Ignition and chemical mechanisms of volume combustion synthesis of titanium diboride

Reaction ignition and chemical mechanisms in volume combustion synthesis of TiB₂ via TiO₂–B₂O₃–Mg precursors were studied using in-situ differential thermal analysis, X-ray diffraction, scanning electron microscopy and thermochemical modeling. Mg–TiO₂ samples ignited at 607 °C through a sudden single step solid-solid reaction while Mg–B₂O₃ samples ignited at 810 °C after melting of magnesium. X-ray diffraction analysis revealed that reduction of TiO₂ occurs in multiple steps and forms intermediate compounds. Results showed that heat released from the first reaction between TiO₂ and Mg ignites the reactions between Mg, Ti and B₂O₃ resulting in the formation of TiB₂. Samples with larger TiO₂ particle size or a higher sample surface to volume ratio showed a two-step reaction behavior and the released heat in the first solid state reaction was insufficient for the propagation of the reaction throughout the sample. In addition, Mg₃B₂O₆ undesired by-product was formed as a result of this two-step reaction.     

Sintering evaluation of doped lanthanum gallate based on thermodilatometry

The sintering behavior of La_0.9Sr_0.1Ga_0.8Mg_0.2O_3-δ oxide-ion conductor was systematically investigated by thermodilatometry. The shrinkage data obtained with heating rates of 4, 7, 10 and 12?°C?min^?1 were analyzed by the constant rate of heating model and by construction of the master sintering curve. Validation of the master sintering curve was carried out by measurements of density in conventionally sintered specimens. Slight anisotropy of shrinkage data was found and changes to the basic equation of density was proposed to account for this effect. Plotting the data determined by the constant rate of heating model versus density allowed an easy identification of the density range of constant activation energy. The activation energy (865?kJ?mol^?1) obtained from the master sintering curve correlates quite well with that (874?kJ?mol^?1) obtained by the constant rate of heating model.     

Enhanced mechanical and sintering properties of MgO-TiO2 ceramic composite via digital light processing

In this work, MgO was applied to modify the titania ceramic slurry, which could realize the high-quality DLP printing of titania ceramic by promoting the grain growth during sintering. Combining the phase and element analysis, it was revealed that the reduced stress concentration and improved mechanical property were attributed to the formation of MgTi₂O₅ in solid-state reaction between MgO and TiO₂. When MgO content increased beyond 10 wt.%, the microstructure pinning effects showed a negative impact on ceramic grain growth. Among all the samples, 5%MgO/TiO₂ has exhibited the best bending strength of 71.9 MPa and the densification of 85%, while its sintering temperature reduced by 200 °C. Meanwhile, the compressive property of representing porous TiO₂ samples reached 18.2 MPa, which was similar to those of porous ceramics produced by conventional manufacturing routes. Overall, MgO-TiO₂ composite ceramic prepared in this study have potential application in field of monolithic catalysts and tissue engineering scaffold.     

Sintering behavior,phase evolution and microwave dielectric properties of thermally stable Li2O-3MgO-mTiO2 ceramics (1≤m≤6)

Li₂O-3MgO-mTiO₂ (1≤m≤6) ceramics were prepared by the solid state reaction method. X-ray diffraction, energy dispersive spectrometer and scanning electron microscopy techniques were used to investigate the phase composition and crystal structure. With increasing m values, the phase structures of ceramics changed as: (Li₂Mg₃TiO₆, m=1)→(Li₂Mg₃Ti₄O₁₂ and Mg₂TiO₄, m=2,3)→(Li₂Mg₃Ti₄O₁₂, m=4)→(Li₂Mg₃Ti₄O₁₂, MgTiO₃ and Li₂MgTi₃O₈, m=5)→(Li₂Mg₃Ti₄O₁₂, MgTiO₃, Li₂MgTi₃O₈ and MgTi₂O₅, m=6). The optimized sintering temperature was lowered from 1275 °C to 1050 °C. When m=5, Li₂O-3MgO-5TiO₂ ceramics showed good microwave dielectric properties at a wide sintering temperature range of 1000–1200 °C, and the best microwave dielectric properties of Q×f=71,726 GHz, ε_r=21.9 and τ_f=−20.9 ppm/°C were obtained at a relatively low sintering temperature of 1050 °C.     

