Densification and mechanical properties of TiC by SPS-effects of holding time,sintering temperature and pressure condition

The influence of sintering temperature, holding time and pressure condition on densification and mechanical properties of bulk titanium carbide (TiC) fabricated by SPS sintering has been systematically investigated. Experimental data demonstrated that relative density and Vickers hardness (H_V) increase with sintering temperature and holding time, but fracture toughness (K_IC) was not significantly influenced by sintering parameters. The H_V and relative density of samples consolidated by SPS technique at 1600 °C for 5 min under 50 MPa pressure (applied entire sintering cycle) reached 30.31 ± 2.23 GPa and 99.90%, respectively. H_V values of ～24–30 GPa and K_IC of ～3.7–5 MPa m^1/2 were obtained in all bulk samples with relative densities of 95.61–99.90% when fabricated under various conditions presented above, without abnormal grain growth. More pronounced effects of pressure condition on grain growth (promoted by grain-boundary diffusion) than on densification were observed. The relationship of fracture toughness and fracture mode is also discussed.     

Enhancing the sintering ability of TiNx by introduction of nitrogen vacancy defects

Highly nitrogen-deficient non-stoichiometric TiN_x powders within nitrogen vacancy defects (0.3<x<0.5) were prepared by mechanical alloying and consolidated by high pressure sintering. The effects of nitrogen vacancy defects, sintering temperature and pressure on densification and grain growth of TiN_x were investigated for improving sintering ability and mechanical properties. Increasing nitrogen vacancy defects promoted densification and grain growth of TiN_x. Nitrogen vacancies accelerated material transport and diffusion during sintering and altered strong covalent bonding nature was believed to result in enhanced sintering ability. Densification of TiN_x was enhanced by increasing temperature and elevating pressure, grain growth was promoted by increasing temperature, whereas restrained by elevating pressure. TiN_x (x=0.32) ceramic with relative density of 99.4% and average grain size of 21 nm was obtained at 1200 °C, 5 GPa and 10 min. Vickers hardness of 22.6 GPa and fracture toughness of 5.0 MPa m^1/2 were achieved.     

Microwave dielectric properties of YAG ceramics prepared by sintering pyrolysised nano-sized powders

YAG ceramics with good dielectric properties were prepared via a modified pyrolysis method, with yttrium nitrate as the yttrium source and combined aluminium sulphate and aluminium nitrate as aluminium sources, and subsequent sintering in a muffle furnace. The effects of the different aluminium sources on the powder characteristic and the impact of sintering temperature, sintering aids (TEOS) and additive (TiO₂) on the dielectric properties of the ceramics were studied. The results show that well-dispersed pure YAG nano-powders can be obtained after calcination at 1000 °C with an aluminium sulphate and aluminium nitrate molar ratio of 1.5:2. The relative density, permittivity (ε_r) and quality factor (Q×f) of the YAG ceramics increase with sintering temperature and TEOS addition. TiO₂ can greatly promote τ_f to near-zero but decreases Q×f. The relative density, ε_r, Q×f and τ_f of the YAG–1 wt% TEOS–1 wt% TiO₂ ceramic obtained at 1520 °C are 97.6%, 9.9, 71, 738 GHz and −30 ppm/°C, respectively.     

Preparation and properties of an Al2O3/Ti(C,N) micro-nano-composite ceramic tool material by microwave sintering

One kind of Al₂O₃/Ti(C,N) micro-nano-composite ceramic tool material with acceptable properties was prepared by microwave sintering. Effects of sintering temperature and holding time on densification, mechanical properties and microstructure were studied. The optimal relative density, fracture toughness and Vickers hardness were 98.4±0.30, 6.72±0.28 MPa m^1/2 and 18.42±0.59 GPa, respectively, which were obtained at 1550 °C for 10 min. Compared to the conventional sintering, the sintering temperature and holding time of microwave sintering were reduced by 14% and 89%, respectively. The microwave sintering made the sizes of some particles keep in nano-scale, which leaded to the formation of intragranular structures. The residual stress in the intragranular structures increased the ratio of grain boundary toughness to grain toughness of matrix (K_cb/K_cg), and thus the micro-Al₂O₃ grains were more inclined to transgranular fracture.     

