Effect of TiO2 additions on densification and mechanical properties of new mulltifunction resistant porcelains using economic raw materials

The aim of this work is to determine the effect of TiO₂ on sintering and mechanical proprieties of new multifunction resistant (MFR) porcelain prepared from local abundant raw materials. Based on a preliminary work, the new selected composition was 30 wt% kaolins (20 wt% kaolin halloysite type + 10 wt% kaolin Tamazart), 45 wt% k-feldspar and 25 wt% quartz and containing different contents of TiO₂ (3, 5 and 8 wt%). The sintering temperatures of mixtures were between 1140 and 1260 °C. Subsequently, the obtained phases in the elaborated samples were investigated by X-ray diffraction and Fourier transform infrared spectroscopy analyses, Raman spectroscopy and SEM analysis. The optimum sintering conditions gave a higher bulk density (2.47 g.cm⁻³) and excellent mechanical properties: The three point flexural strength (3PFS), Vickers micro-hardness (VMH) and apparent porosity (P_A) of porcelains sintered at 1160 °C were 238 MPa, 12.3 GPa and 2%, respectively. This obtained 3PFS value is drastically higher than that achieved for conventional porcelains (ranged between 60 and 80 MPa). Moreover, these two best 3PFS (238 MPa) and VMH (12.3 GPa) values achieved for this new MFR porcelains were considerably higher when compared to those values (3PFS=218 MPa and VMH=6.5 GPa) obtained by others for porcelain −30% ZrO₂ composite, even though their mixtures were hot pressed in vacuum at 970 °C for 2 min. Besides, the maximum value achieved for the new MFR porcelains is nearby that of the flexural strength of porcelain containing 5 wt% TiO₂ and 30 wt % alumina (about 240 MPa). In other words, the presence of 30 wt % alumina in their product well confirm the benefic effect of the used raw materials (saving 30 wt % alumina) on porcelain strengthening.     

Influence of lithium oxide as auxiliary flux on the properties of triaxial porcelain bodies

The influence of Li₂O-content on the properties of standard triaxial porcelains was investigated. Two series of porcelain formulations were produced. The first one comprised seven model formulations with increasing amount of Li₂CO₃ (1–7 wt.%), with respect to the standard triaxial porcelain formulation. The experimental results showed that desirable properties for tableware porcelains can be attained if the Li₂O-content does not exceed ∼1.5 wt.%. In the light of this conclusion, the second series of formulations aimed at producing new porcelains using Li-bearing natural rocks. Under an industrial perspective, the most important finding is that these compositions matured at temperatures 100–120 °C lower than the standard triaxial porcelain formulation and exhibited remarkable resistance at over-firing conditions. The role of increasing Li₂O-content at the different stages of firing is interpreted in the light of its influence on densification, the evolution of crystalline phases and microstructure.     

Mixture behavior and microwave dielectric properties of (1 − x)CaWO4–xTiO2

The microwave dielectric characteristics and mixture behavior of (1 − x)CaWO₄–xTiO₂ ceramics prepared with conventional solid-state route were studied using a network analyzer and X-ray power diffraction, respectively. The CaWO₄ compound had good properties (low permittivity and high quality factor) for microwave applications, but it had a high negative temperature coefficient of resonant frequency (τ_f = −53). Hence, in order to tune the dielectric properties, (1 − x)CaWO₄–xTiO₂ were prepared for different values of x. X-ray powder diffraction and SEM analysis revealed that CaWO₄ and TiO₂ coexisted as a mixture. The mixture formation and dielectric properties could be explained by mixture rule. In particular, at x = 2.6, good microwave dielectric properties were obtained: Qf = 27,000, ɛ_r = 17.48, and τ_f = ∼0 ppm/°C.     