Sintering of Al2O3-SiC composite from sol-gel method with MgO, TiO2 and Y2O3 addition

Al₂O₃-SiC composite ceramics were prepared by pressureless sintering with and without the addition of MgO, TiO₂ and Y₂O₃ as sintering aids. The effects of these compositional variables on final density and hardness were investigated. In the present article at first α-Al₂O₃ and β-SiC nano powders have been synthesized by sol-gel method separately by using AlCl₃, TEOS and saccharose as precursors. Pressureless sintering was carried out in nitrogen atmosphere at 1600 °C and 1630 °C. The addition of 5 vol.% SiC to Al₂O₃ hindered densification. In contrast, the addition of nano MgO and nano TiO₂ to Al₂O₃-5 vol.% SiC composites improved densification but Y₂O₃ did not have positive effect on sintering. Maximum density (97%) was achieved at 1630 °C. Vickers hardness was 17.7 GPa after sintering at 1630 °C. SEM revealed that the SiC particles were well distributed throughout the composite microstructures. The precursors and the resultant powders were characterized by XRD, STA and SEM.     

Low-temperature synthesis of superfine barium strontium titanate powder by the citrate method

Ba_0.6Sr_0.4TiO₃ powder was synthesized by a citrate method. The phase development was examined with respect to calcining temperature and heating rate during the calcining process. The results reveal a crucial role of the heating rate to the formation of a pure perovskite phase at low calcining temperatures. It was found that keeping relatively low heating rates ≤0.7 °C/min during the calcining process after 300 °C was favorable to a sufficient decomposition of (Ba,Sr)₂Ti₂O₅·CO₃ intermediate phase at low temperatures and consequently led to the formation of a pure perovskite phase at 550 °C. Ba_0.6Sr_0.4TiO₃ powder calcined at the temperature under the heating rate of 0.7 °C/min showed a superfine and uniform particle morphology and high sintering reactivity. As a result, the ceramic specimens prepared from the powder attained reasonable relative densities (94–95%) at sintering temperatures of 1250–1270 °C.     

Bioactive glass-ceramics prepared by powder sintering and crystallization of polyphosphate glass containing strontium

Melt-quenching method was employed for obtaining a glass-ceramic with the following composition 42P₂O₅·40CaO·5SrO·10Na₂O·3TiO₂ (mol%) glass. The crystallization and sintering behavior of glass have been studied by using DTA, HSM, XRD, FTIR and SEM methods. It was determined that the surface and volume crystallization mechanisms act simultaneously in bulk glass samples. The comparison of DTA and HSM data revealed that the sintering and crystallization processes are independent. The sintered calcium phosphate glass-ceramic which contained bioactive β-Ca₃(PO₄)₂ and β-Ca₂P₂O₇ phases was successfully prepared. It was determined that during crystallization the primary phase in the precipitate was β-Ca(PO₃)_2. Other phases appearing in the resulting glass-ceramic were: α-Ca₂P₂O₇, γ-Ca₂P₂O₇, Ca₄P₆O₁₉ and CaHPO₄(H₂O)₂. Crystalline phases containing Sr and Ti were not detected. SEM analysis of the glass-ceramic microstructure revealed surface crystallization of glass particles and plate-like morphology of crystal growth. The result of the in vitro bioactivity showed that no apatite layer was formed on the surface of the as-prepared glass-ceramic samples after immersion in the simulated body fluid (SBF).     

Production of TiB2 by SHS and HCl leaching at different temperatures: Characterization and investigation of sintering behavior by SPS

