Showing papers in "International Journal of Applied Ceramic Technology in 2009"
TL;DR: The most efficient way to improve the behavior of pyrocarbon interphase in severe environments is to replace part of PyC by a material displaying a better compatibility, such as SiC itself.
Abstract: Pyrocarbon (PyC), the common interphase for SiC/SiC, is not stable under severe environmental conditions. It could be replaced by boron nitride more resistant to oxidation but poorly compatible with nuclear applications. Other materials, such as ternary carbides seem promising but their use in SiC/SiC has not been demonstrated. The most efficient way to improve the behavior of PyC interphase in severe environments is to replace part of PyC by a material displaying a better compatibility, such as SiC itself. Issues related to the design and behavior of layered interphases are reviewed with a view to demonstrate their interest in high-temperature nuclear reactors.
123 citations
TL;DR: In this article, various approaches of modifying thermal barrier coatings for enhanced protection against CMAS attack were discussed, and a nearly crack-free and reglazed Pd coating provided substantial protection from the CMAS attacks.
Abstract: Electron beam-physical vapor-deposited thermal barrier coatings (TBC) are susceptible to damage due to environmental contaminants such as calcium—magnesium—aluminum—silicon oxide systems (CMAS). This paper discusses various approaches of modifying TBC for enhanced protection against CMAS attack. Methodologies were explored with various coating systems maintaining functionality as nonwetting, sacrificial, and impervious to CMAS attack. In the brief isothermal (1260°C/10 min) tests, a nearly crack-free and reglazed Pd coating provided substantial protection from the CMAS attack. Approaches that provided some minor improvements need further optimization to better assess their viability.
98 citations
TL;DR: In this article, Ba04Sr06TiOO3/MgO composites exhibit a notably enhanced energy density and low dielectric loss, compared with pure BaSr6TiO3 and nanosized MgO powder.
Abstract: (100−x) wt% Ba04Sr06TiO3−x wt% MgO composites (10≤x≤30) were prepared using Ba04Sr06TiO3 powder and nanosized MgO powder (∼60 nm) by a solid-state reaction The energy storage density and dielectric loss were investigated for the purpose of a potential application in solid-state pulse-forming line The results show that Ba04Sr06TiO3/MgO composites exhibit a notably enhanced energy density and low dielectric loss, compared with pure Ba04Sr06TiO3 The enhancement of the energy density is attributed to the notable increase in breakdown strength of the composites and the improvement of dielectric constant stability with regard to electric field In the case of x=30, the samples exhibited a breakdown strength of 331 kV/mm, an energy density of 114 J/cm3, a moderate dielectric constant of 270, and a low dielectric loss of 4 × 10−4
90 citations
TL;DR: In this paper, the influence of the type of the alkaline-earth ion on the phase structure and luminescence properties has been investigated, and the results show that the synthesized powders have a single-phase crystal structure of M2Si5N8:Eu2+ for M =Ca, Sr, and a little amount of BaSi7N10 impurity phase for M=Ba.
Abstract: Eu2+-doped M2Si5N8 (M=Ca, Sr, Ba) orange–red phosphors were successfully prepared by a simple, direct, and efficient solid-state reaction using air-stable MSi2, Eu2O3, and α-Si3N4 as the starting materials under N2–H2 (5%) atmosphere. The influence of the type of the alkaline-earth ion on the phase structure and luminescence properties has been investigated. The results show that the synthesized powders have a single-phase crystal structure of M2Si5N8 for M=Ca, Sr, and a little amount of BaSi7N10 impurity phase for M=Ba. Under the blue light excitation, M2Si5N8:Eu2+ shows a typical broad band emission of Eu2+ ranging from orange to red (585–620 nm) depending on the type of M ion. The emission intensity, conversion efficiency, and thermal stability increase with the sequence of Ca
81 citations
TL;DR: In this article, the effect of particle size distribution of YAG:Ce phosphors on the photoluminescence (PL) properties was investigated and the results demonstrate that the uniform size distribution and particle size affects the packaging performance in white light emitting diode (LED) applications.
