Novel Method to Prepare Electroconductive Titanium Nitride–Aluminum Oxide Nanocomposites

A novel method for the preparation of TiN–Al₂O₃nanocomposites was developed. TiN–Al₂O₃nanocomposite powders were prepared by the direct nitridation of TiO₂–Al₂O₃nanocomposite powders that were derived from the simultaneous hydrolysis of tetra-butyl titanate and precipitation of aluminum nitrate. Dense sintered bodies of these TiN–Al₂O₃nanocomposite powders were obtained by hot pressing at 1450°–1650°C and 30 MPa for 60 min. The resistivity of nanocomposite reaches a minimum (1.5 × 10⁻³Ω·cm) at 25 vol% TiN additions. The percolation concentration of nanocomposite is ∼10 vol% TiN.     

Electroconductive Al2O3–NbN Composites

Electroconductive Al₂O₃–NbN ceramic composites were prepared by hot pressing. Dense sintered bodies of ball-milled Al₂O₃–NbN composite powders were obtained at 1550°C and 30 MPa for 1 h under a nitrogen atmosphere. The bending strength and fracture toughness of the composites were enhanced by incorporating niobium nitride (NbN) particles into the Al₂O₃ matrix. The electrical resistivity of the composites decreased with increasing amount of NbN phase. For a 25 vol% NbN–Al₂O₃ composite, the values of bending strength, fracture toughness, Vickers hardness, and electrical resistivity were 444.2 MPa, 4.59 MPa·m^1/2, 16.62 GPa, and 1.72 × 10⁻²Ω·cm, respectively, making the composite suitable for electrical discharge machining.     

Hydrolysis Deposition of Thin Films of Antimony-Doped Tin Oxide

Thin films of antimony-doped tin oxide have been obtained by a new technique, the so-called hydrolysis deposition method, in which hydrolyzed solids are precipitated from metal fluoride solutions. Mixed solutions of SnF₃ and SbF₃ produce antimony- and fluorine-doped tin oxide films. The amount of antimony can be controlled in a wide range by adjusting the initial fluoride concentrations of the solution. The film containing 2.9 mol% antimony heated at 500°C has an electrical resistivity of 1.0 × 10⁻³Ω·cm, which is lower than previously obtained by wet-chemical techniques.     

Fabrication of TiN/Si3N4 Ceramics by Spark Plasma Sintering of Si3N4 Particles Coated with Nanosized TiN Prepared by Controlled Hydrolysis of Ti(O-i-C3H7)4

TiN-coated Si₃N₄ particles were prepared by depositing TiO₂ on the Si₃N₄ surfaces from Ti(O- i -C₃H₇)₄ solution, the TiO₂ being formed by controlled hydrolysis, then subsequently nitrided with NH₃ gas. A homogeneous TiO₂ coating was achieved by heating a Si₃N₄ suspension containing 1.0 vol% H₂O with the precursor at 40°C. Nitridation successfully produced Si₃N₄ particles coated with 10–20 nm TiN particles. Spark plasma sintering of these TiN/Si₃N₄ particles at 1600°C yielded composite ceramics with a relative density of 96% at 25 vol% TiN and an electrical resistivity of 10⁻³Ω·cm in compositions of 17.5 and 25 vol% TiN/Si₃N₄, making these ceramics suitable for electric discharge machining.     

Chemical Solution Deposition of Columnar-Grained Metallic Lanthanum Nitrate Thin Films

Columnar and (100)-oriented LaNiO₃ thin films were prepared on silicon substrates by a chemical solution deposition (CSD) process using a 0.05 M solution. By reducing the individual layer thickness to 10 nm, columnar LaNiO₃ films with a lateral grain size of ∼120 nm were obtained. The success of this approach required restricting the individual layer thickness to a value below the grain size observed for equiaxed films. This change in microstructure resulted in an improvement in conductivity. The columnar LaNiO₃ film with a thickness of 300 nm showed a resistivity of 4.5 × 10⁻⁵Ω·cm, which is lower by one order of magnitude than that of fine-grain equiaxed films that typically result from CSD methods.     

