الفهرس 
INTRODUCTIONOnly 14 pages are availabe for public view
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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.
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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
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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,
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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
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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
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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
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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
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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
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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
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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
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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
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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