An aqueous-solution based low-temperature pathway to synthesize giant dielectric CaCu3Ti4O12—Highly porous ceramic matrix and submicron sized powder

A simple, cost-effective and environment-friendly pathway for preparing highly porous matrix of giant dielectric material CaCu₃Ti₄O₁₂ (CCTO) through combustion of a completely aqueous precursor solution is presented. The pathway yields phase-pure and impurity-less CCTO ceramic at an ultra-low temperature (700 °C) and is better than traditional solid-state reaction schemes which fail to produce pure phase at as high temperature as 1000 °C (Li, Schwartz, Phys. Rev. B 75, 012104). The porous ceramic matrix on grinding produced CCTO powder having particle size in submicron order with an average size 300 nm. On sintering at 1050 °C for 5 h the powder shows high dielectric constants (>10⁴ at all frequencies from 100 Hz to 100 kHz) and low loss (with 0.05 as the lowest value) which is suitable for device applications. The reaction pathway is expected to be extended to prepare other multifunctional complex perovskite materials.     

Improved dielectric and non-ohmic properties of Ca2Cu2Ti4O12 ceramics prepared by a polymer pyrolysis method

Non-ohmic and dielectric properties of Ca₂Cu₂Ti₄O₁₂ (CaCu₃Ti₄O₁₂/CaTiO₃ composite) ceramics prepared by a polymer pyrolysis method (PP-ceramic samples) are investigated. The PP-ceramics show a highly dense structure and improved non-ohmic and dielectric properties compared to the ceramics obtained by a solid state reaction method (SSR-ceramic samples). ?′ (tan δ) of the PP-ceramic samples is found to be higher (lower) than that of the SSR-ceramic samples. Interestingly, the PP-ceramic sintered at 1050 °C for 10 h exhibits the high ?′ of 2530 with weak frequency dependence below 1 MHz, the low tan δ less than 0.05 in the frequency range of 160 Hz-177 kHz, and the little temperature coefficient, i.e., |Δ?′| ≤ 15 % in the temperature range from −55 to 85 °C. These results indicate that the CaCu₃Ti₄O₁₂/CaTiO₃ composite system prepared by PP method is a promising high-?′ material for practical capacitor application.     

Microwave properties of Ba(Zn1/3Ta2/3)O3 dielectric resonators

Ba(Zn_1/3Ta_2/3)O₃ (BZT) dielectric resonators were prepared by solid-state reaction. The starting materials were BaCO₃, ZnO, and Ta₂O₅ powders with high purity. The double calcined BZT pellets were sintered in air at temperatures of 1575, 1600, 1625, and 1650 °C for 4 h. The X-ray diffraction data allowed the study of the unit cell distortion degree and the presence of the secondary phases. A long-range order with a 2:1 ratio of Ta and Zn cations on the octahedral positions of the perovskite structure was observed with the increase of the sintering temperature. The dielectric constant of BZT resonators measured around 6 GHz was between 26 and 28. High values of Q × f product (120 THz) were obtained for BZT resonators sintered at 1650 °C/4 h. The temperature coefficient of the resonance frequency exhibits positive values less than 6 ppm/°C. The achieved dielectric parameters recommend BZT dielectric resonators for microwave and millimeter wave applications.     

Dense La0.9Sr0.1Ga0.8Mg0.2O2.85 electrolyte for IT-SOFC's: Sintering study and electrochemical characterization

