Thermophysical Properties of Zirconia Toughened Alumina Ceramics with Boron Nitride Nanotubes Addition

Abstract

The demand for high thermal conductivity substrates with electrically insulating materials are increasing with the emerging markets in power electronics and mobile telecommunication device packages. Effective heat transfer in those packages is important to provide high performance and reliability of the product. This paper mainly presents the thermophysical properties of zirconia toughened alumina ceramics with the addition of small amount of boron nitride nanotubes (BNNTs). The effects of the boron nanotubes addition on the sintering behavior, the microstructure and the thermal properties of the yttria-stabilized zirconia toughened alumina (YZTA), nanocomposite ceramics are investigated. The addition of 0.3?wt% boron nitride nanotubes into the YZTA matrix enhanced the thermal diffusivity as well as a mechanical strength. Above all, the addition of boron nitride nanotubes greatly decreased the coefficient of thermal expansion (CTE) of the composites in which the CTE of pure alumina increases with increasing temperatures. Moreover, the BNNTs added YZTA composites revealed a drastic decrease in CTE at high temperature range, 400–800?°C. This enhanced thermal stability of YZTA–BNNT composites may have a potential application to the high temperature structural ceramics and high power semiconductor packaging substrate.     

Biogas reforming over bimetallic PdNi catalysts supported on phosphorus-modified alumina

A series of bimetallic PdNi catalysts supported on alumina modified with different amounts of phosphorus (0.5-5 wt%) were prepared. The effect of phosphorus content on the structure, surface properties and catalytic behavior of supported PdNi catalysts in biogas reforming was studied. The physicochemical properties of the samples were characterized by using different techniques: N₂ adsorption-desorption isotherms, X-ray diffraction (XRD), UV-vis diffuse reflectance spectroscopy (DRS), X-ray photoelectron spectroscopy (XPS), temperature-programmed reduction (TPR), temperature-programmed desorption of ammonia (TPD), thermogravimetric and differential thermal analysis (TG/DTA) and scanning transmission electron microscopy (STEM). The catalytic properties of the catalysts were evaluated in the reaction of reforming of methane with CO₂. It was shown that increasing the P content (≥1 wt%) leads to agglomeration of the metal Ni particles, as well as to increase of the total acidity of the catalysts. Within bimetallic system, the PdNi catalyst with 0.5 wt% phosphorus showed the best performance and stability caused by the presence of highly dispersed nickel particles on the catalyst surface due to the strong interaction between supported species and alumina.     

Optical characterisation of materials and systems for daylighting

The measurement of BRTF (Bi-directional reflectance and transmittance function) is described using a new instrument which is capable of supplying BRTF data and algorithms for use in computer simulations directly on diffuse materials and indirectly on large samples and sub-systems. A high sensitivity and dynamic range is needed to achieve low minimum observable BRTF and the role of angular resolution are discussed with examples. Forward scattering with extended tails is found to dominate pigmented polycarbonate. Slatted blinds are discussed as examples of systems where azimuth is important.     

Thermal characteristics of the lunar surface layer

The thermophysical properties of the fines from the Apollo 12 landing site have been determined as a function of their relevant parameters. These properties include the thermal conductivity, thermal diffusivity, directional reflectance and emittance. The density used was the same as that observed from the returned core-tube samples and so should be close to the true density of the surface layer at the Apollo 12 site. The measured properties are used to calculate the diurnal temperature variation of the moon's surface as well as for several depths below the surface. The maximum surface of 389°K is obtained at lunar noon while the minimum temperature of 86·1°K is obtained at sunrise. It is shown that the most significant effects on temperature, as compared with previous calculations, are caused by using the directional reflectance which controls the amount of solar energy absorption during the day in place of a constant hemispherical reflectance. The results are compared with previous analyses and remote measurements.     

Development of a model for calculating the longwave optical properties and surface temperature of a curved venetian blind

This article is about the development of a mathematical model for calculating the longwave optical properties of a curved venetian blind. The calculated optical properties are used to determine the performance of the glass window installed with a venetian blind in terms of thermal comfort. The blind, whose optical properties are considered nonspecular, is modeled as an effective layer. The effect of slat curvature is included in the developed model. A six surface enclosure formed by two consecutive slats is used to analyze for the longwave optical properties of the effective layer. The longwave optical properties, transmittance, reflectance, absorptance and emittance are developed by using the radiosity method. The steady state energy balance method along with the developed longwave optical properties are used to determine the surface temperature of the effective layer. The empirical expression for the total heat flux from the indoor glass window surface with an adjacent venetian blind is adopted in the developed model. The surface temperature of the blind, which is the key parameter for calculating the thermal performance of glass windows with venetian blinds with respect to thermal comfort, is chosen as the parameter used for the model validation. The predicted surface temperature of the venetian blind is compared with the surface temperature of the venetian blind obtained from the measurement. The agreement between the predicted temperature and the measured temperature is good.     

