Preparation and Dielectric Properties of Low-Temperature-Sinterable (Zr0.8Sn0.2)TiO4 Powder

Low-temperature-sinterable (Zr_0.8Sn_0.2)TiO₄ powders were prepared using 3 mol% Zn(NO₃)₂ additive and then compared with powders prepared using 3 mol% ZnO additive and no additives. Sintering at 1200°C for 2 h produced a dielectric ceramic with ρ= 98.6% of theoretical density (TD), ɛ_r= 38.4, Q × f (GHz) = 42000, and τ _f =−1 ppm/°C. Sintering at 1250°C resulted in an excellent dielectric with ρ= 99% of TD, epsilon_r= 40.9, Q × f (GHz) = 49000, and τ _f =−2 ppm/°C. Scanning electron microscopy showed a microstructure with grains measuring 0.5 to 1 μm, and transmission electron microscopy revealed secondary phase in the grain boundaries.     

Synthesis, Characterization, and Consolidation of Si3N4 Obtained from Ammonolysis of SiCl4

Thermal decomposition of silicon diimide, Si(NH)₂, in vacuum resulted in very-high-purity, fine-particle-size, amorphous Si₃N₄ powders. The amorphous powder was isothermally aged at 50° to 100° intervals from 1000° to 1500°C for phase identification. Examination of ir spectra and X-ray diffraction patterns indicated a slow and gradual transition from an amorphous material to a crystalline α-phase occurring at 1200°C for >4 h and/or 1300° to 1400°C for 2 h. As the temperature was increased to ≥1450°C for 2 h, the crystalline β-phase was observed. Phase nucleation and crystallite morphology in this system were studied by electron microscopy and electron diffraction combined with TG as functions of temperature for the inorganic polymer starting materials. Powders prepared in this manner with 4 wt% Mg₃N₂ added as a sintering aid were hot-pressed to high-density fine-grained bodies with uniform microstructures. The optimum hot-pressing condition was 1650°C for 1 h. Silicon concentration steadily increased as the hot-pressing temperature or time was increased. A method for chemical etching for high-density fine-grained Si₃N₄ is described. Electrical measurements between room temperature and ∼500°C indicated dielectric constant and tan δ values of 8.3±0.03 and 0.65±0.05×10⁻², respectively.     

Molten Salt Synthesis of Magnesium Aluminate (MgAl2O4) Spinel on Ti3AlC2 Substrate

A MgAl₂O₄ (MA) spinel layer was synthesized on Ti₃AlC₂ substrate through the molten salt synthesis (MSS) method. The Ti₃AlC₂ substrate was immersed in MgCl₂·6H₂O powders and treated at 800°, 850°, and 900°C for 4 h in air. A continuous and 10-μm-thick MgAl₂O₄ layer was obtained at 900°C, by which the surface hardness of Ti₃AlC₂ can be effectively improved. The combined scanning electron microscopy observations and crystal morphology simulation further revealed that the as-formed MgAl₂O₄ presents tetragonal bipyramids morphology with (400)-orientation.     

Synthesis and Properties of CaAl2O4-Coated AI2O3 Microcomposite Powders

Novel microcomposite powders, consisting of inert cores (αAL-Al₂O₃) surrounded by reactive cement-based coatings (CaAl₂O₄), were synthesized by a modified Pechini process. The evolution of the crystalline CaAl₂O₄ phase during calcination was studied using multiple analytical techniques, including DRIFTS,¹³C and ²⁷AlMAS FT-NMR, and XRD, for both pure CaAl₂O₄ and CaAl₂O₄-coated Al₂O₃ precursor powders. In both powders, decomposition proceeded via hydrocarbon chain scission and removal of ester groups at low temperatures ( T < 450°C), followed by the formation of inorganic carbonates at higher temperatures ( T > 450°C). These decomposition processes were accelerated by the underlying Al₂O₃ cores. Transmission electron microscopy (TEM) of the fully calcined powders showed that the inert αAL-Al₂O₃ particles were surrounded by relatively uniform CaAl₂O₄ coatings ranging in thickness from approximately 10 to 100 nm.     

Sol–Gel Preparation of BaTi4O9 and Ba2Ti9O20

BaTi₄O₉ and Ba₂Ti₉O₂₀ precursors were prepared via a sol–gel method, using ethylenediaminetetraacetic acid as a chelating agent. The sol–gel precursors were heated at 700°–1200°C in air, and X-ray diffractometry (XRD) was used to determine the phase transformations as a function of temperature. Single-phase BaTi₄O₉ could not be obtained, even after heating the precursors at 1200°C for 2 h, whereas single-phase Ba₂Ti₉O₂₀ (as determined via XRD) was obtained at 1200°C for 2 h. Details of the synthesis and characterization of the resultant products have been given.     

