In Situ TiC/TiB2 Particulate Locally Reinforced Steel Matrix Composites Fabricated Via the SHS Reaction of Ni–Ti–B4C System

The composites synthesized with three kinds of B₄C particles mainly consist of TiC, TiB₂, and the alloy austenite containing Ni element. Ceramic particulate sizes in the composites synthesized with ∼3.5 and ∼45 μm B₄C particles are larger than that synthesized with ∼140 μm B₄C particle. No pores are found between the reinforcing region and matrix in the composites synthesized with ∼3.5 and ∼45 μm B₄C particles, while some large pores exist in the composites synthesized with ∼140 μm B₄C particle. With the decrease of B₄C particle size, the pores in the composites become fewer and the hardness and wear resistance of the composites increase.     

Synthesis and Oxidation Testing of MAX Phase Composites in the Cr–Ti–Al–C Quaternary System

According to the properties determined for the ternary end‐members, MAX phases in the quaternary Cr–Ti–Al–C system could be of interest as protective coatings for nuclear fuel cladding in the case of severe accident conditions. In this study, syntheses of 211 and 312 MAX phase compositions were attempted using pressureless reactions starting from Cr, TiH₂, Al, and C (graphite) powders. It was observed that both the Ti substitution by Cr in Ti₃AlC₂ and the mutual solubility of Ti₂AlC and Cr₂AlC are limited to a few atomic percent. In addition, the remarkable stability of the (Cr_2/3Ti_1/3)₃AlC₂ MAX phase composition was confirmed. Due to the low miscibility of MAX phases in the Cr–Ti–Al–C system, most samples contained substantial amounts of TiC_x and Al–Cr alloys as secondary phases, thus forming composite materials. After sintering, all samples were submitted to a single oxidation test (12 h at 1400°C in air) to identify compositions potentially offering high‐temperature oxidation resistance and so warranting further investigation. In addition to (Cr_0.95Ti_0.05)₂AlC, composite samples containing substantial quantities of Al₈Cr₅ and AlCr₂ formed a stable and passivating Al₂O₃ scale, whereas the other samples were fully oxidized.     

Fabrication and Characterization of In Situ Porous Si3N4‐Si2N2O‐BN Ceramic

Porous Si₃N₄‐Si₂N₂O‐BN ceramic was fabricated at 1750°C using Si₃N₄, BN, and (NH₄)₂HPO₄ as starting materials. During the sintering process, oxygen from the decomposed products of (NH₄)₂HPO₄ would bond Si and N in the liquid phase to form Si₂N₂O. The microstructure and properties of the porous ceramics were investigated. With the (NH₄)₂HPO₄ content varied from 10 to 50 vol.%, porosity of the porous Si₃N₄‐Si₂N₂O‐BN ceramic increased from 43.5% to 51%. The microstructure, mechanical, and dielectric properties was well controlled by adjusting (NH₄)₂HPO₄ contents. The present technique offers a more simple way of synthesizing porous Si₃N₄‐Si₂N₂O‐BN ceramics.     

In Situ Observation of Zirconia Particles at 1200°C by High-Resolution Electron Microscopy

Zirconia (ZrO₂) particles (average diameter, 30 nm) were observed in an in situ heating experiment up to 1200°C using a 400-kV high-resolution electron microscope. Thermal vibration of atoms on a (001) surface plane was observed at 1100°C. At 1200°C, grain growth and sintering phenomena were recorded on a videotape, showing (100) lattice planes migrating on a surface of a particle. Direct observation of the sintering process on a lattice level was accomplished for the first time.     

