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.     

Effect of nanometer barium titanate powders on the dielectric properties of X7R ceramics

High dielectric constant X7R dielectric materials were prepared by doping with nanometer BaTiO₃ additives, with a dielectric constant as high as 5,400 and dielectric loss lower than 1.0% at 25 °C, when they were sintered at 1,240 °C for 4 h. The effects of nanometer BaTiO₃ additives on the microstructure and dielectric properties of X7R ceramics were investigated in this paper. The nanometer BaTiO₃ additives were prepared by sol–gel method with an average particle size of ~65 nm. It was favorable by nanometer BaTiO₃ additives to make the ceramics hold high dielectric constant and still satisfy the requirement of EIA X7R specification. Due to the special nano-effect of the nanometer BaTiO₃ additives, the densification of ceramics were promoted effectively, which resulted in the increase of dielectric constant. The temperature-capacitance characteristics which satisfied X7R specification were attributed to the formation of abundant “core–shell” structure, which gave rise to the abundant spontaneous polarization microarea and the decrease of the tetragonality (c/a ratio).     

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.     

Structural characteristics and dielectric properties of neodymium doped barium titanate

Neodymium (Nd) doped barium titanate powder (Ba_(1−x)Nd _x TiO₃) with x value varying from 0, 0.01, 0.03, 0.05, 0.07, 0.10 and 0.13 was prepared using the sol gel method. The powder samples were calcined at 700 °C and tetragonal phase appeared in the powders before they were sintered at 1250 °C for 3 h. The undoped samples have a polycrystalline tetragonal structure, but Nd doping into the BaTiO₃ caused phase transformation from tetragonal to cubic. The smaller grains (0.35 μm) produced with the addition of Nd is associated to the inhibition of grain growth of samples. The powders for each composition were pressed into pellets and tested as dielectric resonator antenna (DRA). It was found that on the actual antenna circuit, each sample showed a resonance frequency at X-band application and a dielectric constant value in the range of 51.25–56.89 and tangent loss was 0.039–0.045, depending on the concentration of the Nd at room temperature.     

Effect of hot pressing time and temperature on the microstructure and mechanical properties of ZrB<Subscript>2</Subscript>–SiC

Structure–property relations were examined for ZrB₂ containing 30 volume percent SiC particulates. Two grades of ZrB₂ with initial particle sizes of 2 and 6 μm were used. Billets of ZrB₂–SiC were produced by hot pressing at 1850, 1950 or 2050 °C for 45 min. In addition, the material prepared from ZrB₂ with an initial particle size of 2 μm was hot pressed at 2050 °C for 90 and 180 min. Microstructures and mechanical properties were characterized to determine the effects of the initial particle size, hot pressing time, and hot pressing temperature on the final grain size and morphology. The average grain size of the ZrB₂ phase ranged from 2.2 to 4.7 μm. Similarly, the average grain size of the SiC phase ranged from 1.2 to 2.7 μm. Hardness and modulus of elasticity were not affected by the processing conditions with average values of 22 and 505 GPa, respectively. However, flexural strength decreased as grain size increased from a maximum of ∼1050 MPa for the finest grain sizes to ∼700 MPa for the largest grain sizes. Analysis suggested that the strength of ZrB₂–SiC was limited by the size of the SiC inclusions in the ZrB₂ matrix.     

Effect of aluminum addition on electrical properties,dielectric characteristics,and its stability of (Pr,Co, Cr,Y)-added zinc oxide-based varistors

The electrical properties, dielectric characteristics, and its stability against d.c. accelerated aging stress of (Pr, Co, Cr, Y)-added zinc oxide-based varistors were investigated for different aluminum concentrations under a sintering temperature of 1280°C. As the aluminum concentration increased, the average grain size (d) increased in the range of 4.3–5.5 μm and the sintered density increased in the range of 5.63–5.67 g/cm³. As the aluminum concentration increased, the breakdown field decreased in the range of 6327-710 V/cm and the maximum nonlinear coefficient (46.9) was obtained for 0.005 mol% in aluminum concentration, further additions impaired the nonlinear properties. As the aluminum concentration increased, the apparent dielectric constant increased in the range of 500.5–1327.4 and dissipation factor increased in the range of 0.00493–0.0724. The varistor added with 0.001 mol% Al exhibited the highest stability for E-J characteristics in which %ΔE _{1 mA} is +1.4% and %Δα is −5.7%, under stress state of 0.95 E _{1 mA}/150°C/24 h.     