Properties of Silicon Nitride–Molybdenum Disilicide Particulate Ceramic Composites

The physical and mechanical properties of hot-pressed Si₃N₄–MoSi₂ particulate composites containing 15 and 30 vol% MoSi₂ were studied. The average room-temperature four-point bend strength, fracture toughness, and electrical resistivity are 522 MPa, 3.6 MPa·√m, and 6.3 × 10⁵Χ·cm for the 15 vol% MoSi₂ composite, and 487 MPa, 5.3 MPa·√m, and 0.31 Ω·cm for the 30 vol% MoSi₂ composite. The mechanical properties of the composites are very close to those of hot-pressed Si₃N₄ ceramics. The high electrical conductivity of the 30 vol% MoSi₂ composite was attributed to the percolation effect of MoSi₂ particles. Parabolic oxidation behaviors were observed for the 30 vol% MoSi₂ composite during the 1200°C long-term oxidation experiments.     

Development of Reaction-Bonded Electroconductive Silicon Nitride-Titanium Nitride and Resistive Silicon Nitride-Aluminum Oxide Composites

Reaction-bonded Si₃N₄· TiN and Si₃N₄· Al₂O₃ composites were successfully fabricated by heating mixed powder compacts of Si and TiN or Si and Al₂O₃ in a nitrogen atmosphere. The former showed electrical conductivity, owing to the presence of TiN. An electrical resistivity of 2.6 × 10⁻⁵Ω· m was obtained for the Si₃N₄· TiN composite with 70 vol% TiN. The composite with 20 vol% TiN showed an electrical resistivity of 0.22 Ω· m and a bending strength of 460 MPa. On the other hand, the Si₃N₄· Al₂O₃ composite had insulating properties. The use of an appropriate amount of resin binder resulted in a higher green density and, consequently, a higher bending strength. Moreover, electroconductive Si₃N₄· TiN/resistive Si₃N₄· Al₂O₃ complex ceramics could be fabricated by heating green compacts composed of two different portions, one composed of mixed powders of Si and TiN and the other of Si and Al₂O₃. Attainment of such complex ceramics was attributed to the small dimensional change at the nitriding stage, under 0.3% and the similarity of the thermal expansion coefficients of the two composites.     

Electrical and Pyroelectric Properties of Highly (001)-Oriented (Pb0.76Ca0.24)TiO3 Thin Films Grown by a Sol–Gel Process

Highly (001)-oriented (Pb_0.76Ca_0.24)TiO₃ (PCT) thin films were grown on Pt/Ti/SiO₂/Si substrates using a sol–gel process. The PCT film with a highly (001) orientation showed well-saturated hysteresis loops at an applied field of 800 kV/cm, with remanent polarization ( P _r) and coercive electric field ( E _c) values of 23.6 μC/cm² and 225 kV/cm, respectively. At 100 kHz, the dielectric constant and dielectric loss values of these films were 117 and 0.010, respectively. The leakage-current density of the PCT film was 6.15 × 10⁻⁸A/cm² at 5 V. The pyroelectric coefficient ( p ) of the PCT film was measured using a dynamic technique. At room temperature, the p values and figures-of-merit ( F _D) of the PCT film were 185 μC/m²K and 1.79 × 10⁻⁵ Pa^−0.5, respectively.     

Crystallization and Chemical Strengthening of Stuffed β-Quartz Glass-Ceramics

Metastable solid solutions with the β-quartz structure can be crystallized from most glasses in the system SiO₂-Mg(AlO₂)₂-LiAlO₂ as well as from many containing the additional components Zn(AlO₂)₂ Al(AlO₂)₃, Li₂ZnO₂, and Li₂BeO₂. Internal nucleation is afforded by additions of ZrO₂ or TiO₂. Either transparent or opaque crystalline materials can be formed from glasses containing about 70% SiO₂. The transparency is due to a combination of low birefringence in the major stuffed β-quartz phase and minute crystal size. Thermal expansions vary from -20 to +50 × 10⁻⁷/°C. Thermal stability is highly variable. Breakdown products include spinel, cordierite, β-spodumene, willemite, mullite, and cristobalite. Magnesian compositions can be strengthened by a 2Li⁺⇌ Mg²⁺ ionexchange reaction. Abraded flexural strengths range from 30,000 to 160,000 psi.     

