Highly transparent yttrium titanate (Y2Ti2O7) ceramics from co-precipitated powders

Highly transparent yttrium titanate (Y₂Ti₂O₇) ceramics were fabricated by vacuum sintering using co-precipitated powders for the first time. The effects of the powder calcination temperature on the phase composition, morphology of the calcined powders, and on the microstructure and transmittance of the Y₂Ti₂O₇ ceramics were investigated. When the calcination temperature was above 850 °C, pure phase Y₂Ti₂O₇ nanopowders with high sintering activity were obtained. Transparent Y₂Ti₂O₇ ceramics were obtained after vacuum sintered at 1600 °C for 6 h and annealed at 1100 °C for 5 h in air. The highest transmittance reached 73% at 1000 nm when the calcination temperature was 1150 °C. The measured refractive index of Y₂Ti₂O₇ ceramics was higher than 2.24 at the wavelength range of 350–1000 nm, making it a promising candidate for optical devices.     

Mechanical and dielectric properties of 3D printed highly porous ceramics fabricated via stable and durable gel ink

A novel, efficient, versatile strategy was carried out to fabricate highly porous ceramic parts based on the combination of strong colloidal gel ink fabricated with high boiling point organic solvents and DIW technique. The preparation and optimization of inks and the effect of heating temperature on the phase composition, microstructure, mechanical properties and dielectric properties of ceramic parts were systematically investigated. The strong colloidal ink exhibits excellent ambient stability and printability. The sintering temperatures bring about the evolution of phases, structural mechanical properties and dielectric properties of ceramic parts. Ultimately, Si₂N₂O single wall ceramic parts with a frame density of 1.07?1.14 g/cm³ and an apparent porosity of 53.13 ± 1.29% were successful fabricated. The dielectric constant and dielectric loss of Si₂N₂O sample (1650℃) are only 4.24 and 0.0049, respectively. This strategy provides a reference for in-situ synthesis of high-performance porous ceramic components based on the DIW.     

Reactive wetting and interfacial characterization of ZrO2 by SnAgCu-Ti alloy

The reactive wetting behavior of zirconia with SnAgCu-x%Ti (SAC-x%Ti, wt%, x?=?1, 4) alloy was investigated via the sessile drop method in isothermal experiments. As temperatures elevated, the final contact angle decreased and the minimum contact angle of 21° and 7° were obtained at 1000?°C for SAC-1%Ti and SAC-4%Ti droplets, respectively. Kinetic calculations indicated that the spreading of SAC-Ti droplets on zirconia was controlled by interfacial reaction and the wetting activation energy was 108.8?kJ/mol. The reaction products distribution and morphology in droplets were influenced vastly by the addition of Ti. Along with the increase of Ti content from 1% to 4%, a great deal of Ti-Sn intermetallic compounds (IMCs) were generated in droplets, thereby the outline of droplets were transformed from hemispherical into similar trapezoidal due to the limited spreading and fluidity of droplets. Owing to the interfacial reaction between active elements Ti and zirconia and the subsequent formation of the Ti-O layer, the wettability of SAC-Ti/zirconia was greatly promoted. According to transmission electron microscopy (TEM) analysis, the thin Ti-O reaction layer consisted of the Ti₂O, Ti₄O₇, Ti₇O₁₃ and TiO₂ phase.     

Distribution of Tm3+ and Ni2+ in chalcogenide glass ceramics containing Ga2S3 nanocrystals: Influence on photoluminescence properties

The distribution of Tm³⁺ and Ni²⁺ ions is unambiguously exhibited in 80GeS₂-20Ga₂S₃ chalcogenide glass ceramics (GCs) containing Ga₂S₃ nanocrystals (NCs) by using advanced analytical transmission electron microscopy. Distinctively different distribution patterns of Tm³⁺ and Ni²⁺ ions are observed in the GCs obtained by controlled crystallization. The distribution of the dopants imposes strong influence on their optical properties which are revealed by absorption and photoluminescence (PL) spectra. Detailed discussions are given of the mechanisms of the crystallization-induced PL enhancement and quenching of the Tm³⁺ mid-infrared and Ni²⁺ near-infrared emissions, respectively.     

