calorimetry technique for measurements of the thermal conductivity of thin films deposited on substrates. It was used to measure successfully the thermal conductivities of 1000- to 3000-Å-thick aluminum nitride films, aluminum oxide films, etc., which were deposited on a glass substrate. It was also shown to be suitable for developmental measurements on submicron-thick chromatic films deposited on a PET substrate, which are photothermal recording layers, used in the media of CD-R drives of computer systems.     

聚乙烯醇基相变复合材料研究进展

亲水性聚乙烯醇(PVA)具有相对较好的可加工性、安全性及生物可降解性,被广泛用作支撑材料,可将相变材料封装在其三维网状结构中以解决相变组分泄露的问题并使复合材料获得良好的导热性。基于此,综述了分别利用分子间作用力、共价键将PVA基体与相变材料复合的物理共混法及化学接枝法,对比了两种方法用于封装相变材料的优缺点;总结了目前PVA基相变复合材料的类型、成型方法及性能,包括经湿法纺丝、干法纺丝及静电纺丝等方法制备的相变复合纤维,经物理共混法制备的相变复合膜以及经一步法原位复合制备的相变复合多孔材料;分析了复合材料中相变组分及成型工艺等对材料结构及相变蓄热、力学及热稳定性等性能的影响;同时展望了功能化PVA基相变复合材料的研究方向及发展前景。     

Microstructure-guided numerical simulations to predict the thermal performance of a hierarchical cement-based composite material

This paper presents a microstructure-guided numerical homogenization technique to predict the effective thermal conductivity of a hierarchical cement-based material containing phase change material (PCM)-impregnated lightweight aggregates (LWA). Porous inclusions such as LWAs embedded in a cementitious matrix are filled with multiple fluid phases including PCM to obtain desirable thermal properties for building and infrastructure applications. Simulations are carried out on realistic three-dimensional microstructures generated using pore structure information. An inverse analysis procedure is used to extract the intrinsic thermal properties of those microstructural components for which data is not available. The homogenized heat flux is predicted for an imposed temperature gradient from which the effective composite thermal conductivity is computed. The simulated effective composite thermal conductivities are found to correlate very well with experimental measurements for a family of LWA-PCM composites considered in the paper. Comparisons with commonly used analytical homogenization models show that the microstructure-guided simulation approach provides superior results for composites exhibiting large property contrast between phases. By linking the microstructure and thermal properties of hierarchical materials, an efficient framework is available for optimizing the material design to improve thermal efficiency of a wide variety of heterogeneous materials.     

Simultaneous Measurement of Thermophysical Properties and Dielectric Properties of PZT-Based Ferroelectric Ceramics by Thermal Radiation Calorimetry

Simultaneous measurements of thermophysical properties and dielectric properties have been performed for PZT-based ferroelectric ceramics. An apparatus based on thermal radiation calorimetry was used in the present measurements. Anomalies in the thermophysical properties were observed near the ferroelectric-to-paraelectric phase transition temperature. The anomalous peak was at almost the same temperature as the inflection point of the dielectric constant. It was found that modification of PZT with increasing Nb, Mg, Zn, and Sr causes a decrease of the Curie temperature and an increase of the hysteresis phenomena for the phase transition, and the values of the thermal conductivity increase with temperature similar to amorphous materials.     

基于孔尺度的泡沫金属强化相变储热材料传热性能数值模拟

导热系数低是影响相变储热材料应用的主要难题之一,而泡沫金属具有高热导率、高孔隙率以及高比表面积等特性,在相变材料中添加泡沫金属可实现强化传热。该文基于泡沫金属基3D微观结构W-P模型,重点分析了泡沫金属基复合相变材料有效导热系数与泡沫金属孔隙率以及孔径的关系,采用数值模拟方法利用该模型预测并验证了泡沫铝6101添加空气与水的有效导热系数,研究结果表明该模型能够精确预测泡沫金属材料有效导热系数,在此基础上预测了石蜡中添加泡沫铜的有效导热系数,结果表明,泡沫金属可以显著提高相变材料的导热系数,当泡沫铜的孔隙率为97.57%时,复合相变材料的导热系数与纯石蜡相比提高了13倍。研究结果对于相变储热材料的热物性强化研究具有一定参考价值。     

Experimental determination of the thermal conductivity of thin films

Two techniques have been developed to determine experimentally the thermal conductivity of thin solid films of thickness 500 Å or more at low and high temperatures. The first technique is a steady state and is suitable for measurements above room temperature. The method enables the thermal conductivity of eight film specimens to be measured simultaneously. The second technique is a transient one (an adaptation of Ioffe's method for bulk materials) and is suitable for measurements in the temperature range 100–260 K. The two techniques have been used to make measurements of thin films of copper and various crystalline and amorphous semiconductors. The values of the thermal conductivity for thick copper films by both techniques agree quite well with the bulk values.     

