Varistor properties and aging behavior of ZnO-V<Subscript>2</Subscript>O<Subscript>5</Subscript>-Mn<Subscript>3</Subscript>O<Subscript>4</Subscript> ceramics with sintering process

The microstructure, electrical properties, and DC-accelerated aging behavior of the ZnO-V₂O₅-Mn₃O₄ ceramics were investigated at different sintering temperatures of 850–925°C. The microstructure of the ZnO-V₂O₅-Mn₃O₄ ceramics consisted of ZnO grain as a primary phase, and Zn₃(VO₄)₂ which acts as a liquid-phase sintering aid, in addition to Mn-rich phase as secondary phases. The maximum value (3,172 V/cm) and minimum value (977 V/cm) of breakdown field were obtained at sintering temperature of 850 and 900°C, respectively. The nonlinear coefficient exhibited the highest value, reaching 30 at 925°C and the lowest value, reaching 4 at 850°C. The optimum sintering temperature was 900°C, which exhibited not only high nonlinearity with 24 in nonlinear coefficient, but also the high stability, with %ΔE_1mA = −0.9% and %∆α = −12.5% for DC-accelerated aging stress of 0.85 E_1mA/85°C/24 h.     

Effect of replacement of MgO by CaO on sintering,crystallization and properties of MgO–Al<Subscript>2</Subscript>O<Subscript>3</Subscript>–SiO<Subscript>2</Subscript> system glass-ceramics

The effects of replacement of MgO by CaO on the sintering and crystallization behavior of MgO–Al₂O₃–SiO₂ system glass-ceramics were investigated. The results show that with increasing CaO content, the glass transition temperature firstly increased and then decreased, the melting temperature was lowered and the crystallization temperature of the glass-ceramics shifted clearly towards higher temperatures. With the replacement of MgO by less than 3 wt.% CaO, the predominant crystalline phase in the glass-ceramics fired at 900 °C was found to be α-cordierite and the secondary crystalline phase to be μ-cordierite. When the replacement was increased to 10 wt.%, the predominant crystalline phase was found to be anorthite and the secondary phase to be α-cordierite. Both thermal expansion coefficient (TCE) and dielectric constant of samples increases with the replacement of MgO by CaO. The dielectric loss of sample with 5 wt.% CaO fired at 900 °C has the lowest value of 0.08%. Only the sample containing 5 wt.% and10 wt.% CaO (abbreviated as sample C5 and C10) can be fully sintered before 900 °C. Therefore, a dense and low dielectric loss glass-ceramic with predominant crystal phase of α-cordierite and some amount of anorthite was achieved by using fine glass powders (D₅₀ = 3 μm) fired at 875–900 °C. The as-sintered density approaches 98% theoretical density. The flexural strength of sample C5 firstly increases and then decreases with sintering temperature, which closely corresponds to its relative density. The TCE of sample C5 increases with increasing temperature. The dielectric property of sample C5 sintered at different temperatures depends on not only its relative density but also its crystalline phases. The dense and crystallized glass-ceramic C5 exhibits a low sintering temperature (≤900 °C), a fairly low dielectric constant (5.2–5.3), a low dielectric loss (≤10⁻³) at 1 MHz, a low TCE (4.0–4.25 × 10⁻⁶ K⁻¹), very close to that of Si (∼3.5 × 10⁻⁶ K⁻¹), and a higher flexural strength (≥134 MPa), suggesting that it would be a promising material in the electronic packaging field.     

Microwave synthesis and sintering characteristics of CaCu<Subscript>3</Subscript>Ti<Subscript>4</Subscript>O<Subscript>12</Subscript>

CaCu₃Ti₄O₁₂ (CCTO) was synthesized and sintered by microwave processing at 2·45 GHz, 1·1 kW. The optimum calcination temperature using microwave heating was determined to be 950°C for 20 min to obtain cubic CCTO powders. The microwave processed powders were sintered to 94% density at 1000°C/60 min. The microstructural studies carried out on these ceramics revealed the grain size to be in the range 1–7 μm. The dielectric constants for the microwave sintered (1000°C/60 min) ceramics were found to vary from 11000–7700 in the 100 Hz–00 kHz frequency range. Interestingly the dielectric loss had lower values than those sintered by conventional sintering routes and decreases with increase in frequency.     

