Role of sintering on magneto-electric effect in CuFe1.8Cr0.2O4–Ba0.8Pb0.2Ti0.8Zr0.2O3 composite ceramics

Magnetoelectric composites containing CuFe_1.8Cr_0.2O₄–Ba_0.8Pb_0.2Ti_0.8Zr_0.2O₃ phases have been prepared by sintering them at different firing temperatures. The particle size for either phase of the composite was found to increase, whereas porosity decreases with increase in sintering temperature. This is due to the increase in the grain size with increase in sintering temperature. Resistivity of the composite decreases with increase in either sintering temperature or with increase in CuFe_1.8Cr_0.2O₄ content. The variation of dielectric constant (′) with temperature reflects DPT type behaviour. The peak value of dielectric constant (′_max) for a composite decreased with increase in its sintering temperature. The maximum value of the magnetoelectric conversion factor (dE/dH)_max equal to 182.7 μV/(cm*Oe) is obtained for 70% Ba_0.8Pb_0.2Ti_0.8Zr_0.2O₃–30% CuFe_1.8Cr_0.2O₄ composite when sintered at 1000°C.     

The reaction sequence and dielectric properties of BaSm2Ti4O12 ceramics

To establish the correct reaction sequence of BaO–Sm₂O₃–4TiO₂, phases present in different calcining temperatures are identified by X-ray diffraction patterns. When different calcining temperatures are used, the source phase BaO (BaCO₃) consumes below 850°C, the source phases TiO₂ and Sm₂O₃ consume at 1000 and 1150°C; the intermediate phases BaTiO₃, BaTi₄O₉, and Sm₂Ti₂O₇ consume at 1050, 1200, and 1250°C, respectively. The BaSm₂Ti₄O₁₂ phase starts to reveal at the 1100°C-calcined powder. The integrating intensity of BaSm₂Ti₄O₁₂ phase increases with the raising of calcining temperatures, accompanying with the decrease of integrating intensities of the source and intermediate phases. As the sintering temperature increases, the densities, quality values, and dielectric constants of BaSm₂Ti₄O₁₂ ceramics increase and saturate at 1325^oC. The BaSm₂Ti₄O₁₂ ceramics sintered at 1325°C have the properties of Q^*f=5180,_r=81.8, and τ_f=−19.2 ppm/°C.     

The homogeneity range and the microwave dielectric properties of the BaZn2Ti4O11 ceramics

Ceramics with a composition close to BaZn₂Ti₄O₁₁ were synthesized according to various substitutional mechanisms in order to verify an existence of a homogeneity range in the vicinity of this composition. Structural and microstructural investigations showed that the crystal structure of BaZn₂Ti₄O₁₁ was formed in the homogeneity range corresponding to the formula BaZn_2 − xTi₄O_11 − x (0 < x < 0.1). Densely sintered BaZn_2 − xTi₄O_11 − x (0 < x < 0.1) ceramics exhibited a dielectric constant around 30, τ_f = −30 ppm/K and high Q × f values, which increased from 68,000 GHz at x = 0 to 83,000 GHz at x = 0.05. Structurally, the deficiency of Zn in BaZn_2 − xTi₄O_11 − x (0 < x < 0.1) resulted in a slight decrease in the unit-cell volume. The influence of secondary phases in the BaZn₂Ti₄O₁₁-based materials on the microwave dielectric properties was also investigated. A presence of small amounts of ZnO, BaTiO₃, hollandite-type solid solutions (Ba_xZn_xTi_8 − xO₁₆) and BaTi₄O₉ caused a decrease in Q × f values.     

Piezoelectric properties of low temperature sintering in Pb(Zr,Ti)O3–Pb(Zn,Ni)1/3Nb2/3O3 ceramics for piezoelectric transformer applications

In this study, in order to develop low-temperature sintering ceramics for a multilayer piezoelectric transformer application, we explored CuO and Bi₂O₃ as sintering aids at low temperature (900 °C) sintering condition for Sb, Li and Mn-substituted 0.8Pb(Zr_0.48Ti_0.52)O₃–0.16Pb(Zn_1/3Nb_2/3)O₃–0.04Pb(Ni_1/3Nb_2/3)O₃ ceramics. These substituted ceramics have excellent piezoelectric and dielectric properties such as d₃₃  347 pC/N, k_p  0.57 and Q_m  1469 when sintered at 1200 °C. The addition of CuO decreased the sintering temperature through the formation of a liquid phase. However, the piezoelectric properties of the CuO-added ceramics sintered below 900 °C were lower than the desired values. The additional Bi₂O₃ resulted in a significant improvement in the piezoelectric properties. The composition Sb, Li and Mn-substituted 0.8Pb(Zr_0.48Ti_0.52)O₃–0.16Pb(Zn_1/3Nb_2/3)O₃–0.04Pb(Ni_1/3Nb_2/3)O₃ + 0.5 wt% CuO + 0.5 wt% Bi₂O₃ showed the value of k_p = 0.56, Q_m = 1042 (planar mode), d₃₃ = 350 pC/N, when it was sintered at 900 °C for 2 h. These values indicated that the newly developed composition might be suitable for multilayer piezoelectric transformer application.     

