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Abstract In recent years, there have been strong demands for the processing of nonoxide ceramics as structural materials in place of metals and alloys and for use in harsh environments. They have received increased attention because of their unique characteristics. The high proportion of covalent bonds in the carbide and nitrides crystal structures is responsible for the remarkable combination of properties such as high elastic modulus and hardness, high melting points, low thermal expansion, good chemical resistance, high corrosion and wear resistance and high temperature properties. Among the established advanced ceramic materials are Aluminum Nitride (AlN) and Silicon Carbide (SiC), both of which offer features that can benefit specific applications. SiC/AlN ceramics are expected to be beneficial and important for consideration in new applications, such as microwave absorption in highpower amplifiers and microwave components, sensor materials, thermoelectric conversion elements, solar energy absorber and new wide bandgap materials. Problems definition: The limited availability of fossil fuel and nuclear energy carriers, the environmental impact associated with the wide spread application of these fuels, the increasing problems of CO2 and energy security concerns have forced the world to find other alternatives and strengthened interest in alternative, non-petroleum based sources of energy. Moreover, it was estimated that more than 60% of the world energy is lost in vain worldwide, mostly in the form of waste heat. For that, Renewable energy and thermoelectric energy conversion sources have to be introduced into our heat, fuel and electricity market to a much larger extent and at an accelerated rate. Summary 1 9 3 On the other hand, despite of the great importance of the non oxide ceramics especially the carbide and nitride ones as SiC and AlN, they still suffer from some shortages in their reliability, durability and toughness when used in their monolithic form. Such defects will cause their limitation in some aggressive and load bearing applications. In order to compensate these deficiencies, optimizing a new material with enhanced characteristics should be involved. The purposes of this study are to provide a solution of the energy deficiency problems through introducing the concentrated solar radiation to our system. This target can be achieved by optimizing new carbide/nitride ceramic absorber materials work as volumetric solar receiver with high efficiencies and low cost. Carbide/nitride ceramics have become of interest for volumetric receivers for solar tower plant applications. They are capable of producing high outlet air temperatures which in turn enhance the system efficiency. Their durability and lifetime can be extended owing to their aggressive characteristics. Moreover, this non oxide ceramics will act as thermoelectric material with high performance, thermal stress durability and expected high life cycle. The other purpose of this work is to improve the physical, thermal, mechanical and the other properties of the carbide/ nitride (SiC and AlN) ceramics. This goal can be accomplished by the combination of SiC and AlN in one composite structure. This can create a new material with unparalleled properties that can overcome the drawbacks of the two materials by combining their desirable properties. This can be achieved by controlling the ratio of AlN and SiC in this composite in order to compensate their shortages. So that, several SiC/AlN composites and foams structure with different AlN content were produced by the pressureless sintering method and replica technique. A detailed study of the processing and synthesis of the Summary 1 9 4 carbide/nitride composites was provided. Different characteristics in terms of thermal, mechanical, electrical and thermoelectical properties were examined and evaluated. In this work, porous SiC/AlN ceramic foams with different compositions were produced by replica technique for solar energy and high temperature applications. For that purpose, several SiC/AlN composites with different AlN content (0-40 wt%) were produced by pressureless sintering method. Different controlling parameters for the reaction-densification response of the different composites were discussed in terms of sintering additives (0 wt%, 2 wt % Y2O3 and 2.5 wt% Y2O3+Al2O3), sintering temperature (1550, 1650, 1750 and 2080 oC/2h) and different sintering atmospheres (argon/vaccum and nitrogen/vaccum). Phase analysis, densification parameters, and microstructure of the obtained composites were inspected. In addition, technological characteristics in terms of thermal (thermal diffusivity and conductivity, thermal expansion and its coefficient, thermal shock resistance, and thermal stress simulation), mechanical (cold crushing strength), and thermoelectical (electrical resistivity, Seebeck coefficient, figure of merit, and power factor) properties were examined and explained based on their phase analysis and microstructure. Accordingly, the optimum conditions for producing a dense foam struts for the solar receiver foam were optimized. Moreover, factors affecting on producing well- stabilized water based SiC and AlN suspensions, and their rheological properties were controlled and investigated with different solid loading using different dispersing agents with different concentrations. The