Microstructural Effects on the Mechanical Properties of SiC‐15 vol% TiB2 Particulate‐Reinforced Ceramic Composites

The microstructures and mechanical properties were studied for two different SiC ceramics containing 15 vol% of TiB₂ particulates. The first was prepared from commercially available spray‐dried granules and the second by blending individual SiC and TiB₂ powders. The average TiB₂ particle sizes were 2.7 μm for the ceramic prepared from blended powders, which had a uniform distribution of TiB₂, and 2.3 μm for the ceramic prepared from spray‐dried granules, which had a nonuniform distribution of TiB₂ agglomerates. Although the two ceramics had hardness values of 26 GPa, the other properties were different. For example, the fracture toughness was 4.3 MPa·m^1/2 for the ceramic prepared from blended powders compared to 3.1 MPa·m^1/2 for the ceramic prepared from spray‐dried granules. In contrast, the Weibull modulus for the ceramic prepared from spray‐dried granules was 21 compared to 12 for the other. Calculations predicted spontaneous microcracking in the ceramic prepared from spray‐dried granules, which was confirmed by analysis of the microstructure. The presence of microcracks accounted for the higher Weibull modulus, but lower flexural strength, Young's modulus and fracture toughness for the ceramic prepared from spray‐dried granules.     

Intercalated PS/Na+‐MMT Nanocomposites Prepared by Ultrasonically Initiated In Situ Emulsion Polymerization

Summary: A new technique, ultrasonically initiated in situ emulsion polymerization, was employed to prepare intercalated polystyrene/Na⁺‐MMT nanocomposites. FTIR, XRD, and TEM results confirm that the hydrophobic PS can easily intercalate into the galleries of hydrophilic montmorillonite via ultrasonically initiated in situ emulsion polymerization, taking advantages of the multi‐effects of ultrasonic irradiation, such as dispersion, pulverization, activation, and initiation. Properly reducing SDS concentration is beneficial to widen the d‐spacing between clay layers. However, the Na⁺‐MMT amount has little effect on the d‐spacing of nanocomposites. The glass transition temperature of nanocomposites increased as the percentage of clay increased, although the average molecular weight of PS decreased, and the decomposition temperature of the 1obtained nanocomposites moves to higher temperature.

TEM of PS/Na⁺‐MMT nanocomposite prepared by ultrasonically initiated in situ emulsion polymerization.     

Fabrication and Characteristic of Nextel 720 Fiber‐Reinforced Silicon Nitride Matrix Composites by Chemical Vapor Infiltration Process

Most current processes for fiber‐reinforced silicon nitride composites are conducted at very high temperature, which is not possible to use oxide fiber as reinforcement. Here, low‐temperature process of chemical vapor infiltration (CVI) was utilized to fabricate Nextel 720 oxide fiber tow‐reinforced silicon nitride matrix composite with PyC as interphase. The tensile strength was analyzed by Weibull distribution. The microstructure showed that there were two types of interface bonding. The strong interface bonding determined the unexpected low strength of the composites. This indicated that the suitable interface design is the urgent issue for oxide fiber‐reinforced silicon nitride composite by CVI.     

High‐Strength B4C–TaB2 Eutectic Composites Obtained via In Situ by Spark Plasma Sintering

The in situ synthesis/consolidation of B₄C–TaB₂ eutectic composites by spark plasma sintering (SPS) is reported. Samples for the evaluation of bending strength were cut from specimens with diameters of 30 mm. The sample prepared for the three‐point flexural strength test had fibers of tantalum diboride with diameter of 1.3 ± 0.4 μm distributed in the B₄C matrix, thereby reducing composites brittleness and yielding an indentation fracture toughness of up to 4.5 MPa·m^1/2. Furthermore, the Vickers hardness of B₄C–TaB₂ eutectics formed by SPS was as high as 26 GPa at an indentation load of 9.8 N. The flexural strength of the B₄C–TaB₂ system has been reported for the first time. Some steps were identified in the load–displacement curve, suggesting that micro‐ and macrocracking occurred during the flexural test. Ceramic composites with a eutectic structure exhibited a room‐temperature strength of 430 ± 25 MPa. Compared with other eutectic composites of boron carbide with transition‐metal diborides, room‐temperature strength the B₄C–TaB₂ was 40% higher than that of B₄C–TiB₂ ceramics, demonstrating advantage of the in situ synthesis/consolidation of eutectic composites by SPS.     