Cross-property connections for copper–graphite composites

Copper–graphite composite materials in the range of 0–10 vol% of carbon phase were prepared from the mixture of copper and graphite powders by hot isostatic pressing. The microstructure, mechanical (tensile strength, elongation to fracture) and physical (electrical and thermal conductivity) properties of composite samples were investigated, and the cross-property connections were calculated. It was shown that electrical and thermal conductivity cross-property (Lorenz number) is almost constant and increases only slightly (no more than 10 % increase was observed). This implies that in the investigated composition range the Lorenz number of a copper–graphite composite system behaves according to Franz–Wiedemann law for pure metals at constant temperature. On the contrary, the conductivity to tensile strength cross-property connections showed significant linear increase (over 200 % in the investigated composition range) for both electrical conductivity and thermal conductivity of composite materials. The cross-property connections of conductivity to the elongation to fracture exhibit a nonlinear dependence on the volume fraction of graphite.     

Effect of Ag doping on structural, electrical and dielectric properties of TiO2 nanoparticles synthesized by a low temperature hydrothermal method

Silver (Ag) doped thermally stable TiO₂ nanoparticles in the anatase phase have been synthesized by a low temperature hydrothermal method. The formation of anatase phase has been investigated by X-ray diffraction. Thermogravimetry and differential scanning calorimetry have been used for thermal studies. Scanning electron microscopy, transmission electron microscopy and energy dispersive spectroscopy have been used for the morphology and composition studies. Surface areas were studied by the Brunauer-Emmett-Teller method. Dielectric properties were studied for dopant levels of 0.25, 0.5 and 1.0?wt% at 300?K in the frequency range of 42?Hz–5?MHz. The crystallite size, alternating current (AC) conductivity and dielectric properties of undoped TiO₂ nanoparticles were greatly affected by doping with Ag. At high frequencies, the materials showed high AC Conductivity and low dielectric constant.     

Enhancing the thermal conductivity of n-eicosane/silica phase change materials by reduced graphene oxide

Reduced graphene oxide (rGO) sheets prepared by different methods are incorporated to boost the thermal conductivity of organic phase change materials (n-eicosane) in silica microcapsules. Low concentration (1 wt%) of graphene dosing already results in notable increase of the thermal conductivity. The preparation methods of rGO significantly affect the thermal properties of the composite. With 1 wt% dosing, sodium borohydride (NaBH₄) reduced rGO increases the thermal conductivity by 83% and decrease the phase change enthalpy by 6%. On the other hand, the thermal reduced rGO increases the thermal conductivity by 193% but leads to a 15% loss of the phase enthalpy. The difference is attributed to the different surface morphology and functional groups of the rGO sheets.     

Thermo-electrical characterization of Sn–Zn alloys

The thermo-electrical properties of Sn–Zn alloys have been investigated for four different compositions. The SEM micrographs and EDX analysis of the samples have been obtained. The electrical resistivity measurements were performed by using four-point probe technique in the temperature range 300 K–575 K. The resistivity of samples increases linearly with temperature and the electrical conductivity is inversely proportional to temperature. The electrical current preferentially flows through the pure Sn phase having lower resistivity in the matrix. Also, thermal conductivity of samples has been measured by using the radial heat flow method. It has been found that the thermal conductivity decreases with the increasing temperature and composition of Sn. The results were consistent with available experimental results of other studies. Finally, the temperature coefficient of electrical resistivity and the temperature coefficient of thermal conductivity have been determined, which were independent from the composition of Sn and Zn.     

Measurements of thermal conductivity of epoxy impregnated NbTi andNb3Sn windings

The effective thermal conductivity in the transverse direction is one of the less predictable coil parameters. In the framework of the construction of a high-field solenoid (18 T, 100 mm free bore at 4.2 K) at the LASA lab, an experimental apparatus was built for measurement of thermal conductivity of pure materials as well as of composites and coil blocks. The main aim of the experiment is the measurement of the thermal conductance of pieces of real NbTi and Nb₃Sn windings. The authors discuss how the thermal conductivity affects the quench properties of superconducting magnets. The main features of the experimental apparatus are described together with a calculation and a measurement of the thermal loss. Preliminary measurements on some pure materials as well as on a coil block are presented     

An Oscillating Boundary Temperature Method for the Determination of Transient Thermal Conductivity and Internal Heat Generation with a Comparison to a Transient Hot-Wire Method

An oscillating boundary temperature (OBT) method is proposed to simultaneously determine transient thermal properties including thermal conductivity, thermal diffusivity, internal heat generation, and volumetric heat capacity for exothermic solids and semi-solids over a narrow, controlled temperature range by using internal temperature measurements of the thermal wave. A comparison of this method and a transient hot-wire (THW) method is conducted in the presence of heat generation using physical properties which change over time. The advantages and disadvantages of both methods are discussed. The OBT method is potentially useful for the analysis of exothermic solid or semi-solid materials such as hydrating (freshly mixed) cement and concrete, polymers and composites undergoing polymerization reactions, and biological tissues.