Synthesis and microstructure of SrTiO<Subscript>3</Subscript> and BaTiO<Subscript>3</Subscript> ceramics by a reaction-sintering process

Strontium titanate and barium titanate ceramics prepared by a reaction-sintering process were investigated. The mixture of raw materials of stoichiometric SrTiO₃ and BaTiO₃ was pressed and sintered into ceramics without any calcination stage involved. A density 4.99 g/cm³ (97.5% of the theoretic value) was found in SrTiO₃ after 6 h sintering at 1,370 °C. Grains less than 1.5 μm were formed at 1,300–1,330 °C and became 2.2–3.3 μm at 1,350–1,370 °C SrTiO₃. A density 5.89 g/cm³ (97.9% of the theoretic value) was found in BaTiO₃ after 6 h sintering at 1,400 °C. Merged grains were observed in BaTiO₃ and were less than 10 μm after sintered at 1,400 °C.     

Microstructure and electrical properties of Pr<Subscript>6</Subscript>O<Subscript>11</Subscript> doped ZnO-Bi<Subscript>2</Subscript>O<Subscript>3</Subscript>-based varistors

The microstructure and electrical properties of ZnO-Bi₂O₃-based varistor ceramics doped by Pr₆O₁₁ in the content range of 0–5.49 wt% were investigated at different sintering temperatures (1,100, 1,150, 1,175, 1,200 °C). The increase of sintering temperature leads to more dense ceramics, which increases the nonlinear property, whereas it decreases the voltage-gradient and leakage current. With increasing Pr₆O₁₁ content, the breakdown voltage increases because of the decreases of ZnO grain size. The improvement of non linear coefficient together with the decrease of leakage current are related to the uniformly distribution of secondary phases along the grain boundaries of the ZnO. The varistors sintered at 1,175 °C with the 3.37 wt% Pr₆O₁₁ doping possess the best electrical properties: the varistor voltage, nonlinear coefficient, and leakage current are 340 V/mm, 46 and 0.63 μA, respectively.     

Thermoanalytical study of the polymorphism and melting behavior of Cu<Subscript>2</Subscript>V<Subscript>2</Subscript>O<Subscript>7</Subscript>

We have developed a procedure for the synthesis of phase-pure α- and β-Cu₂V₂O₇. Thermal analysis and X-ray diffraction demonstrate that the β-phase (monoclinic structure) exists at low temperatures (stability range 25–610°C), while α-Cu₂V₂O₇ (orthorhombic structure) is stable in the range 610–704°C. The α-phase observed during cooling, in particular at room temperature, is in a metastable state. The melting of the high-temperature phase γ-Cu₂V₂O₇, which forms between 704 and 716°C, has the highest rate in the range 770–785°S and is accompanied by peritectic decomposition and oxygen gas release. Subsequent cooling gives rise to four exothermic peaks, one of which (780.9°C) is attributable to the crystallization of the peritectic melt, one (620.1°C) is due to the γ → α → β phase transformations of Cu₂V₂O₇, and the other two arise from the crystallization of multicomponent low-melting-point eutectics containing α- and β-Cu₂V₂O₇, CuVO₃, and other compounds.     

Effect of MnO<Subscript>2</Subscript> on chemical interaction between CaO and Al<Subscript>2</Subscript>O<Subscript>3</Subscript>

We have studied the effect of manganese dioxide (5–20 wt %) on the formation of calcium monoaluminate in the CaO-Al₂O₃ system during solid-state sintering of five oxide mixtures corresponding to the known calcium aluminates at temperatures from 1100 to 1400°C. X-ray diffraction examination indicated the formation of calcium manganates. The mixture of the calcium manganates melts at 1330°C, promoting the sintering process and the reaction between the calcium oxide and alumina and raising the yield of calcium monoaluminate as the dominant phase. The calcium manganates are shown to be nonreactive with the forming calcium aluminates. The X-ray diffraction results are supported by scanning electron microscopy data for a number of samples.     

Solid state sintering of lime in presence of La<Subscript>2</Subscript>O<Subscript>3</Subscript> and CeO<Subscript>2</Subscript>

The sintering of lime by double calcination process from natural limestone has been conducted with La₂O₃ and CeO₂ additive up to 4 wt.% in the temperature range 1500–1650° C. The results show that the additives enhanced the densification and hydration resistance of sintered lime. Densification is achieved up to 98.5% of the theoretical value with La₂O₃ and CeO₂ addition in lime. Grain growth is substantial when additives are incorporated in lime. The grain size of sintered CaO (1600°C) with 4 wt.% La₂O₃ addition is 82 μm and that for CeO₂ addition is 50 μm. The grains of sintered CaO in presence of additive are angular with pores distributed throughout the matrix. EDX analysis shows that the solid solubility of La₂O₃ and CeO₂ in CaO grain is 2.9 and 1.7 weight %, respectively. The cell dimension of CaO lattice is 4.803 %C. This value decreases with incorporation of La₂O₃ and CeO₂. The better hydration resistance of La₂O₃ added sintered lime compared to that of CeO₂ added one, is related to the bigger grain size of the lime in former case.     