Equiaxed β–Si3N4 ceramics prepared by rapid reaction‐bonding and post‐sintering using TiO2–Y2O3–Al2O3 additives

Sintered reaction‐bonded Si₃N₄ ceramics with equiaxed microstructure were prepared with TiO₂–Y₂O₃–Al₂O₃ additions by rapid nitridation at 1400°C for 2 hours and subsequent post‐sintering at 1850°C for 2 hours under N₂ pressure of 3 MPa. It was found that α–Si₃N₄, β–Si₃N₄, Si₂N₂O, and TiN phases were formed by rapid nitridation of Si powders with single TiO₂ additives. However, the combination of TiO₂ and Y₂O₃–Al₂O₃ additives led to the formation of 100% β–Si₃N₄ phase from the nitridation of Si powders at such low temperature (1400°C), and the removal of Si₂N₂O phase. As a result, dense β–Si₃N₄ ceramics with equiaxed microstructure were obtained after post‐sintering at high temperature.     

Enhanced piezoelectric properties and temperature stability of Bi4Ti3O12-based Aurivillius ceramics via W/Nb substitution

Bi₄Ti₃O₁₂ (BIT), a typical Aurivillius ceramics with high Curie temperature (T_c ? 675 °C), has great potential for high temperature applications. This work provides an effective method of inducing structure distortion, relieving the tetragonal strain of the TiO₆ octahedron and decreasing the concentration of oxygen vacancies to improve the piezoelectricity and temperature stability of BIT ceramics. Bi₄Ti_2.98W_0.01Nb_0.01O₁₂ possesses an optimum piezoelectric coefficient (d₃₃) of 32 pC/N, a high T_c of 655 °C and a large resistivity of 3 × 10⁶ Ω·cm at 500 °C. The maximum d₃₃ reported here is approximately quadruple than that of pure BIT (?7 pC/N). Moreover, the d₃₃ of W/Nb co-doped BIT and the in-situ temperature stability of the compression-mode sensor present a highly stable characteristic in the range of 25–600 °C. These results imply that W/Nb-modified BIT ceramics is a promising candidate for application at high temperatures of up to 600 °C.     

The effect of A-site nonstoichiometry on the microstructure,electric properties,and phase stability of NaNbO3 polycrystalline ceramics

The impact of A-site nonstoichiometry on the microstructure, electric properties, and phase stability of sodium niobate ceramics (Na_1+xNbO₃, x = ?2 to 1 mol %) was investigated. All the components maintained an orthorhombic antiferroelectric (AFE) structure. The grain size increased from 3.9 to 14.3 μm with the variation in x from ?2 to 1. The AFE–FE phase transition electric field dramatically increased from 100 kV cm^?1 at x = 0 to 170 kV cm^?1 at x = ?2, confirming the enlarged energy barrier between AFE Pbma and FE Pmc2₁ phase under external field in A-site deficient components. This is attributed to the lattice compressive stress generated by introducing proper A-site vacancies. Combined results of transmission electron microscopy and Raman spectroscopy indicated that the AFE distortion of Pbma phase was significantly enhanced in A-site deficient components, which jointly contributed to the stability of AFE phase in A-site deficient NaNbO₃ material.     

Effects of Bi2O3-Nb2O5 additives on microstructure and magnetic properties of low-temperature-fired NiCuZn ferrite ceramics

NiCuZn ferrite with superior magnetic performance is vital ceramic material in multilayer chip inductors (MLCI) applications. In this study, low-temperature-sintered Ni_0.22Cu_0.2Zn_0.58Fe₂O₄ ferrite ceramic doped with 1.0?wt% Bi₂O₃-x?wt% Nb₂O₅ (where x?=?0.0, 0.1, 0.2, 0.3, 0.4 and 0.5) was synthesized via solid-state reaction method. Effects of Bi₂O₃-Nb₂O₅ additives on microstructures and magnetic properties of NiCuZn ferrite ceramics sintered at 900?°C were systematically investigated. Results indicate that an appropriate amount of Bi₂O₃-Nb₂O₅ composite additives can significantly promote grain growth and densification of NiCuZn ferrite ceramics when sintered at low temperatures. Specifically, samples doped with 1.0?wt% Bi₂O₃ and 0.4?wt% Nb₂O₅ additives exhibited excellent initial permeability (~ 410 @ 1?MHz), high cutoff frequency (~ 10?MHz), high saturation magnetization (~ 54.92?emu/g), and low coercive force (~ 20.32?Oe). These observations indicate that NiCuZn ferrite ceramics doped with appropriate amounts of Bi₂O₃-Nb₂O₅ additives are great candidate materials for MLCI applications.     