Dielectric Ba(Ti1?xSnx)O3 (x?=?0.13) ceramics, sintered by spark plasma and conventional methods

A two-step sintering approach composed of spark-plasma-sintering (SPS) technique at 1000 °C for 1 min and under a uniaxial pressure of 63 MPa followed by conventional sintering at 1400 °C for 3 h is proposed for synthesis of dense Ba(Ti_0.87Sn_0.13)O₃ ceramics. Starting powders had grain size of about 90 nm and were obtained by co-precipitation. The SPS pellets consist of submicron (300–500 nm) grains. X-ray diffraction analysis of as-prepared Ba(Ti_0.87Sn_0.13)O₃ ceramic shows the occurrence of cubic and tetragonal phase coexistence for the pellets obtained after SPS processing and the presence of only tetragonal phase in the samples after the second (conventional) sintering. Grain uniformity in the final product is high, with average size of ~2 μm. The apparent densities of the sintered pellets at temperature of 1400 °C were ~92% of the theoretical value of Ba(Ti_0.87Sn_0.13)O₃. The ceramics exhibit a high relative dielectric constant of 6,550 and a dielectric loss (tan δ) = 0.078 at Curie temperature of 63 °C and 10 Hz.     

Synthesis of Ba<Subscript>3</Subscript>ZnNb<Subscript>2</Subscript>O<Subscript>9</Subscript>−Sr<Subscript>3</Subscript>ZnNb<Subscript>2</Subscript>O<Subscript>9</Subscript> solid solution and their dielectric properties

Oxides of the type, Ba_3-xSr_xZnNb₂O₉ (0 ≤x ≤3), were synthesized by the solid state route. Oxides calcined at 1000°C show single cubic phase for all the compositions. The cubic lattice parameter (a) decreases with increase in Sr concentration from 4.0938(2) forx = 0 to 4.0067(2) forx = 3. Scanning electron micrographs show maximum grain size for thex = 1 composition (∼ 2 μm) at 1200°C. Disks sintered at 1200°C show dielectric constant variation between 28 and 40 (at 500 kHz) for different values of x with the maximum dielectric constant atx = 1.     

Compositionally graded multilayer Ba<Subscript>x</Subscript>Sr<Subscript>0.95−x</Subscript>Ca<Subscript>0.05</Subscript>TiO<Subscript>3</Subscript> ceramics prepared by tape casting for use as dielectric materials

Compositionally graded multilayer Ba_xSr_0.95−xCa_0.05TiO₃ (BSCT) ceramics were prepared via tape casting method using nanometer powders from co-precipitation. Microstructures and dielectric properties of the BSCT system were investigated. The powders were characterized by using transmission electron microscope and BET surface area measurement. Surface morphologies of the sintered samples and multilayer structure were examined by scanning electron microscopy. BSCT particles were of spherical shape with diameters in the range of 73–93 nm. Their specific surface areas were in the range of 11.7–14.6 m²/g. The graded BSCT ceramics with nine layers laminated in vertical way exhibited a higher sintered density, with an average grain size of 0.4 μm, after sintered at 1,200 °C. Dielectric constant, dielectric loss and tunability of the graded ceramics were 2223.94, 1.5 × 10⁻³ at 2 MHz and 42.9% at 3.0 kV/mm, with good dielectric temperature and frequency stability, which made it a promising candidate used for tunable ceramic capacitors and phase shifters.     