Boron Carbide–Zirconium Boride In Situ Composites by the Reactive Pressureless Sintering of Boron Carbide–Zirconia Mixtures

The heating of B₄C–YTZP (where YTZP denotes yttria-stabilized zirconia polycrystals) mixtures, under an argon atmosphere, generates B₄C–ZrB₂ composites, because of a low-temperature (<1500°C) carbide–oxide reaction. Composites derived from mixtures that include ≥15% YTZP are better sintered than monolithic B₄C that has been fired under the same conditions. Firing to ∼2160°C (1 h dwell) generates specimens with a bulk density of ≥91% of the theoretical density (TD) for cases where the initial mixture includes ≥15% YTZP. Mixtures that include 30% YTZP allow a fired density of ≥97.5% TD to be attained. The behavior of the B₄C–YTZP system is similar to that of the B₄C–TiO₂ system. Dense B₄C–ZrB₂ composites attain a hardness (Vickers) of 30–33 GPa.     

SiC Matrix Structure–Function Composites Prepared by Pressureless Sintering and Their Microwave Attenuation Property in the X Band

For developing excellent microwave attenuation materials in a wide band, two series of SiC‐C (graphite) composites with different C additions were fabricated by pressureless sintering, using α‐SiC and β‐SiC green powder, respectively. β‐SBC composites were more suitable for microwave attenuation materials than α‐SBC composites. β‐SiC composites with 3 wt% C additions exhibited the best microwave absorption: The most significant microwave attenuation was ?40.5 dB, and most other attenuations were above ?30 dB in the whole X band. The composites were prepared with cost‐effective and easily controllable manufacturing process and can be considered as structure–function materials for particular applications.     

Physical and Mechanical Properties of Bulk Ta4AlC3 Ceramic Prepared by an In Situ Reaction Synthesis/Hot-Pressing Method

Bulk Ta₄AlC₃ ceramic was prepared by an in situ reaction synthesis/hot-pressing method using Ta, Al, and C powders as the starting materials. The lattice parameter and a new set of X-ray diffraction data were obtained. The physical and mechanical properties of Ta₄AlC₃ ceramic were investigated. Ta₄AlC₃ is a good electrical and thermal conductor. The flexural strength and fracture toughness are 372 MPa and 7.7 MPa·m^1/2, respectively. Typically, plate-like layered grains contribute to the damage tolerance of Ta₄AlC₃. After indentation up to a 200 N load, no obvious degradation of the residual flexural strength of Ta₄AlC₃ was observed, demonstrating the damage tolerance of this ceramic. Even at above 1200°C in air, Ta₄AlC₃ still retains a high strength and shows excellent thermal shock resistance, which renders it a promising high-temperature structural material.     

Novel Polyamide 6/Polystyrene In Situ Microfibrillar Blends Prepared by Anionic Polymerization of ε‐Caprolactam via Reactive Extrusion

In this study, polyamide 6/polystyrene in situ microfibrillar blends are prepared via anionic polymerization of ε‐caprolactam in a twin screw extruder. Scanning electron microscope analysis reveals that microfibrillated PA6 dispersed phase, which is continuous and preferentially oriented parallel to the extrusion direction, is in situ formed within polystyrene (PS) matrix during reactive extrusion at the content PS equal to 30 and 40 wt%. Mechanical properties analysis shows that the yield strength and elongation at break of PA6/PS (70/30 and 60/40) microfibrillar blends are remarkably increased with respect to those of pure PS. Also, the in situ fibrillation mechanisms are investigated by the analysis of morphological evolution. This work demonstrates a facile and efficient route to fabricate the microfibrillar blends with relatively high contents of polymer microfibrils.

     

Dense, Shaped Ceramic/Metal Composites at lessthan equal to1000°C by the Displacive Compensation of Porosity (DCP) Method

The Displacive Compensation of Porosity method for fabricating dense, shaped ceramic/metal composites at modest temperatures is demonstrated. In this process, liquid-metal/solid-ceramic displacement reactions are used to generate more ceramic (by volume) than is consumed, so that pores within a ceramic preform can be filled with the new ceramic phase (i.e., densification without sintering). Dense, lightweight MgO/Mg-Al composites (74–86 vol% oxide) and higher-melting, co-continuous MgAl₂O₄/Fe-Ni-Al-bearing composites (42–59 vol% oxide) have been produced via the pressureless infiltration and reaction of magnesium-bearing liquids with porous preforms of Al₂O₃ and NiAl₂O₄+Fe, respectively, at temperatures of 900°−1000°C. The composites are relatively tough and retain the shapes and dimensions (to within a few percent) of the starting preforms.     

