Strengthening of transparent spinel/Si3N4 nanocomposites

In response to a need for improving the mechanical properties of optically transparent ceramics, the nanocomposites approach is used to strengthen transparent magnesium-aluminate spinel with Si₃N₄ nanodispersoids. The as-processed nanocomposites are found to be >70% transparent in the critical infrared wavelength range of 3-4.5 μm. Mie scattering combined with absorption by the Si₃N₄ nanodispersoids explains quantitatively the IR transmission behavior of these nanocomposites. The nanocomposites are also found to be transparent in the visible region. Upon heat treatment (1000 °C for 4 h in air), the optical properties of the nanocomposites remain unchanged. However, the heat treatment results in a 29% increase in the average strength, accompanied by almost doubling of the Weibull modulus, and an 85% increase in the indentation toughness. The improvements in mechanical properties after the heat treatment in these nanocomposites are explained qualitatively, based on generally accepted arguments involving surface-oxidation-induced surface compression and flaw-healing. While further work is needed to fully understand and exploit these effects, this first report on transparent nanocomposites could have broad implications for the creation of mechanically robust, transparent ceramics of the future.     

Effects of alumina additions on sintering behavior of Ce0.8Sm0.2O1.9 ceramics synthesized by Pechini method

The effects of alumina additions on the densification behavior and grain growth of Ce_0.8Sm_0.2O_1.9 ceramics were investigated by the use of powders synthesized by Pechini method. Both the sintered density and grain size were found to increase rapidly up to 1 mol% Al₂O₃, and then to decrease with further additions. A pinning effect on grain growth was found at a higher Al₂O₃ content above 2 mol%. In the dopant content range of 0–5 mol%, 1 mol% Al₂O₃ was the optimum doping level in promoting densification and grain growth of Ce_0.8Sm_0.2O_1.9 ceramics.     

Bulk WC-Al2O3 composites prepared by spark plasma sintering

WC and WC-Al₂O₃ materials without metallic binder addition were densified by spark plasma sintering in the range of 1800-1900 °C. The densification behavior, phase constitution, microstructure and mechanical properties of pure WC and WC-Al₂O₃ composite were investigated. The addition of Al₂O₃ facilitates sintering and increases the fracture toughness of the composites to a certain extent. An interesting phenomenon is found that a proper content of Al₂O₃ additive helps to limit the formation of W₂C phase in sintered WC materials. The pure WC specimen possesses a hardness (HV₁₀) of 25.71 GPa, fracture toughness of 4.54 MPa·m^1/2, and transverse fracture strength of 862 MPa, while those of WC-6.8 vol.% Al₂O₃ composites are 24.48 GPa, 6.01 MPa·m^1/2, and 1245 MPa respectively. The higher fracture toughness and transverse fracture strength of WC-6.8 vol.% Al₂O₃ are thought to result from the reduction of W₂C phase, the crack-bridging by Al₂O₃ particles and the local change in fracture mode from intergranular to transgranular.     

An evaluation of Al2O3-ZrO2 composites produced by coprecipitation method

The objective of this work is to produce Al₂O₃-ZrO₂ composite from nano-sized powders processed by coprecipitation method. Al₂O₃ and mixture of Al₂O₃ + 10 wt.% ZrO₂ precipitated successfully by chemical route from aluminum sulfate and zirconium sulfate were pressed under uniaxial compression of 170 MPa and sintered at 1600 °C for 1 h. SEM investigations revealed that, pure alumina sample has a microstructure with coarse grains which anisotropically grown up to 30-40 μm in size. In alumina-zirconia composite, the structure consists of very fine equiaxed grains of typically 2 μm in which zirconia precipitates were uniformly dispersed. By adding zirconia to alumina, hardness and indentation fracture toughness were increased from 11.6 GPa to 16.8 GPa and from 3.2 MPa m^1/2 to 4.9 MPa m^1/2, respectively. Improvement in fracture toughness was attributed to bridging effects of zirconia particles as well as transformation toughening.     

Sintering effect on electrical properties of ZnO-V2O5-MnO2-Nb2O5 ceramics

The microstructure and electrical properties of quaternary ZnO-V₂O₅-MnO₂-Nb₂O₅ ceramics were investigated at different sintering temperature (875-950 °C). The average grain size increased from 4.4 μm to 9.6 μm with increasing sintering temperature. The breakdown field decreased from 6991 V/cm to 943 V/cm with increasing sintering temperature. Proper sintering for quaternary ZnO-V₂O₅-MnO₂-Nb₂O₅ ceramics led to surprisingly high nonlinear coefficient (50). The donor concentration increased from 3.33 × 10¹⁷ cm⁻³ to 7.64 × 10¹⁷ cm⁻³ with increasing sintering temperature and the barrier height exhibited the maximum value (1.07 eV) at 900 °C.     