Effect of sintering and poling conditions on the properties of 0·125Pb (Mg1/3Nb2/3)O3−0.875Pb(Zr0·5Ti0·5)O3 ceramics doped with 4PbO. B2O3

The influence of sintering and poling conditions on dielectric properties and microstructures of the system 0·125Pb(Mg_1/3Nb_2/3)O₃−0·875Pb (Zr_0·5Ti_0·5)O₃ was investigated. Specimens were prepared by the conventional mixed-oxide technique. On account of eliminating the pyrochlore phase and lowering the sintering temperature, the calcined 0·125PZT−0·875PMN ceramic was doped with 4PbO.B₂O₃ glass powder. The 4PbO.B₂O₃ glass frit not only has a low flow temperature, but also a high polarizability. Additions of 4PbO.B₂O₃ to the perovskite 0·125PMN–0·875PZT solid solution will form a liquid phase, which served as a densification aid for the ceramics. With additions of 0·2 wt% glass frit, densities in excess of 98% of theoretical were obtained after sintering at 115°C. By variation of the fabrication processes, the influence of sintering and poling conditions on the properties of the ceramics was studied.     

Sintering behavior and microwave dielectric properties of Bi2O3–ZnO–Nb2O5-based ceramics sintered under air and N2 atmosphere

The sintering behavior and dielectric properties of the monoclinic zirconolite-like structure compound Bi₂(Zn_1/3Nb_2/3)₂O₇ (BZN) and Bi₂(Zn_1/3Nb_2/3−xV_x)₂O₇ (BZNV, x = 0.001) sintered under air and N₂ atmosphere were investigated. The pure phase were obtained between 810 and 990 °C both for BZN and BZNV ceramics. The substitution of V₂O₅ and N₂ atmosphere accelerated the densification of ceramics slightly. The influences on microwave dielectric properties from different atmosphere were discussed in this work. The best microwave properties of BZN ceramics were obtained at 900 °C under N₂ atmosphere with _r = 76.1, Q = 850 and Q_f = 3260 GHz while the best properties of BZNV ceramics were got at 930 °C under air atmosphere with _r = 76.7, Q = 890 and Q_f = 3580 GHz. The temperature coefficient of resonant frequency τ_f was not obviously influenced by the different atmospheres. For BZN ceramics the τ_f was −79.8 ppm/°C while τ_f is −87.5 ppm/°C for BZNV ceramics.     

Preparation and dielectric properties of B2O3–Li2O-doped BaZr0.35Ti0.65O3 ceramics sintered at a low temperature

BaZr_0.35Ti_0.65O₃ (BZT) ceramics have been fabricated via a traditional ceramic process at a relatively low sintering temperature using liquid-phase sintering aids B₂O₃ and Li₂O. The dielectric properties of BZT ceramics have been investigated with the emphasis placed on the dielectric properties under an applied dc electric field. The temperature-dependent dielectric constant reveals that the pure BZT and B₂O₃–Li₂O-doped BZT ceramics all have a typical relaxor behavior and diffuse phase transition characteristics. The temperature-dependent dielectric constant under the applied dc electric field shows that the Curie temperature is slightly shifted to higher temperature and the peaks are suppressed and broadened. The dielectric loss is still under 0.005 and tunability is above 20% at an applied dc electric field of 30 kV/cm.     

Sintering of Si3N4 with liquid in the system Ce2O3---AIN---SiO2

Liquid phase sintering of Si₃N₄ with melts from the system Ce₂O₃---AIN---SiO₂ has been studied. The glass forming region in this system and the reaction products formed during sintering at 1750–1800°C were analysed. Sintering of Si₃N₄ with two melt compositions selected from outside the glass forming region yields fully dense Si₃N₄. Post sintering treatment at 1300°C resulted in devitrification with consequent improvement of high temperature mechanical properties. The mechanical properties of Si₃N₄ sintered with liquids in the system Ce₂O₃---AIN---SiO₂ were found to be inferior to those of liquids selected from Y₂O₃---AIN---SiO₂, but superior to those selected from the system MgO---AIN---SiO₂.     