This paper presents the sintering behaviour of a La_0.9Sr_0.1Ga_0.8Mg_0.2O_2.85 coral-like microstructure powder. This is prepared by a successive freeze-drying and self-ignition process followed by calcination at 1200 °C during 1 h. This synthesis method gives great uniformity of the powder and allows shaping into compacts without requiring a grinding step. The grain size distribution (between 0.5 and 4 μm) favours a good sintering behaviour: open porosity disappear at 1400 °C and relative densities over 99% can be achieved after 6 h at 1450 °C. The same powder can also be sintered into a thin disc of ∼100 μm thickness. The characterization of the dense material by impedance spectroscopy shows that the activation energies below and above 600 °C are 1.0 eV and 0.7 eV, respectively. The conductivity at 800 °C is ∼0.11 S cm⁻¹. Special attention is devoted to the temperature range between 200 °C and 400 °C, where the intragrain and intergrain contributions can be distinguished. The analysis of the parameters describing the intragrain constant phase element in the equivalent circuit suggests that, above 325 °C, the system evolves from a distribution of relaxation time to only one relaxation time. The analysis of the data by the complexes permittivity show that ionic oxide conduction mechanism would occur in two steps. In the first, an oxygen vacancy would be released and, in the second, the migration of the ionic oxide would take place in the material.     

Dielectric and microwave properties of ZrO2 doped CaO-SiO2-B2O3 ceramic matrix composites

The behavior of dielectric and microwave properties against sintering temperature has been carried out on CaO-SiO₂-B₂O₃ ceramic matrix composites with ZrO₂ addition. The results indicated that ZrO₂ addition was advantageous to improve the dielectric and microwave properties. X-ray diffraction (XRD) patterns show that the major crystalline β-CaSiO₃ and a little SiO₂ phase existed at the temperature ranging from 950 °C to 1050 °C. At 0.5 wt% ZrO₂, CaO-SiO₂-B₂O₃ ceramic matrix composites sintered at 1000 °C possess good dielectric properties: ?_r = 5.85, tan δ = 1.59 × 10⁻⁴ (1 MHz) and excellent microwave properties: ?_r = 5.52, Q · f = 28,487 GHz (11.11 GHz). The permittivity of Zr-doped CaO-SiO₂-B₂O₃ ceramic matrix composites exhibited very little temperature dependence, which was less than ±2% over the temperature range of −50 to 150 °C. Moreover, the ZrO₂-doped CaO-SiO₂-B₂O₃ ceramic matrix composites have low permittivity below 5.5 over a wide frequency range from 20 Hz to 1 MHz.     

Sinterability and microwave dielectric properties of nano structured 0.95MgTiO3-0.05CaTiO3 synthesised by top down and bottom up approaches

0.95MgTiO₃-0.05CaTiO₃ (MCT) nano powders were synthesised using sol-gel method and high energy ball milling (HEBM). Synthesised powders were characterised using X-ray diffraction analysis to ensure phase purity and HRTEM to determine the fine microstructural features like particle size, interplanar spacing, etc. The powder pellets were heat treated to study the sinterability and microwave dielectric properties and these properties were then compared with the microwave dielectric properties of micron sized sample. Nano powder synthesised using HEBM shows better dielectric properties, sinterability and gets densified to 90% of theoretical density (TD) at 1200 °C/2 h. Dielectric resonators prepared using chemically synthesised nano powder showed poor sinterability and microwave dielectric properties, but, dielectric properties of HEBM samples were very near to that of solid state synthesised samples. Sintered HEBM powders retain the microwave dielectric properties almost to the same level as the solid state synthesised powder with considerable lowering of sintering temperature.     

Dielectric properties of soft chemical method derived CaCu3Ti4O12 thin films onto Pt/TiO2/Si(1 0 0) substrates

Calcium copper titanate, CaCu₃Ti₄O₁₂ (CCTO), thin film has been deposited by the soft chemical method on Pt/Ti/SiO₂/Si (1 0 0) substrates at 700 °C for 2 h. The peaks were indexed as cubic phase belonging to the Im−3 space group. The film exhibited a duplex microstructure consisting of large grains of 130 nm in length and regions of fine grains (less than 80 nm). The CCTO film capacitor showed a dielectric loss of 0.031 and a dielectric permittivity of 1020 at 1 MHz. The J-V behavior is completely symmetrical, regardless of whether the conduction is limited by interfacial barriers or by bulk-like mechanisms. Based on impedance analyses, the equivalent circuit of CCTO film consisting of a resistor connected in series with two resistor-capacitor (RC) elements.     