Bismuth titanates candidates for high permittivity LTCC

Bi₂O₃–TiO₂ composites are known to possess attractive microwave dielectric properties. However, producing LTCC analogues with equally promising dielectric properties is problematic. Here, we show that judicious choice of both TiO₂ starting powders and dopants can produce composites with excellent properties. Three TiO₂ powders were evaluated: 1 μm-anatase, 1 μm-rutile and a nanosized (30 nm) mixture of 75–25 anatase-rutile. The best dielectric properties were obtained by using uncalcined nanosized anatase/rutile with Bi₂O₃ powder. By doping this Bi₂O₃–TiO₂ powder mixture with 0.112 wt.% CuO dielectric properties of Q × f = 9000 GHz, ɛ_r = 80 and τ_f = 0 ppm/K (at 300 K) were obtained at a sintering temperature of 915 °C.     

Low sintering temperature of (Zn0.65Mg0.35)TiO3–xCaTiO3-based dielectric with controlled temperature coefficient

(Zn_0.65Mg_0.35)TiO₃–xCaTiO₃-based dielectric ceramics sintered at low temperature thanks to ZnO–B₂O₃ glass phase addition are investigated. The effects of such additions on the dilatometric curves, the microstructure, the phase composition and the dielectric properties have been carefully examined. It is shown that the sintering temperature is significantly lowered to 930 °C by the addition of 2 wt.% of ZnO–B₂O₃ glass phase. The temperature coefficient of permittivity (τ_?) could be controlled by varying the CaTiO₃ content and lead to near zero τ_? value. As an optimal composition, (Zn_0.65Mg_0.35)TiO₃ + 7%CaTiO₃, co-sinterable with silver electrodes at 930 °C, exhibits at 1 MHz, a relative permittivity of ?_r = 21, a temperature coefficient of the permittivity τ_? of ?4 ppm/°C and low dielectric losses (tan(δ) < 10^?3). These interesting properties make this system promising to manufacture Ag-based electrodes multilayer dielectric devices.     

Structures and microwave dielectric properties of Ca(1−x)Nd2x/3TiO3 ceramics

Ca_(1?x)Nd_2x/3TiO₃ microwave dielectric ceramics were prepared by the mixed oxide route; powders were calcined at 1100 °C and sintered at 1450–1500 °C. High density, single phase products were obtained for all compositions. Grain sizes ranged from 1 μm to 100 μm. There was evidence of significant discontinuous grain growth in mid range compositions; all ceramics were characterised by complex domain structures. With increasing Nd content there was a evidence of a transition from an orthorhombic Pbnm structure to a monoclinic C2/m structure. This was accompanied by a decrease in relative permittivity (?_r) from 180 to 78, and decrease in the temperature coefficient of resonant frequency (τ_f) from +770 ppm K^?1 to +200 ppm K^?1. The product of dielectric Q value and resonant frequency (Q × f) varied in a grossly non-systematic way, exhibiting a peak at 13,000 GHz in Ca_0.7Nd_0.2TiO₃.     

Synergic effect of cation doping and phase composition on the photocatalytic performance of TiO2 under visible light

The synergic effect of cation doping and phase composition for the further improvement of the photocatalytic activity of TiO₂ under visible light is reported for the first time. Fe^3 + and Sn^4 + co-doped TiO₂ with optimized phase composition were synthesized through a simple soft-chemical solution method. The visible-light-driven photocatalytic activity of Fe^3 + and Sn^4 + co-doped TiO₂ was 5 times of that of Evonik P25 TiO₂ using degradation of methylene blue as model reaction. The synthesized photocatalysts were characterized by powder X-ray diffraction, UV–Vis diffuse reflectance spectroscopy, X-ray photoelectron spectroscopy, ¹¹⁹Sn Mössbauer spectroscopy, and X-ray absorption fine structure spectroscopy. It is indicated that Sn^4 + doping can facilitate the phase transition from anatase to rutile. The different ratios of anatase and rutile can be achieved by tuning the amount of Sn^4 + doped into the lattice. Furthermore, the doping of Sn^4 + into TiO₂ lattice can stabilize the phase composition when Fe^3 + is co-doped. In the Fe^3 + and Sn^4 + co-doped TiO₂, Sn^4 + is mainly used to tune and stabilize the phase composition of TiO₂ and Fe^3 + acts as a doping cation to narrow the band gap of TiO₂. Both band gap and phase composition of TiO₂ can be tuned effectively by the simultaneous introduction of Fe^3 + and Sn^4 +. The synergic effect of optimized phase composition (anatase/rutile = 25/75) and narrowed band gap should be the two main reasons for the promoted photocatalytic activity of TiO₂ under visible light.     