Characterization of Material for Low-Temperature Sintered Multilayer Ceramic Substrates

The sintering temperature of multilayer ceramic substrates must decrease to 1000° or below to avoid melting the conductors (Pd-Ag, Au, or Cu) during sintering. In this study, SiO₂, CaO, B₂O₃, and MgO were used as additives to Al₂O₃ to decrease the firing temperature by liquid-phase sintering. Compositions with 18.0 and 22.5 wt% B₂O₃ were sintered at around 1000° in an air atmosphere to yield dense ceramics with good properties: relative dielectric contant between 6 to 7 (1 MHz), tan δ≤× 3 × 10⁻⁴ (1 MHz), insulating resistivity > 10¹⁴ω cm, coefficient of thermal expansion ∼ 7.0 × 10⁻⁶/°, and thermal conductivity ∼ 4.1 W/(m · K).     

Microwave Dielectric Loss of Titanium Oxide

The dielectric loss (tan δ) of titanium dioxide (TiO₂) disks has been measured at a frequency of 3 GHz. High-purity TiO₂ sintered to almost-full density exhibits a very high tan δ, which is interpreted to be due to oxygen deficiency. To counter this, doping with stable divalent and trivalent cations, such as Mg and Al, leads to a low tan δ, probably by preventing Ti⁴⁺ reduction. The tan δ of polycrystalline TiO₂ doped with divalent and trivalent ions with ionic radii in the range of 0.5–0.95 Å at 3 GHz can be very low: 6 × 10⁻⁵ ( Q ∼ 17 000) at a temperature of 300 K. The tan δ of undoped pure TiO₂ disks increases when the disks are cooled from 300 K to ∼100 K. At temperatures <100 K, the tan δ decreases rapidly, which is interpreted as carrier freeze-out. The tan δ for all the high- Q doped TiO₂ polycrystalline samples smoothly decrease to ∼5 × 10⁻⁶ ( Q ∼ 200 000) at 15 K, comparable to that of single crystals.     

Effects of TiO2 Seeding Layer on Crystalline Orientation and Ferroelectric Properties of Bi3.15Nd0.85Ti3O12 Thin Films Fabricated by a Sol–Gel Method

The effect of a 20-nm thick TiO₂ seeding layer on the growth of a Bi_3.15Nd_0.85Ti₃O₁₂ (BNT) thin film on Pt(111) thin-film substrates has been studied. Under otherwise identical deposition process conditions, the BNT film could be turned from a highly random orientation to a (200) preference orientation by adding the seeding layer. Field-emission scanning electron microscope result reveals that the BNT thin film with the TiO₂ seeding layer is composed of fine grains with smaller sizes about 80–150 nm in diameter. The P _r and E _c values of the BNT thin film and BNT film with the TiO₂ seeding layer were 36 and 16 μC/cm², and 96.9 and 92 kV/cm at a voltage of 12 V, respectively. The fatigue test exhibited a very strong fatigue endurance up to 10⁹ cycles for both films. The leakage current densities were generally in the order of 10⁻⁶–10⁻⁵ A/cm² for both samples.     