Abstract: The synthesis of cerium-doped yttrium aluminum garnet (YAG:Ce) phosphor of different sizes with uniform size distribution was carried out using solid-state reaction followed by grinding and sieving method. The effect of particle size distribution of YAG:Ce phosphors on the photoluminescence (PL) properties was investigated. The results demonstrate that the uniform size distribution and particle size affects the packaging performance in white light emitting diode (LED) applications. The YAG:Ce phosphors with different particle sizes were packaged in white LEDs using different amounts of each phosphors in order to get similar efficiency as that of commercially available YAG:Ce phosphors. It was observed that minimum amount of phosphor material is required for smaller particle size for getting the similar efficiency as that exhibited by commercially available YAG:Ce phosphors. The results are particularly interesting in view of reducing the cost of current LEDs by lowering the amount of phosphors without compromising the efficiencies of final LED package. A systematic study of YAG:Ce phosphors on the packing performance in white LEDs is reported.
68 citations
TL;DR: In this paper, three kinds of coating structures for these composites were applied in order to improve their ablation resistance: pure silicon carbide coatings, ZrB 2 -SiC mixture coatings and ZrC-SiC alternating multilayer coatings.
Abstract: C/ZrB 2 -SiC composites were fabricated by polymer infiltration and pyrolysis combined with slurry impregnation method. Three kinds of coating structures for these composites were applied in order to improve their ablation resistance: pure silicon carbide coatings, ZrB 2 -SiC mixture coatings, and ZrC-SiC alternating multilayer coatings. The ablation experiments were carried out on an oxyacetylene torch flame with a temperature of about 3000°C. The ZrC-SiC alternating multilayer showed the best ablation resistance. The linear erosion rate for ZrC-SiC alternating multilayer coatings is half of that for ZrB 2 -SiC mixture and pure SiC coatings. A model was put forward to account for such a result.
64 citations
TL;DR: In this paper, the in-plane tensile stress-strain, tensile creep, and after-creep retained tensile properties of SiC-SiC composites reinforced with different fiber types were evaluated with an emphasis on obtaining simple or first-order microstructural design guidelines for these inplane mechanical properties.
Abstract: In-plane tensile stress-strain, tensile creep, and after-creep retained tensile properties of melt-infiltrated SiC-SiC composites reinforced with different fiber types were evaluated with an emphasis on obtaining simple or first-order microstructural design guidelines for these in-plane mechanical properties. Using the mini-matrix approach to model stress-strain behavior and the results of this study, three basic general design criteria for stress and strain limits are formulated, namely a design stress limit, a design total strain limit, and an after-creep design retained strength limit. It is shown that these criteria can be useful for designing components for high temperature applications.
63 citations
TL;DR: In this paper, pre-precursor-based composite coatings were developed as barrier coatings on steel against oxidation and corrosion using a silazane in the system SiN as a polymer material and BN particles as a passive filler.
Abstract: Precursor-based composite coatings were developed as barrier coatings on steel against oxidation and corrosion using a silazane in the system SiN as a polymer material and BN particles as a passive filler. After thermal treatment in air up to 800°C, dense and very well adhered ceramic composite SiNO/BN coatings with a thickness of 12 μm were achieved. These were investigated by means of scanning electron microscopy, energy-dispersive spectrum, glow discharge optical emission spectroscopy, and adhesion measurements. Static oxidation tests on coated mild steel substrates up to 700°C displayed parabolic oxidation kinetics and a reduced weight gain by two orders in magnitude compared with uncoated substrates.
61 citations
TL;DR: In this paper, an austenitic TRIP-steel/Mg-PSZ-composite honeycomb structures are formed with different mixing proportions due to ceramic extrusion at room temperature.
Abstract: Porous materials have received extensive attention for energy absorption in the last few years. In terms of this study, austenitic TRIP-steel/Mg-PSZ-composite honeycomb structures are formed with different mixing proportions due to ceramic extrusion at room temperature. Their specific energy absorption SEA as well as their compression strength have been registered as a function of the compressive strain. X-ray diffractometry (XRD), electron backscatter diffraction (EBSD) as well as electron dispersive X-ray (EDX) analysis support the microstructure characterization. The zirconia addition has mainly contributed as a hard phase in a ductile TRIP steel matrix and has reinforced the composite material up to a compressive strain of about 24%.