Thermoelectric Ca0.9Yb0.1MnO3−δ grain growth controlled by spark plasma sintering

n-Type Ca_0.9Yb_0.1MnO_3?δ thermoelectric (TE) powders were prepared by solid state synthesis (SSS) and co-precipitation method (Cop). The bulk TE materials were consolidated using conventional sintering (CS) and spark plasma sintering (SPS) respectively. The shrinkage behavior, as well as the sample densification strongly depends on the starting particle size. Consequently, the bulk samples from normal powder (SSS) and nano-powder (Cop) were prepared with similar density by using different sintering temperatures, of 1400 °C and 1200 °C, then 1200 and 950 °C for CS and SPS respectively. Such a decrease (up to 200 °C) of the sintering temperature is a consequent progress in terms of engineering for applications. Another advantage of the co-precipitation process compared to the conventional solid state synthesis is that, due to the small particle sizes and the decreased sintering temperature, grain growth was limited and TE properties were enhanced. The interest of the SPS process was also evidenced and we are presenting here the structural and microstructural investigations. In addition, the thermoelectric properties of samples prepared with two different processes were studied with the figure of merit of 0.18 at 750 °C.     

Microstructure and mechanical properties of ZTA composites fabricated by oscillatory pressure sintering

The influence of sintering temperature on the microstructure and mechanical properties of Al₂O₃?20 wt% ZrO₂ composites fabricated by oscillatory pressure sintering (OPS) was investigated by means of X-ray diffraction, scanning electron microscopy, three-point bending test and Vickers indentation. Results were compared to specimens obtained by conventional hot pressing (HP) under a similar sintering schedule. The optimum oscillatory pressure sintering temperature was found to be 1600 °C; almost fully dense materials (99.94% of theoretical density) with homogeneous microstructure could be achieved. The highest flexural strength, fracture toughness and hardness of such composites reached 1145 MPa, 5.74 MPa m^1/2 and 19.08 GPa when sintered at 1600 °C, respectively. Furthermore, the oscillatory pressure sintering temperature could be decreased by more than 50 °C as compared with the HP method, OPS favouring enhanced grain boundary sliding, plastic deformation and diffusion in the sintering process.     

HfB2-CNTs composites with enhanced mechanical properties prepared by spark plasma sintering

HfB₂-x vol%CNTs (x=0, 5, 10, and 15) composites are prepared by spark plasma sintering. The influence of CNTs content and sintering temperature on densification, microstructure and mechanical properties is studied. Compared with pure HfB₂ ceramic, the sinterability of HfB₂-CNTs composites is remarkably improved by the addition of CNTs. Appropriate addition of CNTs (10 vol%) and sintering temperature (1800 °C) can achieve the highest mechanical properties: the hardness, flexural strength and fracture toughness are measured to be 21.8±0.5 GPa, 894±60 MPa, and 7.8±0.2 MPa m^1/2, respectively. This is contributed to the optimal combination of the relative density, grain size and the dispersion of CNTs. The crack deflection, CNTs debonding and pull-out are observed and supposed to exhaust more fracture energy during the fracture process.     

Influence of sintering temperature on dielectric properties and crystal structure of corundum-structured Mg4Ta2O9 ceramics at microwave frequencies

Microwave dielectric properties of corundum-structured Mg₄Ta₂O₉ ceramics were investigated as a function of sintering temperatures by an aqueous sol–gel process. Crystal structure and microstructure were examined by X-ray diffraction (XRD) technique and field emission scanning electron microscopy (FE-SEM). Sintering characteristics and microwave dielectric properties of Mg₄Ta₂O₉ ceramics were studied as a function of sintering temperature from 1250 °C to 1450 °C. With increasing sintering temperature, the density, ε_r and Qf values increased, saturating at 1300 °C with excellent microwave properties of ε_r=11.9, Qf=195,000 GHz and τ_f=?47 ppm/°C. Evaluation of dielectric properties of Mg₄Ta₂O₉ ceramics were also analyzed by means of first principle calculation method and ionic polarizability theory.     

