• Journal of the Korean Ceramic Society
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• v.45 no.7
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• pp.405-410
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• 2008

Ceramics are widely used as plasma resistant materials in semiconductor industries. However, the plasma erosion resistance has not been properly evaluated in terms of microstructural changes during the exposure to plasma. In this study, microstructure developments of $Al_2O_3$ were investigated under the fluorine plasma conditions. In polycrystalline alumina, uniform erosion throughout the specimen as well as spatially distributed local erosion were observed. Local erosion was much more severe in lower purity alumina. In contrast to the polycrystalline alumina, only uniform erosion was observed in single crystalline sapphire. These specimens, however, had practically the same erosion depth, which results in the incorrectly similar plasma resistance. This implies that the plasma erosion resistance of ceramics should be evaluated in terms of the microstructural changes, as well as the conventionally accepted erosion depth.

• Journal of the Korean Ceramic Society
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• v.33 no.2
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• pp.235-241
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• 1996

Self-Propagating high temperature synthesis(SHS) technique was used to synthesize the spinel phase of MgAl2O from MgO and Al powder. Processing factors such as mixing time preheating temperature and ignition catalyst were varied to determine the optimum condition to form MgAl2O4 phase. The reaction products were heat treated at the temperature range of 120$0^{\circ}C$ and 150$0^{\circ}C$. to observe phase transformation of unreacted materials. Processing factors such as 48 hrs-mixing 80$0^{\circ}C$-preheating and 20wt% KNO3-ignition catalyst were effective of the formation of MgAl2O spinel. An activation energy 49.7kcal/mol. was calculated to form a MaAl2O4 spinel from unreacted materials.

• Proceedings of the Korean Powder Metallurgy Institute Conference
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• 2002.04b
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• pp.11-11
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• 2002

Plastic deformation was observed by TEM around the intragranular SiC particles in the $Al_2O_3$ matrix for $Al_2O_3/SiC$ nanocomposite system. The dislocations are generated at selected planes and there is a tendency for the dislocations to form a subgrain boundary structure with low-angel grain boundaries and networks. In this study, dislocation generated in the $Al_2O_3$ matrix during cooling down from sintering temperatures by the highly localized thermal stresses within and/or around SiC particles caused from the thermal expansion mismatch between $Al_2O_3$ matrix and SiC particle was observed. In monolithic $Al_2O_3$ and $Al_2O_3/SiC$ microcomposite system. These phenomena is closely related to the plastic relaxation of the elastic stress and strain energy associated with both thermal misfitting inclusions and creep behaviors. The plastic relaxation behavior was explained by combination of yield stress and internal stress.

• Journal of the Korean Ceramic Society
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• v.25 no.2
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• pp.136-142
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• 1988

β-Al2O3, which is used for solid electrolyte membrances in sodium-sulfur batteries, was prepared by sol-gel process. Sodium-n-propoxide NaOC3H7 and aluminum-isopropoxide Al(OC3H7)3 were hydrolyzated in the solution at pH 3, pH 7, pH 9 and pH 11, respectively. The sol-gel processed samples were calcined at several temperature steps, respectively and analysed by thermal analyser(DT-TGA), infrared spectrum analyser and X-ray diffraction analyser. The gelling rate of solution at pH 7 was much higher than that of the solution at pH 3. Thermal exchanging behavior of the gels at pH 3 were similar to Na2O·Al2O3·6H2O and, above pH 7, were similar to Na2O·Al2O3·3H2O. When samples' composition ratio was 9.13 : 90.87 [NaOC3H7:Al(OC3H7)3] at pH 7, β-Al2O3 was formed at 1100℃.

• Journal of the Korean Ceramic Society
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• v.29 no.5
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• pp.347-356
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• 1992

Various kaolin samples with different alumina content were prepared from calcined admixture of kaolin and ammonium sulfate by varying the treatment time in sulfuric acid. Samples were nitridated under N2 or N2-H2 atmosphere with changing the amount of added carbon, the reaction time and temperature. As the alumina content lowered, the size of kaolin particles decreased and the specific surface area increased. XRD analysis indicated that ${\alpha}$-quartz remained by decomposition of halloysite and meta-halloysite. Experimental results of nitridation behavior are summerized as follows; 1) Nitridation under N2 atmosphere. With the increase of C/SiO2 ratio and with the decrease of Al2O3 content, disappearance of XRD pattern peaks of mullite, ${\alpha}$-quartz and ${\alpha}$-Al2O3 were accelerated at 1300$^{\circ}C$. SiC was the main phase in the reaction product of acid-treated kaolin samples nitridated at 1300$^{\circ}C$ for 10 hours regardless of C/SiO2 ratio. But the XRD peak intensities of ${\beta}$-Si3N4, ${\beta}$-sialon and SiC did not show much difference when untreated raw kaolin was fired at the same condition. When the ratio of C/SiO2 was 3.5, ${\beta}$-sialon and ${\beta}$-Si3N4 existed in the reaction product of about 22% alumina containing kaolin sample fired at 1350$^{\circ}C$ for 7 hours. Only ${\beta}$-sialon existed in the same sample fired at 1400$^{\circ}C$ for 10 hours. ${\beta}$-sialon was obtained from all of the acid-treated kaolin samples fired at 1400$^{\circ}C$ for 40 hours, but AlN and SiC remained in the untreated kaolin sample. Z value of the ${\beta}$-sialon obtained from the 22% alumina containing kaolin sample fired at 1400$^{\circ}C$ for 40 hours was about 1.3(XRD) and 1.5(EDS). 2) Nitridation under 80N2+2OH2 mixed gas atmosphere with the C/SiO2 ratio of 1 Mullite was not found, but ${\alpha}$-Si3N4, and ${\beta}$-sialon were present in the reaction product of about 22% alumina containing kaolin sample fired at 1300$^{\circ}C$ for 10 hours. When untreated kaolin sample was nitridated at the same condition, mullite remained. AlN and SiC were not found in the reaction product of about 22% alumina containing kaolin sample fired at 1350$^{\circ}C$ for 5 hours. On the other hand, AlN and SiC remained in the product of untreated kaolin fired at the same condition.