Synthesis and Characterization of Barium Lanthanum Titanates

Ternary compounds in the system BaO—TiO₂—La₂O₃ were prepared by the solid-state reaction technique at temperatures between 1300° and 1400°C using precursor oxides as the starting materials. In an alternative processing technique, BaTiO₃ was reacted with appropriate proportions of prefabricated lanthanum titanates at 1350°C to obtain the compounds. Two compounds were identified in the TiO₂-rich region of the system. The X-ray powder diffraction pattern of a compound with a chemical composition BaLa₂Ti₃O₁₀ (BaO·La₂O₃·3TiO₂) is indexed on the basis of an orthorhombic unit cell with a = 7.665 × 10⁻¹ nm, b = 28.524 × 10⁻¹ nm, and c = 3.876 × 10⁻¹ nm. The other compound, which has a chemical composition Ba₄La₈Ti₁₇O₅₀ (BaO·La₂O₃·4.25TiO₂) occurs in a narrow homogeneity range within the system. The X-ray powder diffraction pattern of the compound is indexed on the basis of an orthorhombic unit cell with a = 12.317 × 10⁻¹ nm, b = 22.394 × 10⁻¹ nm, and c = 3.881 × 10⁻¹ nm. Both the compounds are compatible with BaTiO₃ and form pseudobinary joins with BaTiO₃ in the system BaO—TiO₂—La₂O₃.     

Solid-State Diffusive Amorphization in TiO2/ZrO2 Bilayers

Thin films of crystalline TiO₂ were deposited on self-assembled organic monolayers from aqueous TiCl₄ solutions at 80°C; partially crystalline ZrO₂ films were deposited on top of the TiO₂ layers from Zr(SO₄)₂ solutions at 70°C. In the absence of a ZrO₂ film, the TiO₂ films had the anatase structure and underwent grain coarsening on annealing at temperatures up to 800°C; in the absence of a TiO₂ film, the ZrO₂ films crystallized to the tetragonal polymorph at 500°C. However, the TiO₂ and ZrO₂ bilayers underwent solid-state diffusive amorphization at 500°C, and ZrTiO₄ crystallization could be observed only at temperatures of 550°C or higher. This result implies that metastable amorphous ZrTiO₄ is energetically favorable compared to two-phase mixtures of crystalline TiO₂ and ZrO₂, but that crystallization of ZrTiO₄ involves a high activation barrier.     

Preliminary Observations on Phase Relations in the "V2O3–FeO" and V2O3–TiO2 Systems from 1400°C to 1600°C in Reducing Atmospheres

Phase relations within the "V₂O₃–FeO" and V₂O₃–TiO₂ oxide systems were determined using the quench technique. Experimental conditions were as follows: partial oxygen pressures of 3.02 × 10⁻¹⁰, 2.99 × 10⁻⁹, and 2.31 × 10⁻⁸ atm at 1400°, 1500°, and 1600°C, respectively. Analysis techniques that were used to determine the phase relations within the reacted samples included X-ray diffractometry, electron probe microanalysis (energy-dispersive spectroscopy and wavelength-dispersive spectroscopy), and optical microscopy. The solid-solution phases M₂O₃, M₃O₅, and higher Magneli phases (M _n O_{2 n −1}, where M = V, Ti) were identified in the V₂O₃–TiO₂ system. In the "V₂O₃–FeO" system, the solid-solution phases M₂O₃ and M₃O₄ (where M = V, Ti), as well as liquid, were identified.     

Electrical Characterization of Polymethylsiloxane/MoSi2-Derived Composite Ceramics

High-temperature-resistant ceramics in the system Mo-Si-O-C were prepared from polymethylsiloxanes loaded with MoSi₂ filler. Irreversible structural rearrangement reactions in the polymer-derived Si-O-C matrix phase and at the filler-matrix interface during heat treatment were detected by in situ electrical resistance measurements and impedance spectroscopy. At low temperatures a large difference in thermal expansion between the polymer and the filler resulted in an increase of resistance from 0.05 to 560 Ω·cm when the temperature was raised from 20° to 680°C. An increase of filler particle contact distances resulted in a high positive temperature coefficient (PTC_200°C= 4.64 K⁻¹). At high temperatures carburization reaction of the filler on the particle surface resulted in contact formation which gave rise to a resistance lower than 10⁻³Ω·cm above 1000°C.     