Effect of glass additions on the sintering and microwave properties of composite dielectric ceramics based on BaO–Ln2O3–TiO2 (Ln = Nd, La)

Ten weight percent BBZS (Bi₂O₃, B₂O₃, ZnO and SiO₂) glass was added to x(Ba₄Nd_9.333Ti₁₈O₅₄) − (1 − x)(BaLa₄Ti₄O₁₅) (BNLT, 0 ≤ x ≤ 1) composite dielectric ceramics to lower their sintering temperature whilst retaining microwave properties useful for low temperature co-fired ceramic and antenna core technology. With the addition of 10 wt% BBZS glass, dense BNLT composite ceramics were produced at temperatures between 950 and 1140 °C, depending on composition (x), an average reduction of sintering temperature by 350 °C. X-ray diffraction, scanning and transmission electron microscopy and Raman spectroscopy studies revealed that there was limited inter-reaction between BLT/BNT and the BBZS glass. Microwave property measurement showed that the addition of BBZS glass to BNLT ceramics had a negligible effect on _r and τ_f, although deterioration in the measured quality factor (Qf) was observed. The optimised composition (xBNT − (1 − x)BLT)/0.1BBZS (x = 0.75) had _r  61, τ_f  38 ppm/°C and Qf  2305 GHz.     

Relationships between crystal structure and microwave dielectric properties of (Zn1/3B2/3)xTi1 − xO2 (B = Nb, Ta) ceramics

Dependence of microwave dielectric properties on the crystal structure of (Zn_1/3B_2/3⁵⁺)_xTi_1 − xO₂ (B⁵⁺ = Nb, Ta) ceramics was investigated as a function of Zn_1/3B_2/3⁵⁺O₂ (B⁵⁺ = Nb, Ta) content (0.4 ≤ x ≤ 0.7). Dielectric constant (K) and the temperature coefficient of resonant frequency (TCF) of sintered specimens were strongly dependent on the structural characteristics of oxygen octahedra in rutile structure. Cation rattling and the distortion of oxygen octahedra were dependent on the bond length ratio of apical (d_apical)/equatorial (d_equatorial) of oxygen octahedra. The quality factor (Qf) was dependent on the reduction of Ti ion as well as the microstructure of the sintered specimens.     

Grain growth in TiO2-added ZnO–Bi2O3–CoO–MnO ceramics prepared by chemical processing

The effect of TiO₂ on the grain growth of the ZnO–Bi₂O₃–CoO–MnO ceramic system prepared by chemical coprecipitation, was studied between 1150 and 1300 °C in air. Bi₂O₃ melts during firing, and then TiO₂ dissolves into Bi₂O₃-rich liquid. TiO₂ initially reacts with Bi₂O₃ to form Bi₄Ti₃O₁₂. Above ≈1050 °C, Bi₄Ti₃O₁₂ reacts with ZnO to form Zn₂TiO₄ spinel phase. The kinetic study of grain growth carried out using the expression Gⁿ–G_oⁿ=K_o·t·exp(−Q/RT) gave grain exponent (n) value as 6 and the apparent activation energy (Q) as 226.46 kJ/mol. 1.00 mol% TiO₂ addition increased the grain growth exponent value from 6 to 7 and apparent activation energy with 1.00 mol% TiO₂ addition was found to be 197.10 kJ/mol. The ZnO grain size gradually increases with increasing TiO₂ content. Addition of TiO₂ may increase the reactivity of the Bi₂O₃-rich liquid towards the ZnO grain, thus affecting the ZnO grain growth.     

Thermoelectric characterization of NaxMx/2Ti1−x/2O2 (M=Co, Ni and Fe) polycrystalline materials

Layered -titanate materials, Na_xM_x/2Ti_1−x/2O₂ (M=Co, Ni and Fe, x=0.2–0.4), were synthesized by flux reactions, and electrical properties of polycrystalline products were measured at 300–800 °C. After sintering at 1250 °C in Ar, all products show n-type thermoelectric behavior. The values of both d.c. conductivity and Seebeck coefficient of polycrystalline Na_0.4Ni_0.2Ti_0.8O₂ were ca. 7×10³ S/m and ca. −193 μV/K around 700 °C, respectively. The measured thermal conductivity of layered -titanate materials has lower value than conductive oxide materials. It was ca. 1.5 Wm⁻¹ K⁻¹ at 800 °C. The estimated thermoelectric figure-of-merit, Z, of Na_0.4Ni_0.2Ti_0.8O₂ and Na_0.4Co_0.2Ti_0.8O₂ was about 1.9×10⁻⁴ and 1.2×10⁻⁴ K⁻¹ around 700 °C, respectively.     