Sintering behavior and dielectric properties of polycrystalline CaCu3Ti4O12

Sintering behavior and dielectric properties of CaCu₃Ti₄O₁₂ (CCTO) have been studied. CCTO phase was formed at 950 °C. A two-step sintering method was employed. The pressed green body of CaCu₃Ti₄O₁₂ was first heated to 1,080 °C to achieve an intermediate density (>75% of theoretical density), then cooled down and held at 950 °C for 0–20 h until it was fairly dense. A slight grain growth was observed for 20 h holding time with a significant increase in density from 81% to 92% of theoretical density. The average grain size of dense ceramics was ~6 μm and the bulk density was 4.7 g/cm³. The normally sintered specimen at 1,070 °C exhibited slightly higher density (4.8 g/cm³, 95%) with bimodal grain size. Normal sintering also yielded higher dielectric constants than the specimens processed by the two-step sintering.     

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.     

Effects of Nb<Subscript>2</Subscript>O<Subscript>5</Subscript> doping on the microstructure and the dielectric temperature characteristics of barium titanate ceramics

In this work, the effects of Nb₂O₅ addition on the dielectric properties and phase formation of BaTiO₃ were investigated. A core–shell structure was formed for Nb-doped BaTiO₃ resulted from a low diffusivity of Nb⁵⁺ ions into BaTiO₃ when grain growth was inhibited. In the case of 0.3–4.8 mol% Nb₂O₅ additions, two dielectric constant peaks were observed. The Curie dielectric peak was determined by the ferroelectric-paraelectric transition of grain core, whereas the secondary broad peak at lower temperature was due to strong chemical inhomogeneity in Nb-doped BaTiO₃ ceramics. The dielectric constant peak at Curie temperature was markedly depressed with the addition of Nb₂O₅. On the other hand, the secondary dielectric constant peak was enhanced when sintered above 1280 °C for higher Nb₂O₅ concentrations (≥1.2 mol%). The Curie temperature was shifted to higher temperatures, whereas the transition temperature corresponding to the secondary peak moved to lower temperatures as increasing the amount of Nb₂O₅ more than 1.2 mol%. The decrease of this lower transition temperature was assumed to be closely related with the secondary phase formation when Nb concentration greater than 1.2 mol%. From XRD analyses, a large amount of secondary phases was observed when Nb₂O₅ amount exceeded 1.2 mol%. The coefficients of thermal expansion of Nb-doped BaTiO₃ were increased with increasing Nb₂O₅ contents, resulting in large internal stress between cores and shells. Therefore, the shift of Curie temperature to higher temperatures was attributed to internal stress resulting from the formation of a core–shell structure and a large amount of secondary phase grains.     

Effect of Nb<Subscript>2</Subscript>O<Subscript>5</Subscript> additions on the power loss of NiZn ferrites

Nickel–zinc ferrite system, Ni_0.65Zn_0.35Fe₂O₄ + x Nb₂O₅ where x varies from 0.0 wt% to 1.5 wt% in steps of 0.3 wt%, has been prepared by conventional ceramic technique. The samples were sintered at 1250 °C for 4 h in air atmosphere followed by natural cooling. The power loss and microstructures of these materials are examined. Microstructures reveal that niobium oxide additions promoted grain growth with an increase in grain size from 4 μm to 13.2 μm with the increase in niobium concentration. The measured power loss at frequencies from 100 kHz to 10 MHz under different exciting flux densities from 5 mT to 30 mT was found to be low up to 3 MHz, thus making the materials suitable for power applications up to this frequency. In the total power loss, hysteresis loss is predominant below 500 kHz and eddy current loss component is much higher at higher frequencies.     

