Influence of Pyridine–Borane on the Sintering Behavior and Properties of α-Silicon Carbide

In the present study, α-SiC powder is coated with pyridineborane (BH₃·C₅H₅N), a liquid molecular compound, which forms a boron carbonitride (BC_3.5N) layer by heat treatment at 1000°C under argon. The precipitation method leads to an improved chemical homogeneity in the compacted powder resulting in enhanced densification and significant reduction in grain growth during subsequent sintering at temperatures exceeding 2070°C. Thus, small average grain sizes of d ₅₀= 1.3 μm and a narrow grain size distribution ( d ₁₀= 0.6 μm, d ₉₀= 2.2 μm) are detected in the liquid-phase-processed sample sintered at 2200°C for 0.5 h in argon. Final densities of at least 98% of theoretical could be obtained by pressureless sintering at 2100°C. These results as well as the microstructural distribution of the sintering aids in the densified samples are discussed.     

Machining-Induced Surface Residual Stress Behavior in Al2O3–SiC Nanocomposites

The machining and subsequent annealing behavior of an Al₂O₃-SiC nanocomposite (A1₂O₃+ 5 vol% 0.2 μm SiC particles) was compared to that of single-phase A1₂O₃. The machining-induced residual line force was determined by measuring the extent of elastic bending in thin disk specimens, and the surface roughness was evaluated by profilometry. The results showed that, when the two materials were subjected to the same grinding conditions, they developed compressive residual stresses and surface roughness values of similar magnitude. The maximum thickness of the residual stress layers was estimated to be ∼ 10 μm for the A1₂O₃ and ∼12 μm for the nanocomposite. A direct linear correlation was observed between the residual force and the surface roughness for different machining treatments. Annealing of the machined samples produced complete relaxation of residual stresses in the single-phase Al₂O₃, whereas only partial stress relaxation occurred for the nanocomposite.     

Piezoelectric Multilayer Ceramic/Polymer Composite Transducer with 2–2 Connectivity

A multilayer piezoelectric ceramic/polymer composite with 2–2 connectivity was fabricated by thermoplastic green machining after co-extrusion. The multilayer ceramic body was composed of piezoelectrically active lead zirconate titanate (PZN)–lead zinc niobate (PZN)-lead zirconate titanate (PZT) layers and electrically conducting PZN–PZT/Ag layers. After co-extruding the thermoplastic body, which consisted of five piezoelectric layers interspersed with four conducting layers, it was computer numeric-controlled machined to create periodic channels within it. Following binder burnout and sintering, an 18 vol% array of 190 μm thin PZT slabs with a channel size of 880 μm was fabricated. The channels were filled with epoxy in order to fabricate a PZN–PZT/epoxy composite with 2–2 connectivity. The piezoelectric coefficient (effective d ₃₃) and hydrostatic figure of merit ( d _h× g _h) of the PZN–PZT/epoxy composite were 1200 pC/N and 20 130 × 10⁻¹⁵ m²/N, respectively. These excellent piezoelectric characteristics as well as the relatively simple fabrication procedure will contribute in widening the application range of the piezoelectric transducers.     

Preparation of Hydroxyapatite Fibers by Electrospinning Technique

Hydroxyapatite (Ca₁₀(PO₄)₆(OH)₂, HA) fibers were prepared by electrospinning a precursor mixture of Ca(NO₃)₂·4H₂O and (C₂H₅O)₃PO with a polymer additive, followed by a thermal treatment. The X-ray diffraction (XRD) analysis of the annealed composite fibers revealed that pure HA phase could be obtained by annealing at 600°C for 1 h. The scanning electron microscopy (SEM) analysis showed the surface of as-electrospun composite fibers with an average diameter of 50 μm was smooth due to the amorphous nature of the polymer. However, the surface of the calcined HA fibers was rough because of the complete removal of the polymer. The pure HA fibers obtained by electrospinning in this work were up to 10 mm in length and 10–30 μm in diameter and the hydroxyapatite grain size was ∼1 μm in the HA fibers.     

Low-Temperature Sintering of Lead Zinc Niobate–Lead Zirconate Titanate Ceramics Using Nano-Sized Powder Prepared by the Stirred Media Milling Process

Lead zinc niobate–lead zirconate titanate (PZN–PZT) nano-sized powders with a diameter of ∼35 nm were fabricated by a high-energy stirred media mill using 50 μm diameter zirconia beads as the milling media at a rotation speed of 4000 rpm for 1 h. The sintering temperature of PZN–PZT was greatly reduced, and a fully densified bulk body was obtained at only 750°C when stirred media milled nanopowder was used. The control of evaporation of lead oxide was very important to obtain high electrical properties due to the increased surface area of nano-sized powders. The ferroelectric hysteresis, piezoelectric d ₃₃ coefficient, and dielectric properties of sintered ceramics using nanopowder were measured and compared with the values obtained from a sintered specimen using conventional milled powders. Remanent polarization, d ₃₃ coefficient, and relative dielectric constant of 750°C sintered stirred media milled powders containing 2% of excess PbO and 1% of 4PbO–B₂O₃ liquid phase were 10.3 μC/cm², 277 pC/N, and 1310, respectively.     

