Preparation of PTCR ceramics in the BaO–Nb2O5–TiO2 system

The PTCR effect was investigated in the ferroelectric BaNb₂O₆ phase doped with TiO₂. Composite ceramics formed after sintering in a reducing atmosphere and subsequent reoxidation show the PTCR effect at around 70 and 300°C, respectively. Both PTCR anomalies are associated with the formation of high resistivity grain boundaries after controlled oxidation of reduced constituent phases.     

Dielectric Characteristics of Ceramics in BaO–Nd2O3–TiO2–Ta2O5 System

Dielectric ceramics in the BaO–Nd₂O₃–TiO₂–Ta₂O₅ system were prepared and characterized. The ceramics with tungsten–bronze structure based on the compositions Ba₂NdTi₂Ta₃O₁₅ and Ba₅NdTi₃Ta₇O₃₀ had a high dielectric constant (>100) with a lower frequency-dependency when complete densification was achieved; a low dielectric loss was obtained in the former.     

High permittivity and low loss dielectric ceramics in the BaO–La2O3–TiO2–Ta2O5 system

Dielectric ceramics were synthesized and characterized in the BaO–La₂O₃–TiO₂–Ta₂O₅ quaternary system for the three typical compositions: Ba₃La₃Ti₅Ta₅O₃₀, Ba₄La₂Ti₄Ta₆O₃₀ and Ba₅LaTi₃Ta₇O₃₀, which formed the filled tungsten-bronze structures. The present ceramics indicated high dielectric constant ε (127.7–148.1) and low dielectric loss tanδ (in the order of 10⁻⁴–10⁻³ at 1 MHz). Meanwhile, the temperature coefficient of dielectric constant τ_ε varied from −728 to −1347 ppm/°C with increasing Ba and Ta and decreasing La and Ti concentration in the temperature range of 20–85 °C. The present ceramics are promising candidates for high-ε and low loss dielectric ceramics, and the suppression of τ_ε should be the primary issue in the future work.     

Microwave dielectric properties of BaO–4.3TiO2–0.5ZnO ceramics with BaCu(B2O5)

Effect of BaCu(B₂O₅) (BCB) addition on microwave dielectric properties and sintering behaviors of BaO–4.3TiO₂–0.5ZnO system (BTZ) ceramics were investigated to develop middle-k dielectric composition with low sintering temperatures. When a small amount of BCB was added to BTZ system, the sintering temperature can be lowered from 1100 °C to 900 °C due to the formation of BCB liquid phase. The system added with 7 wt% BCB was sintered at 900 °C for 2 h and ?_r of 31, Q × f of 18,200 GHz and τ_f of 3.8 ppm/°C were obtained. The suitability of BTZ ceramics for tape casting and cofiring with Ag electrodes was investigated, and no evidence of chemical reaction between Ag and ceramics was observed. The dielectric properties of the stacked multilayer plate without any electrodes were also measured. The result shows that the as-prepared BTZ ceramics are suitable for low-temperature co-fired ceramics applications.     

BaO–TiO2 microwave ceramics

In this paper, the structure and dielectric properties of BaO–TiO₂ system ceramics were studied. By adding ZnO and Nb₂O₅ as sintering agents to the raw materials, the BaO–TiO₂ system ceramics were sintered at a temperature of 1260 °C for 2 h and have superior dielectric properties at 1 GHz: quality factor Q=12,500, relative dielectric constant ε_r≈37, temperature coefficient of dielectric constant α_ε=0±30 ppm/°C. XRD pattern shows that the main crystal phase of the ceramics is Ba₂Ti₉O₂₀, accompanied by a small number of additional phases: BaTi₄O₉, Ba₄Ti₁₃Zn₇O₃₄, Ba₄Ti₁₃O₃₀ and Ti₂Nb₁₀O₂₉, etc. The initial Ba/Ti ratio has a great effect on the dielectric properties of the ceramics, which can be explained by the variance in the formation of phases due to different Ba/Ti ratios.     

Liquid Immiscibility Phenomena in Melts of the ZrO2–FeO–Fe2O3 System

The ZrO₂–FeO _x melts prepared by induction melting in a cold crucible in air are investigated experimentally. It is found that the immiscibility region exists in the liquid phase. The theoretical fusibility curves (calculated according to the authors' DIATRIS 1.2 program) for the ZrO₂–FeO–Fe₂O₃ system are compared with the experimental temperatures of the formation of the secondary phase, liquidus and solidus lines, and X-ray microprobe spectroscopic and X-ray powder diffraction data for quenched and slowly cooled samples. It is demonstrated that the immiscibility region in the liquid phase in an air atmosphere is observed in the temperature range 1870–2230°C and the concentration range 34–82 wt % ZrO₂.     