61 citations
TL;DR: In this article, the authors used multimodal boron carbide mixtures to achieve 75% green density with high Weibull modulus, and the volume fraction of residual Si in the composites was in the 8−10% range.
Abstract: The presence of unreacted, free silicon lowers the mechanical properties of reaction-bonded boron carbide. The fraction of free silicon can be reduced by increasing the green density of the initial boron carbide performs. The use of multimodal boron carbide mixtures allows attaining 75% green density. After reaction bonding with molten silicon, the composites consist of four phases, namely the original B 4 C particles, the B 12 (B,C,Si) 3 phase, product of the dissolution―precipitation process, β-SiC, and residual Si. The volume fraction of residual Si in the composites is in the 8―10% range. The infiltrated composites display elevated values of the mechanical properties with a high Weibull modulus.
58 citations
TL;DR: In this paper, the authors introduced silver-doped titanium oxide nanofibers produced by electrospinning technique, which can eliminate >92% of the methylene blue dye within 10min only.
Abstract: In the present study, we are introducing silver-doped titanium oxide nanofibers produced by electrospinning technique. Calcination of dry nanofiber mats consisting of silver nitrate–titanium isopropoxide/PVAc in air at 600°C for 1 h leads to produce Ag-doped titania nanofibers. Two dyes have been invoked to check the photocatalytic ability of the produced nanofibers; methylene blue dihydrate and methyl red. The obtained results indicated that the silver-doped titanium oxide nanofibers can eliminate >92% of the methylene blue dye within 10 min only. In a case of methyl red, almost the dye was decayed (93%) within 3 h.
TL;DR: A steel-making slag (basic oxygen furnace) obtained from the Kardemir steelworks was modified to be used as a clinker additive in the cement industry as discussed by the authors, which is an attractive alternative for cement manufacturers, because calcined material is expensive, and slag substitutes are very often cheaper, in addition to its low or zero-greenhouse gas emission.
Abstract: A steel-making slag (basic oxygen furnace) obtained from the Kardemir Steelworks was modified to be used as a clinker additive in the cement industry. This study confirmed that the compressive strength values of concretes produced by addition of these steel-making slags up to 30mass% were within the values of Grade-325 and Grade-425 steel-making slag cement. This is an attractive alternative for cement manufacturers, because calcined material is expensive, and slag substitutes are very often cheaper, in addition to its low or zero-greenhouse-gas emission.
TL;DR: In this article, a joint process consisting of pressureless sintering and chemical vapor infiltration (CVI) was developed to prepare porous Si3N4 ceramics with controlled microstructure.
Abstract: The joint process consisting of pressureless sintering and chemical vapor infiltration (CVI) was developed to prepare porous Si3N4 ceramics with controlled microstructure. Lu2O3 and phenolic resin acted as sintering aid and pore-forming agent, respectively. The 5 wt% Lu2O3-doped ceramics using 12–57 vol% phenolic resin attained a porosity ranging from 46% to 53%. With increasing the resin content, the average pore size increased from 1 to 2 μm. The porous ceramic infiltrated with CVI Si3N4 had an improved microstructure. The decreased pore size and porosity led to an increase in flexural strength, and the densified surface led to an improved surface hardness.
TL;DR: In this article, the performance of slurry cast melt-infiltrated SiC matrix composites consisting of Sylramic-iBN fibers with a wide variety of fiber architectures was compared.
Abstract: The matrix cracking behavior of slurry cast melt-infiltrated SiC matrix composites consisting of Sylramic-iBN fibers with a wide variety of fiber architectures were compared. The fiber architectures included 2D woven, braided, 3D orthogonal, and angle interlock architectures. Acoustic emission was used to monitor in-plane matrix cracking during unload–reload tensile tests. Two key parameters were found to control matrix-cracking behavior: the fiber volume fraction in the loading direction and the area of the weakest portion of the structure, that is, the largest tow in the architecture perpendicular to the loading direction. Empirical models that support these results are presented and discussed.
TL;DR: In this article, three hot-pressed ZrB 2 -based ultra-high-temperature ceramic composites were joined to Cu-clad-Mo using AgCuTi brazes.