High Strain, 〈001〉 Textured 0.675Pb(Mg1/3Nb2/3)O3–0.325PbTiO3 Ceramics: Templated Grain Growth and Piezoelectric Properties

Lead magnesium niobate–lead titanate, 0.675Pb(Mg_1/3Nb_2/3)O₃–0.325PbTiO₃ (PMN–32.5PT) ceramics were textured (grain-oriented) in the 〈001〉-crystallographic direction by the templated grain growth process. The textured PMN–32.5PT ceramics were produced by orienting {001}-SrTiO₃ (ST) platelets (∼10 μm in diameter and ∼2-μm thickness) in a submicron PMN–32.5PT matrix. The templated growth of 〈001〉-oriented PMN–32.5PT grains on the ST platelets resulted in textured ceramics with ∼70% Lotgering factor and >98% theoretical density. Unlike most lead-based ceramics, excess PbO was not needed for sintering or grain growth. Based on unipolar stain-field measurements at 0.2 Hz, the textured samples displayed >0.3% strain at 50 kV/cm. Low-field d ₃₃-coefficients of >1600 pC/N (<5 kV/cm) were measured directly from unipolar measurements. The low drive field d ₃₃-piezoelectric coefficient of the highly textured samples is two times greater than polycrystalline PMN–32.5PT.     

Thermal Expansion of Polycrystalline Ti3SiC2 in the 25°–1400°C Temperature Range

We measured the volume thermal expansion of Ti₃SiC₂ from 25° to 1400°C using high-temperature X-ray diffraction using a resistive heated cell. A piece of molybdenum foil with a 250 μm hole contained the sample material (Ti₃SiC₂+Pt). Thermal expansion of the polycrystalline sample was measured under a constant argon flow to prevent oxidation of Ti₃SiC₂ and the molybdenum heater. From the lattice parameters of platinum (internal standard), we calculated the temperature by using thermal expansion data published in the literature. The molar volume change of Ti₃SiC₂ as a function of temperature in °C is given by: V _M (cm³/mol)=43.20 (2)+9.0 (5) × 10⁻⁴ T +1.8(4) × 10⁻⁷ T ². The temperature variation of the volumetric thermal expansion coefficient is given by: α_v (°C⁻¹)=2.095 (1) × 10⁻⁵+7.700 (1) × 10⁻⁹ T . Furthermore, the results indicate that the thermal expansion anisotropy of Ti₃SiC₂ is quite mild in accordance with previous work.     

Controlled Oxygen Partial Pressure Sintering of (Pb,La)(Zr,Ti)O3 Ceramics

Transparent PLZT(7/60/40) ceramics with large piezoelectric coefficients were obtained using a two-step sintering process with controlled oxygen partial pressure. Specifically, low-oxygen-pressure and low-temperature sintering were used in the first step, followed by a high-oxygen-pressure, high-temperature sintering cycle. High-density ceramics with small grain sizes of about 3 µm were prepared. As a result, k _p= 0.71, k ₃₃= 0.78, d ₃₃= 850 × 10^-12 C/N, and a transparency of 15% (λ= 610 nm, thickness of 1 mm) have been achieved; 20% improvement of d ₃₃ was gained compared to conventional processed PLZT ceramics ( d ₃₃= 710 × 10^-12 C/N).     

Influence of Sintering Temperature on Grain Growth and Phase Structure of Compositionally Optimized High-Performance Li/Ta-Modified (Na,K)NbO3 Ceramics

Microstructure characteristics, phase transition, and electrical properties of (Na_0.535K_0.485)_0.926Li_0.074(Nb_0.942Ta_0.058)O₃ (NKN-LT) lead-free piezoelectric ceramics prepared by normal sintering are investigated with an emphasis on the influence of sintering temperature. Some abnormal coarse grains of 20–30 μm in diameter are formed in a matrix consisting of about 2 μm fine grains when the sintering temperature was relatively low (980°C). However, only normally grown grains were observed when the sintering temperature was increased to 1020°C. On the other hand, orthorhombic and tetragonal phases coexisted in the ceramics sintered at 980°–1000°C, whereas the tetragonal phase becomes dominant when sintered above 1020°C. For the ceramics sintered at 1000°C, the piezoelectric constant d ₃₃ is enhanced to 276 pC/N, which is a high value for the Li- and Ta-modified (Na,K)NbO₃ ceramics system. The other piezoelectric and ferroelectric properties are as follows: planar electromechanical coupling factor k _p=46.2%, thickness electromechanical coupling factor k _t=36%, mechanical quality factor Q _m=18, remnant polarization P _r=21.1 μC/cm², and coercive field E _c=1.85 kV/mm.     