final SiC/AlN foam structures were produced by replica technique based on the optimum conditions released from suspension and sintering. Evaluation and characterization of the final foam structure in terms of (XRD phase analysis, dimension change, Summary 1 9 5 microstructure evaluation, density and cellularity (ppi) measurement were inspected and analyzed. The results showed that: Increasing sintering temperature of the investigated compositions to 2080 oC/2h with using 2.5% alumina and yttria as sintering additives has promoted the sintering process and densification behavior through liquid phase and solid solution formation. Sintering of SiC/AlN composites in Ar/vacuum atmosphere gave the highest density (80 % of theoretical density), homogenous microstructure and complete formation of 2Hss SiC/AlN solid solution reaction. Complete single phase 2Hss SiC/AlN solid solution was formed and its content increased with the increment of AlN wt% and the sintering temperature. In addition, 2H- 6H SiC/AlN solid solution was formed in composites with AlN content higher than 20 Wt%. On the other side, sintering of composites in N2/vacuum atmosphere gave poor density (56.5 % of theoretical density). Besides, the produced composites did not form a single phase or complete solid solution. Nitrogen atmosphere restricted the formation of completely dissolved 2Hss solid solution reaction. XRD analysis and microstructure examination confirmed the β→α-SiC transformation in Ar/vaccum atmosphere through the appearance of 4H SiC with elongated grain morphology. On the contrary, nitrogen atmosphere inhibited the β→α-SiC transformation and prevents its occurrence. Consequently, β-SiC retains its cubic crystal structure. Its microstructure gave a combination of hexagonal and cubic grains of SiC without any appearance of elongated form grains. The densification parameters in terms of apparent porosity and bulk density and also the relative density of the different composites were enhanced by increasing the sintering temperature using sintering additives in controlled atmosphere (Ar/vaccum). At lower sintering temperature, the densification Summary 1 9 6 parameters were enhanced with increasing the AlN content due the formation of liquid phase. However at high sintering temperature (2080oC), the densification parameters behavior was inversed. They deteriorated with the increment of AlN wt%. This demeanor was explained by the mass transfer by vapor condensation which is dominant in AlN rich composites at high sintering temperature (>2000 oC) as compatible with literature. AlN vaporizes more easily than SiC, and mass transfer readily occurs by vapor condensation at high temperature. Moreover, it was found that the dropping of density and porosity values of composites sintered in N2 compared to Ar atmosphere was due to the higher mass loss accompanied by sintering in N2 atmosphere. Microstructure of the pressureless sintered SiC/AlN composites was investigated. It showed uniform and gradual distribution of AlN and SiC from composite 0 (0% AlN) to composite 4 (40% AlN). The matrix for all composites showed a homogeneous distribution resulting from the good achieved homogeneity and from using small sized particles. It was found that insertion of AlN to SiC ceramic extremely affected on its microstructure. With addition of AlN, composites predominantly showed elongated grain morphology. Furthermore, addition of yttria and alumina sintering additives to SiC/AlN composites promoted and enhanced both the densification process and the formation of solid solution. Composites sintered in argon atmosphere gave more favorable microstructure than that sintered in nitrogen one in terms of grain morphology, solid solution formation, porosity and homogeneity of the matrix. The cold crushing strength (CCS) measured at room temperature for the different SiC/AlN composites with 2.5% Y+A sintered at 2080 °C/2h in argon/ vacuum atmosphere was inspected. Results indicated a gradual Summary 1 9 7 decrease in the CCS values with increasing the amount of added AlN from 10 wt% until reached to its minimal value for 40% AlN composite. This behavior was in good agreement with the densification parameters trend of the sintered samples. CCS values of the SiC/AlN composites were higher than that of other reported SiC/AlN composites in literature. This was attributed to solid solution formation, good bonding and diffusion between SiC and AlN. Thermal diffusivity and conductivity of the produced composites were measured and analyzed. Different SiC/AlN composites gave good thermal diffusivity and conductivity values. Thermal conductivity values were dependent of the weight percentage (wt. %) of AlN and the porosity level. They decreased in a linear manner with increasing wt. % of AlN up to 40 % and increasing the porosity percentage. Thermal expansion (TE) and coefficient of thermal expansion (CTE) of the reaction sintered composites followed the same sequence and behavior. They were linearly increased with temperature up to 1000 oC. CTE of different SiC/AlN composites was mainly dependent on temperature and AlN wt%. Nevertheless, it attractively recorded low and negative values at high temperatures especially composites with higher SiC content. Thermal shock resistance (TSR) of the carbide/nitride composites was evaluated in terms of strength difference and microstructure of shocked and non-shocked samples. The