Abstract: Three hot-pressed ZrB 2 -based ultra-high-temperature ceramic composites (UHTCC), ZrB 2 -SiC P (ZS), ZrB 2 -SiC P -C (ZSC), and ZrB 2 -SCS9A (SiC fiber)-SiC P (ZSS), were joined to Cu-clad-Mo using AgCuTi brazes (T L ∼1073-1173K) and Pd-base brazes (TL∼ 1493-1513K). More extensive chemical interactions occurred in Pd-base joints than in AgCuTi-base joints. The Pd-braze region displayed higher hardness in joints made using ZS than ZSS and ZSC. Residual stress calculations point toward negative strain energy up to ∼ 23% clad layer thickness because α Cu-clad-Mo α ZS , strain energy is positive, and it increases with increasing thickness. Projected reductions in the thermal resistance highlight the benefits of joining the UHTCC to Cu-clad-Mo.
TL;DR: In this article, the phase structure was identified by X-ray diffraction and Fourier transform infrared spectroscopy, and the bioactivity of zirconia nanotubes at different annealing temperatures was assessed by simulated body fluids immersion experiments.
Abstract: Zirconia nanotube arrays with a diameter of 300-350 nm and a length of 100 pm were prepared by anodization of zirconium in an organic electrolyte containing NH 4 F. The zirconia layers were annealed at 200―800°C in air. The morphologies of the specimens were observed using scanning electron microscopy. The phase structure was identified by X-ray diffraction and Fourier transform infrared spectroscopy. The bioactivity of zirconia nanotubes at different annealing temperatures was assessed by simulated body fluids immersion experiments. Results show that the nanotube arrays were stable when annealed at 600°C. The tubal layers containing zirconia with a low crystallinity and amorphous structure are more bioactive compared with a well-crystallized structure.
TL;DR: In this paper, thermal barrier oxides with a lower thermal conductivity than yttria-stabilized zirconia ceramics were synthesized with the chemical-coprecipitation and calcination method.
Abstract: (Yb x Gd 1―x ) 2 Zr 2 O 7 (0≤x≤ 1.0) ceramic powders synthesized with the chemical-coprecipitation and calcination method were pressureless-sintered at 1550―1700°C to develop new thermal barrier oxides with a lower thermal conductivity than yttria-stabilized zirconia ceramics. (Yb x Gd 1―x ) 2 Zr 2 O 7 ceramics exhibit a defective fluorite-type structure. The linear thermal expansion coefficients of (Yb x Gd 1―x ) 2 Zr 2 O 7 ceramics increase with increasing temperature from room temperature to 1400°C. The measured thermal conductivity of (Yb x Gd 1 ―x ) 2 Zr 2 O 7 ceramics first gradually decrease with increasing temperature and then slightly increase above 800"C because of the increased radiation contribution. YbGdZr 2 O 7 ceramics have the lowest thermal conductivity among all the composition combinations studied.
TL;DR: An AlPO4-SiO2 powder with a composition of Al:P:Si=1.5:1:0.1 was synthesized by the sol-gel method using aluminum nitrate, phosphate acid, and tetraethoxysilane as discussed by the authors.
Abstract: An AlPO4–SiO2 powder with a composition of Al:P:Si=1.5:1:0.1 was synthesized by the sol–gel method using aluminum nitrate, phosphate acid, and tetraethoxysilane. The structural evolution of this material was characterized by thermal gravimetric analysis-differential scanning calorimetry, Fourier transform infrared, and X-ray diffraction. By adding silica in AlPO4, the sinterability of the AlPO4 was enhanced because of the reactions between excess alumina and silica to form mullite. The sintered composites have a high strength and good dielectric properties at 10 GHz. Because of the formation of mullite at high temperatures, the composites showed a hydrophobic property. These unique properties indicate that the sintered AlPO4–mullite composites are suitable for the radome application.
TL;DR: In this paper, the sintering temperature of CaMgSi2O6 (CMS) ceramics was reduced below the melting point of Ag using lowmelting LBS and LMZBS glasses.