Effect of grain size and Cu-rich phase on the electric properties of CaCu3Ti4O12 ceramics

The CaCu₃Ti₄O₁₂ ceramics were prepared by the traditional solid-state reaction method under different sintering conditions. The XRD patterns show that crystal structures of the samples are basically single-phase pseudo-cubic, except little second phases of CuO and Cu₂O in the samples sintered in air at 1050 and 1100 °C, respectively, for 12 h. The SEM results indicate that the pellet sintered at 1100 °C for 12 h possess larger grain size and more Cu-rich phases at the grain boundaries than the pellet sintered at 1050 °C for 12 h. It is interesting that the pellet sintered at 1050 °C under the pressure of 5 Gpa for 3 h shows smaller grain size (~1 μm) and no Cu-rich phases due to the higher pressure during the sintering process. The results show that the grain size has a reverse effect on the values of the permittivity and the values of breakdown electric field (E _b) and nonlinear coefficient. The pellet sintered at 1100 °C for 12 h exhibits a higher permittivity, but with a lower breakdown electric field (E _b) and a lower nonlinear coefficient due to larger grain size. The pellet sintered at 1050 °C under the pressure of 5 Gpa for 3 h exhibits a lower permittivity, but with a higher breakdown electric field (E _b) and a higher nonlinear coefficient due to smaller grain size. The Cu-rich phases at grain boundaries can raise the resistance of the grain boundary leading to the lower dielectric loss tangent, which has been supported by the results of impedance spectroscopy analysis.     

Semiconducting ceramics produced using nanocrystalline barium titanate powder

Positive temperature coefficient of resistance ceramics of composition (Ba_0.89Ca_0.08Pb_0.03)TiO₃ + Y₂O₃ + MnO + SiO₂ have been produced using barium titanate powder with an average crystallite size of 125 nm prepared by calcining barium titanyl oxalate at 900°C. The effect of firing temperature on their microstructure and electrical properties has been studied. The results demonstrate that the ceramics possess semiconducting properties starting at a firing temperature of 1205–1215°C. The room-temperature resistivity of the ceramics has a minimum at t _firing ≈ 1245–1250°C. The samples sintered at 1250–1260°C have the largest positive temperature coefficient of resistance. The highest electric strength (360 V/mm at ρ_25°C = 290 Ω cm) is offered by the thermistor materials sintered at 1260°C, which is 60–70°C below the firing temperature of analogous ceramics produced by solid-state reaction.     

Effect of BaTiO<Subscript>3</Subscript> heat treatment on the microstructure and dielectric properties of BaTiO<Subscript>3</Subscript>-based ceramics

We have studied the effect of heat treatment of the starting BaTiO₃ powder on the dielectric properties and microstructure of X7R-type BaTiO₃-based ceramics. The results demonstrate that annealing of BaTiO₃ stabilizes the degree of tetragonality in the crystal lattice of the ceramics. Microstructural analysis shows that the annealing temperature has no effect on the average grain size of the ceramics. Increasing the BaTiO₃ annealing temperature increases the dielectric permittivity of the core phase and reduces the temperature coefficient of capacitance (TCC). We obtained an X7R-type BaTiO₃-based ceramic material (BaTiO₃ annealing temperature, 1150°C; firing temperature, 1160°C) with the following properties: ɛ_25°C = 2230, TCC = ±12% (−55 to 125°C), and tanδ_25°C = 0.013.     

Dielectric properties of Erbium doped CaCu<Subscript>3</Subscript>Ti<Subscript>4</Subscript>O<Subscript>12</Subscript> prepared by sol–gel self combustion method

The dielectric properties of Erbium doped CaCu₃Ti_(4–x)Er_xO_(12–δ) with x = 0, 0.05, 0.1 were synthesized by the sol–gel self combustion method. XRD (X-ray powder diffraction) analysis confirmed the formation of single-phase material in the samples calcined at 800 °C. Crystal structure does not change on doping with Erbium and it remains cubic in all the three compositions studied. It is found that lattice parameter increases slightly with Erbium doping. The surface morphology of CaCu₃Ti_(4–x)Er_xO_(12–δ) powders sintered at 950 °C in air for 3 h was observed using high resolution—scanning electron microscope and it shows that the grain size is in the range of 1–8 μm for these samples. Energy dispersive X-ray spectroscopy pattern confirmed the presence of Erbium with 1.9 and 4.86 atomic percentages with doping concentration. The dielectric characteristics of CaCu₃Ti_(4–x)Er_xO_(12–δ) were studied by LCR meter in the frequency range (100 Hz–1 MHz) at various temperatures (RT to 500 °C). Interestingly, the dielectric constant increases and dielectric loss had lower values than those of undoped CCTO.     