Effect of CuO on the sintering temperature and piezoelectric properties of MnO2-doped 0.75Pb(Zr0.47Ti0.53)O3-0.25Pb(Zn1/3Nb2/3)O3 ceramics

The sintering temperature of 0.75Pb(Zr_0.47Ti_0.53)O₃-0.25Pb(Zn_1/3Nb_2/3)O₃ ceramics containing 1.5 mol% MnO₂ was decreased from 930 to 850 °C with the addition of CuO. The CuO reacted with the PbO and formed a liquid phase during the sintering, which assisted the densification of the specimens. Most of the Cu²⁺ ions existed in the CuO second phase, thereby preventing any possible hardening effect from the Cu²⁺ ions. Variations of the k_p, Q_m, ?₃^T/?₀ and d₃₃ values with CuO were similar to that of the relative density. The specimen containing 0.5 mol% CuO sintered at 850 °C showed the good piezoelectric properties of k_p = 0.5, Q_m = 1000, ?₃^T/?₀ = 1750 and d₃₃ = 300 pC/N.     

Effects of K2O-Li2O doping on surface and catalytic properties of Fe2O3/Cr2O3 system

The effects of K₂O and Li₂O-doping (0.5, 0.75 and 1.5 mol%) of Fe₂O₃/Cr₂O₃ system on its surface and the catalytic properties were investigated. Pure and differently doped solids were calcined in air at 400-600 °C. The formula of the un-doped calcined solid was 0.85Fe₂O₃:0.15Cr₂O₃. The techniques employed were TGA, DTA, XRD, N₂ adsorption at −196 °C and catalytic oxidation of CO oxidation by O₂ at 200-300 °C. The results revealed that DTA curves of pure mixed solids consisted of one endothermic peak and two exothermic peaks. Pure and doped mixed solids calcined at 400 °C are amorphous in nature and turned to α-Fe₂O₃ upon heating at 500 and 600 °C. K₂O and Li₂O doping conducted at 500 or 600 °C modified the degree of crystallinity and crystallite size of all phases present which consisted of a mixture of nanocrystalline α- and γ-Fe₂O₃ together with K₂FeO₄ and LiFe₅O₈ phases. However, the heavily Li₂O-doped sample consisted only of LiFe₅O₈ phase. The specific surface area of the system investigated decreased to an extent proportional to the amount of K₂O and Li₂O added. On the other hand, the catalytic activity was found to increase by increasing the amount of K₂O and Li₂O added. The maximum increase in the catalytic activity, expressed as the reaction rate constant (k) measured at 200 °C, attained 30.8% and 26.5% for K₂O and Li₂O doping, respectively. The doping process did not modify the activation energy of the catalyzed reaction but rather increased the concentration of the active sites without changing their energetic nature.     

Electric and dielectric behavior of CaCu3Ti4O12-based thin films obtained by soft chemical method

CaCu₃Ti₄O₁₂ (stoichiometric) and Ca_1.1Cu_2.9Ti₄O₁₂ (non-stoichiometric) thin films have been prepared by the soft chemical method on Pt/Ti/SiO₂/Si substrates, and their electrical and dielectric properties have been compared as a function of the annealing temperature. The crystalline structure and the surface morphology of the films were markedly affected by the annealing temperature and excess calcium. The films show frequency-independent dielectric properties at room temperature which is similar to those properties obtained in single-crystal or epitaxial thin films. The room temperature dielectric constant of the 570-nm-thick CCTO thin films annealed at 600 °C at 10 kHz was found to be 124. The best non-ohmic behavior (α = 12.6) presented by the film with excess calcium annealed at 500 °C. Resistive hysteresis on the I-V curves was observed which indicates these films can be used in resistance random access memory (ReRAM).     