Synthesis and characterization of 0.3 Vf TiC–Ti3SiC2 and 0.3 Vf SiC–Ti3SiC2 composites

In this work, two composite compositions—one with 30% (v/v) SiC, the other with 30% (v/v) TiC, balance Ti₃SiC₂—were synthesized and characterized. Fully dense samples were fabricated by hot isostatically pressing Ti, SiC and C powders for 8 h at 1500 or 1600 °C and a pressure of 200 MPa. Both TiC and SiC lower grain boundary mobility in Ti₃SiC₂. Coarsening of the SiC particles was also observed. At comparable grain sizes, all composites tested were weaker in flexure than the unreinforced Ti₃SiC₂ matrix, with the reduction in strength being the worst for the SiC composites. This reduction in strength is most probably due to thermal expansion mismatches between the matrix and reinforcement phases. The composite samples were exceptionally damage tolerant; in one case a 100 N Vickers indentation (in a 1.5-mm thick bar) did not reduce the flexural strength as compared to an unindented or as-fabricated samples. The same is true for thermal shock resistance; quenching samples from 1400 °C in room temperature water, resulted in strength reductions that were 12% at best and 50% at worst. In the 25–1000 °C temperature range, the thermal expansion coefficients of the two composites were indistinguishable at 8.2×10⁻⁶ K⁻¹. The Vickers hardness values depended on load; at 100 N, the hardnesses were ≈15 GPa; at 300 N, they asymptote to 7–8 GPa. For the most part, very few cracks emanate from the corners of the Vickers indents even at loads as high as 500 N. In the few cases where cracks did initiate, fracture toughness values were crudely estimated to lie in the 5–7.5 MPa √m range.     

Effect of sintering temperature on the structure and properties of cerium-doped 0.94(Bi0.5Na0.5)TiO3–0.06BaTiO3 piezoelectric ceramics

Phase structure, microstructure, dielectric and piezoelectric properties of 0.4 wt% CeO₂ doped 0.94(Bi_0.5Na_0.5)TiO₃–0.06BaTiO₃ (Ce-BNT6BT) ceramics sintered in the temperature range from 1120 to 1200 °C have been investigated as a candidate for lead-free piezoelectric ceramics. Tetragonal phase played an important role in improvement of electrical properties and the density of the ceramics. Dielectric constant decreased slightly with the increase of sintering temperature in ferroelectric region but a reverse phenomenon occurred in antiferroelectric and paraelectric regions, suggesting that interfacial polarizations were improved with the increase of sintering temperature and domain walls of ferroelectricity became active after depolarization. At room temperature, Ce-BNT6BT ceramics sintered at 1180 °C showed good performances: dielectric constant was 914 at 1 kHz, thick coupling factor k_t was 0.52, and the ratio of k_t/k_p was 2.3. The ceramics were suitable for narrowband filters and ultrasonic transducers in commercial applications.     

Study on phase formation and sintering kinetics of BaTi0.6Zr0.4O3 powder synthesized through modified chemical route

BaTi_0.6Zr_0.4O₃ powder was prepared from barium oxalate hydrate, zirconium oxy-hydroxide and titanium dioxide precursors. Barium oxalate hydrate and zirconium oxy-hydroxide were precipitated from nitrate solution onto the surface of suspended TiO₂. Phase formation behaviour of the materials was extensively studied using XRD. BaTiO₃ (BT) and BaZrO₃ (BZ) start forming separately in the system upon calcinations in the temperature range 600–700 °C. BT–BZ solid solution then forms by diffusion of BT into BZ from 1050 °C onwards. The precursor completely transforms into BaTi_0.6Zr_0.4O₃ (BTZ) at 1200 °C for 2 h calcination. The activation energy (AE) of BT (134 kJ mol⁻¹) formation was found to be less than that of BZ (167.5 kJ mol⁻¹) formation. BTZ formation requires 503.6 kJ mol⁻¹ of energy. The sintering kinetics of the powder was studied using thermal analyzer. The mean activation energy for sintering was found to be 550 kJ mol⁻¹.     