Microwave dielectric properties of Mn-doped BaO–(Nd0.7,Sm0.3)2O3–4TiO2 ceramic sintered in a reducing atmosphere

The effect of MnCO₃ doped from 0 to 55 mol% into BaO–(Nd_0.7Sm_0.3)₂O₃–4TiO₂ (BNST) sintered in a reducing atmosphere on the microstructure and electrical properties was studied. Mn³⁺ completely substituted into Ti⁴⁺-sites of BNST to form a solid solution, so there is no second phase until 42 mol% which is the maximum solubility. Mn (<42 mol%)-doped BNST sintered in a reducing atmosphere is in a semi-conducing state because the concentration of free electron is higher than that of the acceptors. On the other hand, when Mn content doped into BNST exceeds a critical value (>43 mol%), the second Mn-rich phase due to excess of Mn³⁺ substituted into Ti⁴⁺-site, corresponding to original BaO–(Nd_0.7Sm_0.3)₂O₃–4TiO₂ (1 1 4) phase, is formed. Mn (>43 mol%)-doped BNST sintered in a reducing atmosphere is in an insulating state because the concentration of the acceptors is higher than that of liberated free electron, so the insulation resistance becomes high and tan δ becomes low. The formation of the second Mn-rich phase affects Q × f factor and temperature coefficient of capacitance (T.C.C.) of BNST significantly.     

Experimental determination of the liquidus line in the high Bi2O3 region in the TiO2–Bi2O3 system

Liquidus line in the high-Bi₂O₃-containing region of the TiO₂–Bi₂O₃ system was determined experimentally. The equilibrating and quenching technique with subsequent electron probe microanalyser (SEM-EDS) microanalysis were employed. Based on the data, liquidus line was constructed between 60 and 92 mol% Bi₂O₃. The current results showed a higher solubility of Bi₂O₃ in the liquid phase in equilibrium with the Bi₄Ti₃O₁₂ compound compared with the existing phase diagram. In addition, differential scanning calorimetry (DSC) was used to estimate the transformations covering the composition range from 60 to 95 mol% Bi₂O₃. Further, the phase diagram of the TiO₂–Bi₂O₃ system was calculated using a quasichemical model for the liquid phase. The thermodynamic properties of the intermediate compounds were estimated from the data of TiO₂ and Bi₂O₃ pure solids.     

Influence of Li2O–B2O3–SiO2 glass on the sintering behavior and microwave dielectric properties of BaO–0.15ZnO–4TiO2 ceramics

This paper reports the investigation of the performance of Li₂O–B₂O₃–SiO₂ (LBS) glass as a sintering aid to lower the sintering temperature of BaO–0.15ZnO–4TiO₂ (BZT) ceramics, as well as the detailed study on the sintering behavior, phase evolution, microstructure and microwave dielectric properties of the resulting BZT ceramics. The addition of LBS glass significantly lowers the sintering temperature of the BZT ceramics from 1150 °C to 875–925 °C. Small amount of LBS glass promotes the densification of BZT ceramic and improves the dielectric properties. However, excessive LBS addition leads to the precipitation of glass phase and growth of abnormal grain, deteriorating the dielectric properties of the BZT ceramic. The BZT ceramic with 5 wt% LBS addition sintered at 900 °C shows excellent microwave dielectric properties: ε_r=27.88, Q×f=14,795 GHz.     

Production of Chemically Pure Zirconia-Based Nanoceramics in the ZrO2(Y2O3)–Al2O3 System for Restorative Dentistry

Theoretical Foundations of Chemical Engineering - The synthesis technology of a chemically pure nanodisperse precursor powder (10–12 nm) based on a tetragonal solid zirconia (t-ZrO2) solution...     