Synthesis of fine-grain Ba0.96La0.04TiO3 dielectric ceramics by different routes for multilayer ceramic capacitors

Spherical Ba_0.96La_0.04TiO₃ powders were synthesized by a wet chemical method-based on a precipitation process, able of controlling uniformity and particle size. After sintering, fine-grain Ba_0.96La_0.04TiO₃ ceramics with good dielectric properties were obtained. Different synthesis routes of Ba_0.96La_0.04TiO₃ ceramics were used, and their effects on microstructure and dielectric properties were examined. Results showed that the ceramics samples prepared by La³⁺ and Ba²⁺ added together (abbreviated as, BLT-T) resulted in a mixture of large and small grains, sharp permittivity peaks, and high dielectrics loss. By comparison, another ceramics samples prepared by La³⁺ and Ba²⁺ added separately (BLT-S) exhibited uniform grain size and broadened/flattened permittivity peaks. The high value of ε_r (3255) and low tan δ (0.01) at room temperature and negligible temperature coefficient of capacitance from − 35 to 85 °C suggested the suitability of the above materials for multilayer capacitor applications.     

The dielectric properties for (Nb,In,B) co-doped rutile TiO2 ceramics

Recently, colossal permittivities (~10⁵) and low loss factors (<0.1) were reported in (Nb+In) co-doped rutile TiO₂ ceramics, which have attracted considerable attention. In this work, (Nb,In,B) co-doped rutile TiO₂ ceramics were investigated for achieving temperature- and frequency- stable dielectric properties in TiO₂ based colossal dielectric ceramics. The (Nb,In,B) co-doped rutile TiO₂ ceramics were prepared by conventional solid-state reaction method. The microstructures, dielectric properties and complex impedance of 1 mol.% (Nb+In) co-doped rutile TiO₂ (TINO) and xwt% B₂O₃ (x=0.5, 1, 2 and 4) doped TINO were systematically investigated and compared. It was found that by doping B₂O₃ the sintering temperature of TINO ceramics can be reduced by 100 °C. Meanwhile, the dielectric loss of TINO ceramics was decreased by doping B₂O₃. In the ₂wt% B₂O₃ doped TINO ceramics, the dielectric permittivity kept a high value of >2.0×10⁵ and the dielectric loss was lower than 0.1 in a frequency range of 10²−10⁵ Hz and a temperature range of 25–200 °C.     

Colossal permittivity in (In1/2Nb1/2)xTi1−xO2 ceramics prepared by a glycine nitrate process

(In + Nb) co-doped TiO₂ nanoparticles with very low dopant concentrations were prepared using a glycine nitrate process. A pure rutile—TiO₂ phase with a dense microstructure and homogeneous dispersion of dopants was achieved. By doping TiO₂ with 1.5% (In + Nb) ions, a very high dielectric permittivity of ε′ = 42,376 and low loss tangents of tanδ = 0.06 (at room temperature) were achieved. The large conduction activation energy at the grain boundary decreased with decreasing dopant concentration. The colossal permittivity was primarily attributed to the internal barrier layer capacitor (IBLC) effect. The dominant effect of interfacial polarization at the non–Ohmic sample–electrode contact was observed when the dopant concentration was ≤1.0 mol%. Interestingly, the sample–electrode contact and resistive–outer surface layer effects, i.e., surface barrier layer capacitor (SBLC) effect, has also an effect on the colossal dielectric response in (In + Nb) co-doped TiO₂ ceramics.     