Experimental and theoretical studies of thermal conductivity,viscosity and heat transfer coefficient of titania and alumina nanofluids

Thermal conductivity, viscosity and heat transfer coefficient of water-based alumina and titania nanofluids have been investigated. The thermal conductivity of alumina nanofluids follow the prediction of Maxwell model, whilst that of titania nanofluids is slightly lower than model prediction because of high concentration of stabilisers. None of investigated nanofluids show anomalously high thermal conductivity enhancement frequently reported in literature. The viscosity of alumina and titania nanofluids was higher than the prediction of Einstein–Batchelor model due to aggregation. Heat transfer coefficients measured in nanofluids flowing through the straight pipes are in a very good agreement with heat transfer coefficients predicted from classical correlation developed for simple fluids. Experimental heat transfer coefficients in both nanofluids as well as corresponding wall temperatures agree within ±10% with the values obtained from numerical simulations employing homogeneous flow model with effective thermo-physical properties of nanofluids. These results clearly shows that titania and alumina nano-fluids do not show unusual enhancement of thermal conductivity nor heat transfer coefficients in pipe flow frequently reported in literature.     

Solar sintering of cordierite-based ceramics at low temperatures

Solar furnaces allow materials processing at much higher heating and/or cooling rates compared with those in the conventional industrial and laboratory processes using electric furnaces. During the course of our recent works using solar furnace, we demonstrated usability of solar furnace for sintering-consolidation of oxide ceramics (alumina) and non-oxide ceramics (WC with Co additive) as well as for producing raw material powders including carbide and carbonitride of transition metals and silicon. Being encouraged by these earlier solar-consolidation experiment results obtained for ceramics with relatively simple composition, we decided to continue this line of solar-sintering experiments for other industrial ceramics with more complicated composition. In this work, two types of commercial ceramic powder mixtures, BL7 and RP7, supplied by Rauschert Portuguesa Lda, were subjected to sintering under concentrated solar beam at 950 °C for 20 min. No evidence of formation of cordierite (2MgO · 2Al₂O₃ · 5SiO₂) phase was detected for the present solar sintered specimens whereas evidence of cordierite phase formation was detected for the RP7 powders heated to 950 °C in a laboratory electric furnace. As such, the present results implied that, for the preparation of consolidated cordierite using solar furnace, control over the heating rate is of critical importance as well as selection of the starting materials and setting of the processing temperature.     

Effect of modelling solar radiation on the cooling performance of radiant floors

When modelling buildings, solar radiation has a large impact on the thermal balance because it usually heats the rooms. In radiant systems that are used for heating and cooling buildings, solar radiation has a large influence both on indoor temperatures and on the efficiency of the radiant system.Many analyses have already been carried out in order to study how beam and diffuse radiation can be distributed in a room. One of the most difficult issues, when modelling room thermal balance, is how to simulate the solar radiation when it enters the room, which in turn depends on the reflectance characteristics of the surface finishing elements.In this study, four different radiation models have been applied in order to solve an overall detailed, dynamic thermal balance in a room with pipes embedded in the floor. Two of the models are detailed; the other two consider the radiation entering the room to be diffuse radiation. As for the behaviour of the impinging solar radiation on the covering materials in a room, measurements have been carried out to determine the reflectance coefficients, which will be used in simulations for characteristic materials used in buildings.Results of the simulations show that a simplified model, which considers solar radiation as uniformly distributed in a room, cannot be used for a detailed comfort analysis; however, when looking at the cooling output of a radiant floor system at the design stage, a simplified model can predict energy transfer to a certain level of accuracy. Moreover, results coming from combined measurements and simulations show that the reflectance characteristic of the covering materials does not affect the cooling capacity of the radiant floor systems, since the most important parameter for cooling performance is the thermal conductivity of the covering layer.     