• Journal of the Korean Ceramic Society
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• v.31 no.6
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• pp.678-684
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• 1994

The surface of alumina is capable of acquiring a change when it is in an aqueous solution. This surface change will have a strong influence on the surrounding ions, particularly those of opposite change known as the counter ions. A site-binding model of the {{{{ gamma }}-alumina/KCl(aq) interface was used to calculated theoretical surface ionization constants and P.Z.C.(Point of zero change) of {{{{ gamma }}-alumina. This paper was carried out to investigate the effect of calcination temperature on the acidic and electrical properties of pure {{{{ gamma }}-alumina prepared by the precipitation method from the Al(NO3)3.9H2O and NH4OH. From the experimental data it was shown that {{{{ gamma }}-alumina have a mainly Br nsted acid site. However, the acidity of {{{{ gamma }}-alumina decreased with increasing calcination temperature at strength Ho +9.3. The surface charge density of {{{{ gamma }}-alumina was increased with electrolyte ionic strength and calcination temperature.

• Transactions of the Korean Society of Mechanical Engineers
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• v.14 no.3
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• pp.547-553
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• 1990

With alumina ceramics to copper brazement of cylindrical shape, the thermal stress analysis was carried out by finite element method. Elastic and plastic behaviour was considered to copper, but only elastic behaviour was considered to alumina. Also material properties of alumina and copper were considered in not constant values but variable functions dependent on temperature. The result of analysis is shown that maximum tensile longitudinal stress is occurred at perimeter of alumina side interface and maximum compressive radial and tangential stresses are occurred at center of alumina side interface. Because of bending effect, tensile raidial and tangential stresses are occurred at near bottom of alumina, far from interface.

• Journal of the Korean Ceramic Society
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• v.35 no.10
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• pp.1085-1093
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• 1998

Amorphous alumina powder prepared by the fast calcination of aluminum trihydroxide(Al(OH)3 gibbsite) for 0.5 second at 580$^{\circ}C$ was investigated rehydration propeties. Phase composition crystal size and mor-phology surface area pore volume and pore size distribution of pesudo-boehmite and bayerite crystals changed with temperature time water/alumina ratio and particle size when amorphous alumina rehydrated with water. Phase compositions were examined with XRD and DTA and crystal sized morphologies were investigaed with SEM and TEM. Also rehydration properties of amorphous alumina were in-vestigated by measuring the surface area pore volume and pore size distribution.

• Korean Journal of Materials Research
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• v.24 no.1
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• pp.48-52
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• 2014

The Mg-enriched magnesium aluminum silicate (MAS) glass is known for its higher mechanical strength and chemical resistance. Among such glasses, cordierite ($Mg_2Al_4Si_5O_{18}$) is well known to have a low thermal expansion and low melting point. Polycrystalline engineering ceramics such as alumina can be strengthened by a surface modification with low thermal expansion materials. The present study involves the synthesis of cordierite by a sol-gel process and investigates the effect of glass penetration on the surface of alumina. The cordierite powders were prepared from $Al(OC_3H_7)_3$, $Mg(OC_2H_5)_2$ and tetraethyl orthosilicate by hydrolysis and condensation reaction. The cordierite powders were characterized by X-ray diffraction (XRD, Rigaku), scanning electron microscope (SEM, JEOL: JSM-5610), energy dispersive spectroscopy (EDS, JEOL: JSM-5610), and universal testing machine (UTM, INSTRON). The X-ray diffraction patterns showed that the synthesized particles were ${\mu}$-cordierite calcined at $1100^{\circ}C$ for 1 h. The shape of synthesized cordierite was changed from ${\mu}$-cordierite to ${\alpha}$-cordierite with increasing calcination temperature. Synthesized cordierite was used for surface modification of alumina. Cordierite powders penetrated deeply into the alumina sample along grain boundaries with increasing temperature. The results of surface modification tests showed that the strength of the prepared alumina sample increased after surface modification. The strength of a surface modified with synthesized cordierite increased the most, to about 134.6MPa.

• Journal of the Korean Ceramic Society
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• v.34 no.6
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• pp.591-600
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• 1997

Al2O3/Al composites were produced by displacement reaction method, which was carried out by immersing the sintered silica preform, which was prepared from fused silica powder, in molten aluminum. Because the molten aluminum did not penetrate into the silica preform with higher than 20% of porosity when the displacement reaction was accomplished at 100$0^{\circ}C$ for 10 hours in air atmosphere, the optimum range of sintering temperature of silica preform was from 135$0^{\circ}C$ to 140$0^{\circ}C$. The microstructure of this Al2O3/Al composites showed three-dimentionally co-continuous alumina, which provides wear resistance and high stiffness, and aluminium which acts as a toughnening phase. The grain size of the alumina in composites did not change with the particle size of the silica preform. The exact shape of the preform was retained and a net-shaped composite was produced. The representative Al2O3/Al composite prepared in this study showed 3.30mg/㎤ of bulk density, 350-430 MPa of flexural strength, 7.0 MPa.m1/2 of fracture toughness, and good machinability.