A VOx/meso-TiO2 catalyst for methanol oxidation to dimethoxymethane

Mesoporous TiO₂ was prepared by simply controlling the hydrolysis of Ti(OBu)₄ with the help of acetic acid. The mesoporous TiO₂ had a well-crystallized anatase phase and a high surface area of 290 m² g⁻¹ with a pore size of about 4 nm. The anatase phase and the mesoporous structure were maintained in the VO_x/TiO₂ catalyst with a monolayer dispersion of V₂O₅, however, the surface area decreased to 126 m² g⁻¹. The catalyst was highly active and selective for methanol oxidation, giving about 55% conversion of methanol and 85% selectivity to dimethoxymethane at 423 K.     

X-ray diffraction,dielectric, pyroelectric,piezoelectric and Raman spectroscopy studies on (Ba0.95Ca0.05)0.8875Bi0.075TiO3 ceramic

The (Ba_0.95Ca_0.05)_0.8875Bi_0.075TiO₃ ceramic composition was prepared using the conventional mixed-oxide technique. X-ray diffraction at room temperature and dielectric permittivity in the temperature range from 85 to 450 K and frequency range from 10² to 2 × 10⁵ Hz, respectively, were studied. The X-ray spectra were investigated by profile refinement technique with the use of specialized software at room temperature, the (Ba_0.95Ca_0.05)_0.8875Bi_0.075TiO₃ composition crystallizes in quadratic perovskite structure. The dielectric measurements show classical ferroelectric behavior. The pyroelectric and piezoelectric results confirm the dielectric measurements. The pyroelectric coefficient is about 69.2 nC/cm² K at the transition temperature (T_C = 367 K). The piezoelectric constant is d₃₁ = 31.1 pC/N and the electromechanical coupling factor is k_P = 0.14679. Raman spectra of (Ba_0.95Ca_0.05)_0.8875Bi_0.075TiO₃ ceramic were taken at various temperatures and measured over the wave number range from 50 to 1000 cm^?1. All the Raman bands were assigned as the transitional modes of Ba²⁺, Ca²⁺, Bi³⁺ and Ti⁴⁺ cations. The temperature evolution of Raman spectra across the transition shows an important evolution characterizing the disorder of the high temperature phase.     

Photoacoustic thermal characterization of electrical porcelains: effect of alumina additions on thermal diffusivity and elastic constants

The open-photoacoustic-cell technique to measure thermal diffusivity is described in detail. We have applied it to the measurement of thermal diffusivity of porcelain samples with four different alumina additions in the range 0–15 wt% and fired at four temperatures in the range 1270–1350°C. Thermal diffusivity is shown to vary from 4·1×10⁻⁷ m² s⁻¹ for a classical triaxial porcelain to 6·4×10⁻⁷ m² s⁻¹ for 15 wt% alumina addition made mainly at the expense of quartz when the firing temperature was 1325°C. The values of thermal diffusivity are found to be well correlated with the values of the shear and Young's modulus. Good correlation was also observed among the measured values for thermal diffusivity and those for density and mullite to quartz ratio. We conclude that the open-photoacoustic-cell technique has enough sensitivity to detect small changes in composition and microstructure of materials as complex as porcelains.     