Antibacterial and photocatalytic active transparent TiO2 crystallized CaO–BaO–B2O3–Al2O3–TiO2–ZnO glass nanocomposites

Transparent TiO₂ crystallized 5CaO–10BaO–65B₂O₃–Al₂O₃–20TiO₂–10ZnO (CBBATZ) glass nanocomposites were fabricated using melt-quenching technique followed by specific heat treatments. As-quenched glass samples were provided three different heat treatments at 630°C for 3, 5, and 10 hours in order to obtain different amounts of TiO₂ nanocrystals in the glass. The presence of rutile phase of TiO₂ nanocrystals in glass was confirmed by X-ray diffraction. The glass nanocomposite heat treated for 10 hours showed a hydrophobic nature with contact angle of 90.90°. Contact angle decreased from 90.90 to 22.20°, when irradiated under ultraviolet (UV) radiation for 45 minutes. This photoinduced hydrophilicity showed a photocatalytic and self-cleaning properties of glass nanocomposite. During photocatalytic ink test, the maximum change in color of Resurin (Rz) ink and 60% degradation in absorbance of ink within 150 minutes under UV radiation were found for glass nanocomposite heat treated at 10 hours. Also, 78% degradation in absorbance of methylene blue dye (pollutant) within 180 minutes under UV irradiation was found for glass naocomposite heat-treated at 10 hours. Antibacterial performance of transparent glass nanocomposite against Escherichia coli was evaluated as well. More than 95% of the bacterial cells were degraded with glass nanocomposite heat-treated at 10 hours. CBBATZ glass nanocomposite found to impart the antibacterial effect through generation of reactive oxygen species (ROS) in aqueous medium. ROS species which was confirmed in the bacterial cell through intracellular ROS generation kit. During evaluation of mechanical properties using nanoindentation technique, the values of hardness and reduced modulus increased by ~26% and 10%, respectively, for glass nanocomposite heat-treated at 10 hours as compared to as-quenched glass.     

Effect of TiO2 addition amount on BaO–CaO–Al2O3 -SiO2 glass-bonded Al2O3 ceramics

A new type of glass-ceramic BaO–CaO–Al₂O₃–SiO₂ (BCAS) was developed to join Al₂O₃ ceramics, adding TiO₂ to the glass-ceramics can promote the crystallization behavior of the glass-ceramics. Through the observation of the joints, rutile TiO₂ whiskers can grow on the surface of Al₂O₃ ceramics, and the grown TiO₂ whiskers are one-dimensional needle-like whiskers growing in different directions in the joints, providing mechanical support for the joints. The aspect ratio of TiO₂ whiskers was changed by controlling the addition of TiO₂, and the crystallization behavior and microstructure of the joints were studied. The experimental results show that when the amount of TiO₂ added is 10% (wt%), the density of TiO₂ whiskers in the joint is the largest, the strengthening effect on the joint is the best, and the shear strength can reach 94.33 MPa.     

Thermodynamic calculation of Ms in ZrO2–CeO2–Y2O3 system

The difference of Gibbs free energy between tetragonal and monoclinic phases in ZrO₂–CeO₂–Y₂O₃ as a function of composition and temperature is thermodynamically calculated from the three related binary systems. In 8 mol% CeO₂–0.5 mol% Y₂O₃–ZrO₂, the equilibrium temperature between tetragonal and monoclinic phases, T₀, is obtained as 832.5 K and the M_s temperature of this alloy with a mean grain size of 0.90 μm is calculated as 249.9 K using the approach derived by Hsu et al. [J. Mater. Sci., 18(1983)3206; 20(1985)23; Acta Metall., 37(1989)3091; Acta Metall. Mater., 39(1991)1045; Mater. Trans. JIM, 37(1996)1284], which is in good agreement with the experimental one of 253 K.     

Insights into the ternary eutectic microstructure formed in intercolony regions in Al2O3–ZrO2(Y2O3) system

Al₂O₃/ZrO₂(Y₂O₃) pseudo-binary eutectic melt may solidify in the dendritic form at the low G/v ratio (thermal gradient G divided by growth rate v) associated with the Y₂O₃ microsegregation along the monovariant line. Especially, in the system with high Y₂O₃ content (≥4.5 mol%), an Al₂O₃/ZrO₂/YAG ternary eutectic in situ microstructure growing with a “plane front” will appear at the boundaries of dendritic structures. The volume fraction of the ternary eutectic increases with the increase in Y₂O₃ content. It was found that the amount of ternary eutectic can be well predicted by the Scheil equation (or nonequilibrium lever rule).     