Low-temperature sintering of SiC reticulated porous ceramics with MgO–Al<Subscript>2</Subscript>O<Subscript>3</Subscript>–SiO<Subscript>2</Subscript> additives as sintering aids

SiC reticulated porous ceramics (SiC RPCs) was fabricated with polymer replicas method by using MgO–Al₂O₃–SiO₂ additives as sintering aids at 1,000∼1,450 °C. The MgO–Al₂O₃–SiO₂ additives were from alumina, kaolin and Talc powders. By employing various experimental techniques, zeta potential, viscosity and rheological measurements, the dispersion of mixed powders (SiC, Al₂O₃, talc and kaolin) in aqueous media using silica sol as a binder was studied. The pH value of the optimum dispersion was found to be around pH 10 for the mixtures. The optimum condition of the slurry suitable for impregnating the polymeric sponge was obtained. At the same time, the influence of the sintering temperature and holding time on the properties of SiC RPCs was investigated. According to the properties of SiC RPCs, the optimal sintering temperature was chosen at 1,300 °C, which was lower than that with Al₂O₃–SiO₂ additives as sintering aids.     

Templated synthesis of mesoporous aluminas by <Emphasis Type="Italic">graft</Emphasis> copolymer and their CO<Subscript>2</Subscript> adsorption capacities

Mesoporous aluminas were synthesized via a sol–gel process by templating an amphiphilic graft copolymer, PVC–g–POEM, consisting of a poly(vinyl chloride) (PVC) backbone and poly(oxyethylene methacrylate) (POEM) side chains. The mesoporous structures of aluminas with large surface areas were confirmed by X-ray diffraction, transmission electron microscopy, and nitrogen adsorption/desorption analysis. Aluminas synthesized with PVC–g–POEM graft copolymer exhibited higher CO₂ adsorption capacities (0.7 mol CO₂/kg sorbent) than aluminas synthesized without graft copolymer (0.6 mol CO₂/kg sorbent). The adsorption capacity of alumina strongly depends on its structure and calcination temperature; amorphous (400 °C) > γ phase (800 °C) > α phase (1000 °C).     

Synthesis and variable temperature electrical conductivity studies of highly ordered TiO<Subscript>2</Subscript> nanotubes

Rafts of aligned, high aspect ratio TiO₂ nanotubes were fabricated by an electrochemical anodization method and their axial electrical conductivities were determined over the temperature range 225–400 °C. Length, outer diameter, and wall thickness of the nanotubes were approximately 60–80 μm, 160 nm, and 30 nm, respectively. Transmission electron microscopy studies confirmed that the TiO₂ nanotubes were initially amorphous, and became polycrystalline anatase after heat treatment at temperatures as low as 250 °C in air. The activation energy for conductivity over the temperature range 250–350 °C was found to be 0.87 eV. The conductivity values are comparable to those of nanocrystalline and nanoporous anatase thin films reported in literature.     

Fabrication of NaZr<Subscript>2</Subscript>(PO<Subscript>4</Subscript>)<Subscript>3</Subscript>-type ceramic materials by spark plasma sintering

Ceramic materials based on Ca_0.5Zr₂(PO₄)₃ and NaFeNb(PO₄)₃, structural analogs of NaZr₂(PO₄)₃ (NZP), were prepared by spark plasma sintering. At sintering temperatures of 1100–1200 and 880°C and sintering times of 12 and 3 min, the relative densities reached were 99.1 and 99.9%, respectively. According to X-ray diffraction data, the sintering process caused no changes in phase composition. The ceramics had a dense, homogeneous microstructure and ranged in grain size from 0.5 to 2.5 μm.     