Ordered mesoporous carbon–TiO2 materials for improved electrochemical performance of lithium ion battery

A series of ordered mesoporous carbon–TiO₂ (OMCT) materials with various weight percentages of TiO₂ (50–75 wt%) were synthesized by evaporation-induced self-assembly and in-situ crystallization at various calcination temperatures (600–1200 °C) to evaluate the Li-ion storage performance. The OMCT has ordered 2D hexagonal mesoporous structures and the TiO₂ nanocrystals with different phases are embedded into the frameworks of carbonaceous matrix. The reversible capacity of OMCT is highly dependent on the phase and content of TiO₂, and the anatase TiO₂ is a superior crystalline phase to rutile and TiN for Li-ion insertion. The OMCT₆₅ which contains 35 wt% carbon and 65 wt% TiO₂ shows a high capacity of 500 mAh g^?1 at 0.1C after 80 cycles. In addition, OMCT₆₅ exhibits a good cyclability and rate capability. The reversible capacity remains at 98 mAh g^?1 at a high rate of 5C, and then recoveries to 520 mAh g^?1 at 0.1C after 105 cycles. The excellent reversible capacity and rate capability of OMCT₆₅ are attributed to the embedment of well-dispersed anatase TiO₂ nanocrystals into the specific porous structure of OMCT, which can not only facilitate the fast Li-ion charge transport but can also strengthen the carbon–TiO₂ co-constructing channels for lithiated reactions.     

CaCu3Ti4O12 ceramics from co-precipitation method: Dielectric properties of pellets and thick films

Dielectric properties of CaCu₃Ti₄O₁₂ (CCTO)-based ceramics and thick films (e ～50 μm) prepared from powders synthesized by a soft chemistry method (co-precipitation) are presented and discussed. The characteristics of pellets and thick films are compared.The pellets exhibit high values of the dielectric permittivity (?_r ～1.4 × 10⁵) and relatively small dielectric losses (tan δ ～0.16) at 1 kHz and room temperature. These properties are independent of the nature of the metallization of the electrodes. In addition, the dielectric permittivity decreases when the diameter of the electrodes of the pellets increases, while the losses remain constant. This result, which is strongly related to the nature of the dielectric material in between the electrodes, constitutes a strong indication that the high dielectric permittivity values observed in this material are not related to an interfacial (electrode material) related mechanism but is an internal barrier layer capacitor (IBLC) type.Very high values of the dielectric permittivity of CCTO thick films are measured (?_r ～5 × 10⁴). The differences in dielectric permittivity between thick films and dense pellets may be attributed to the difference in grain size due to different CuO contents, and to the different reactivity of the materials.     

Polypropylene/carbon nanotube nano/microcellular structures with high dielectric permittivity,low dielectric loss,and low percolation threshold

Nano/microcellular polypropylene/multiwalled carbon nanotube (MWCNT) composites exhibiting higher electrical conductivity, lower electrical percolation, higher dielectric permittivity, and lower dielectric loss are reported. Nanocomposite foams with relative densities (ρ_R) of 1.0–0.1, cell sizes of 70 nm–70 μm, and cell densities of 3 × 10⁷–2 × 10¹⁴ cells cm⁻³ are achieved, providing a platform to assess the evolution of electrical properties with foaming degree. The electrical percolation threshold decreases more than fivefold, from 0.50 down to 0.09 vol.%, as the volume expansion increases through foaming. The electrical conductivity increases up to two orders of magnitude in the nanocellular nanocomposites (1.0 > ρ_R > ∼0.6). In the proper microcellular range (ρ_R ≈ 0.45), the introduction of cellular structure decreases the dielectric loss up to five orders of magnitude, while the decrease in dielectric permittivity is only 2–4 times. Thus, microcellular composites containing only ∼0.34 vol.% MWCNT present a frequency-independent high dielectric permittivity (∼30) and very low dielectric loss (∼0.06). The improvements in such properties are correlated to the microstructural evolution caused by foaming action (biaxial stretching) and volume exclusion. High conductivity foams have applications in electromagnetic shielding and high dielectric foams can be developed for charge storage applications.     