Synergetic effects of SiC and Csf in ZrB2-based ceramic composites. Part I: Densification behavior

The effects of processing variables on densification behavior of hot pressed ZrB₂-based composites, reinforced with SiC particles and short carbon fibers (C_sf), were studied. A design of experiment approach, Taguchi methodology, was used to investigate the characteristics of ZrB₂–SiC–C_sf composites concentrated upon the hot pressing parameters (sintering temperature, dwell time and applied pressure) as well as the composition (vol% SiC/vol% C_sf). The analysis of variance recognized the sintering temperature and SiC/C_sf ratio as the most effective variables on the relative density of hot pressed composites. The microstructural investigations showed that C_sf can act as a sintering aid and eliminate the oxide impurities (e.g. B₂O₃, ZrO₂ and SiO₂) from the surfaces of raw materials. A fully dense composite was achieved by adding 10 vol% C_sf and 20 vol% SiC to the ZrB₂ matrix via hot pressing at 1850 °C for 30 min under a pressure of 16 MPa. Moreover, the in-situ formation of interfacial ZrC, which also improves the sinterability of ZrB₂-based composites, was studied by energy-dispersive X-ray spectroscopy analysis and verified thermodynamically.     

Sintering behavior and microwave dielectric properties of Ca1-xBixW1-xVxO4 ceramics

Structure, sintering behavior and microwave dielectric properties of ceramics have been investigated by x-ray powder diffraction (XRD) and scanning electron microscopy (SEM) in this paper. The microwave dielectric properties of the ceramics were studied with a network analyzer at the frequency of about 6–11 GHz. The sintering temperature and microwave dielectric properties could be successfully tuned in a wide window simultaneously by adjusting the A–O bond characteristics. The sintering temperature of CaWO₄ was successfully reduced from 1100 °C to about 950 °C by BiVO₄ addition. Approximately 95%–96% theoretical density could be obtained after sintering at 950 °C for 2 h. All samples exhibit single Scheelite structure (I4₁/a) phase. The dielectric constant increased, whereas the Q×f value decreased, with the increase of x. The τ_f value changed from negative to positive with the increases of x. Combined excellent microwave dielectric properties with ε_r=22. 1, Q×f=16,730 GHz and τ_f=2.39 ppm/°C could be obtained after sintered at the 950 °C for 2 h for x=0.3 compositions.     

De-sintering of Ba5Nb4O15 ceramic and its influence on microwave characteristics

The sintering behaviour of a cation-deficient perovskite, Ba₅Nb₄O₁₅, is investigated in the present study. The highest density can be achieved through pressureless sintering at 1250 °C is only 93%. A further increase in sintering temperature results in a decrease in density; for example, the density is only 82% after sintering at 1435 °C. The density decrease, de-sintering, can be related to the formation of abnormal grains ( > 1250 °C) and the reduction of niobium ions ( > 1400 °C). The permittivity of sintered Ba₅Nb₄O₁₅ specimen shows strong dependence on density; however, the quality factor (Qxf) increases to 45,000 with the increase of sintering temperature. The increase in quality factor is attributed to the ordering of cations in the perovskite structure.     