Abstract: CaMgSi2O6 (CMS) ceramics prepared by the solid-state ceramic route have a sintering temperature of 1300°C/2 h. The sintering temperature of CMS was reduced below the melting point of Ag using low-melting LBS and LMZBS glasses. In the case of CMS+15 wt% LMZBS sintered at 900°C/2 h, the dielectric properties obtained were ɛr=8.2, Qu×f=32,000 GHz (10.15 GHz), and τf=–48 ppm/°C. The CMS+15 wt% LBS composite, sintered at 925°C/2 h, showed ɛr=8, Qu×f=15,000 GHz (10.17 GHz), and τf=–49 ppm/°C. The chemical compatibility of Ag with the ceramic–glass composites was also investigated for low-temperature co-fired ceramic applications.
TL;DR: In this paper, the behavior of polishing and glazing residues as cement constituents is reported, with reference to EN 197-1 requirements and the results compared with ordinary Portland cement.
Abstract: Waste generated in ceramic tiles manufacturing is not usually recycled inside the productive plant, but rather disposed to landfill. This paper deals with ceramic residues from polishing and glazing processes, as constituents for innovative blended cements. New binders made up of 75% CEM I 52.5 R and 25% residues were chemically, physically, and mechanically characterized with reference to EN 197-1 requirements and the results compared with ordinary Portland cement. Mechanical strength development and microstructure of the relevant mortar have been investigated up to 90 days of curing, and the behavior of polishing and glazing residues as cement constituents is reported.
TL;DR: In this paper, Strontium titanate (SrTiO3) filled polytetrafluroethylene (PTFE) composites were fabricated through sigma mixing (SM), extrusion (E), calendering (C) followed by hotpressing (H) (SMECH) process.
Abstract: Strontium titanate (SrTiO3) filled polytetrafluroethylene (PTFE) substrates were fabricated through sigma mixing (SM), extrusion (E), calendering (C) followed by hotpressing (H) (SMECH) process. The filler dispersion in the polymer matrix and the surface morphology were studied using scanning electron microscopic (SEM) technique. The permittivity (ɛ′r) and loss tangent (tan δ) of the composites were measured using precision impedance and vector network analyzers. The experimental permittivity of filled composites was compared with theoretically predicted values. Dimensionally stable substrate having a permittivity of 13.1 and a loss tangent of 0.0055 at X-band was prepared in the PTFE/SrTiO3 composite system.
TL;DR: In this paper, the fabrication and terahertz wave properties of alumina microphotonic crystals with a diamond structure were investigated, and a complete photonic band gap was observed from 0.40 to 0.47 THz and showed good agreement with the simulation of a plane wave expansion method.
Abstract: The fabrication and terahertz wave properties of alumina microphotonic crystals with a diamond structure were investigated. Acrylic diamond structures with alumina particles' dispersion were formed using microstereolithography. Fabricated precursors were dewaxed and sintered in air. The electromagnetic wave properties were measured by terahertz time-domain spectroscopy. A complete photonic band gap was observed from 0.40 to 0.47 THz, and showed good agreement with the simulation of a plane wave expansion method. Moreover, a localized mode was observed by introducing a plane defect between twinned diamond structures. The localized mode was analyzed using transmission line modeling simulation.
TL;DR: In this article, the authors describe the fabrication process used to create the precise channel and jet structures used in these LTCC-based coolers, as well as some of the challenges associated with these processes, including the erosion of the copper coolers by the coolant, a requirement for the use of deionized water within the system, and a significant CTE mismatch between the diode bar and the metal cooler.
Abstract: A number of emerging applications of low-temperature co-fired ceramic (LTCC) require embedded fluidic structure within the co-fired ceramic and or precise external dimensional tolerances. These structures enable the control of fluids for cooling, sensing, and biomedical applications, and variations in their geometry from the design can have a significant impact on the overall performance of the devices. One example of this type of application is a multilayer cooler developed recently by the authors for cooling laser diode bars. In many laser systems, laser diodes are the primary emitters, or assemblies of these diode bars are used to pump traditional laser crystals such as Nd:YLF. Assemblies of these diodes require large amounts of electrical current for proper operation, and the device operating temperature must be carefully controlled in order to avoid a shift in the output wavelength. These diodes are packaged into water-cooled assemblies and by their nature dissipate enormous amounts of heat, with waste heat fluxes on the order of 2000 W/cm2. The traditional solution to this problem has been the development of copper multilayer coolers. Assemblies of laser diodes are then formed by stacking these diode bars and coolers. Several problems exist with this approach including the erosion of the copper coolers by the coolant, a requirement for the use of deionized water within the system, and a significant CTE mismatch between the diode bar and the metal cooler. Diodes are bonded to these metal structures and liquid coolant is circulated through the metal layers in order to cool the diode bar. In contrast, the coolers developed by the authors utilize fluid channels and jets formed within LTCC as well as embedded cavity structures to control the flow of a high-velocity liquid and actively cool the laser diode bars mounted on the surface of the LTCC.† The dimensional tolerances of these cooler assemblies and complex shapes that are used to control the fluid can have a significant impact on the overall performance of the laser system. This paper describes the fabrication process used to create the precise channel and jet structures used in these LTCC-based coolers, as well as some of the challenges associated with these processes.