Spark-plasma-sintering temperature dependence of structural and piezoelectric properties of BNT–BT<Subscript>0.08</Subscript> nanostructured ceramics

(Bi_0.5Na_0.5)TiO₃ doped with 8 mol% BaTiO₃ powder prepared by sol–gel was compacted and sintered by spark-plasma-sintering method. The influence of spark-plasma-sintering temperature on the densification and piezoelectric properties of these ceramics was studied. Starting from BNT–BT_0.08 powder gel with a microstructure consisting of particles with average size of 50 nm, ceramics with grain size of 60–90 nm and density of about 98.9–99.6% of the theoretical density were obtained by spark-plasma-sintering at 800–900 °C. Increasing the sintering temperature by SPS from 800 to 900 °C lead to the increase of d ₃₃, k _p, ε₃₃^T and, decrease of Q _m. Typical d ₃₃ and k _p values of BNT–BT_0.08 ceramics sintered by spark-plasma-sintering at 900 °C were 8 and 0.029, respectively.     

Microstructure and dielectric properties of BaTiO3-based X7R ceramics

The relationship between the microstructures and dielectric properties of BaTiO₃-based X7R ceramics has been investigated at different calcination temperatures. The XRD and SEM results show that calcinations of BaTiO₃ raw powders increase the grain size and stabilize the tetragonality (c/a ratio) of the ceramics. The grain growth caused by the calcination prevents the doped ions from diffusing into the interior of the grains, and then increases the volume fraction of the tetragonal phased core. This process greatly increases the dielectric constant by improving the ferroelectricity. As a result, the relaxation mechanism of the domain reorientation generates high loss tangent. The BaTiO₃ ceramics with X7R specifications were prepared at the calcination temperature of 1200 °C and the sintering temperature of 1240 °C, whose dielectric properties were ɛ _r ≥ 4500, Δɛ _r/ɛ _r25 ≤ ± 10%(−55∼125 °C), tanδ ≤ 0.012(25 °C), respectively.     

A sol–gel-derived α-Al<Subscript>2</Subscript>O<Subscript>3</Subscript> crystal interlayer modified 316L porous stainless steel to support TiO<Subscript>2</Subscript>, SiO<Subscript>2</Subscript>, and TiO<Subscript>2</Subscript>–SiO<Subscript>2</Subscript> hybrid membranes

A homogeneous α-Al₂O₃ crystal membrane was fabricated by the sol–gel technique on 316L porous stainless steel (PSS) substrate with an average pore size of 1.0 μm. The preparation process was optimized by carefully choosing the binder, the concentrations of the casting solutions and the sintering temperatures of the membranes. Compared to methylcellulose and polyethylene glycol 20000, polyvinyl alcohol 1750 was found to be the most effective binder to fabricate a homogeneously structured Al₂O₃ membrane without defects. The concentration to prepare an uniform coverage membrane with a thickness of ~10 μm was 0.032 mol/L. When sintered at 1000 °C, γ-Al₂O₃ membrane with ~3 μm grains was obtained. When sintered at 1200 °C, γ-Al₂O₃ completely transformed into α-Al₂O₃ and the grains grew to ~5 μm. Accordingly, the process was applied to a bigger pore-sized PSS with an average pore size of 1.5 μm to fabricate an α-Al₂O₃ intermediate layer to initially modify its surface. A single α-Al₂O₃ crystal layer with a thickness of ~5 μm and an average pore size of 0.7 μm was achieved. Subsequently, TiO₂, SiO₂, and TiO₂–SiO₂ hybrid membranes were tried on the modified PSS. Defect-free microfiltration membranes with average pore sizes of ~0.3 μm were readily fabricated. The results indicate that the sol–gel method is promising to initially modify the PSS substrates and the sol–gel-derived α-Al₂O₃ crystal layer is an appropriate intermediate layer to modify the PSS and to support smaller grain-sized top membranes.