Effect of hot corrosive environment on ceramics

We investigated the effect of moisture on the high-temperature stability of several ceramics at 1700 °C in an atmosphere of oxygen/water vapor (O₂/H₂O). The melt growth composite (MGC) which is composed of alumina/yttrium-aluminum-garnet, Al₂O₃/YAG, was the most stable among ceramics at 1700 °C in this atmosphere, showing only a slight change in microstructure, flexural strength and volume after an exposure for 200 h. Thus, MGCs are among the most promising ceramics for structural applications at ultra-high temperatures.     

Thin-film silicon solar cells on mullite substrates

Here we review the different methods used to create thin-film silicon solar cells on the most suitable ceramic substrates, namely alumina and mullite. The chemical vapor deposition (CVD) process on bare ceramics, the CVD on glassy layers (CVD-OGL) process, and the aluminum-induced crystallization (AIC) technique are reported and compared in terms of grain size, grain distribution and crystallographic orientation. The electrical quality of such layers was investigated through their open-circuit voltage before and after hydrogenation. Values up to 410 mV were measured on n⁺p mesa cell structures on ceramics.     

Analyzing the effect of the longwave emissivity and solar reflectance of building envelopes on energy-saving in buildings in various climates

A dynamic computer simulation is carried out in the climates of 35 cities distributed around the world. The variation of the annual air-conditioning energy loads due to changes in the longwave emissivity and the solar reflectance of the building envelopes is studied to find the most appropriate exterior building finishes in various climates (including a tropical climate, a subtropical climate, a mountain plateau climate, a frigid-temperate climate and a temperate climate). Both the longwave emissivity and the solar reflectance are set from 0.1 to 0.9 with an interval of 0.1 in the simulation. The annual air-conditioning energy loads trends of each city are listed in a chart. The results show that both the longwave emissivity and the solar reflectance of building envelopes play significant roles in energy-saving for buildings. In tropical climates, the optical parameters of the building exterior surface affect the building energy-saving most significantly. In the mountain plateau climates and the subarctic climates, the impacts on energy-saving in buildings due to changes in the longwave emissivity and the solar reflectance are still considerable, but in the temperate continental climates and the temperate maritime climates, only limited effects are seen.     

Concentration fields in the solidification processing of metal matrix composites

A numerical investigation of the solidification of a binary alloy (Al-1.0 wt.% Cu) around cylindrical fibers with different fiber layouts and thermophysical properties was undertaken to gain insight into the processing of fiber-reinforced metal matrix composites. The focus of this study was on solute transport and redistribution during the solidification process, and the resulting concentration fields in the solidified alloy matrix. Change of phase in the alloy was formulated using a modified version of the temperature-transforming method for the energy equation. A source term that accounts for the solute rejection at the interface was incorporated into the solute concentration equation to model solute redistribution at the interface. Detailed results were obtained from the numerical simulations of low-(alumina) and high-(copper) conductivity fibers in inline and staggered configurations. Effects of the fiber pitch (longitudinal spacing) and transverse spacing were investigated. Higher concentrations of solute were seen to accumulate around copper fibers than for alumina fibers. With an initial, uniform concentration of 1.0 wt.% Cu in the melt, the maximum-recorded solute concentration in the domain for alumina fibers was 1.26% while that for copper fibers was 3.11%. For inline fibers, increasing the fiber pitch beyond a critical value did not change the overall shape of the local solute distribution around the fibers: the critical pitch for alumina fibers was found to be roughly 2.5 fiber diameters while that for copper was 2 fiber diameters.     

An application of thermal microscopes to the measurement of thermal effusivity of films

The application of thermal microscopes to the measurement of local thermal properties has drawn considerable scientific interest. We report on the application of a thermal microscope to the measurement of thermal effusivity for films comprising alumina deposited on a substrate, which were fabricated by an electrophoretic deposition method. The measured data was analyzed to consider the undulations on the sample surface The thermal effusivity of these samples was approximately 1×10³ Js^?0.5m^?2K^?1; this value is smaller than that for dense alumina because the alumina grain makes contact with a point. © 2008 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20227     

Preparation of calcium chloride-anodized aluminum composite for water vapor sorption

In the present study, we propose a novel solid sorbent, which is composed of anodized aluminum and calcium chloride, for water sorption chillers and heat pumps. Aluminum is electrolyzed in acid baths with the result that a thin and porous aluminum oxide film is formed on its surface. The anodic alumina films prepared using an electrolytic bath of sulfuric acid have an average thickness from 8 to 100 μm and mean pore diameter from 6 to 22 nm. The physical properties of anodized aluminum can be controlled by electrolytic conditions such as current density, processing time and temperature. A calcium chloride-anodized aluminum composite sorbent has been prepared by a solution impregnating method. The amount of calcium chloride impregnated on the alumina films reaches up to 16.1 wt%, and the deposition of calcium chloride in the alumina layer has been confirmed by X-ray analysis. The experimental results shows that bare anodized aluminum adsorbs very little water vapor, but the prepared calcium chloride-anodized aluminum composite is active against water vapor even in the low pressure range. Hence, this composite is a promising sorbent for adsorption chillers and heat pumps using water as working fluid.     