Study of (1 − x)(Mg0.6Zn0.4)0.95Co0.05TiO3–xCa0.61Nd0.26TiO3 microwave dielectrics

The microwave dielectric properties and the microstructures of the (1 − x)(Mg_0.6Zn_0.4)_0.95Co_0.05TiO₃–xCa_0.61Nd_0.26TiO₃ ceramic system were investigated. In order to achieve a temperature-stable material, we studied a method of combining a positive temperature coefficient material with a negative one. Ca_0.61Nd_0.26TiO₃ has a large positive temperature coefficient of resonant frequency. (Mg_0.6Zn_0.4)_0.95Co_0.05TiO₃ possesses a negative temperature coefficient of resonant frequency. By appropriately adjusting the x value in the (1 − x)(Mg_0.6Zn_0.4)_0.95Co_0.05TiO₃–xCa_0.61Nd_0.26TiO₃ ceramic system, a near-zero τ_f value can be obtained. A new microwave dielectric material of 0.8(Mg_0.6Zn_0.4)_0.95Co_0.05TiO₃–0.2Ca_0.61Nd_0.26TiO₃ possesses the excellent dielectric properties of a dielectric constant of 28.6, a Q × f value of 80,600 GHz and a temperature coefficient of resonant frequency of 4.1 ppm/°C and has a lower sintering temperature of 1250 °C.     

Effect of sintering on microstructure and mechanical properties of nano-TiO2 dispersed Al65Cu20Ti15 amorphous/nanocrystalline matrix composite

Mechanically alloyed Al₆₅Cu₂₀Ti₁₅ amorphous alloy powder with or without 10 wt% nano-TiO₂ dispersion was consolidated by isothermal spark plasma sintering in the range 200–500 °C with pressure up to 50 MPa. Selected samples were separately cold compacted with 50 MPa pressure and sintered at 500 °C using controlled atmosphere resistance and microwave heating furnaces. Phase and microstructural evolution at appropriate stages of mechanical alloying/blending and sintering was monitored by X-ray diffraction and scanning and transmission electron microscopy. Measurement and comparison of relevant properties (density/porosity, microhardness and yield strength) of the sintered compacts suggest that spark plasma sintering is the most appropriate technique for developing nano-TiO₂ dispersed amorphous/nanocrystalline Al₆₅Cu₂₀Ti₁₅ matrix composite for structural application.     

Low-temperature sintering and dielectric properties of high-permittivity microwave (Ca, Nd)TiO3 ceramics

The effect of H₃BO₃-CuO-Li₂CO₃ combined additives on the sintering temperature, microstructure and microwave dielectric properties of (Ca_0.61Nd_0.26) (Ti_0.98Sn_0.02)O₃ (CNTS) ceramics was investigated. The H₃BO₃-CuO-Li₂CO₃ combined additives lowered the sintering temperature of CNTS ceramics effectively from 1300 to 950 °C. This may be due to the interim liquid-phase of Li₂O-CuO-B₂O₃, which were formed in the sintering process. (Li_0.5Nd_0.5)TiO₃ (LNT) demonstrated an effective compensation in τ_f value of the low-fired CNTS ceramics. The 0.4CNTS-0.6LNT ceramics with 5 wt% (H₃BO₃-CuO)-0.5 wt% Li₂CO₃ sintered at 900 °C for 2 h shows excellent dielectric properties: ?_r = 90.6, Q × f = 3400 GHz, and τ_f = 9 ppm/°C. Also, the LTCC material is compatible with Ag electrode.     

Microstructure, dielectric and ferroelectric properties of 0.94Bi0.5Na0.5TiO3-0.06BaTiO3 (NBTB) and 0.05BiFeO3-0.95NBTB ceramics: Effect of sintering atmosphere

0.94Bi_0.5Na_0.5TiO₃-0.06BaTiO₃ (NBTB) and 0.05BiFeO₃-0.95NBTB (BF-doped NBTB) lead-free ceramics were prepared by solid state reaction method. The ceramics were sintered at 1180 °C for 2 h in O₂ and N₂. All ceramics exhibited a single phase of perovskite structure. Relative amount of tetragonal phase was related to the sintering atmospheres. Both grain size and shape were influenced by the sintering atmospheres. Sintering the ceramics in N₂ weakened their dielectric anomalies corresponding to the transition from ferroelectric phase to the so-called “intermediate phase”. When the NBTB and BF-doped NBTB ceramics were sintered in N₂, their maximum dielectric constant and the degree of diffuseness of the transition from the “intermediate phase” to paraelectric phase increased, but their Curie temperatures decreased. The difference in dielectric properties of the ceramics sintered in different atmospheres was closely related to the difference in oxygen vacancy concentration. The correlation between ferroelectric properties and sintering atmospheres is associated with a competing effect among oxygen vacancy concentration, A-site vacancy concentration and percentage of tetragonal phase.     