Colloidal Filtration of Silicon Nitride Aqueous Slips,Part II: Slip Casting and Pressure Casting Performance

In a previous part, the rheological behaviour of silicon nitride aqueous slips was optimized by dispersing with TMAH up to pH 11·5 using different mixing procedures and including different concentrations of sintering aids (Al₂O₃ and Y₂O₃). In this part, the obtention of pressureless sintered silicon nitride bodies by colloidal filtration techniques is studied. The kinetics of the different compositions is studied for both slip casting and pressure casting. The pressure casting kinetics is up to 20 times faster than that of slip casting, which allows the scale-up of the process for a low cost production, The obtained green density is slightly >58%th for slip casting and 57–55%th for pressure casting, depending on the applied pressure. This small difference does not influence sintered density. At 1750°C/2h, a final density around 96%th is obtained. The sintering conditions are studies considering the time, temperature, atmosphere and sintering bed. The best results are obtained when the sintering bed has the same composition to that of the sample to be sintered. The room temperature properties of the sintered materials show a K_IC value higher than 6 MPam^1/2, comparable to those found in the literature for pressure sintered materials.     

Influence of sintering temperature on energy storage properties of BaTiO3–(Sr1−1.5xBix) TiO3 ceramics

The effect of sintering temperature on microstructure, dielectric properties and energy storage properties of BaTiO₃–(Sr_1?1.5xBi_x)TiO₃ (x = 0.09) (BT–SBT) ceramics was investigated. The sintering temperature has pronounced influence on the grain size, shrinkage, and dielectric properties of the BT–SBT ceramics. With increasing sintering temperature, the dielectric constant increases largely. However, the increasing tendency of the dielectric breakdown strength (BDS) is less noticeable but become more evident with the consideration of Weibull modulus. For the BT-SBT ceramics, the unreleased energy density decreases and the electric field stability of the energy storage efficiency enhances with the increase of sintering temperature.     

Enhanced dielectric and piezoelectric properties in Li/Sb-modified (Na,K)NbO3 ceramics by optimizing sintering temperature

In this paper, lead-free (Na_0.474K_0.474Li_0.052)(Nb_0.948Sb_0.052)O₃ ceramics were synthesized by a conventional solid-state reaction route. The effects of sintering temperature on the crystal structure, microstructure, densification, dielectric properties, and ferroelectric properties of the KNNLS ceramics were addressed. X-ray diffraction patterns and Raman spectrum indicated a transition from orthorhombic to tetragonal phase during the sintering temperature region. This transition is attributed to the migration of Li between the matrix grain and grain boundary. Scanning electron microscopy study revealed increased grain size and enhanced densification with increasing sintering temperature. The density of the ceramics sintered at 1080 °C reached a maximum value of 4.22 g/cm³. KNNLS ceramics sintered at an optimum temperature of 1080 °C exhibited high piezoelectric properties, that is 242 pC/N for d₃₃, 0.42 for k_p and 18.2 μC/cm² for P_r.     

Low temperature firing of BiSbO4 microwave dielectric ceramic with B2O3–CuO addition

The influence of B₂O₃–CuO addition on the sintering behavior, phase composition, microstructure and microwave dielectric properties of BiSbO₄ ceramic have been investigated. The BiSbO₄ ceramics can be well densified to approach above 95% theoretical density in the sintering temperature range from 840 to 960 °C as the addition amount of B₂O₃–CuO increases from 0.6 to 1.2 wt.%. Sintered ceramic samples were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The microwave permittivity ?_r saturated at 19–20 and Qf values varied between 33,000 and 46,000 GHz while temperature coefficient of resonant frequency shifting between ?70 and ?60 ppm/°C at sintering temperature around 930 °C. Lowering sintering temperature of BiSbO₄ ceramics makes it possible for application in low temperature co-fired ceramic technology.     