Preparation of Undoped Lead Titanate Ceramics via Sol-Gel Processing

Crack-free, undoped PbTiO₃ ceramics were fabricated successfully using sol–gel-synthesized powder prepared from chelated titanium alkoxide and lead acetate. The sintered ceramics, 8.3 mm in diameter and 6–8 mm thick, were 96% dense. In the present study, PbTiO₃ ceramics with excess lead (Pb:Ti = 1.1:1.0) had large grains, averaging 14.3 μm. Lower-lead ceramics (Pb:Ti = 1.0:1.0 and 0.9:1.0) had smaller grains, averaging 1.8 μm. The PbTiO₃ ceramics with a high lead content cracked during sintering at 1150°C, whereas the other ceramics did not crack. Excess lead, in a more-than-stoichiometric ratio, promoted grain growth and caused disintegration of the ceramics. Therefore, uncracked PbTiO₃ ceramics apparently can be fabricated by avoiding excess lead, possibly because restricted grain growth in low-lead ceramics causes low residual stress over many small grains during transition. The electrical properties measured in the present study for PbTiO₃ ceramics with a Pb:Ti ratio of 1:1 are d ₃₃= 35 pC/N, K ₃= 64, k _p= 0.59, and k _t≈ 0.     

Thermal Expansion of Homogeneous and Gradient Solid Solutions in the System KZr2(PO4)3─KTi2(PO4)3

Low-thermal-expansion ceramics having arbitrary thermal expansion coefficients were synthesized from homogeneous solid solutions in the system KZr₂(PO₄)₃─KTi₂(PO₄)₃ (KZP–KTP). Dense and strong ceramics were fabricated by sintering at 1100° to 1200°C with 2 wt% MgO. The thermal expansion coefficient increased from 0 to +3 × 10⁻⁶/°C with increasing x in KZr_{2 − x}Ti_x (PO₄)₃ (KZTP). In addition, a functionally gradient material with respect to thermal expansion was prepared by forming a series of KZTP solid solutions in a single ceramic body. By heating a pile of KZP and KTP ceramics in contact with each other, KZP and KTP bonded together to form a KZTP gradient solid solution near the interface.     

Textured PMN–PT and PMN–PZT

A promising way to improve the performance of piezoelectric ceramics is grain orientation by templated grain growth. In this work lead-based piezoelectric ceramics Pb(Mg_1/3Nb_2/3)_0.68Ti_0.32O₃ (PMN–32PT) and Pb(Mg_1/3Nb_2/3)_0.42(Ti_0.638Zr_0.362)_0.58O₃ (PMN–37PT–21PZ) ceramics were textured via templated grain growth process. For texturization (001)-oriented BaTiO₃ (BT) platelets (approximately 10 μm × 10 μm × 2 μm) were utilized as templates. The texturized ceramics were accomplished by aligning the templates by tape casting. The template growth into the matrix resulted in textured ceramics with Lotgering factors between 0.94 and 0.99 for both compositions. Consequences of the texture are enhanced dielectric and piezoelectric properties. Unipolar strain-field measurements of textured ceramics showed 0.25% strain s ₃₃ at 3 kV/mm. Large signal d ₃₃^* of up to 878 pm/V were determined directly from strain measurements. Compared with randomly oriented ceramics in texturized samples unipolar strain s ₃₃ and large signal d ₃₃^* was enhanced by a factor of up to 1.8.     

Near-Net-Shape Fabrication of Ti3AlC2-Based Composites

A porous ceramic preform was fabricated by printing a powder blend of TiC, TiO₂, and dextrin. The presintered preforms contained a bimodal pore size distribution with intra-agglomerate pores ( d ₅₀≈0.7 μm) and inter-agglomerate pores ( d ₅₀≈30 μm), which were subsequently infiltrated by aluminum melt spontaneously in argon above 1050°C. A redox reaction at 1400°C resulted in the formation of dense Ti–Al–O–C composites mainly composed of Ti₃AlC₂, TiAl₃, Al, and Al₂O₃, which attained a bending strength of 320 MPa, a Young's modulus of 184 GPa, and a Vicker's hardness of 2.5 GPa.     