Influence of the thermo-chemical conditions on the carbo-chromization process of α-Fe matrices obtained by powder sintering technique

We report on the superficial layer formation resulting from the carburization followed by chromization of α-Fe samples obtained by powder sintering technique. The carburization and chromization were carried out by thermal diffusion between 880-980 °C and 950-1050 °C in a solid powder mixture of charcoal/BaCO₃ and ferrochromium/alumina/NH₄Cl, respectively. The obtained layers were investigated using X-ray diffraction, optical microscopy, Vickers micro-hardness technique and scanning electron microscopy. The results show that the layers are of micrometric size and consist mostly of chromium carbides of different phases. These phases as well as the thickness of the layers are closely related to the treatment temperature used for carburization and to the temperature and Cr initial concentration in the mixture used for chromization. For highly reactive carbo-chromization conditions (high concentration of Cr, and high carburization and chromization temperatures) the superficial layer is constituted of two chromium carbide sub-layers (Cr₃C₂/Cr₇C₃) separated by a sharp interface. The thickness and hardness of the coating layer reached 45 μm and 2300 HV, respectively. Such coating could be used for tools that have to be abrasion and oxygen resistant at high temperatures.     

Complex impedance analysis of (Y2O3 + CeO2)-YCr0.5Mn0.5O3 composite NTC ceramics

Ceramic compositions based on (aY₂O₃ + bCeO₂)-0.4YCr_0.5Mn_0.5O₃ (a + b = 0.6) were prepared by conventional solid state reaction at 1200 °C, and sintered under air atmosphere at 1600 °C. For 0 ≤ a < 0.6, XRD patterns have shown that the major phases presented in the calcined powders are Y₂O₃, CeO₂ and orthorhombic perovskite YCr_0.5Mn_0.5O₃ phase, respectively. SEM and EDAX observations confirm the YCr_0.5Mn_0.5O₃ phases mostly exist at the grain, whereas the Y₂O₃ and CeO₂ phases mainly exist at the grain boundaries. Complex impedance analysis shows that, for 0 < a ≤ 0.6, single semicircular arc whose shape does not show any change with temperature. Nevertheless, for a = 0, two overlapping semicircular arcs are observed at and above 300 °C. The grain boundary properties exhibit thermistor parameters with a negative temperature coefficient characteristic. The relaxation behavior and conduction for the grain boundary could be due to a space-charge relaxation mechanism and oxygen vacancies, respectively.     

Properties of praseodymium-doped bismuth potassium titanate (Bi0.5K0.5TiO3) synthesised using the soft combustion technique

Bismuth potassium titanate (Bi_0.5K_0.5TiO₃; BKT) and praseodymium-doped BKT (Bi_0.5(1−x)Pr_xK_0.5TiO₃; BPKT) powders were synthesised using the soft combustion technique. Fine particles of 10-100 nm of BKT and BPKT were produced. A single phase BKT was obtained with a minimum of 0.5 mol of glycine. Various compounds of Bi_0.5(1−x)Pr_xK_0.5TiO₃ where x = 0.01, 0.03, 0.05, 0.10, 0.15 and 0.20 were prepared. Pure BKT and BPKT powders were obtained after calcination at 800 °C for 3 h. After sintering at 1050 °C for 5 h, pure BKT and BPKT pellets were obtained for x = 0 and 0.01. However, for BPKT with x = 0.03, 0.05, 0.10, 0.15 and 0.20, a minor amount of Bi₄Ti₃O₁₂ (BIT) secondary phase was present after sintering at 1050 °C for 5 h. The crystallite size and grain size of all the samples followed similar trends, first increasing from x = 0 (undoped BKT) to x = 0.05 and then decreasing above x = 0.05. Among the undoped and doped samples, BPKT with x = 0.05 had the highest dielectric properties (?_r = 713.87) due to its large crystallite size (68.66 nm), large grain size (∼435 nm) and high relative density (93.39%).     