Effects of Sr2+ substitution on the crystal structure,Raman spectra,bond valence and microwave dielectric properties of Ba3-xSrx(VO4)2 solid solutions

The effects of Sr²⁺ substitution for Ba²⁺ on microwave dielectric properties and crystal structure of Ba_3-xSr_x(VO₄)₂ (0 ≤ x ≤ 3, BSVO) solid solution were investigated. Such Sr²⁺ substitution contributes to significant reduction in sintering temperature from 1400 °C to 1150 °C. Both permittivity (∑_r) and quality factor (Q × f) values decreased with increasing x value, which was determined to be related with the descending values of average polarizability and packing fraction, whereas the increase in τ_f value was explained by the decreased average VO bond length, A-site bond valence. BSVO ceramics possessed encouraging dielectric performances with ∑_r = 12.2–15.6 ± 0.1, Q × f = 44,340 - 62,000 ± 800 GHz, and τ_f = 24.5–64.5 ± 0.2 ppm/°C. Low-temperature sintering was manipulated by adding B₂O₃ as sintering additive for the representative Sr₃V₂O₈ (SVO) ceramic and only 1 wt.% B₂O₃ addition successfully contributed to a 21.7% decrease in sintering temperature to 900 °C, showing good chemical compatibility with silver electrodes, which render BSVO series and SVO ceramics potential candidates in multilayer electronic devices fabrication.     

Microwave dielectric properties of low firing temperature stable scheelite structured (Ca,Bi)(Mo,V)O4 solid solution ceramics for LTCC applications

In the present work, a systematic study on microwave properties of Ca_1-xBi_xMo_1-xV_xO₄ (0.2 ≤ x ≤ 0.5) solid solution ceramics synthesized by using the traditional solid-state reaction method was conducted. A scheelite structured solid solution was formed in the composition range 0.2 ≤ x ≤ 0.5. We successfully prepared a microwave dielectric ceramic Ca_0.66Bi_0.34Mo_0.66V_0.34O₄ with a temperature coefficient of resonant frequency (TCF) near to zero and a low sintering temperature by using (Bi, V) substituted (Ca, Mo) in CaMoO₄ to form a solid solution. The Ca_0.66Bi_0.34Mo_0.66V_0.34O₄ ceramic can be well sintered at only 870 °C and exhibits good microwave dielectric properties with a permittivity (ε_r) ?21.9, a Qf ?18,150 GHz (at 7.2 GHz) (Q = quality factor = 1/dielectric loss; f = resonant frequency), a TCF ? + 0.1 ppm/°C. The chemical compatibility with silver indicated that the Ca_0.66Bi_0.34Mo_0.66V_0.34O₄ ceramic might be a good candidate for the LTCC applications.     

Mechanical properties and pre-oxidation behavior of spark plasma sintered B4C ceramics using (Ti3SiC2+CeO2/La2O3) as sintering aid

B₄C ceramic with the addition of 5 wt % (Ti₃SiC₂+ CeO₂/La₂O₃) as sintering aids was fabricated by spark plasma sintering at a relatively low temperature of 1650 °C for 5 min at 80 MPa. The phase composition, microstructures, and comprehensive mechanical properties of the ceramics were studied in detail. The existence of reinforced second phase particles, the refinement of the matrix grains, the formation of residual stress along the grain boundaries and the appearance of the mixed fracture mode had a synergetic strengthening effect on the mechanical properties. The flexural strength, fracture toughness and Vickers hardness of B₄C ceramics reached 565.2 ± 21.8/551.0 ± 25.2 MPa, 6.28 ± 0.01/6.41 ± 0.12 MPa·m^0.5, and 28.51 ± 0.86/27.23 ± 1.08 GPa, respectively. In addition, to reduce the crack sensitivity of the ceramic, the ceramics were pre-oxidized at 800 °C for different durations. The flexural strength was increased by approximately 13.4% after the ceramic was oxidized at 800 °C for 45 min due to the crack-healing effect induced by the oxide glass B₂O₃ on the ceramic surface.