Evidence for the Microwave Effect During Hybrid Sintering

A microwave/conventional hybrid furnace has been used to sinter three ceramics with different microwave absorption characteristics under pure conventional and a range of microwave/conventional hybrid heating regimes. The precursor powder particle size was also varied for each material. In each case it was ensured that every sample within a series had an identical thermal history in terms of its temperature/time profile. An increase in both the onset of densification and the final density achieved was observed with an increasing fraction of microwave energy used during sintering, the effect being greatest for the materials that absorbed microwaves most readily. Twenty-three percent greater densification was observed for submicron zinc oxide powder, the material with the largest microwave absorption capability, when sintered using hybrid heating involving 1 kW of microwave power compared with pure conventional power under otherwise identical conditions. For the ceramic with the lowest microwave absorption characteristic, alumina, the increase in densification was extremely small; partially stabilized zirconia, a moderate microwave absorber, was intermediate between the two. Temperature gradients within the samples, a potential cause of the effect, were assessed using two different approaches and found to be too small to explain the results. Hence, it is believed that clear evidence has been found to support the existence of a genuine "microwave effect."     

Direct and hybrid microwave solid state synthesis of CaCu3Ti4O12 ceramic: Microstructures and dielectric properties

CaCu₃Ti₄O₁₂ (CCTO) electroceramic possesses unusual giant dielectric permittivity up to ε?=?10⁴ at low frequency range and room temperature. CCTO dielectric properties strongly depend on its microstructure therefore it is essential to pay attention to the processing techniques which impact grain size and microstructure. In this work, direct and hybrid microwave solid state synthesis was specifically designed and used for the synthesis of CCTO. The microwave process was also compared to the conventional process which involves usual infrared heating. The structural (XRD) and microstructural (SEM) characterizations indicate that microwave synthesis is particularly efficient to get rapidly pure CCTO powder. The fully automated 915?MHz single-mode microwave cavity used for hybrid synthesis allows a perfect control of the temperature distribution and heating rate. Therefore hybrid microwave synthesis leads to a fine, mono-disperse and practically pure CCTO powder in the range of 300 – 500?nm. The advantages of the hybrid microwave heating method are discussed and compared to the conventional and direct microwave heating processes. From the powders synthesized by the different routes, dense compacts were sintered in air at 1050?°C in a conventional furnace. Microstructural characterizations reveal abnormal grain growth during sintering which levels dielectric properties. All exhibit a giant dielectric constant ε?>?10³ at room temperature which decreases drastically to ε?=?90 at 10?K. Those properties are discussed according to the well-established Internal Barrier Layer Capacitor (IBLC) model.     

Grain Growth in Microwave-Annealed Alumina

Normal grain growth in dense, fine-grained, aluminum oxide-0.1 wt% MgO was studied under both conventional furnace and 28-GHz microwave furnace annealing conditions. The microstructural changes that occurred were the same for both sets of samples; soap bubble microstructures were observed and the aspect ratios and shape factors did not change during the anneals. The kinetics of grain growth were greatly increased by the 28-GHz microwave anneals; e.g., the grain growth rate at 1500°C in the microwave furnace was the same as the rate at 1700°C in the conventional furnace. Also, the activation energy for grain growth was reduced by the microwave anneal from 590 kJ/mol (conventional) to 480 kJ/mol (microwave). Finally, these results demonstrate that a "microwave effect" can exist in a dense ceramic body and that no free pore-solid interfaces are necessary.     

Microwave Sintering of Alumina at 2.45 GHz

The sintering kinetics and microstructural evolution of alumina tubes (∼17 mm length, ∼9 mm inner diameter, and ∼11 mm outer diameter) were studied by conventional and microwave heating at 2.45 GHz. Temperature during microwave heating was measured with an infrared pyrometer and was calibrated to ±10°C. With no hold at sintering temperature, microwave-sintered samples reached 95% density at 1350°C versus 1600°C for conventionally heated samples. The activation energy for microwave sintering was 85 ± 10 kJ/mol, whereas the activation energy for conventionally sintered samples was 520 ± 14 kJ/mol. Despite the difference in temperature, grains grew from ∼1.0 μm at 86% density to ∼2.6 μm at 98% density for both conventionally sintered and microwave-sintered samples. The grain size/density trajectory was independent of the heating source. It is concluded that the enhanced densification with microwave heating is not a consequence of fast-firing and therefore is not a result in the change in the relative rates of surface and grain boundary diffusion in the presence of microwave energy.     