Fabrication of Transparent, Sintered Sc2O3 Ceramics

We report here the fabrication of transparent Sc₂O₃ ceramics via vacuum sintering. The starting Sc₂O₃ powders are pyrolyzed from a basic sulfate precursor (Sc(OH)_2.6(SO₄)_0.2·H₂O) precipitated from scandium sulfate solution with hexamethylenetetramine as the precipitant. Thermal decomposition behavior of the precursor is studied via differential thermal analysis/thermogravimetry, Fourier transform infrared spectroscopy, X-ray diffractometry, and elemental analysis. Sinterability of the Sc₂O₃ powders is studied via dilatometry. Microstructure evolution of the ceramic during sintering is investigated via field emission scanning electron microscopy. The best calcination temperature for the precursor is 1100°C, at which the resultant Sc₂O₃ powder is ultrafine (∼85 nm), well dispersed, and almost free from residual sulfur contamination. With this reactive powder, transparent Sc₂O₃ ceramics having an average grain size of ∼9 μm and showing a visible wavelength transmittance of ∼60–62% (∼76% of that of Sc₂O₃ single crystal) have been fabricated via vacuum sintering at a relatively low temperature of 1700°C for 4 h.     

Effect of Al(OH)3 Addition on β-LiAlO2 Crystal Growth

Crystal growth of rod-shaped β-LiAlO₂ was previously reported by us, and the rod-shaped β-LiAlO₂ crystals were 1.5 μ in diameter and 10 to 15 μm long. In the present study needle-shaped β-LiAlO₂ crystals which were thinner and had larger aspect ratios (length/diameter) than the rodshaped β-LiAlO₂ crystals were grown by using LiOH–Al₂O₃–Al(OH)₃–NaOH as the raw material. These crystals were 0.7 to 1 μm in diameter, 9 to 13 μm long, and had aspect ratios of about 10 to 13.     

Influence of Impurity Particles on the Fracture of UO2

Three types of second-phase precipitates were identified by scanning electron microscopy of helical specimens of nominally pure UO₂, i.e. (1) bonded or weakly bonded complex Si particles precipitated during sintering from impurities in the starting material, (2) more strongly bonded Al₂O₃-UO₂ surface particles that formed on the external surfaces during sintering, and (3) metallic Fe or U phases. The influence of these particles on the room-temperature fracture stress, σ _f , was investigated. Particles of types (1) and (2) were as large as 300 μm and caused up to 100% reduction in σ _f , whereas the metallic phases had no significant effect. The critical-strain-energy-release rate, G_c , determined from the measured flaw sizes and associated fracture stresses, was ∼3600 ergs/cm². Purification procedures that reduced the impurity particle sizes and number density improved the strength significantly.     

Use of Crack Branching Data for Measuring Near-Surface Residual Stresses in Tempered Glass

An analytical procedure based on fracture mechanics is used to obtain the amount of residual stress in glass from measurements on the fracture surface. The technique utilizes the measurement of microcrack branching distances, known as the mirror — mist boundary, which occur at a critical crack branching stress intensity (K _m ) value. This procedurre shows that σ _A r _m ^1/2 Y _F (θ) =σ _R r ^1/2 _m +Ψ₀, where σ _A is the applied stress, r _m is the microcrack branching radius, σ _R is the residual stress, Y _F ( θ ) is the crack-border correction factor, and Ψ₀ is a material constant based on K _m . Thus, the equation is that of a straight line with the slope equal to the magnitude of the residual stress. Data for tempered glass from the literature are used to demonstrate the applicability of the technique.     

Elimination of large Pores During Gas-Pressure Sintering of β'-Sialon

The effect of N₂-gas pressure on the liquid filling of large pores (20 to 120 μm in diameter) is studied in sintered β'-sialon ( z = 1). The initially sintered sialon with large pores is sintered again and infitrated by a liquid (41A1₂O₃-41Y₂O₃-14Si₃Y₄-4AIN (wt%)) at 17800°C for various times under 0.1-, 0.3-, and 0.5-MPa (1-, 3-, and 5-atm) N₂. The liquid fills large interconnected pores; the size of the pores filled with liquid increases with N₂-gas pressure and time. In some liquid pockets, gas bubbles are formed and subsequently disappear during prolonged sintering treatment. The liquid-filling be havior with sintering pressure and time is explained by gas pressure in the pore and thermal decomposition of the material. The benefit of gas-pressure sintering for the elimination of large pores is assessed.     

Flexible Ceramics in the System KZr2(PO4)3–KAlSi2O6 Prepared by Mimicking the Microstructure of Itacolumite

A ceramic composite mimicking the pervasively cracked microstructure of flexible sandstone (itacolumite) was successfully synthesized by sintering two ceramic materials with different thermal expansion coefficients. A combination of granular KZr₂(PO₄)₃ (high thermal expansion) and powdered KAlSi₂O₆ (low thermal expansion) resulted in a material with a jigsaw-like three-dimensional cracking microstructure similar to that of itacolumite. The synthesized composite was found to exhibit ductile deformation.     

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.