Ultra-Fine Ba2Ti9O20 Powders Synthesized by Inverse Microemulsion Processing and their Microwave Dielectric Properties

A double–inverse microemulsion (IME) process is used for synthesizing nano-sized Ba₂Ti₉O₂₀ powders. The crystallization of powders thus obtained and the microwave dielectric properties of the sintered materials were examined. The IME-derived powders are of nano-size (∼21.5 nm) and possess high activity. The BaTi₅O₁₁, intermediate phase resulted when the IME-derived powders were calcined at 800°C (4 h) in air. However, high-density Ba₂Ti₉O₂₀ materials with a pure triclinic phase (Hollandite like) can still be obtained by sintering such a BaTi₅O₁₁ dominated powders at 1250°C/4 h. The phase transformation kinetics for the IME-derived powders were markedly enhanced when air was replaced by O₂ during the calcinations and sintering processes. Both the calcination and densification temperatures were reduced by around 50°C compared with the process undertaken in air. The microwave dielectric properties of sintered materials increase with the density of the samples, resulting in a large dielectric constant ( K ≅39) and high-quality factor ( Q × f ≅28 000 GHz) for samples possessing a density higher than 95% theoretical density, regardless of the sintering atmosphere. Overfiring dissociates Ba₂Ti₉O₂₀ materials and results in a poor-quality factor.     

Single-Calcination Synthesis of Pyrochlore Free Pb(Mg1/3Nb2/3)O3 Powders Using Particle-Coating Method

Using two-step particle-coating method, pyrochlore-free Pb(Mg_1/3Nb_2/3)O₃ (PMN) powders have been successfully synthesized by a single calcination step at a relatively lower calcined temperature of 850°C. The XRD and EDS results confirmed that the Mg–citric acid polymeric complex coatings effectively prevent direct contact between PbO and Nb₂O₅ and thus avoid the formation of pyrochlore phase. The coated powders were calcined directly without the ball-milling procedure at 850°C. The pyrochlore-free PMN powders obtained showed uniform and even grain size. The results showed that this method is an attractive method for the synthesis of PMN-based composite powders.     

Preparation of Ca3Co4O9 and Improvement of its Thermoelectric Properties by Spark Plasma Sintering

The precursor powders of Ca₃Co₄O₉ were synthesized by a sol–gel method. The results of X-ray diffraction and thermogravimetric and differential thermal analyses patterns indicate that pure Ca₃Co₄O₉ powders could be obtained by calcining the precursor at 800°C for 2 h. High dense Ca₃Co₄O₉ ceramic samples (∼99% of theoretical density) were prepared by the spark plasma sintering (SPS) method. Compared with the conventional sintering (CS), the SPS samples exhibit much higher electrical conductivity and power factor which are respectively about 118 S/cm and 3.51 × 10⁻⁴ W·(m·K²)⁻¹. The SPS method is greatly effective for improving the thermoelectric properties of Ca₃Co₄O₉ oxide ceramics.     

Fabrication and Photoluminescence Characteristics of Eu3+-Doped Lu2O3 Transparent Ceramics

A novel co-precipitation process was adopted for the preparation of highly sinterable europium-doped lutetia powders using ammonium hydroxide (NH₃·H₂O) and ammonium hydrogen carbonate (NH₄HCO₃) as the mixed precipitant. The resultant powders calcined at 1000°C for 2 h showed good dispersity and excellent sinterability. Highly transparent polycrystalline lutetia ceramics with a relative density of ∼99.9% were fabricated by pressureless sintering in flowing H₂ atmosphere at 1850°C for 6 h without any additives. The average grain sizes of the transparent material were estimated to be 50–60 μm. Optical in-line transmittance in the visible wavelength region for Lu₂O₃ ceramics (1 mm in thickness) reached 80%. The luminescence and decay behavior of the obtained transparent plate and the corresponding nanophosphors were also investigated.     

Single-Phase MnFe2O4 Powders Obtained by the Polymerized Complex Method

The present investigation reports on a simple technique for the preparation of single-phase MnFe₂O₄ nanocrystalline powders using the polymerized complex method, starting from manganese carbonate and iron nitrate. A mixed aqueous solution with citric acid, ethylene glycol, Fe, and Mn ions was polymerized. The phases formation, the homogeneity, and the structure of the obtained powders have been investigated by thermogravimetry, X-ray diffraction (XRD), scanning and transmission electron microscopy, and Mössbauer spectroscopy measurements. The magnetic hysteresis loop behavior was measured at 5 K in a vibrating sample magnetometer. XRD results demonstrated that thermally induced crystallization of cubic MnFe₂O₄ from the Mn–Fe polymeric precursor occurred at temperatures as low as 400°C. Pure single-phase MnFe₂O₄ nanocrystallites without any impurity or amorphous phases were obtained when the precursor was treated at 600°C for 1 h.     