Electromechanical properties of Ba(Zr0.2Ti0.8)O3 ceramics prepared by spark plasma sintering

Ba(Zr_0.2Ti_0.8)O₃ (BZT20) ceramics were prepared by spark plasma sintering (SPS) and conventional sintering. The dynamic field-induced displacement and small-signal remnant piezoelectric constant measured by a resonant–antiresonant frequency method were evaluated. By normal sintering, the density, grain size, and dielectric constant of the ceramics increased with sintering temperature. The BZT20 ceramics prepared by SPS were characterized by linear field-induced strain. In response to the application of post-annealing at 1300 °C, BZT20 ceramics exhibited linear strain loop and high field-induced strain corresponding to dynamic strain/field d₃₃ at 20 kV/cm of 290 pm/V. The remnant piezoelectric properties of the BZT20 ceramics were found to largely depend on the preparation conditions, including the sintering temperature and annealing temperature. The BZT20 ceramics prepared by SPS and post-annealed at 1300 °C showed Q_m and k_p values of 325 and 25.1 (%), respectively.     

Microwave dielectric properties and sintering behaviors of scheelite compound CaMoO4

The microwave dielectric properties, sintering behaviors of scheelite compound CaMoO₄ were investigated using dilatometry, X-ray diffraction, scanning electron microscopy and network analyzer. To improve the sintering property of CaMoO₄, samples with different Ca/Mo ratio were prepared. The bulk densities of CaMo_(x)O₄ (for x = 1.02, 1.05, 1.08) samples were higher than those of pure CaMoO₄ over all temperature range. The well-sintered CaMo_(x)O₄ (for x = 1.02) sample with ∼95.7% of the theoretical density shows Q × f value of 71,000 GHz and dielectric constant (ɛ_r) = 10.3. The effects of liquid phase formation on sintering process and dielectric properties were discussed. Sample with hot-press sintering was also prepared in order to investigate dielectric properties of fully dense CaMoO₄, which exhibited quality factor (Q × f), ∼55,000 GHz and dielectric constant (ɛ_r) = 11.7.     

Two-step sintering assisted consolidation of bulk titania nano-ceramics by spark plasma sintering

This work investigates the feasibility to the fabrication of high density of nano-grained titanium dioxide ceramics by two-step sintering process under spark plasma conditions. First step is carried out by fast-heating-rate sintering in order to obtain an initial high density and a second step is held at a lower temperature by isothermal sintering aiming to increase the density without obvious grain growth. Experiments are conducted to determine the appropriate temperatures for each step. The temperature range between 840 and 850 °C is effective for the first step sintering (T₁) due to its highest densification rate. The isothermal sintering is then carried out at 810–830 °C (T₂) for various hours in order to avoid the abnormal grain growth and improve the density at the same time. TiO₂ bulk ceramics with nano-sized grain were fabricated successfully by combination of spark plasma sintering (SPS) with the two step sintering method (TSS). Titanium dioxide nano-ceramics with >95% theoretical density could be obtained with T₁ and T₂ taken as 850 °C and 830 °C, respectively. The relative grain growth quotient D/D_o for the used starting powder was almost ～1. Further XRD investigations indicated no obvious anatase–rutile transition was found in the prepared nano-crystalline ceramics.     

Effects of sintering parameters and Nd doping on the microwave dielectric properties of Y2O3 ceramics

Y₂O₃ ceramics with good dielectric properties were prepared via co-precipitation reaction and subsequent sintering in a muffle furnace. The effects of Nd doping and sintering temperature on microwave dielectric properties were studied. With the increase in sintering temperature, the density, quality factor (Q×f), and dielectric constant (ε_r) values of pure Y₂O₃ ceramics increased to the maximum and then gradually decreased. The Y₂O₃ ceramics sintered at 1500 °C for 4 h showed optimal dielectric properties: ε_r=10.76, Q×f=82, 188 GHz, and τ_f=−54.4 ppm/°C. With the addition of Nd dopant, the Q×f values, ε_r, and τ_f of the Nd: Y₂O₃ ceramics apparently increased, but excessive amount degraded the quality factor. The Y₂O₃ ceramics with 2 at% Nd₂O₃ sintered at 1460 °C displayed good microwave dielectric properties: ε_r=10.4, Q×f=94, 149 GHz and τ_f=−46.2 ppm/°C.