In-situ growth of needlelike LaAl11O18 for reinforcement of alumina composites

In situ growth of needlelike LaAl₁₁O₁₈ grains reinforcing Al₂O₃ composites can be fabricated by a coprecipitation method using La(NO₃)₃√6H₂O and Al(NO₃)₃√9H₂O as starting materials. The new two-step process involved firstly preparing needlelike LaAl₁₁O₁₈ grains distributed homogeneously in Al₂O₃ powder and then pressureless sintering the composite powders. The Al₂O₃/25 vol.%LaAl₁₁O₁₈ samples pressureless sintered at 1550°C for 4 h achieve relative density up to 96.5% and exhibit a bending strength of 420±30 MPa and a fracture toughness of 4.3±0.4 MPa m^1/2.     

Gas pressure sintering of Al2O3/TiCN composite

Al₂O₃/TiCN composites have been fabricated by gas pressure sintering, which overcomes the limitations of hot pressing. The densification behavior and mechanical properties of the Al₂O₃ gas pressure sintered with 30 wt.% TiCN at different temperatures have been investigated. The gas pressure sintered Al₂O₃–30 wt.%TiCN composite achieved a relative density of 99.5%, a bending strength of 772 MPa, a hardness of 19.6 GPa, and a fracture toughness of 5.82 MPa·M^1/2.     

Piezoelectric properties of Pb(Mn1/3Nb2/3)O3–PbZrO3–PbTiO3 ceramics with sintering aid of 2CaO–Fe2O3 compound

Since the electromechanical devices move towards enhanced power density, high mechanical quality factor (Q_m) and electromechanical coupling factor (k_p) are commonly needed for the high powered piezoelectric transformer with Q_m≥2000 and k_p=0.60. Although Pb(Mn_1/3Nb_2/3)O₃–PbZrO₃–PbTiO₃ (PMnN–PZ–PT) ceramic system has potential for piezoelectric transformer application, further improvements of Q_m and k_p are needed. Addition of 2CaO–Fe₂O₃ has been proved to have many beneficial effects on Pb(Zr,Ti)O₃ ceramics. Therefore, 2CaO–Fe₂O₃ is used as additive in order to improve the piezoelectric properties in this study. The piezoelectric properties, density and microstructures of 0.07Pb(Mn_1/3Nb_2/3)O₃–0.468PbZrO₃–0.462PbTiO₃ (PMnN–PZ–PT) piezoelectric ceramics with 2CaO–Fe₂O₃ additive sintered at 1100 and 1250 °C have been studied. When sintering temperature is 1250 °C, Q_m has the maximum 2150 with 0.3 wt.% 2CaO–Fe₂O₃ addition. The k_p more than 0.6 is observed for samples sintered at 1100 °C. The addition of 2CaO–Fe₂O₃ can significantly enhance the densification of PMnN–PZ–PT ceramics when the sintering temperature is 1250 °C. The grain growth occurred with the amount of 2CaO–Fe₂O₃ at both sintering temperatures.     

Facile ultrasonic-assisted synthesis of micro–nanosheet structure Bi4Ti3O12/g-C3N4 composites with enhanced photocatalytic activity on organic pollutants

The Bi₄Ti₃O₁₂/g-C₃N₄ composites with microsheet and nanosheet structure were prepared through facile ultrasonic-assisted method. The SEM and TEM results suggested that the nanosheets g-C₃N₄ were stacked on the surface of regular Bi₄Ti₃O₁₂ sheets. Comparing with pure Bi₄Ti₃O₁₂ and g-C₃N₄, the Bi₄Ti₃O₁₂/g-C₃N₄ composites showed significant enhancement in photocatalytic efficiency for the degradation of RhB in solution. With the mass ratio of g-C₃N₄ increasing to 10 wt%, the Bi₄Ti₃O₁₂/g-C₃N₄-10% presented the best photocatalytic activity. Its photocatalysis reaction constant was approximately 2 times higher than the single component Bi₄Ti₃O₁₂ or g-C₃N₄. Meanwhile, good stability and durability for the Bi₄Ti₃O₁₂/g-C₃N₄-10% were confirmed by the recycling experiment and FT-IR analysis. The possible mechanism for the improvements was the matched band positions and the effective separation of photo-excited electrons (e^-) and holes (h⁺). Furthermore, based on the results of active species trapping, photo-generated holes (h⁺) and superoxide radical (·O₂^-) could be the main radicals in reaction.     