Characteristics of BaO-B2O3-SiO2 nano glass powders prepared by flame spray pyrolysis as the sintering agent of BaTiO3 ceramics

Nanosized BaO-B₂O₃-SiO₂ glass powders are directly prepared by flame spray pyrolysis. The mean size of the BaO-B₂O₃-SiO₂ glass powders with amorphous phase and spherical shape is 30 nm. The effects of glass powders on the sintering characteristics of the BaTiO₃ pellet formed from the nanosized BaTiO₃ powders are investigated. The mean size and BET surface area of the BaTiO₃ powders prepared by spray pyrolysis are 110 nm and 9.1 m²/g. The BaTiO₃ pellet with glass additive has large grain size with several microns, dense structure and pure tetragonal crystal structure at a sintering temperature of 1000 °C. The XRD pattern of the pellet has distinct split of (2 0 0) and (0 0 2) peaks at 2θ ≈ 44.95°. The dielectric constant of the pellet without glass additive is 2180. However, the dielectric constants of the pellets with 1, 3, 5 and 7 wt% glass additive with respect to BaTiO₃ are 2496, 2514, 2700 and 2225, respectively.     

Sintering behaviors, magnetic and electric properties of Bi-Zn co-doped Co2Y ferrites

The Bi and Zn substitution effects on the sintering behaviors, magnetic and electric properties of hexagonal ferrites with a composition of 2(Ba_1−xBi_xO)·2(Zn_yCo_0.8−yCu_0.2O)·6(Fe_2−x/3Zn_x/3O₃) were investigated. The results showed that the addition of Bi and Zn can significantly promote Co₂Y densification. The Y phase may be triggered to decompose into M and spinel phases at high sintering temperatures (above 1050 °C) for samples with excess Bi (x = 0.2) substitution, which resulted in densification and magnetic properties degradation. Co₂Y ferrites with x = 0.1 and y = 0.4 sintered at 1050 °C show a relative density of 94%, a high initial permeability of 4.5, a quality factor (Q) of 50.     

Characterization of paramagnetic KHo(WO4)2 nanocrystals: Synthesized by polymeric mixed-metal precursor sol-gel method

Nanocrystalline KHo(WO₄)₂ (KHW) particles were successfully synthesized via conventional Pechini sol-gel method. Prepared precursor gel was calcined at 250, 550, 600, 650 and 700 °C, and the resulting samples were analyzed with TG-DTA, powder X-ray diffraction, FT-IR, Raman, FESEM, TEM, UV-Vis-NIR (diffuse reflectance spectrum (DRS)), fluorescence and vibrating sample magnetometer (VSM). Thermal degradation of derived gel was observed up to 400 °C and phase formation starts from 550 °C. The product phase formation at higher annealing temperature was investigated by means of powder XRD. Organic liberation in the samples with respect to temperature was analyzed using FT-IR spectrum. Raman spectrum reveals the formation of tungsten ribbons as well as the quality of the samples while increasing the calcination temperature. The nano size of the synthesized particles was confirmed with FESEM and TEM micrographs. Reflectance and emission studies reveal the corresponding absorption and emission properties of trivalent state holmium ion. Paramagnetic behavior of the derived KHW was confirmed with VSM results.     

Dielectric relaxation behaviors of pure and Pr6O11-doped CaCu3Ti4O12 ceramics in high temperature range