Control of refractive index by annealing to achieve high figure of merit for TiO2/Ag/TiO2 multilayer films

We modified the refractive index (n) of TiO₂ by annealing at various temperatures to obtain a high figure of merit (FOM) for TiO₂/Ag/TiO₂ (45 nm/17 nm/45 nm) multilayer films deposited on glass substrates. Unlike the as-deposited and 300 °C-annealed TiO₂ films, the 600 °C-annealed sample was crystallized in the anatase phase. The as-deposited TiO₂/Ag/as-deposited TiO₂ multilayer film exhibited a transmittance of 94.6% at 550 nm, whereas that of the as-deposited TiO₂/Ag/600 °C-annealed TiO₂ (lower) multilayer film was 96.6%. At 550 nm, n increased from 2.293 to 2.336 with increasing temperature. The carrier concentration, mobility, and sheet resistance varied with increasing annealing temperature. The samples exhibited smooth surfaces with a root-mean-square roughness of 0.37–1.09 nm. The 600 °C-annealed multilayer yielded the highest Haacke's FOM of 193.9×10⁻³ Ω⁻¹.     

Dielectric properties of barium-titanate sintered from tribophysically activated powders

It has been well known that the tribophysical (mechanical) activation could be used as a method for modification of physicochemical properties of dispersed systems such as polycrystalline mixture of oxides. In this study, we consider the properties of BaTiO₃ obtained from tribophysically activated initial powders. The mixture of 50 mol% BaCO₃ and 50 mol% TiO₂ powders was tribophysically activated in high energy vibromill during 0, 3, 30, 90 and 180 min, calcinated at 800°C for 1 h and reaction sintered at 1100, 1200 and 1300°C for 2 h. The surface specific areas, densities (green and sintered), phase compositions and dielectric properties were evaluated.     

The effects of deposition temperature on structure and dielectric properties of Ba0.6Sr0.4TiO3 thin films produced by pulsed laser deposition

The effects of deposition temperature on orientation, surface morphology and dielectric properties of the thin films for Ba_0.6Sr_0.4TiO₃ thin films deposited on Pt/Ti/SiO₂/Si substrates by pulsed laser deposition were investigated. X-ray diffraction patterns revealed a (2 1 0) preferred orientation for all the films. With rising substrate temperature from 650 °C to 700 °C, the crystallinity and crystal grain size of the films increase, the relative dielectric constant increases, but the dielectric losses have not obvious difference. The film deposited at 350 °C and annealed at 700 °C has strongly improved roughness and dielectric permittivity compared with the film only deposited directly at 700 °C. Three distinct relaxation processes within tan(δ) were found for the Ba_xSr_1?xTiO₃ film: a broadened process of the film relaxation, an intermediate peak which originates from Maxwell–Wagner–Sillars polarization, and an extremely slow process ascribed to leak current. The complex dielectric permittivity and loss can be fitted by an improved Cole–Cole model corresponding to a stretched relaxation function.     

Zn2+ doped TiO2/C with enhanced sodium-ion storage properties

To improve the performance of anatase TiO₂ as an anode material for sodium-ion batteries, Zn²⁺-doped TiO₂/C composites are synthesized by a co-precipitation method. The results of XRD, EPR and XPS demonstrate that Zn²⁺ occupies at the Ti⁴⁺ site of TiO₂ to form a solid-solution, resulting in an expansion of lattice and an increase of Ti³⁺ content. The expansion of lattice can enhance the stability of the crystal structure of TiO₂. The increase of Ti³⁺ content can improve the conductivity of TiO₂. Therefore, Ti_0.94Zn_0.06O₂/C delivers a reversible capacity of 160 mA h g⁻¹ with a capacity retention of 96% after 100 cycles at 5 C. Even charged/discharged at 10 C, this sample still exhibits a reversible capacity of 117 mA h g⁻¹, comparing to 86 mA h g⁻¹ for TiO₂/C. The enhanced electrochemical performances can be ascribed to the improvement of the conductivity and the structural stability of TiO₂ due to Zn²⁺-doping. Therefore, Ti_0.94Zn_0.06O₂/C is an attractive anode material of sodium-ion batteries.