Effect of addition of Ba(W0.5Cu0.5)O3 in Pb(Mg1/3Nb2/3)O3-Pb(Zn1/3Nb2/3)O3-Pb(Zr0.52Ti0.48)O3 ceramics on the sintering temperature, electrical properties and phase transition

In this work, we report on the Pb(Mg_1/3Nb_2/3)O₃-Pb(Zn_1/3Nb_2/3)O₃-Pb(Zr_0.52Ti_0.48)O₃ (PMN-PZN-PZT) ceramics with Ba(W_0.5Cu_0.5)O₃ as the sintering aid that was manufactured in order to develop the low-temperature sintering materials for piezoelectric device applications. The phase transition, microstructure, dielectric, piezoelectric properties, and the temperature stability of the ceramics were investigated. The results showed that the addition of Ba(W_0.5Cu_0.5)O₃ significantly improved the sintering temperature of PMN-PZN-PZT ceramics and could lower the sintering temperature from 1005 to 920 °C. Besides, the obtained Ba(W_0.5Cu_0.5)O₃-doped ceramics sintered at 920 °C have optimized electrical properties, which are listed as follows: (K_p = 0.63, Q_m = 1415 and d₃₃ = 351 pC/N), and high depolarization temperature above 320 °C. These results indicated that this material was a promising candidate for high-power multilayer piezoelectric device applications.     

Toughened ZrO2 ceramics sintered with a La2O3-rich glass as additive

In this study, the influence of the glass addition and sintering parameters on the densification and mechanical properties of tetragonal zirconia polycrystals (3Y-TZP) ceramics were evaluated. High-purity tetragonal ZrO₂ powder and La₂O₃-rich glass were used as starting powders. Two compositions based on ZrO₂ and containing 5 wt.% and 10 wt.% of La₂O₃-rich glass were studied in this work. The starting powders were mixed/milled by planetary milling, dried at 90 °C for 24 h, sieved through a 60 mesh screen and uniaxially cold pressed under 80 MPa. The samples were sintered in air at 1200 °C, 1300 °C, 1400 °C for 60 min and at 1450 °C for 120 min, with heating and cooling rates of 10 °C/min. Sintered samples were characterized by relative density, X-ray diffraction (XRD) and scanning electron microscopy (SEM). Hardness and fracture toughness were obtained by Vickers indentation method. Dense sintered samples were obtained for all conditions. Furthermore, only tetragonal-ZrO₂ was identified as crystalline phase in sintered samples, independently of the conditions studied. Samples sintered at 1300 °C for 60 min presented the optimal mechanical properties with hardness and fracture toughness values near to 12 GPa and 8.5 MPa m^1/2, respectively.     

Solid-state synthesis and spark plasma sintering of SrZrO3 ceramics

SrZrO₃ powders are obtained by solid state reaction from SrCO₃ and ZrO₂ precursors, without involving intermediate calcination and grinding steps. The resulted powders are essentially within a single phase, with sub-micron average crystallite size. Pellets of these powders show a relatively poor sintering behavior, when fired up to 1600 °C. Alternatively, spark plasma sintering technique is used in order to obtain nearly 100% dense samples at the expense of excessive grain coarsening (i.e., up to 5 μm in diameter). Crystalline structure, composition and morphology of the specimens obtained in this work are investigated by X-ray diffraction, scanning and transmission electron microscopy together with energy dispersive X-ray spectroscopy.     

Nano-sized Y2O3:Eu hollow spheres with enhanced photoluminescence properties

Nano-sized Y₂O₃:Eu³⁺ hollow spheres were fabricated via a facile strategy including preparation of the hollow precursor and a later calcination. Moreover, the growth process of these hollow spheres was monitored by time-dependent experiments and their luminescence properties were also intensively studied. The products exhibit strong red emitting at 613 nm under ultraviolet excitation and control experiments were carried out to optimize the synthetic conditions. It was found 850 °C calcination with 9 mol% doping level could give out the best photoluminescence performance. Moreover, a possible mechanism for the enhanced PL performance was also proposed based on the FT-IR investigation.     

Mechanical properties evaluation of hot-pressed Si3N4–SiC(w) composites

Silicon nitride based ceramic composites have been prepared using an alternative development route, aiming to reach properties for structural applications. The prepared compositions were silicon nitride based matrix with aluminum nitride and yttrium oxide, as sintering aids, reinforced with silicon carbide whiskers. The powders were grounded/homogenized directly as received. Samples were obtained by uniaxial hot-pressing and, physical (specific mass, X-ray diffraction and microstructural analysis) and mechanical (microhardness, fracture toughness and flexural strength) properties were determined. The resulting high fracture toughness values were related to crack deflection and crack bridging toughening mechanisms, observed by SEM.     