Synthesis and characterization of Al<Subscript>2</Subscript>O<Subscript>3</Subscript>–ZrO<Subscript>2</Subscript> nanocomposite powder by sucrose process

Nanocrystalline alumina–zirconia powders were prepared by a modified chemical route using sucrose, polyvinyl alcohol (PVA) and metal nitrates followed by a post calcination process. The process involved dehydration of Al³⁺–Zr⁴⁺ ions-sucrose–PVA solution to a highly viscous liquid which on decomposition process produced a black precursor material. The obtained precursor were then calcined at various temperatures: 1,050, 1,100, 1,150, 1,200 and 1,250 °C for different soaking times (1, 2, 4 h) in air. The formation of a nanocomposite composed of α-alumina (~20 nm) and tetragonal (t) zirconia (~19 nm) crystallites were confirmed for the sample calcined at 1,200 °C for 2 h, based on our XRD and TEM results. However, for the samples calcined below 1,150 °C the composite formed were composed of metastable alumina (γ, δ, θ) as well as t-zirconia phases. Interestingly, the zirconia phase retained its tetragonal structure for all the samples calcined above 1,050 °C. This is possibly related to the “size effect” and reduction of surface enthalpy of the zirconia crystallites surrounded by Al³⁺ cations.     

Effect of lithium magnesium zinc borosilicate glass addition on densification temperature and dielectric properties of Mg<Subscript>2</Subscript>SiO<Subscript>4</Subscript> ceramics

Mg₂SiO₄ (Forsterite) ceramics were synthesized by solid state route. The effect of lithium magnesium zinc borosilicate (LMZBS) glass addition on the densification temperature and microwave dielectric properties of forsterite ceramics was investigated. The crystal structure and microstructure of ceramic–glass composites were studied by X-ray diffraction and scanning electron microscopic techniques. The dielectric properties of the sintered samples were measured in the microwave frequency range by the resonance method. Addition of 0.5 wt% LMZBS glass improved densification with ε _r = 7.3 and Qxf = 121,200 GHz. Addition of 15 wt% LMZBS glass lowered the sintering temperature to about 950 °C with ε _r = 6.75 and Qxf = 30,600 GHz. The reactivity of 15 wt% LMZBS glass added forsterite with silver was also studied. The result shows that forsterite doped with suitable amount of LMZBS glass is a possible material for LTCC and microwave substrate applications.     

Structure and properties of Ba(Zr<Subscript>0.2</Subscript>Ti<Subscript>0.8</Subscript>)O<Subscript>3</Subscript> ceramics prepared by spark plasma sintering

Ba(Zr_0.2Ti_0.8)O₃ (BZT) ceramics are prepared from spray-dried powder by spark plasma sintering (SPS) and by normal sintering. By the application of SPS, ceramics with >96% relative densities could be obtained by sintering at 1,100 °C for 5 min in air atmosphere. The pellet as sintered by SPS at 1,100 °C was black and conductive. Although SPS was carried out in air atmosphere, the samples were deoxidized by heating the carbon die. By post-annealing at 1,000 °C for 12 h in air, the pellet was oxidized and became white and insulating. Grain growth was suppressed in the ceramics prepared by SPS, and the average grain size was 0.52 μm. The starting powder contained 1.90% carbon, mainly as binder, and the SPS-prepared ceramics and ordinary prepared ceramics contained 0.15 and 0.024% carbon, respectively. The BZT ceramics obtained by SPS and the subsequent annealing at 1,000 °C for 12 h exhibited a mild temperature dependence of their dielectric constant. The field-induced displacement of the BZT ceramics was less hysteretic and smaller than that of the ceramics sintered by the conventional method.     

Far infrared reflectance of sintered Zn<Subscript>2</Subscript>TiO<Subscript>4</Subscript>

Zinc-titanate ceramics were obtained by initial mechanical activation in a high-energy planetary mill for 15 min followed by sintering at temperatures 900–1100 °C for 2 h. Room temperature far infrared reflectivity spectra for all samples were measured in the range 100–1200 cm⁻¹. The same ionic oscillators were present in the measured spectra, but their intensities increased with the sintering temperature in correlation with the increase in sample density and microstructure changes. Optical parameters were determined for seven oscillators belonging to the spinel structure using the four-parameter model of coupled oscillators. Born effective charges were calculated from the transversal/longitudinal splitting.     