Sub-micron sized TiB₂ ceramic powders were prepared via self-propagating high-temperature synthesis (SHS) followed by HCl leaching at different temperatures. Purified powders obtained using optimum process parameters were consolidated by field assisted sintering technology/spark plasma sintering (FAST/SPS) technique. Phase and microstructural analyses of both the powder and sintered samples were carried out by X-ray diffractometer (XRD) and scanning electron microscope (SEM). The chemical analyses and particle size measurements of the specimen were conducted by inductively coupled plasma-mass spectrometry (ICP-MS) and dynamic light scattering (DLS) techniques. The final properties of the sintered sample were determined in terms of density and microhardness. The effects of different HCl leaching temperatures on the formation, microstructure, particle size, purity and sintering behavior of the SHS-produced TiB₂ powders were investigated. The SHS reaction of TiO₂-B₂O₃-Mg powders as a starting mixture yielded MgO, Mg₃(BO₃)₂ and Mg beside the desired phase TiB₂. All three magnesium containing by-products were completely removed by performing hot HCl leaching. TiB₂ powders after SHS reaction and leaching with 9.3 M HCl for 30 min at 80 °C revealed a minimum purity of 98.4% and a homogenous particle size distribution with an average particle size of 536 nm. In the ultimate SPS experiment which was conducted at 1500 °C for 5 min under a pressure of 50 MPa, a relative density of 94.9% and a micro-hardness value of 24.56 GPa were achieved.     

Sintering behavior of Cr2O3–Al2O3 ceramics

We investigated the sintering behavior of Cr₂O₃–Al₂O₃ ceramic materials. In our observation of the isothermal shrinkage behavior of Cr₂O₃–Al₂O₃ ceramic, the activation energy of sintering reaction was measured to be 102 kJ/mol, that is, the near value of the activation energy of diffusion of Al ions in Al₂O₃ single crystal. Therefore the diffusion of cations is believed to control the sintering behavior of this material. With the addition of TiO₂, (the compound chosen to accelerate the diffusion of cations) to Cr₂O₃–Al₂O₃, the sintering behavior was accelerated.     

Effect of TiO2 addition on thermal and mechanical properties of Y-Si-Al-O-N glasses

Y-Si-Al-O-N glasses are intergranular phases in silicon nitride based ceramics in which the composition and volume fraction of oxynitride glass phases determine the sintering/shrinkage behaviour. Several investigations on oxynitride glass formation and properties have shown that addition of nitrogen increases glass transition and softening temperatures, viscosity, elastic modulus and hardness. In the present study, effect of TiO₂ addition on thermal and mechanical properties of Y-Si-Al-O-N glasses is investigated since the most typical Si₃N₄ ceramics for bearing applications are fabricated using a Si₃N₄-Y₂O₃-Al₂O₃-TiO₂-AlN system. Addition of TiO₂ is effective in preparing Y-Si-Al-O-N glasses with lower glass transition temperatures and with higher hardness.     

The influence of sintering aids for Nd(Mg0.5Ti0.5)O3 microwave dielectric ceramics properties

The influence of various sintering aids on the microwave dielectric properties and the structure of Nd(Mg_0.5Ti_0.5)O₃ ceramics were investigated systematically. B₂O₃, Bi₂O₃, and V₂O₅ were selected as liquid-phase sintering aids to lower the sintering temperature. The sintered Nd(Mg_0.5Ti_0.5)O₃ ceramics are characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), and microwave dielectric properties. The sintering temperature of Nd(Mg_0.5Ti_0.5)O₃ microwave dielectric ceramics is generally high, about 1500 °C. However, the sintering temperature was significantly lowered about 175 °C from 1500 °C to 1325 °C by incorporating in 10 mol% B₂O₃ and revealed the optimum microwave dielectric properties of dielectric constant (_r) value of 26.2, a quality factor (Q × f) value of 61,307 (at 9.63 GHz), and τ_f value of −45.5 ppm/°C. NdVO₄ secondary phase was observed at 10 mol% V₂O₅ addition in the sintering temperature range of 1300–1325 °C, which led the degradation in microwave dielectric properties. The microwave dielectric properties as well as grain sizes, grain morphology, and bulk density were greatly dependent on sintering temperature and various sintering aids. In this study, it is found that Nd(Mg_0.5Ti_0.5)O₃ incorporated with 10 mol% B₂O₃ with lower sintering temperature and excellent dielectric microwave properties may be suggested for application in microwave communication devices. The use of liquid-phase sintering, the liquid formed during firing normally remains as a grain boundary phase on cooling. This grain boundary phase can cause a deterioration of the microwave properties. Therefore, the selection of a suitable sintering aid is extremely important.     