different composites were capable of withstanding exposure to 20 consecutive thermal shock/water quenching cycles at temperature of 700 °C without destruction or failure. Investigation of compression strength of the inspected composites before and after shock treatment revealed that, the thermal shock behavior of the samples belong to Summary 1 9 8 regime I with retention of about 70% of their original strength. Composites with higher SiC and lower AlN contents gave the best thermal shock resistance. Microstructure characterization of the tested composites did not show any change in the grains morphology or any fracture appearance. Thermal stress resulted from the exposure of different sintered SiC/AlN composites to continuous heating with different temperatures up to 1500 oC are calculated and simulated by the finite element simulation software DEFORM 3D. Different SiC/AlN composites showed uniform transition and distribution of heat with splendid thermal stress durability. There was no indication or formation of hot spots or spalling of the different composites at any investigated temperature. Thermal stress values increased gradually with temperature and AlN content. Influence of the incident thermal stress on the strength of the different composites was null at low temperatures and negligible at higher ones. Thermoelectric properties of the proposed composites in terms of electrical resistivity, Seebeck coefficient, figure of merit and power factor were measured and calculated in an attempt to develop materials with high thermoelectric energy conversion beside their high indicated thermal properties. It was found that SiC/AlN composites were p-type semiconductors with low electrical resistivity and high thermoelectric properties at high temperatures. Electrical and thermoelectric properties were found to be dependent on AlN content, microstructure, and temperature. Composite with 30% AlN gave the lowest electrical resistivity of 1.5 x105 μOhm x cm, the highest Seebeck coefficient of 370 μ V/K and the best thermoelectric efficiency at the temperature of 1000 K. The highly attractive thermoelectric properties of SiC/AlN composites at high temperatures were strongly nominated them to be used effectively in solar energy and Summary 1 9 9 thermoelectric applications. This means that, SiC/AlN composites will not be used only as a successful volumetric solar receiver, but also as effective thermoelectric material. In order to achieve and produced the proposed porous SiC/AlN structures, several phases were studied and analyzed such as: surface treatment of AlN powder, zeta potential measurement, rheological measurement, foaming using replica technique and evaluation of the final foams structure. Surface treatment of AlN powder with 3wt% aluminum dihydrogen phosphate [Al (H2PO4)3] was enough and succeeded to protect the AlN surface from any hydrolysis for further aqueous suspension treatment. Treated-AlN powder did not undergo to any hydrolysis at any investigated temperature (room temperature, 80 and 100oC). There was no any noticeable change in its pH value or formation of any other phases than AlN. Treated - AlN was stable in water. The pH of its suspensions remained nearly constant even after 48 hours of contact with water. Zeta potential of the different starting powders (treated-AlN, non-treated AlN, α-SiC , β-SiC, Y2O3 and Al2O3) were revealed as a function of pH without and with addition of different dispersing agents (Dolapix A88, Dolapix PC 75, Dolapix CE 64 and Darvan C-N). It was found that raw materials with absence of dispersants gave poor zeta potential values and unstable suspension. However, using different dispersing agent especially (Dolapix PC 75) enhanced the negative zeta potential values (become more negative) along the entire pH range. Dolapix PC 75 gave the highest negative zeta potential value and was selected to prepare a concentrated stable suspension. Protected- AlN gave higher negative zeta potential value than the unprotected- AlN. Moreover, it was noticed that with a higher basic media at Summary 2 0 0 pH of 8, the zeta potential of the samples with the addition of Dolapix PC 75 increased and became more negative. Hence, the selected pH for producing stable concentrated suspension was pH = 8. Rheological measurements: Several factors affecting on the stability of suspension and the rheological properties of the investigated SiC/AlN composites were discussed as (effect of dispersing agent concentration and effect of solid loading). Influence of different dispersing agent concentrations (0.5-3 % Dolapix PC 75) on the flow behavior of the different water based powder suspensions having 30 vol% solid loading was investigated. It was revealed that by using 2 wt.