Microwave dielectric properties of Ba<Subscript>5</Subscript>Nb<Subscript>4</Subscript>O<Subscript>15</Subscript> ceramic by molten salt method

Ba₅Nb₄O₁₅ powders were synthesized by molten-salt method in NaCl–KCl flux at a low temperature of 650–900 °C for 2 h, which is lower than that of the conventional solid-state reaction. This simple process involved mixing of the raw materials and salts in a certain proportion. Subsequent calcination of the mixtures led to Ba₅Nb₄O₁₅ powders at 650–900 °C. XRD and SEM techniques were used to characterize the phase and morphology of the fabricated Ba₅Nb₄O₁₅ powders, respectively. After sintering at 1,300 °C for 2 h, the densified Ba₅Nb₄O₁₅ ceramics with good microwave dielectric properties of ε_r = 39.2, Q × f approximated as 27,200 GHz and τ _f  = 72 ppm/°C have been obtained.     

Dielectric and piezoelectric properties of MnO<Subscript>2</Subscript>-doped K<Subscript>0.5</Subscript>Na<Subscript>0.5</Subscript>Nb<Subscript>0.92</Subscript>Sb<Subscript>0.08</Subscript>O<Subscript>3</Subscript> lead-free ceramics

Lead-free MnO₂-doped K_0.5Na_0.5Nb_0.92Sb_0.08O₃ ceramics have been fabricated by a conventional ceramic technique and their dielectric and piezoelectric properties have been studied. Our results show that a small amount of MnO₂ (0.5–1.0 mol%) is enough to improve the densification of the ceramics and decrease the sintering temperature of the ceramics. The co-effects of MnO₂ doping and Sb-substitution lead to significant improvements in the ferroelectric and piezoelectric properties. The K_0.5Na_0.5Nb_0.92Sb_0.08O₃ ceramic with 0.5 mol%MnO₂ doping possesses optimum propeties: d ₃₃ = 187 pC/N, k _P = 47.2%, ε _r = 980, tanδ = 2.71% and T _c = 287 °C. Due to high tetragonal-orthorhombic phase transition temperature (T _O-T ~ 150 °C), the K_0.5Na_0.5Nb_0.92Sb_0.08O₃ ceramic with 0.5 mol%MnO₂ doping exhibits a good thermal stability of piezoelectric properties.     

Low temperature preparation of the Zn<Subscript>2</Subscript>SiO<Subscript>4</Subscript> ceramics with the addition of BaO and B<Subscript>2</Subscript>O<Subscript>3</Subscript>

The Zn₂SiO₄ ceramics with the addition of BaO and B₂O₃ are fabricated by traditional solid-state preparation process at a sintering temperature of 900 °C. The introduction of BaO and B₂O₃ to the binary system ZnO-SiO₂ is achieved by adding 10 and 20 wt. % flux BB to the mixed ZnO-SiO₂ ceramic powders pre-sintered at 1,100 °C, respectively. The chemical composition of the flux BB (50 wt.%BaO-50 wt.% B₂O₃) is located at a liquid phase zone with a temperature range of about 869–900 °C in the binary diagram BaO-B₂O₃. In addition, the introduction of BaO and B₂O₃ to the binary system ZnO-SiO₂ is also achieved by the means of a chemical combination of H₂SiO₃, H₃BO₃, ZnO and Ba(OH)₂·8H₂O, which can result in the formation of the hydrated barium borates with low melting characteristics. In turn, by the liquid sintering aid of the barium borate melts, the preparation process of the Zn₂SiO₄ ceramics can be further simplified. In the two preparation methods, the Zn₂SiO₄ ceramics with the 1.5–2.0 ZnO/SiO₂ molar ratios and the addition of a 10 wt. % flux BB can show good dielectric properties whereas the bending strength mainly depends on the microstructure of the Zn₂SiO₄ ceramics and SiO₂ content in the composition of the specimen.     

ZrO<Subscript>2</Subscript> foams for porous radiant burners

In this work, Y₂O₃-stabilized ZrO₂ (YSZ) foams with low relative density were developed through the replication method, for application as porous radiant burners. The ceramic foams were produced by impregnation of open-cell polyurethane foams with aqueous suspensions and different fractions of raw materials: ZrO₂–8% Y₂O₃ (8YSZ) powder, and additives. The materials were milled for 10–40 min. The impregnated foams were dried and submitted to a heat treatment for polyurethane elimination at 1000 °C for 1 h, with subsequent sintering of the remaining ceramic structure at 1600 °C for 2 h, which resulted in YSZ foams with low relative density (0.07). The structural analysis revealed a cellular structure with an average mechanical strength of 95.6 kPa. The radiation efficiency (>19%) was obtained by tests with different air/fuel ratio. The ceramic matrixes exhibited high performance and structural integrity at high operation temperatures (1400 °C).