A novel temperature-stable Ba2-xCaxMgTi5O13 microwave dielectric ceramic

The Ba_2-xCa_xMgTi₅O₁₃ (0 ≤ x ≤ 0.3) microwave dielectric ceramics were for the first time prepared via a conventional solid-state reaction method. A small amount of Ca²⁺ can dissolve into the lattice by forming solid solutions with a monoclinic structure (C2/m) and further influence the sintering behavior, grain growth and microwave dielectric properties of Ba_2-xCa_xMgTi₅O₁₃ ceramics. Both increase of ε_r and decrease of Qxf with x should be associated with increased lattice distortion and uneven grain growth although the sample density and the ratio of the ionic polarizability to the molar volume show little variation. Moreover, the A-site bond valence and τ_f indicate a close relation in current study, such that the Ca²⁺substitution can induce an increase of τ_f values. The optimum microwave dielectric properties of ε_r ∼ 29.3, Qxf ∼ 30,870 GHz (6.5 GHz), and a near-zero τ_f ∼ +2.1 ppm/°C can be contained in the x = 0.15 ceramic sintered at 1160 °C.     

Effect of temperature and heating rate on the sintering performance of SiC-Al2O3-Dy2O3 and SiC-Al2O3-Yb2O3 systems

Samples of SiC+10 vol%(Al₂O₃+Dy₂O₃) and SiC+10 vol%(Al₂O₃+Yb₂O₃) mixtures were obtained by cold isostatic pressing and sintered for one hour in a dilatometer at 1800 °C and 1900 °C, applying heating rates of 10, 20 and 30 °C/min. The results of the complete sintering cycle indicated that the heating rates do not significantly influence the shrinkage, but that temperature and total sintering time may be relevant factors. The compacts sintered at 1900 °C shrank on average 9% more than those sintered at 1800 °C, and it was found that the sintering time can be reduced by 40–50% at faster heating rates. The maximum shrinkage rates occurred at temperatures lower than those of the sintering thresholds for the two mixtures, two temperatures and three heating rates. It was also found that after formation of the liquid, the mechanisms of particle rearrangement and solution-precipitation were not as fast as reported in the literature, even at high heating rates, for example 30 °C/min, but they are responsible for much of the shrinkage occurring throughout the sintering cycle.     

Role of different rare earth oxides on the reaction sintering of magnesium aluminate spinel

Role of three rare earth oxides, viz., La₂O₃, CeO₂ and Yb₂O₃ on reaction sintering of magnesium aluminate spinel having molar ratio of MgO:Al₂O₃?=?1:2 from its solid oxide precursors was investigated in static and dynamic heating conditions. Effect of these additives (3?wt%) on densification behavior, phase assemblage and microstructure development were studied in the temperatures of 1500–1700?°C. Yb₂O₃ enhanced the sintering of spinel, while La₂O₃ and CeO₂ negatively impacted the sintering of magnesium aluminate spinel which can be discerned from the shrinkage curve of TMA as well as from static firing regime. This is ascribed to the formation of secondary phases in La₂O₃ and CeO₂ containing samples which have different crystalline structures to that of spinel. This anisotropy due to different crystallinity hindered the pore shrinkage and pore removal and thereby retarded the densification. Whereas, the cubic structure of the secondary phase formed in Yb₂O₃ containing sample which is isotropic with the crystalline orientation of the parental spinel phase assisted the densification.     

On the spinel formation in Co1 − xO/Co2TiO4 composites via reactive sintering, exsolution and oxidation

The formation mechanism and microstructural development of the spinel phases in the Co_1 − xO/Co₂TiO₄ composites upon reactive sintering the Co_1 − xO and TiO₂ powders (9:1 molar ratio) at 1450 °C and during subsequent cooling in air were studied by X-ray diffraction and analytical electron microscopy. The Co₂TiO₄ spinel occurred as inter- and intragranular particles in the matrix of Ti-doped Co_1 − xO grains with a rock salt-type structure during reactive sintering. The submicron sized Co₂TiO₄ particles were able to detach from grain boundaries in order to reach an energetically favorable parallel orientation with respect to the host Co_1 − xO grains via a Brownian-type rotation/coalescence process. Upon cooling in air, secondary Co₂TiO₄ nanoparticles were precipitated and the Ti-doped Co_1 − xO host was partially oxidized as Co_3 − δO₄ spinel by rapid diffusion along the {1 1 1} and {1 0 0}-decorated interphase interface and the free surface of the composites.     

