Materials Science Forum Vols. 743-744

Abstract: nfluence of physical boundary conditions on the thermoelectric transportation coefficients has been analyzed starting form Onsager equations. Four boundary conditions have been considered: electric short, i.e, the chemical potential difference is zero; electric open, or electric current free; isothermal, i.e., no temperature difference; adiabatic, or heat flux free. Four kinds of thermoelectric equations have been derived with different boundary conditions. It was found that the influence of boundary cannot be ignored when figure-of-merit is near and larger than 1.0. This results could be useful in designing thermoelectric device with high performance thermoelectric materials.

Abstract: In the present study, the glass microsphere dispersed Bi-Sb thermoelectric materials have been fabricated through mechanical alloying followed by pressureless sintering. The phase composition and the microstructure were investigated by X-ray diffraction (XRD) and scanning electron microscopy (SEM) analysis. Electrical conductivity, Seebeck coefficient and thermal conductivity were measured in the temperature range of 77~300 K. The ZT values were calculated according to the measurement results. The results showed that the electrical conductivity, Seebeck coefficient and thermal conductivity decreased by adding glass microsphere into Bi-Sb thermoelectric materials. However, the optimum ZT value of 0.24 was obtained at 260 K, which was increased 10% than that of the Bi-Sb matrix. So it is confirmed that the thermoelectric performance of Bi-Sb-based materials can be improved by adding moderate glass microspheres.

Abstract: In this investigation, the multi-walled carbon nanotubes (MWCNTs) were dispersed in an interpenetrating polymer networks (IPNs) based on acrylate and cycloaliphatic epoxy resin (CER). The influences of the external electric field on the MWCNTs dispersion and the microstructure of host matrix were evaluated by means of optical microscopy, scanning electric microscopy (SEM) and atomic force microscopy (AFM), respectively. The microscopy measurements showed that the distribution of the MWCNTs depended strongly on the properties of the applied electric field. Applying AC electric field to the liquid MWCNTs/thermoset systems during curing stage could redistribute the MWCNTs, which arranged them in chain-like structures and oriented fibrous inclusions parallel to the applied electric field. However, the similar phenomenon was not observed in DC electric field. From the observations of AFM measurement, it was found that the utilization of the external electric field resulted in the nanostructured twophase structures in the resulting MWCNTs/thermoset nanocomposites. These novel electric-field-induced morphology transformations were mainly attributed to the curing process under the applied electric fields. The relationships between the microstructures and various physical properties of nanocomposites were also presented in this paper. The resulting nanocomposites displayed the interesting dielectric properties and the thermal stability properties, which significantly depended on their special microstructures of inclusions and the host matrix.

Abstract: Significant progress has been made in thermoelectric materials during the last decades and it is found that thermoelectric thin film materials have high thermoelectric conversion efficiency. ZnO based thermoelectric materials, such as ZnO:Al (AZO), are considered as the most promising oxide materials for high-temperature, nontoxic and low-cost thermoelectric application. In this work, the effects of annealing temperature on the thermoelectric properties of AZO thin films prepared by direct current magnetron sputtering were investigated. The results indicate that the Seebeck coefficient of AZO thin films increases and the resistivity decreases as increasing of annealing temperature. Among the prepared AZO films in this work, the maximum absolute value of Seebeck coefficient is 460 μV/K and the minimum resistivity is 3.25×10^-4 Ω·m. The sample annealed at 773 K has a maximum power factor value of 1.46×10^-4 W/mK² at 620 K with a moderate Seebeck coefficient of-355 μV/K and a electrical conductivity of 1.16×10³ S/m.

Abstract: A TEG composed of p-type higher manganese silicide and n-type magnesium silicide-stannide was evaluated by theoretical simulation based on finite element method and steady-state approximation. The geometry factors, heat flux, power output and the thermal electrical conversion efficiency of the TEG were calculated by applying the measured thermoelectric parameters of each leg into the simulation tool. Furthermore, the contact effect on the performance of the TEG was analyzed by separately introducing a contact layer between the thermoelectric legs and the metal layers having specific electrical and thermal conductivity. It was found that the different cross-sectional areas were required for the p-and n-type legs to achieve maximum module output or conversion efficiency. In ideal contact state, a promising efficiency of 8.29% can be obtained at a given temperature gradient. On the other hand, the performance of the TEG might be seriously deteriorated if the electrical or/and thermal resistance of the contact layer increased.