Pure and Pr₆O₁₁-doped CaCu₃Ti₄O₁₂ (CCTO) ceramics were prepared by conventional solid-state reaction method. The compositions and structures were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The influences of Pr-ion concentration on dielectric properties of CCTO were measured in the ranges of 60 Hz-3 MHz and 290-490 K. The third phase of Ca₂CuO₃ was observed from the XRD of CCTO ceramics. From SEM, the grain size was decreased obviously with high valence Pr-ion (mixing valence of Pr³⁺ and Pr⁴⁺) substituting Ca²⁺. The room temperature dielectric constant of Pr-doped CCTO ceramics, sintered at 1323 K, was an order of magnitude lower than the pure CCTO ceramics due to the grain size decreasing and Schottky potential increasing. The dielectric spectra of Pr-doped CCTO were flatter than that of pure CCTO. The loss tangent of Pr-doped CCTO ceramics was less than 0.20 in 2 × 10²-10⁵ Hz region below 440 K. The complex impedance spectra of pure and Pr-doped CCTOs were fitted by ZView. From low to high frequency, three semicircles were observed corresponding to three different conducting regions: electrode interface, grain boundary and grain. By fitting the resistors R and capacitors C, the activation energies of grain boundary and electrode contact were calculated. All doped CCTOs showed higher activation energies of grain boundary and electrode than those of pure CCTO ceramics, which were concordant with the decreasing of dielectric constant after Pr₆O₁₁ doping.     

Magnetic and FTIR studies of BixY3−xFe5O12 (x = 0, 1, 2) powders prepared by the metal organic decomposition method

The crystallization process of bismuth substituted yttrium iron garnet (Bi_xY_3−xFe₅O₁₂; x = 0, 1, 2) powder prepared by the metal-organic decomposition method has been studied with various sintering temperatures. The pure garnet phase was observed for the x = 1 powder at 900 °C sintering temperature, whereas the x = 0, 2 powder showed secondary phases. The x = 0 powder showed a similar crystallization process to that of the solid state reaction method. For the x =1, 2 powders, it is proposed that the lowering of the crystallization temperature is due to the lowered stability of the intermediate phase. The infrared spectroscopy and magnetic properties were also investigated. The pure garnet phase showed three absorption bands located at 563, 598, 655 cm⁻¹ that shifted to 555, 588, 639 cm⁻¹ along with an increase of bismuth concentration. The maximum values of saturation and remanence magnetization and the minimum value of coercivity were observed for the x = 1 powder sintered at 900 °C, which were 20.8 emu/g, 2.67 emu/g, and 31.9 Oe, respectively.     

CaCu3Ti4O12 ceramics from basic co-precipitation (BCP) method: Fabrication and properties

High dielectric CaCu₃Ti₄O₁₂ (CCTO) ceramics have been successfully prepared by a novel basic co-precipitation (BCP) method. Compared with the conventional solid-state and/or soft chemistry methods, the BCP method has many advantages such as relatively lower sintering temperature, shorter sintering time and lower costs. The XRD patterns confirm the formation of CCTO crystal phase in the as-prepared samples. Influences of initial ingredients and sintering condition on phase composition, microstructure and dielectric property have been investigated through series of trials. The correlation between the process of the grain growth and dielectric properties of final products has been explored. The final products exhibit the dielectric constants higher than 10,000 and the dielectric losses lower than 0.15 at 1 KHz.     

Synthesis of (Bi0.5Na0.5)TiO3 (BNT) and Pr doped BNT using the soft combustion technique and its properties

In this work, bismuth sodium titanate (Bi_0.5Na_0.5)TiO₃ (BNT) and praseodymium (Pr)-doped BNT were successfully produced using the soft combustion technique. The effects of Pr doping on stoichiometry, microstructure, density and dielectric properties were studied. Pure Pr-doped BNT was obtained in all samples containing 5, 10 and 20 mol% Pr after calcination at 800 °C for 3 h. The produced powders were then pressed into pellets and sintered at 1100 °C for 3 h. The very similar ionic radii of Pr³⁺ with Bi³⁺ and Na⁺ made it possible to substitute both Bi and Na. The crystallite size and grain size decreased with increasing Pr amount because Pr acted as grain growth inhibitor, both for calcined powders and for sintered pellets. Maximum density was obtained in 5 mol% Pr-doped BNT, beyond which density decreased. The maximum dielectric constant of 756 was obtained in 5 mol% Pr-doped BNT and decreased at higher levels of Pr doping. Pr doped into BNT also caused a decrease in dielectric loss.