TL;DR: In this paper, a brief review related with dielectric properties of BaTiO3/epoxy composites is presented, where the composites were obtained using the dipping technique.
Abstract: A brief review related with dielectric properties of BaTiO3/epoxy composites is presented. The composites were obtained using the dipping technique. To facilitate the mixing and modify the filler surface, a solvent and a surface coupling agent were used. Intermediate and low concentrations of solvent and silane improved microstructure and dielectric properties of the composite material, whereas higher concentrations led to composites of poor quality. Finally, a model using finite elements was used, in order to predict the composite permittivity in relation to the percentage of filler. Model results were compared with the effective medium theory and experimental results.
TL;DR: In this paper, a one-pot processing method was used to produce SiOC glass monoliths possessing hierarchical porosity by using periodic mesoporous organosilica (PMO) particles embedded into a foamed siloxane preceramic polymer.
Abstract: SiOC glass monoliths possessing hierarchical porosity were produced by a one-pot processing method. Periodic mesoporous organosilica (PMO) particles were embedded into a foamed siloxane preceramic polymer. After pyrolysis at 1000°C in inert atmosphere, open celled, permeable SiOC ceramic monoliths with a high amount of pores, ranging in size from hundred of micrometers to a few nanometers, were obtained. The components possessed a specific surface area of 137 m2/g, indicating the retention of most of the mesopores after the pyrolytic conversion of the PMO precursor particles. These fillers converted to truncated rhombic dodecahedral SiOC mesoporous micron-sized grains, homogeneously distributed throughout the SiOC cellular matrix. The produced porous ceramics possessed compression strength of about 1.7 MPa, which is adequate for their use in several engineering applications.
TL;DR: In this article, laser heating was used to melt sintered ceramics, as part of a novel thermal analysis technique for probing extremely high temperature phase transformations, and the feasibility of the technique for detecting melting transitions in zirconium carbide was demonstrated.
Abstract: Commercial ZrC(0.96) powder (ABCR, Karlsruhe, Germany) was densified by spark plasma sintering to greater than 96% relative density at temperatures of 1900-2180 degrees C, applied pressures of 40-100 MPa, and soak time of 6-30 min. Effects of process parameters on microstructure were assessed by ceramography. High temperature (>2000 degrees C) was more instrumental in full densification than was high pressure, and excessive ramp rate resulted in high residual porosity. Grain coarsening was promoted by prolonging the isothermal dwell.
Laser heating was used to melt sintered ceramics, as part of a novel thermal analysis technique for probing extremely high temperature phase transformations. Temperatures well in excess of the expected melting temperature of ZrC and up to 4000 K were achieved. The feasibility of the technique for detecting melting transitions in zirconium carbide was demonstrated, and solidus and liquidus temperatures within 50-80 K of predicted values were measured. Post-melting analysis of laser-melted specimens revealed dendritic microstructure and composition consistent with single phase ZrC.
TL;DR: In this paper, the activation energy associated with the electrical relaxation determined from the electric modulus spectra was found to be 1.76 eV, close to that of dc conductivity, indicating that the same species took part in both the processes.