Using fiber optics to tap the sun's power

This paper discusses a solar energy collection system in which optical fibers are used to transport energy from a single-stage and a double-stage, three-dimensional Compound Parabolic Concentrator (CPC). After developing a thermo-mathematical model for the assembly of CPC and fibers, numerical simulations are used to optimize the system design. The modules filled with plastic and glass are shown to perform considerably better than those filled with air. A two-stage system performs better than a single-stage module. CPC surface reflectance improves the yield but an increase in fiber length decreases the performance.     

Beta-alumina electrolyte for use in sodium/sulphur batteries Part 2. Manufacture and use

Part 1 of this two-part review was concerned with the fundamental chemical and physical properties of sodium beta-alumina. In Part 2 we consider the ceramic science and technology involved in the manufacture of electrolyte tubes and their use in sodium/sulphur batteries.The specification set for β-alumina tubes is first outlined, followed by a review of the fabrication procedures which have been employed. The problems of quality control and the elimination of flaws are emphasised. Mechanical and thermal expansion data which have been measured for β- alumina ceramics are discussed. The surface interaction effects between the electrolyte and the electrode materials (sodium and sulphur) are considered, and it is suggested that wetting effects are important in determining both the cell resistance and the life span of the electrolyte in use.The failure modes of β-alumina electrolyte tubes fall into three categories — electrical breakdown, mechanical shock, and thermo-mechanical failure — but in a real situation these may be interactive. Electrical breakdown may be either a result of localised high current densities during recharge or a consequence of too high an imposed voltage at top of charge (dielectric breakdown). Published work on these failure modes is reviewed. Finally, the present state-of-the-art in manufacturing β-alumina electrolyte tubes is summarised, and the need for further research and development on improving their durability is noted. As one component of a system — the Na/S cell — the electrolyte cannot be viewed in isolation and its durability is a function both of its perfection of manufacture and the conditions of use in the cell.     

Alumina filtering ceramic based on aluminum oxide melted in solar furnace and phosphate binding agents

The effect of different cellulating agents in the form of reprocessed Claus catalyst, gypsum, and coke on the physical and technical characteristics and permeability of alumina filtering ceramics based on aluminum oxide melted in a solar furnace and phosphate binding agents is studied. Samples with high permeability that are nearly monostructural with regard to the transport pore size distribution are produced.     

Thermophysical properties of supercritical H2 from Molecular Dynamics simulations

The ability to predict thermophysical properties of molecular hydrogen with high accuracy, especially at high pressures, is crucial to design and to operate processes involving compressed hydrogen. Molecular simulations comprise an adequate tool to investigate both thermodynamic and transport properties of different molecular systems using a single potential energy surface model. Such a potential is called a force field. Here we propose a new single-site force field for pure hydrogen using a Mie potential to describe the intermolecular interactions. The proposed force field yields better predictions of thermodynamic properties when compared to other available force fields that use Lennard-Jones interaction potential. The new force field is also able to predict transport properties with reasonable accuracy.     

Design criteria in PCM wall thermal storage

Performance criteria for a wall with an enclosed phase-change material (PCM) are discussed. Qualitative conclusions about how best to choose the PCM are derived and illustrated by means of computer simulations. Material properties dominating the heat-transfer process are identified and relations between them are determined.     

Microstructure and optical properties of silicon nitride thin films as radiative cooling materials

SiN_X thin films were prepared by the RF plasma-enhanced chemical vapor deposition method. Composition, structure, surface morphology and optical properties of the thin films were analyzed by X-ray fluorescence, IR transmittance, IR reflectance and SEM. The results show that the composition of the films is SiN_0.35. Nitrogen atoms take part in the reaction with silicon atoms and Si–N bonds are formed. There are also some Si–H and N–H bonds in the films. The films have very low hemispherical IR reflectance across the full 8–13 μm band and high hemispherical reflectance elsewhere, which indicates that silicon nitride films can be used as good radiative cooling materials. The surface morphology and growth mechanisms of the films were also explained.