Sintering of Partially Stabilized Zirconia by Microwave Heating Using ZnO–MnO2–Al2O3 Plates in a Domestic Microwave Oven

Partially stabilized zirconia (PSZ) powders were fully densified by microwave heating using a domestic microwave oven. Pressed powder compacts of PSZ were sandwiched between two ZnO–MnO₂–Al₂O₃ ceramic plates and put into the microwave oven. In the first step, PSZ green pellets were heated by self-heating of ZnO–MnO₂–Al₂O₃ ceramics (1000°C). In the second step, the heated PSZ pellets absorbed microwave energy and self-heated up to a higher temperature (1250°C), leading to densification. The density of PSZ obtained by heating in the microwave oven for 16 min was 5.7 g/cm³, which was approximately equal to the density of bodies sintered at 1300°C for 4 h or 1400°C for 16 min by the conventional method. The average grain size of the sample obtained by this method was larger than the average grain size of samples sintered by the conventional method with a similar heating process.     

Densification and microstructure evolution of Y-Tetragonal Zirconia Polycrystal powder during direct and hybrid microwave sintering in a single-mode cavity

The densification and microstructure changes of 2 mol% yttria-stabilized zirconia nanopowder have been investigated during direct and hybrid microwave sintering. Microwave heating tests were achieved in a resonant single-mode cavity at 2.45 GHz and a cylindrical SiC susceptor was used for hybrid sintering experiments. Constant heating rate runs (25 °C/min) were controlled by adjusting the position of a sliding piston at constant forward microwave power. The temperature on the upper surface of the specimen was measured with an infrared camera. The final densities and the microstructures observed by SEM were compared to those of conventionally sintered materials. Homogeneous microstructures have been obtained by hybrid heating whereas direct microwave heating led to rather heterogeneous microstructures due to thermal gradients. Nevertheless, microwave-sintered materials always exhibited higher final densities for a given sintering temperature. This significant enhancement of the densification process was particularly observed in the intermediate sintering stage (1200–1350 °C range). Besides, grain growth was found to be mainly influenced by the sintering temperature rather than by the heating mode.     

Microwave Sintering of Solid Oxide Fuel Cell Materials: I, Zirconia-8 mol% Yttria

The successful microwave sintering of zirconia demonstrates the necessity to understand both the materials and electromagnetic field aspects of microwave processing. It was difficult to produce crack-free parts in the multimode microwave furnace employed in this investigation. Nonuniformities in the microwave field, and dielectric properties that increased rapidly with temperature, produced "hot spot" in the parts, which led to differential sintering and subsequent cracking. To produce crack-free sintered parts, an indirect heating method was developed that eliminated the severe differential heating. Using this indirect heating method, it was demonstrated that the sintering temperature of zirconia could be lowered from 1375° to 1200°C by microwave processing and that the resulting grain size was finer.     

Effect of Heating Rate on the Sintering Behavior and the Piezoelectric Properties of Lead Zirconate Titanate Ceramics

The effect of heating rate on the sintering behavior and the piezoelectric properties of lead zirconate titanate (PZT) ceramics was investigated. Two different types of PZT (pure and doped with Nb₂O₅) were sintered at 1150°C for 2 h with a wide range of heating rate (0.5°–100°C/min). The densification of pure PZT was improved significantly by increasing the heating rate. The improvement was attributed to the suppression of PbO volatilization and grain coarsening during heating. In contrast, the densification behavior of a PZT specimen doped with Nb₂O₅ was not much influenced by the heating rate. These densification behaviors affected the piezoelectric properties of the specimens. The piezoelectric properties of pure PZT were enhanced significantly by increasing the heating rate, while those of doped specimens were improved only moderately.     

Comparison of Microwave Hybrid and Conventional Heating of Preceramic Polymers to Form Silicon Carbide and Silicon Oxycarbide Ceramics

Six different preceramic polymers were pyrolyzed via conventional and microwave hybrid heating; these polymers provide a range of carbon content and local atomic coordination. The products were compared with each other using X-ray diffractometry and transmission electron microscopy. Nanocrystalline β-SiC was the principal crystal phase detected, and the amount and size of the nanocrystals increased as the processing temperature increased. Differences were observed in the amount and size of the β-SiC nanocrystals and the graphitization of residual carbon between the microwave hybrid heating and the conventional oven heating of polycarbosilanes. Conventional heating of a high-carbon polysiloxane in an oven (in flowing argon) produced a greater amount of β-SiC from carbothermal reduction at high temperature. Microwave hybrid heating led to better β-SiC nanocrystal development for polyureasilazane.     