New dielectric material system of La(Mg1/2Ti1/2)O3–Ca0.6La0.8/3TiO3 for microwave applications

The microstructure and microwave dielectric properties of xLa(Mg_1/2Ti_1/2)O₃–(1 − x)Ca_0.6La_0.8/3TiO₃ ceramics system with ZnO additions (0.5 wt.%) investigated by the conventional solid-state route have been studied. Doping with ZnO (0.5 wt.%) can effectively promote the densification and the dielectric properties of xLa(Mg_1/2Ti_1/2)O₃–(1 − x)Ca_0.6La_0.8/3TiO₃ ceramics. 0.6La(Mg_1/2Ti_1/2)O₃–0.4Ca_0.6La_0.8/3TiO₃ ceramics with 0.5 wt.% ZnO addition possess a dielectric constant (_r) of 43.6, a Q × f value of 48,000 (at 8 GHz) and a temperature coefficient of resonant frequency (τ_f) of −1 ppm/°C sintering at 1475 °C. As the content of La(Mg_1/2Ti_1/2)O₃ increases, the highest Q × f value of 62,900 (GHz) for x = 0.8 is achieved at the sintering temperature 1475 °C. A parallel-coupled line band-pass filter is designed and simulated using the proposed dielectric to study its performance.     

Characterization and sintering of BaZrO3 nanoparticles synthesized through a single-step combustion process

Barium zirconate (BaZrO₃) nanoparticles synthesized by a self-sustained single-step combustion process is reported in this paper. In this process, a phase pure nanopowder of BaZrO₃ has been obtained by the combustion of an aqueous solution containing Ba and Zr ions by using citric acid as complexing agent and liquor ammonia as fuel, thus giving rise to phase pure BaZrO₃ nanopowder in a single-step combustion without any further calcination. The X-ray diffraction studies have shown that the as-prepared powder was single phase, crystalline, and has a cubic perovskite structure (ABO₃) with a lattice constant a = 4.19 Å. The average particle size calculated from FWHM is 30 nm. The phase purity of BaZrO₃ nanopowder has been examined using differential thermal analysis (DTA), thermo gravimetric analysis (TGA) and Fourier transform of infrared spectroscopy (FTIR). The transmission electron microscopic investigation has shown that the particle size of the as-prepared powder was in the range 30–50 nm with a mean size of 40 nm. The nano BaZrO₃ has been sintered to a density of 99% of the theoretical density at 1650 °C in 2 h without the use of any sintering aids. The morphology of the sintered pellets has been studied with scanning electron microscopy (SEM). The dielectric constant (_r) and loss factor (tan δ) values obtained at 10 MHz for a well-sintered barium zirconate pellet has been found to be 32.2 and 1 × 10⁻⁴, respectively, at room temperature.     

Cu + TiB2 composite filler for brazing Al2O3 and Ti-6Al-4V alloy

Al₂O₃ and Ti-6Al-4V alloy were brazed using Cu + TiB₂ composite filler, which manufactured by mechanical milling of Cu and TiB₂ powders. Typical interface microstructure of joint was Al₂O₃/Ti₄(Cu,Al)₂O/Ti₂Cu + Ti₃Al + Ti₂(Cu,Al)/Ti₂(Cu,Al) + AlCu₂Ti/Ti₂Cu + AlCu₂Ti + Ti₃Al + Ti₂(Cu,Al) + TiB/Ti(s.s) + Ti₂Cu/Ti-6Al-4V alloy. Based on temperature- and time-dependent compositional change, the formation of intermetallics in joint was basically divided into four stages: formation of interfacial Ti₄(Cu,Al)₂O in Al₂O₃ side, formation of Ti₂Cu, Ti₃Al, TiB, Ti₂Cu, and AlCu₂Ti in layers II and IV, formation of Ti₂(Cu,Al) and AlCu₂Ti in layer III, formation of Ti + Ti₂Cu hypereutectoid organization adjacent to Ti-6Al-4V alloy. TiB in situ synthesized in joint not only acted as low thermal expansion coefficient reinforcement to improve the mechanical properties at room temperature, but also as skeleton ceramic of joint to increase high temperature mechanical properties of Al₂O₃/Ti-6Al-4V alloy joint increasing. When the joint containing 30 vol.% TiB brazed at 930 °C and 10 min of holding time, the maximum room temperature shear strength of joint was 96.76 MPa, and the high temperature shear strength of joint was 115.16 MPa at 800 °C.