Sintering mechanisms of Yttria with different additives

The sintering behaviour of conventional yttria powder was investigated, with emphasis on the effect of sintering additives such as B₂O₃, YF₃, Al₂O₃, ZrO₂, and TiO₂, etc. at sintering temperatures from 1000 °C to 1600 °C. Powder shrinkage behaviour was analysed using a dilatometer. The powder sintering mechanisms were identified at different temperatures using powder isothermal shrinkage curves. This analysis showed that the sintering additives B₂O₃ and YF₃ could improve yttria sintering by changing the diffusion/sintering mechanisms at certain temperatures, while sintering additives TiO₂, Al₂O₃ and ZrO₂ appeared to retard the powder densification at temperatures around 1000 °C and are more suitable when used at temperatures in excess of 1300 °C. The powder with La₂O₃ added had the slowest densification rate throughout the test temperatures in this experiment and was also found to be more suitable when used at temperatures higher than 1550 °C.     

Fabrication and microstructure of porous SiC ceramics with Al2O3 and CeO2 as sintering additives

In the present study, porous silicon carbide ceramics were prepared via spark plasma sintering at relatively low temperatures using Al₂O₃ and CeO₂ as sintering additives. Sacrificial template was selected as the pore forming mechanism, and gelcasting was used to fix the slurry in a short time. The evolution process of the microstructures during different steps was observed by SEM. The influence of the sintering temperature and sintering additives on the shrinkage and porosity of the samples was studied. The microstructures of different samples were characterized, and the mechanical properties were also evaluated.     

Dielectric behaviour of (Mg0·95Ca0·05)TiO3 for application in terahertz resonator

Abstract

The 0·15 THz resonator based on the (Mg_0·95Ca _0·05)TiO₃ (abbreviated as 95MCT hereafter) ceramic was designed, and the dielectric property of 95MCT for application has been studied. La₂O₃ and Nb₂O₅ were selected as liquid sintering aids to lower the sintering temperature. X-ray diffraction patterns indicated that MgTi₂O₅ secondary phase could be effectively suppressed by La₂O₃ and Nb₂O₅ additions. When the Nb₂O₅+La₂O₃ codoping content was 0·25 wt-%, the ceramic could be densified at 1320°C and also has good dielectric behaviours of Q_f?=?69720 GHz (6·8 GHz), ?_r?=?20·18 and τ_f?=??7·56 ppm °C^?1. The terahertz resonator designed at 0·15 THz exhibited that with the increasing height of inner cylinder, the two modes’ resonance frequencies decreased.     

Effect of Bi2O3 additives on sintering and microwave dielectric behavior of La(Mg0.5Ti0.5)O3 ceramics

Bi₂O₃ was selected as liquid phase sintering aid to lower the sintering temperature of La(Mg_0.5Ti_0.5)O₃ ceramics. The sintering temperature of La(Mg_0.5Ti_0.5)O₃ ceramics is generally high, about 1600 °C. However, the sintering temperature was significantly lowered about 275 °C from 1600 °C to 1325 °C by incorporating in 15 mol% Bi₂O₃ and revealed the optimum microwave dielectric properties of dielectric constant (?_r) value of 40.1, a quality factor (Q × f) value of 60,231 GHz, and the temperature coefficient (τ_f) value of 70.1 ppm/°C. During all addition ranges, the relative dielectric constants (?_r) were different and ranged from 32.0 to 41.9, the quality factors (Q × f) were distributed in the range of 928–60,231 GHz, and the temperature coefficient (τ_f) varies from 0.3 ppm/°C to 70.3 ppm/°C. Noticeably, a nearly zero τ_f can be found for doping 5 mol% Bi₂O₃ sintering at 1325 °C. It implies that nearly zero τ_f can be achieved by appropriately adjusting the amount of Bi₂O₃ additions and sintering temperature for La(Mg_0.5Ti_0.5)O₃ ceramics.