Preparation, microstructure and microwave dielectric properties of ZrxTi1−xO4 (x=0.40–0.60) ceramics

Zr_xTi_1−xO₄ (x=0.40–0.60) ceramics sintered without additives were prepared from powders made by the coprecipitation of metal salts from aqueous solutions in order to investigate the existence range of a homogeneous phase and the relationships between composition, microstructure and the dielectric properties. XRD, TEM, SEM, EDS, and the dielectric measurements were used to characterize the products. A homogeneous solid solution was obtained. Its crystal structure was isomorphous with ZrTiO₄. The variation of the lattice parameters with TiO₂ content was discussed. The optimum sintering temperature of samples was dependent of composition. TiO₂ suppressed the densification and acted as a grain growth enhancer during the sintering process. With the increase in TiO₂ content the relative densities of the sintered bodies decrease, while the grain sizes increase. The dielectric properties at microwave frequency (1.8 GHz) in this system, especially Q value, were poor, due to low densification, impurities and lattice defects. The dielectric constant _r and Q value exhibited a significant dependence on the relative density and composition. Both _r and Q increased with the increase in relative density, but they were primarily influenced by the composition and the effect of the relative density could be ignored when the relative density was greater than 90% theoretical. _r increased slightly with increasing TiO₂ content, while Q value decreased.     

Effect of Bi2O3 Addition on the Sintering Temperature and Microwave Dielectric Properties of Zn2SiO4 Ceramics

Bi₂O₃ was added to a nominal composition of Zn_1.8SiO_3.8 (ZS) ceramics to decrease their sintering temperature. When the Bi₂O₃ content was <8.0 mol%, a porous microstructure with Bi₄(SiO₄)₃ and SiO₂ second phases was developed in the specimen sintered at 885°C. However, when the Bi₂O₃ content exceeded 8.0 mol%, a liquid phase, which formed during sintering at temperatures below 900°C, assisted the densification of the ZS ceramics. Good microwave dielectric properties of Q × f =12,600 GHz, ɛ_r=7.6, and τ_f=−22 ppm/°C were obtained from the specimen with 8.0 mol% Bi₂O₃ sintered at 885°C for 2 h.     

Synthesis of platelike CaTiO3 particles by a topochemical microcrystal conversion method and fabrication of textured microwave dielectric ceramics

Platelike CaTiO₃ particles with an orthorhombic perovskite structure have been synthesized by topochemical microcrystal conversion (TMC) from platelike precursor particles of the layer-structured CaBi₄Ti₄O₁₅ at 950 °C. The CaTiO₃ particles inherited and retained the shape of the precursor particles with a thickness of approximately 0.3 μm, and a width of 2–6 μm. XRD analysis showed that in the TMC reaction, the crystallographic {0 0 1} plane of CaBi₄Ti₄O₁₅ is converted into the {1 0 0} plane of CaTiO₃. Using the platelike CaTiO₃ particles as templates in the templated grain growth method, dense {1 0 0} grain-oriented CaTiO₃ ceramics having a {1 0 0} orientation could be fabricated at sintering temperatures between 1350 and 1500 °C. The maximum orientation factor reached 99.7% at 10% of template. It was found that texturing improves microwave dielectric low-loss properties, providing a 1.55 times higher Q_f value of 9310 GHz in textured ceramics compared to that of 6005 GHz in non-textured ceramics.     

Preparation of porous aluminium oxide (Al2O3) hollow fibre membranes by a combined phase-inversion and sintering method

Al₂O₃ hollow fibre membranes were prepared by a combined phase-inversion and sintering method. An organic binder solution (dope) containing suspended aluminium oxide (Al₂O₃) powders, either in mono size or a distributed size, is spun to a hollow fibre precursor, which is then sintered at elevated temperatures. In spinning the hollow fibre precursor, polyethersulfone (PESf), N-methyl-2-pyrrolidone (NMP) and polyvinyl pyrrolidone (PVP) were used as a polymer binder, a solvent and an additive, respectively. The Al₂O₃ hollow fibre membranes prepared were characterized using a scanning electron microscope (SEM) and gas permeation techniques. Effects of Al₂O₃ particle size and size distribution, the sintering temperature and Al₂O₃/PESf ratio on the structure and performance of the resulting membranes were studied extensively. The prepared Al₂O₃ hollow fibre membranes retains its asymmetric structure (mainly resulted from the phase inversion technique) even after the sintering process. Preparation of the Al₂O₃ hollow fibre membrane with a high mechanical strength and moderate permeation characteristics is feasible if the Al₂O₃ powders with a distributed particle size in the spinning (dope) solution is employed.