Abstract: Glasses in the system CaO-Bi2O3-B2O3 (in molar ratio) have been prepared using melt-quenching route. Ion transport characteristics were investigated for this glass using electric modulus, ac conductivity and impedance measurements. The ac conductivity was rationalized using Almond-West power law. Dielectric relaxation has been analyzed based on the behavior of electric modulus behavior. The activation energy associated with the electrical relaxation determined from the electric modulus spectra was found to be 1.76 eV, close to that the activation energy for dc conductivity (1.71 eV) indicating that the same species took part in both the processes. The stretched exponent beta (0.5-0.6) is invariant with temperature for the present glasses.
TL;DR: In this paper, the effect of low-loss glasses on the microwave dielectric properties of Sm2Si2O7 was investigated and the results showed that the results show that with suitable amounts of selected glasses is a promising material for microwave substrate applications.
Abstract: The Sm2Si2O7 ceramics were synthesized by solid-state ceramic route. The calcination and sintering temperature of Sm2Si2O7 were optimized for the best properties. The crystal structure and microstructure of the ceramic were studied by X-ray diffraction and scanning electron microscopic methods. The low frequency dielectric properties were studied at 1 MHz. The dielectric properties of the ceramic were measured in the microwave frequency range by the cavity perturbation method. Sm2Si2O7 has ɛr=12.5, tan δ=8 × 10−4, and τɛ=+63 ppm/°C at 1 MHz and a relative permittivity of 10 and tan δ of 6 × 10−3 in the microwave frequency range. The effect of addition of various low loss glasses such as 50ZnO–50B2O3 (ZB), 60ZnO–30B2O3–10SiO2 (ZBS), 27B2O3–35Bi2O3–6SiO2–32ZnO (BBSZ), 22.2MgO–22.2Al2O3–55.5SiO2 (MAS), 35.1Li2O–31.7B2O3–33.2SiO2 (LBS) and 20Li2O–20MgO–20ZnO–20B2O3–20SiO2 (LMZBS) on the microwave dielectric properties of Sm2Si2O7 was also investigated. It was found that the addition of 15 wt% LBS glass reduced the sintering temperature from 1375°C to 975°C with ɛr=9.89 and tan δ=0.024. Fifteen weight percent LMZBS glass ceramic decreased the sintering temperature to 950°C with ɛr=9.09 and tan δ=0.009. The results show that Sm2Si2O7 mixed with suitable amounts of selected glasses is a promising material for microwave substrate applications.
TL;DR: In this paper, the use of diffuse optical spectral reflectance as a nondestructive tool to characterize the microstructure of EB-PVD thermal barrier coatings (TBCs) has been investigated and the contributions of intercolumnar gaps and intracolumnar pores distinguished.
Abstract: The use of diffuse optical spectral reflectance as a nondestructive tool to characterize the microstructure of electron beam physical vapor deposition (EB-PVD) thermal barrier coatings (TBCs) has been investigated and the contributions of intercolumnar gaps and intracolumnar pores distinguished. It is shown that the reflectance is controlled by the refractive index mismatch and that the optical scattering coefficient depends on the thickness of the TBC due to the porosity distribution through the thickness of the coating. The sensitivity of the reflectance to the porosity suggests that optical reflectance can be used to characterize the microstructure of EB-PVD TBC for both quality control and nondestructive evaluation purposes.
TL;DR: In this paper, the influence of different heating rates on phase and microstructural evolution during sintering was investigated in depth on the above best green bodies and it was shown that a low-rate thermal cycle leads to a significant reduction of the α-Al 2 O 3 crystallization temperature and promotes a more effective particle rearrangement during phase transformation.
Abstract: Deagglomeration of a nanocrystalline transition alumina performed using different techniques was first demonstrated to be active in the achievement of a better powder compaction ability under uniaxial pressing and consequently in the development of a highly dense and homogeneous microstructure during pressureless sintering. A major effect, however, was associated to the heating rate chosen during the densification cycle. In fact, the influence of different heating rates (10°C/min or 1 °C/min) on phase and microstructural evolution during sintering was investigated in depth on the above best green bodies. A low-rate thermal cycle leads to a significant reduction of the α-Al 2 O 3 crystallization temperature and promotes a more effective particle rearrangement during phase transformation. As a consequence, in the low-rate treated material, it was possible to avoid the development of a vermicular structure as usually expected during the densification of a transition alumina and to yield a more homogenously fired microstructure.