Spark Plasma Sintering of Alumina

A systematic study of various spark plasma sintering (SPS) parameters, namely temperature, holding time, heating rate, pressure, and pulse sequence, was conducted to investigate their effect on the densification, grain-growth kinetics, hardness, and fracture toughness of a commercially available submicrometer-sized Al₂O₃ powder. The obtained experimental data clearly show that the SPS process enhances both densification and grain growth. Thus, Al₂O₃ could be fully densified at a much lower temperature (1150°C), within a much shorter time (minutes), than in more conventional sintering processes. It is suggested that the densification is enhanced in the initial part of the sintering cycle by a local spark-discharge process in the vicinity of contacting particles, and that both grain-boundary diffusion and grain-boundary migration are enhanced by the electrical field originating from the pulsed direct current used for heating the sample. Both the diffusion and the migration that promote the grain growth were found to be strongly dependent on temperature, implying that it is possible to retain the original fine-grained structure in fully densified bodies by avoiding a too high sintering temperature. Hardness values in the range 21–22 GPa and fracture toughness values of 3.5 ± 0.5 MPa·m^1/2 were found for the compacts containing submicrometer-sized Al₂O₃ grains.     

Grain Growth in Sintered Uranium Dioxide: I, Equiaxed Grain Growth

Grain growth was investigated in a UO₂ sinter of 94%) theoretical density over the temperature range 1555° to 2440°C. The results were in close, but not exact, agreement with a theoretical expression describing grain growth with a poly-crystalline matrix. For the material studied the mean grain diameter D (μm) after annealing for t hours at a temperature T (°K) was given by the equation

where D₀ and K₀ are, respectively, the initial grain size and a proportionality constant. Uranium metal was found in all specimens annealed above 2000°C. This was taken as evidence that the UO₂ lattice can be oxygen-deficient at high temperatures.     

Mullite/Alumina Particulate Composites by Infiltration Processing

A method has been developed for incorporating mullite into alumina by infiltrating an alumina preform with a prehydrolyzed ethyl silicate solution, followed by heating to decompose the infiltrant, form mullite, and densify the mullite/alumina composite. It has been found that the major portion of the weight loss of gelled ethyl silicate sols occurs in the 250° to 350°C range. Mullite formed in infiltrated bodies at ∼1475°C and specimens containing 12 to 15 vol% mullite reached 98.5% of the theoretical density after heating for 2 h at 1650°C. The mullite was found to be well dispersed within the alumina matrix and its presence decreased grain growth in the alumina by more than an order of magnitude.     

Mechanical Properties of Pure, Dense Magnesium Oxide as a Function of Temperature and Grain Size

The elastic modulus and transverse bend strength of pure, dense magnesia were determined as a function of grain size (1 to 190μ) and temperature (30° to 1500°C). The elastic modulus was essentially independent of grain size over the temperature range covered. The transverse bend strength for magnesia revealed a 1/6 power dependency versus grain size at low temperatures, i.e. S_TB= 50,000 G^−1/6 At temperatures above approximately 600°C, the strength of specimens of all grain sizes tested decreased with temperature and the grain size dependence, of strength also decreased. Nonlinear load-deflection behavior was observed at temperatures above approximately 700°C and etch pit observations of dislocations produced during deformation and fracture of large-grain magnesia specimens revealed extensive slip in grains adjacent to the fracture area.     

Rapid Formation and Growth of Bixbyite-Type (In0.67Fe0.33)2O3 by 28 GHz Microwave Irradiation

(In_0.67Fe_0.33)₂O₃ with the bixbyite structure was synthesized via 28 GHz microwave irradiation, using multimode microwave heating equipment. Indium sesquioxide strongly absorbs 28 GHz microwaves, and this strong coupling with microwave energy can be used to drive a reaction with iron sesquioxide. A mixture of In₂O₃ and α-Fe₂O₃ powders (In:Fe ratio of 2:1) was irradiated with microwaves at a frequency of 28 GHz. The mixture was heated to 1400°C during the microwave irradiation. The formation of a solid solution was completed within a minute, which indicated a drastic enhancement of the reaction rate. Scanning electron microscopy revealed remarkable grain growth under microwave irradiation.     

Two-Step Sintering of Nanocrystalline ZnO Compacts: Effect of Temperature on Densification and Grain Growth

Two-step sintering (TSS) was applied on nanocrystalline zinc oxide (ZnO) to control the accelerated grain growth occurring during the final stage of sintering. The grain size of a high-density (>98%) ZnO compact produced by the TSS was smaller than 1 μm, while the grain size of those formed by the conventional sintering method was ∼4 μm. The results showed that the temperature of both sintering steps plays a significant role in densification and grain growth of the nanocrystalline ZnO compacts. Several TSS regimes were analyzed. Based on the results obtained, the optimum regime consisted of heating at 800°C (step 1) and 750°C (step 2), resulting in the formation of a structure containing submicrometer grains (0.68 μm). Heating at 850°C (step 1) and then at 750°C (step 2) resulted in densification and grain growth similar to the conventional sintering process. Lower temperatures, e.g., 800°C (step 1) and 700°C (step 2), resulted in exhaustion of the densification at a relative density of 86%, above which the grains continued to grow. Thermogravimetric analysis results were used to propose a mechanism for sintering of the samples with transmission electron micrographs showing the junctions that pin the boundaries of growing grains and the triple-point drags that result in the grain-boundary curvature.     