% Dolapix PC 75, the suspension was dominated by repulsive forces, thus it was stabilized, consequently the viscosity decreased. Accordingly, well-dispersed suspensions were prepared from the addition of 2 wt% of Dolapix PC 75. Moreover, increasing the suspension viscosity was found to be increased with increasing the solid loading with maintaining the stability of the suspension; i.e. no sedimentation or aggregation occurred for the particles. Consequently, this behavior gave indication for the possibility of obtaining a highly dense ceramic body. According to the different solid loading behavior of the different SiC and AlN particles, higher solid loading of 50 vol% was chosen as the optimum concentration for obtaining SiC/AlN ceramic foams with highly dense struts. Finally, the final foam structure of the different SiC/AlN compositions were prepared by replica technique using the optimum condition released from the stable suspension treatment and sintered at the optimum sintering conditions stated from the pressureless sintering of different SiC/AlN composites. The final produced sintered SiC/AlN foams with different AlN contents were evaluated and analyzed through different characterizations (XRD phase Summary 2 0 1 analysis, dimension change up on consolidation, microstructure evaluation, density and cellularity (ppi). X-ray diffraction (XRD) analysis of the resulted foams with different SiC and AlN contents sintered at 2080o C/2h in Ar/vacuum atmosphere was demonstrated. Sintered SiC/AlN foams gave the same phases that attained by their counterpart’s composites at the same sintering conditions. The only observed difference was that, the splitted peaks formed in SiC/AlN composites concerning the 2H-6H solid solution were completely converted to sharp single 2 Hss SiC/AlN solid solutions peaks in the SiC/AlN ceramic foams. Besides, the intensity of the formed solid solution peaks became more sharp and thin. This behavior reflected the more enhanced sintering behavior attained by the foams structure and confirmed the formation of complete single phase SiC/AlN solid solution reaction Geometric density, porosity and relative density of the different sintered foams with different SiC/AlN composition were estimated. Results showed that with increasing AlN content, density decreased and porosity increased. Porosity represented both of cell density and the micro pores formed in the foam struts. So that, its percentage increased with increasing the AlN content. Geometric density of the different foams was ranging from 0.414 to 0.265 g/cm3. Relative porosity and cell density was in the range of 87.5- 92%. Linear shrinkage of the different carbide/nitride foams was completely proportional with the investigated density of the samples. For using different AlN content, linear shrinkage increased with decreasing the AlN wt%. The mean pore size (mm) and cellularity (PPI) of the different prepared SiC/AlN foams were investigated and calculated from analysis of their Summary 2 0 2 optical microscope micrographs. It was found that the average pores size of the different carbide/nitride foams are ranging between 0.29 mm for 100% SiC and 0.17 mm for 30% AlN. Cellularity of the different foam increased with increasing AlN content from 84 PPI for 100% SiC to 140 PPI for 30% AlN. The relative porosity increased with the cellularity of foam. Increasing AlN content led to increasing the cellularity and decreasing the mean pore size of the investigated foam system. SiC/AlN foam with 30% AlN gave the highest porosity and cellularity with the lowest mean pore size with expected highest surface area. Microstructure investigation of the foam struts gave the same behavior of their counterpart composites with uniform distribution and elongated grains morphology. Moreover, they gave homogenous open cell microstructure. Their struts consisted of dense and well sintered material. The welldeveloped channels of the connected open pore structure give higher filtration ability of these porous ceramics. It was also found that increasing AlN content led to increasing cell density and decreasing struts density of the investigated SiC/AlN ceramic foams. Finally, it can be concluded that: 1- Sintering of different SiC/AlN composites at 2080oC/2h in Argon/vacuum atmosphere with addition of 2.5 % Y+A was the optimum conditions for producing near fully dense SiC/AlN structures constituting the final foam struts of the proposed volumetric solar receiver. 2- Investigation of the different composites properties and behaviors under different conditions revealed that high AlN content will not be preferable for obtaining highly efficient structure. So that, SiC/AlN composites with 0- 30 wt% AlN were considered the most suitable compositions for producing Summary 2 0 3 highly efficient foam structure. SiC/AlN composite with 40% AlN (4 YA) was excluded. 3-The comparison of different physical, mechanical, thermal and thermoelectric characteristics of the investigated carbide/nitride has revealed that, the combination of the two non oxide ceramics in one composite structure has sharply enhanced their monolithic shortage and gave new material with splendid characteristics and promised applications. Besides, different properties and behaviors of SiC/AlN ceramic system could be controlled by controlling their composition i.e. selecting the proper content of AlN in the composite, 4-The best conditions reached for producing well dispersed and stable suspensions with different SiC and AlN contents were achieved at pH =8 using 2wt% Dolapix PC 75 dispersing agent and 50 vol% of solid loading. 5- Several SiC/AlN foam structures with different AlN content was successfully produced by the replica technique 6- It is possible to control and tailor the required cell density, PPI and the foam surface area by controlling the AlN content based on the required applications. 7- The highly attractive characteristics of the investigated SiC/AlN ceramic structure can nominate them to be used effectively in solar energy and thermoelectric applications at high temperatures with expected high life cycle |