Fabrication and microstructure of Al2O3–ZrO2(3Y)–SiC nanocomposites

Al₂O₃–ZrO₂(3Y)–SiC composite powder was prepared by the heterogeneous precipitation method. Calcinating temperature of the powder was important to obtain dense sintered body. The nanocomposites were got by hot-pressing, and addition of ZrO₂ did not raise the sintering temperature. Some Al₂O₃ grain shape was elongated, and Al₂O₃ grain size was about μm. Nano SiC particles were observed uniformly distributing throughout the composites, and most of them were located within the matrix grains. Because SiC particles located within ZrO₂ grains influenced the phase transformation of ZrO₂, the sintering of nanocomposites, which controlled grain size and transformable ZrO₂ amount, become important to get high performance. The strength of 80 wt% Al₂O₃–15 wt% ZrO₂–5 wt% SiC nanocomposites was 555 MPa, and toughness was 3·8 MPa m^1/2, which were higher than those of monolithic Al₂O₃ ceramics. ©     

Phase diagram study and thermodynamic modeling of the Na2O–Al2O3–ZrO2 system

The phase diagram of the Na₂O–Al₂O₃–ZrO₂ system was experimentally studied at 1500°C–1650°C by a classical equilibration/quenching method and differential thermal analysis followed by X-ray diffraction phase analysis and electron probe micro-analysis. A sealed Pt crucible was utilized to prevent the volatile loss of Na₂O during high-temperature phase equilibrium experiments and the hydration upon quenching. The phase diagram of the Na₂O–Al₂O₃–ZrO₂ system was revealed for the first time. Based on the present experimental data and available binary modeling results in literature, the thermodynamic modeling of the ternary system was performed using the Calculation of Phase Diagram method and the phase diagram of the entire the Na₂O–Al₂O₃–ZrO₂ system was constructed and the optimized thermodynamic properties for all solids and liquid phase within the ternary system were obtained.     

Structure and viscosity of CaO–Al2O3–B2O3–BaO slags with varying mass ratio of BaO to CaO

The structure of CaO–Al₂O₃–B₂O₃–BaO glassy slags with varying mass ratio of BaO to CaO has been investigated by Raman spectroscopy, ¹¹B and ²⁷Al magic angle spinning nuclear magnetic resonance (MAS-NMR) spectroscopy and atomic pair distribution function (PDF). ¹¹B MAS-NMR spectra reveal the dominant coordination of boron as trigonal. Both simulations on ¹¹B MAS-NMR spectra and Raman spectroscopy indicate the presence of orthoborate as the primary borate group with a few borate groups with one bridging oxygen and minor four-coordinated boron sites. ²⁷Al MAS-NMR and PDF show the Al coordination as tetrahedral. Raman spectral study shows that the transverse vibration of Al^IV–O–Al^IV and Al^IV–O–B^III, stretching vibration of aluminate structural units and vibration of orthoborate and pyroborate structural groups. A broader distribution of Al–O bond lengths in PDF also supports the enhanced network connectivity. Viscosity measurements show the increase in viscosity of molten slags with increasing mass ratio of BaO to CaO, which further attributes to the enhanced degree of polymerization of the aluminate network.     

Phase analysis and microwave dielectric properties of BaO–Nd2O3–5TiO2 composite ceramics using variable size TiO2 reagents

There are significant inconsistencies in published literature surrounding the phase analysis and physical properties of ceramics with the nominal composition BaO–Nd₂O₃–5TiO₂ (BNT125). A careful phase analysis investigation of BNT125 ceramics using variable size TiO₂ reagents was therefore undertaken using XRD, FESEM and EPMA with corresponding dielectric properties characterised over 2–3 GHz. Three distinct phases were consistently formed: Ba_6?3xNd_8+2xTi₁₈O₅₄ (x ～ 0.67), Ba₂Ti₉O₂₀ and TiO₂. The use of nano-scale TiO₂ reagents significantly reduced porosity and improved the dielectric properties of the composite ceramics, while markedly reducing processing times. Structural and crystal chemical indications as to the origin of this system's physical properties are discussed, with these results providing new insights into optimisation paths for microwave dielectric materials of this type.     

Thick film cermet of ZrO2–Y2O3–TiO2/Ni: anodic polarization study

Thick films of [(ZrO₂)_0·92(Y₂O₃)_0·08]_0·9(TiO₂)_0·1/Ni (35 vol%) have been deposited onto yttria stabilized zirconia (YSZ) substrates. The cermets were prepared by mixing a YSZ commercial powder, TiO₂, NiO and a controlled amount of carbon. Thixotropic slips with a solids content of 60 wt% were obtained. After sintering at 1300°C in air, without introduction any reducing atmosphere, the thick film cermets show good adherence and chemical and mechanical compatibility with YSZ electrolyte. The hydrogen oxidation on the cermets, using a three-electrode cell, has been studied. The polarization resistance is consisted of two separated contributions, one might be associated to the typical proton charge transfer, and the second one, might be originated from the adsorption reaction of electroactive species. The anodic polarization curve exhibited a maximum of current density for an overpotential close to that of the Ni–NiO system, which could be associated to the oxidation of hydrogen on NiO.