Evidence for non-thermal microwave effect in processing of tailing-based glass-ceramics

Tailing-based glass-ceramics were crystallized via conventional and microwave heating at 720 °C for 30 min and 820 °C without holding and compared in order to obtain evidence for a non-thermal microwave effect. The comparative analytical results showed that the microstructural uniformity was greatly enhanced and the crystallization activation energy was significantly reduced by microwave processing compared to conventional heating, suggesting accelerated grain growth during crystallization. Microwave radiation affected the crystal orientation and induced corresponding changes in the SiO bond lengths and SiOSi angles, thus enhancing the formation of the diopside crystal structure. In addition, the samples under microwave processing exhibited superior physicochemical performance, including greater relative density, bending strength, microhardness, and resistance to acids and alkali, compared to conventionally processed samples. All these results provided positive evidence supporting the existence of a genuine microwave non-thermal effect, allowing deepened understanding for fine control over microwave-assisted metallurgy.     

Anisotropic Grain Growth and Microstructural Evolution of Dense Mullite above 1550°C

Mullite powder with a nearly stoichiometric composition was hot-pressed at 1550°C to produce an almost fully dense microstructure of fine, nearly uniaxial grains. The grain growth of the dense mullite was investigated during subsequent annealing at temperatures in the range of 1550–1750°C. Grain growth was relatively slow at 1550°C and the microstructure remained nearly equiaxial. Annealing at temperatures above the eutectic temperature (∼1590°C) produced fairly rapid anisotropic grain growth. At 1750°C, the anisotropic grain growth can be divided into two stages. In the first stage, the initial microstructure with an anisometric shape factor of 1.7 evolved rapidly into a microstructure with a shape factor of 2.7, consisting of a significant fraction of highly elongated grains. In the second stage, the microstructure evolved slowly into a system consisting of somewhat "blocky" grains with a shape factor of 2.2. The Al₂O₃ content of the mullite grains increased slightly and reached an equilibrium value during the first stage of anisotropic grain growth. For the samples annealed at 1750°C, the indentation fracture toughness (2.5 ± 0.2 MPa · m^1/2) was almost independent of the anisometric shape factor. The interaction between the indentation cracks and the microstructure showed a predominantly transgranular mode of crack propagation. The data indicate that while a network of highly elongated grains can be developed by the present approach, some further manipulation of the grain boundary chemistry is required for an improvement of the fracture toughness.     

Sintering Behavior of Beryllium Oxide

The sintering behavior of beryllia in a reducing atmosphere was studied between 1500° and 2100°C. Above 1700°C. the firing temperature had only a small effect on the density after heating for 24 hours. Examination of the time dependence of sintering showed that at 1700°C. during the first 3 to 5 hours there was a large increase in the density of the body accompanied by a simultaneous rapid rate of grain growth. Firing for longer times resulted in more moderate increases in both density and grain growth. The grain-growth characteristics of beryllia were unchanged by most oxide additions although compacts of higher density resulted. Oxide additives which formed a liquid phase at the sintering temperature enhanced both the sinterability and grain growth of beryllia.     

Formation of Varistor Characteristics by the Grain-Boundary Penetration of ZnO-PrOx Liquid into ZnO Ceramics

Penetration of a liquid (ZnO-PrO _x ) into the grain boundaries of sintered, cobalt-doped ZnO pellets resulted in varistors with breakdown voltages per grain boundary in the 1-2 V range and nonlinearity coefficients of 22-37. The varistors were fabricated by spreading a thin layer of Pr₆O₁₁ powder paste on the surface of ZnO pellets and heating to various temperatures (1200°-1400°C) and times (0-60 min). Comparing the varistor properties per grain boundary (e.g., threshold voltage, donor concentration, and barrier height) of liquid penetration to those of conventional method indicated the individual grain boundaries were electrically activated when the samples were heat-treated above liquid-phase formation temperature.     

Effect of Microwave Processing on the Crystallization and Energy Density of BaO‐Na2O‐Nb2O5‐SiO2‐B2O3 Glass‐Ceramics

Barium sodium niobate (BNN) glass‐ceramics were successfully synthesized through a controlled crystallization method, using both a conventional and a microwave hybrid heating process. The dielectric properties of glass‐ceramics devitrified at different temperatures and conditions were measured. It was found that the dielectric constant increased with higher crystallization temperature, from 750°C to 1000°C, and that growth of the crystalline phase above 900°C was essential to enhancing the relative permittivity and overall energy storage properties of the material. The highest energy storage was found for materials crystallized conventionally at 1000°C with a discharge energy density of 0.13 J/cm³ at a maximum field of 100 kV/cm. Rapid microwave heating was found to not give significant enhancement in dielectric properties, and coarsening of the ferroelectric crystals was found to be critical for higher energy storage.