Preparation and Sintering Behavior of Fine-Grained A12O3-SiO2 Composite

A fine, uniform A1₂O₃-SiO₂ powder was prepared by heterocoagulation of narrow Al₂O₃ and SiO₂ powders. This composite powder was dispersed, compacted, and fired in air at 900° to 1580°C for 1 to 13 h. Full density was achieved at 1550°C with the formation of a mullite phase. Relative densities of 83% and 98% (0.3 μm grain size) were measured for samples sintered at 1200°C for 13 h and at 1400°C for 1 h, respectively.     

Equilibria of SiF4 with SiO2, Be2SiO4, and BeO in Molten Li2BeF4

Equilibrium partial pressures of SiF₄ were measured for the reactions 2SiO₂( c )+2BeF₂( d )⇋SiF₄( g )+Be₂SiO₄( c ) (log P _siF4(mm) = [8.790 - 7620/ T ] ±0.06(500°–640°C)) and Be₂SiO₄( c ) +2BeF₂( d )⇋SiF₄( g ) +4BeO( c )(log P _siF4(mm) = [9.530–9400/T] ±0.04 (700°–780°C)), wherein BeF₂ was present in solution with LiF as molten Li₂BeF₄. The solubility of SiF₄ was low (∼0.04 mol kg^-1 atm^-1) in the melt. The results for the first equilibrium were combined with available thermochemical data to calculate improved Δ H_f and Δ G_f values for phenacite (–497.57 ±2.2 and –470.22±2.2 kcal, respectively, at 298°K). The few measurements above 700°C for the second equilibrium are consistent with the temperature of the subsolidus decomposition of phenacite to BeO and SiO₂ and with the heat of this decomposition as determined by Holm and Kleppa. Below 700°C, the pressures of SiF₄ generated showed an increasing positive deviation from the expression given for the equilibrium involving Be₂SiO₄ and BeO. This deviation might have been caused by the formation of an unidentified phase below 700°C which replaced the BeO; it more likely resulted from a metastable equilibrium involving BeO and SiO₂.     

Spark Plasma Sintering of Nano-Sized TiN Prepared from TiO2 by Controlled Hydrolysis of TiCl4 and Ti(O-i-C3H7)4 Solution

Nano-sized TiO₂ powders were prepared by controlled hydrolysis of TiCl₄ and Ti(O-i-C₃H₇)₄ solutions and nitrided in flowing NH₃ gas at 700°–1000°C to form TiN. Nano-sized TiN was densified by spark plasma sintering at 1300°–1600°C to produce TiN ceramics with a relative density of 98% at 1600°C. The microstructure of the etched ceramic surface was observed by SEM, which revealed the formation of uniformly sized 1–2 μm grains in the TiCl₄-derived product and 10–20 μm in the Ti(O-i-C₃H₇)₄-derived TiN. The electric resisitivity and Vickers micro-hardness of the TiN ceramics was also measured.     

Solubility and Crystal Growth of ZnAl2,O4 and AI2O3 in Molten PbF2 Solutions

The solubility of ZnAl₂O₄ (gahnite spinel) and NZOI (corundum) in their respective molten PbF₂ solutions was determined by a sealed-tube quenching method between 900° and 1250°C. Differential thermal analysis was used below 900°C. The solubilities (crystal-constituent/ PbF₂ weight ratio) at 1200°C were 0.151 for A1₂O₃ and 0.108 for ZnAl₂O₄, whereas at 900°C they were 0.093 and 0.048, respectively. These results are compared with those of single-crystal growth experiments and the crystalline products are described.     

Factors Affecting the Electrical Conductivity of Donor-Doped Bi4Ti3O12 Piezoelectric Ceramics

High-temperature piezoelectric ceramics based on W⁶⁺-doped Bi₄Ti₃O₁₂ (W-BIT) were prepared by both the conventional mixing oxides and the chemical coprecipitation methods. Sintering was carried out between 800° and 1150°C in air. A rapid densification, >99% of the theoretical density (rho_th) at 900°C/2 h, took place in the chemically prepared W⁶⁺-doped Bi₄Ti₃O₁₂ ceramics, whereas conventionally prepared BIT-based materials achieved a lower maximum density, ∼94% of rho_th, at higher temperature (1050°C). The microstructure study revealed a platelike morphology in both materials. Platelike grains were larger in the conventionally prepared W-BIT-based materials. The sintering behavior could be related both to the agglomeration state of the calcined powders and to the enlargement of the platelets at high temperature. The W⁶⁺-doped BIT materials showed an electrical conductivity value 2-3 orders of magnitude lower than undoped samples. The electrical conductivity increased exponentially with the aspect ratio of the platelike grains. The addition of excess TiO₂ produced a further decrease of the electrical conductivity.     