Effects of Oxygen Partial Pressure on the Nucleation Behavior and Morphology of Chemically-Vapor-Deposited Zirconia on Hi-Nicalon Fiber and Si

A ZrO₂ coating was prepared on Hi-Nicalon fiber and single-crystal Si by chemical vapor deposition (CVD) using ZrCl₄, CO₂, and H₂ as precursors at 1050°C. The effects of oxygen partial pressure on the nucleation behavior of the CVD-ZrO₂ coating were systematically studied by intentionally varying the controlled amount of O₂ into the CVD chamber. Characterization results suggested that the number density of tetragonal ZrO₂ nuclei apparently decreased with increasing the oxygen partial pressure from 4 × 10⁻³ to 1.6 Pa. Also, the coating layer became more columnar and contained larger monoclinic ZrO₂ grains. The observed relationships between the oxygen partial pressure and the nucleation and morphologic characteristics of the ZrO₂ coating were attributed to the grain size and oxygen deficiency effects, which have been previously reported to cause the stabilization of the tetragonal ZrO₂ phase in bulk ZrO₂ specimens.     

Effect of Processing Parameters on Physical Properties of Cadmium Stannate Thin Films Prepared by a Dip-Coating Technique

Thin films of dicadmium stannate spinel (Cd₂SnO₄) were deposited on glass substrates using a dip-coating technique. The films were transparent to visible light (90%) and electrically conductive. X-ray diffractometry showed that annealed films consisted of a single cubic spinel phase only when they were prepared from a solution with the composition of Cd:Sn = 2.5 and fired at a temperature of 400°–500°C. The Cd:Sn ratio, the firing temperature, and the post-deposition annealing sequence were crucial for the formation of a single phase, which is vital to obtain optimal optical and electrical properties. A resistivity as low as 3.3 × 10⁻⁴Ω·cm could be obtained after annealing.     

Preparation of a Highly Conductive Al2O3/TiN Interlayer Nanocomposite through Selective Matrix Grain Growth

An electroconductive TiN/Al₂O₃ nanocomposite was prepared by a selective matrix grain growth method, using a powder mixture of submicrosized α-Al₂O₃, nanosized γ-Al₂O₃, and TiN nanoparticles synthesized through an in situ nitridation process. During sintering, a self-concentration of TiN nanoparticles at the matrix grain boundary occurred, as a result of the selective growth of large α-Al₂O₃ matrix grains. Under suitable sintering conditions, a typical interlayer nanostructure with a continuous nanosized TiN interlayer was formed along the Al₂O₃ matrix grain boundary, and the electroconducting behavior of the material was significantly improved. Twelve volume percent TiN/Al₂O₃ nanocomposite with such an interlayer nanostructure showed an unprecedentedly low resistivity of 8 × 10⁻³Ω·cm, which was more than two orders lower than the TiN/Al₂O₃ nanocomposite without such an interlayer nanostructure.     

Synthesis and Characterization of a Remarkable Ceramic: Ti3SiC2

Polycrystalline bulk samples of Ti₃SiC₂ were fabricated by reactively hot-pressing Ti, graphite, and SiC powders at 40 MPa and 1600°C for 4 h. This compound has remarkable properties. Its compressive strength, measured at room temperature, was 600 MPa, and dropped to 260 MPa at 1300°C in air. Although the room-temperature failure was brittle, the high-temperature load-displacement curve shows significant plastic behavior. The oxidation is parabolic and at 1000° and 1400°C the parabolic rate constants were, respectively, 2 × 10⁻⁸ and 2 × 10⁻⁵ kg²-m⁻⁴.s⁻¹. The activation energy for oxidation is thus =300 kJ/mol. The room-temperature electrical conductivity is 4.5 × 10⁶Ω⁻¹.m⁻¹, roughly twice that of pure Ti. The thermal expansion coefficient in the temperature range 25° to 1000°C, the room-temperature thermal conductivity, and the heat capacity are respectively, 10 × 10⁻⁶°C⁻¹, 43 W/(m.K), and 588 J/(kgK). With a hardness of 4 GPa and a Young's modulus of 320 GPa, it is relatively soft, but reasonably stiff. Furthermore, Ti₃SiC₂ does not appear to be susceptible to thermal shock; quenching from 1400°C into water does not affect the postquench bend strength. As significantly, this compound is as readily machinable as graphite. Scanning electron microscopy of polished and fractured surfaces leaves little doubt as to its layered nature.