Abstract: Microstructure engineering of thermoelectric materials can resolve the conflicts of electrical and thermal transports. Especially, one-dimensional structure can obviously improve the thermoelectric figure of merit because of its crystal anisotropy and strong quantum confinement effect. In this paper, the Te nanowires, one-dimensional core-shell heterostructure of Te/Bi and Te/Bi₂Te₃ were controlled synthesized by microwave assisted chemical synthesis. The effect of PVP concentration and reductant dropping rate on the microstructure of the Te nanowires were investigated. The experimental results showed that with increasing the amount of PVP, the Te nanowires got less crystallinity and its surface become more rough due to its steric hindrance effect. With decreasing reductant dropping rate, the longer and thiner Te nanowires were obtained. Epitaxial growth can describe the relation of core Te and shell Bi (or Bi₂Te₃). It has been found that Bi shell uniformly surrounded around Te nanowires core, but Bi₂Te₃ sheets were perpendicular to the c-axis of Te nanowires. The different core-shell heterostructure structure can be obtained by adjusting reaction conditions and controlling diffusion kinetics of Te and Bi.

Abstract: The alkali-activated slag mortar (AASM) has disadvantages of high shrinkage and therefore low crack resistance. In this paper, water quenched raw slag was used to replace sand for making AASM, and effects of crumb rubber on the drying shrinkage and plastic shrinkage cracking of AASM were investigated. The experimental results showed that the incorporation of crumb rubber into AASM reduced the drying shrinkage of AASM and improves the plastic shrinkage cracking resistance. Compared with the referenced mortar, after the 90-day drying the shrinkage of AASM containing 3% rubber decreased by 28.5% and crack index decreased by 64%. However, the addition of higher dosage of crumb rubber may bring negative effect on the flexural strength and the compressive strength of AASM. It is therefore proposed that the optimum rubber content be around 3% in order to balance both the mechanical performance and the plastic and the drying shrinkage.

Abstract: The key influences of foamed concrete and the optimum preparation technology were studied. The performance of foamed concrete was analyzed by the compressive strength testing, SEM. Results show that the flowability of cement paste is good when water-cement ratio is 0.4. The amount of foam added in sulphoaluminate cement is not able to exceed 3.5L/kg. However, the dilution multiple of foaming agent is near concentration and the mixing time depends on the foam quantity. When the water-cement ratio is 0.4, the foam quantity is 3 L/kg and the foaming agent is diluted 30 times. When the mixing time is 60s, the dry density is around 380kg/m3 and the 7d compressive strength reaches to 0.9MPa.

Abstract: Calcium carbide Slag is from CaC₂ hydrolysis reaction and will do harm to land and make pollution. Calcium carbide slag can be a substitute for limestone to produce clinker with a high portion of CaO as an excellent calcium raw material. As a kind of industrial wastes, the properties of calcium carbide slag differentiate from that of natural limestone. In the present investigation, the modern analysis methods of XRF, XRD, DTA/TG, petrographic analysis were used to compare carbide slag and limestone, and the results showed that the main chemical compositions of the calcium carbide slag were basically the same with that of natural limestone. Comparing with limestone materials, calcium carbide slag had a higher content of CaO, and the main mineral phase constituent of limestone was CaCO₃, whereas the main mineral of calcium carbide slag was Ca (OH)₂ with a lower decomposition temperature. It has been found that under the same temperature the amount of C₃S in the clinker of calcium carbide slag batching was slightly less than that of limestone batching.

Abstract: In this paper, the artificial aggregates were prepared by limestone and industrial wastes in accordance with the ratio of cement raw meal, to obtain the recycling concrete replacing natural aggregates. Cement clinker could be regenerated by these recycling concrete and the aim of recycling could be realized. After these concrete was cured for 90 days, then was crushed and grinded, the phase transformation of recycling concrete in recalcination would been studied by XRD and so on. The experimental results showed that the composition of the recycling concrete met the demand of clinker modulus and C₂S, C₃S, C₄AF, C₃A were well crystallized at 1400 °C. So these recycling concrete could be directly calcined. The calcium hydroxide dehydrated around the temperature of 500°C. The calcium silicate hydrate was dehydrated gradually between the temperature of 400°C and 1000°C. At the same time, C₂S crystallized during the temperature range. C₄AF and C₃A stared to crystallize around the temperature of 1000°C. Due to the composition of these concrete was complex, the lowest eutectic temperature dropped. C₃S started to crystallize around the temperature of 1250°C and the best crystallization was achieved at the temperature of 1400°C.