Synthesis of Transparent Nd-doped HfO2-Y2O3 Ceramics Using HIP

A Nd-doped HfO₂-Y₂O₃ ceramic having excellent transmittance was synthesized by HIPing, using high-purity powders (>99.99 wt%) of Y₂O₃, Nd₂O₃, and HfO₂. The mixed powder compacts of these powders were sintered at 1650°C for 1 h under vacuum and HIPed at 1700°C for 3 h under 196 MPa of Ar. The specimen after HIPing consisted of uniform grains measuring about 30 μm and having pore-free structure. The optical transmittance of 1 at.% Nd-doped 2.6 mol% HfO₂-Y₂O₃ ceramics ranging between visible and infrared wavelength was almost equivalent or superior to that of a Nd:Y₂O₃ single crystal grown by the Verneuil method.     

Hot Isostatic Processing of Shocked Si3N4 Powder

To enhance the sinter ability of Si₃N₄, powders mixed with 0, 2, and 5 wt% Y₂O₃ were explosively shock-treated. Compacts of these powders were encapsulated in 96% silica glass containers and isostatically hot-pressed. The shocked Si₃N₄ with 5 wt% Y₂O₃ was pressed to a density of 3.09 g/cm³ (95.4% of theoretical) at 1400°C under 430 MPa for 3 h, whereas the unshocked material attained only 82.4% of theoretical density under the same hot isostatic pressing conditions.     

Zirconium Aluminum Carbides: New Precursors for Synthesizing ZrO2–Al2O3 Composites

ZrO₂–Al₂O₃ nanocrystalline powders have been synthesized by oxidizing ternary Zr₂Al₃C₄ powders. The simultaneous oxidation of Al and Zr in Zr₂Al₃C₄ results in homogeneous mixture of ZrO₂ and Al₂O₃ at nanoscale. Bulk nano- and submicro-composites were prepared by hot-pressing as-oxidized powders at 1100°–1500°C. The composition and microstructure evolution during sintering was investigated by XRD, Raman spectroscopy, SEM, and TEM. The crystallite size of ZrO₂ in the composites increased from 7.5 nm for as-oxidized powders to about 0.5 μm at 1500°C, while the tetragonal polymorph gradually converted to monolithic one with increasing crystallite size. The Al₂O₃ in the composites transformed from an amorphous phase in as oxidized powders to θ phase at 1100°C and α phase at higher temperatures. The hardness of the composite increased from 2.0 GPa at 1100°C to 13.5 GPa at 1400°C due to the increase of density.     

Equilibrium Cation Distribution in NiAl2O4, CuAl2O4, and ZnAl2O4 Spinels

Stoichiometric NiAl₂O₄, CuAl₂O₄, and ZnAl₂O₄ spinels were prepared and equilibrated at temperatures from 600° to 1400°C. The parameters u and x , denoting the oxygen position and fraction of divalent cations on tetrahedral sites, respectively, were determined from a detailed X-ray diffraction analysis. In NiAl₂O₄, x increased from 0.07 at 595° to 0.26 at 1391°C; in CuAl₂O₄, x decreased from 0.68 at 613° to 0.64 at 1195°C; and in ZnAl₂O₄, x decreased from 0.96 at 905° to 0.94 at 1197°C. The form of the temperature dependence of x could not be described using theoretically based equations advanced in the literature. A more general equation which allows for a non-distributional contribution to the configurational entropy was derived and observed to properly describe the temperature dependence; the results indicate that short-range order is of definite significance in these intermediate aluminate spinels.     

Fabrication and Microstructure Characterization of Ti3SiC2 Synthesized from Ti/Si/2TiC Powders Using the Pulse Discharge Sintering (PDS) Technique

Ti/Si/2TiC powders were prepared using a mixture method (M) and a mechanical alloying (MA) method to fabricate Ti₃SiC₂ at 1200°–1400°C using a pulse discharge sintering (PDS) technique. The results showed that the Ti₃SiC₂ samples with <5 wt% TiC could be rapidly synthesized from the M powders; however, the TiC content was always >18 wt% in the MA samples. Further sintering of the M powder showed that the purity of Ti₃SiC₂ could be improved to >97 wt% at 1250°–1300°C, which is ∼200°–300°C lower than that of sintered Ti/Si/C and Ti/SiC/C powders using the hot isostatic pressing (HIPing) technique. The microstructure of Ti₃SiC₂ also could be controlled using three types of powders, i.e., fine, coarse, or duplex-grained, within the sintering temperature range. In comparison with Ti/Si/C and Ti/SiC/C mixture powders, it has been suggested that high-purity Ti₃SiC₂ could be rapidly synthesized by sintering the Ti/Si/TiC powder mixture at relatively lower temperature using the PDS technique.