A Novel Method of Synthesis of Nanostructured Aluminum Nitride Through Sol‐Gel Route by In Situ Generation of Nitrogen

Nanostructured Aluminum Nitride (AlN) has been prepared by carbothermal reduction followed by nitridation (CTRN) of alumina gel over a temperature range 1200°C–1350°C and time period of 30 min to 3 h. Before heat treatment the gel is repeatedly evacuated and purged with ammonia. The nanopores of the gel are filled with ammonia which acts as a source of in situ nitrogen at heat‐treatment temperature. Dextrose also decomposes at the reduction temperature and generates ultrafine carbon. The stability diagram of the carbon saturated Al–N–O system is constructed and it shows that extremely low partial pressure of oxygen is required for the stability of AlN. The ultrafine carbon as well as hydrogen from the cracking of ammonia is not sufficient to create the extremely low partial pressure of oxygen required for the stabilization of AlN. So the sample is heat treated in charcoal boat in nitrogen atmosphere to achieve an extremely low partial pressure of oxygen required for the formation of AlN. The material has been characterized through XRD, FESEM, and HRTEM analyses. The spherical particle size of AlN is obtained ～21 nm.     

ZrB2–SiC–G Composite Prepared by Spark Plasma Sintering of In‐Situ Synthesized ZrB2–SiC–C Composite Powders

To avoid introduction of milling media during ball‐milling process and ensure uniform distribution of SiC and graphite in ZrB₂ matrix, ultrafine ZrB₂–SiC–C composite powders were in‐situ synthesized using inorganic–organic hybrid precursors of Zr(OPr)₄, Si(OC₂H₅)₄, H₃BO₃, and excessive C₆H₁₄O₆ as source of zirconium, silicon, boron, and carbon, respectively. To inhabit grain growth, the ZrB₂–SiC–C composite powders were densified by spark plasma sintering (SPS) at 1950°C for 10 min with the heating rate of 100°C/min. The precursor powders were investigated by thermogravimetric analysis–differential scanning calorimetry and Fourier transform infrared spectroscopy. The ceramic powders were analyzed by X‐ray diffraction, X‐ray photoelectron spectroscopy, and scanning electron microscopy. The lamellar substance was found and determined as graphite nanosheet by scanning electron microscopy, Raman spectrum, and X‐ray diffraction. The SiC grains and graphite nanosheets distributed in ZrB₂ matrix uniformly and the grain sizes of ZrB₂ and SiC were about 5 μm and 2 μm, respectively. The carbon converted into graphite nanosheets under high temperature during the process of SPS. The presence of graphite nanosheets alters the load‐displacement curves in the fracture process of ZrB₂–SiC–G composite. A novel way was explored to prepare ZrB₂–SiC–G composite by SPS of in‐situ synthesized ZrB₂–SiC–C composite powders.     

Effects of pH Value on Composition Structure and Catalytic Activity of Pt‐SnOx/C Prepared by Ethylene Glycol Method

Pt‐SnO_x nanoparticles were synthesized by the ethylene glycol (EG) method in solution of H₂PtCl₆ and SnCl₂, with the same concentrations of Pt and Sn, but different pH values. The pH value after the end of platinum reduction reaction was not changed any more, except that a certain amount of water was added to deposit the Pt‐SnO_x nanoparticles on the carbon support. The pre‐nanocatalysts were characterized by X‐ray photoelectron spectroscopy (XPS) to investigate the contents of Pt and Sn, and their catalytic activities for ethanol electrooxidation were tested by cyclic voltammetry (CV). The result was that the Sn contents were increasing as the Pt/Sn atomic ratios of 2.2, 2.6, 5.1, 7.4, 8.7, with the decreasing end pH values of 4.5, 5.0, 5.5, 6.5, 7.5, and the Pt contents became less than the addition in the preparation solution while the end pH values were <5.5, but the catalytic activities for ethanol electrooxidation were not so much regularly changed. Besides, from the end pH value of 5.5 to the increasing 9.0, all the platinum nanoparticles could be completely deposited on the carbon support, under the condition that only a certain amount of water was added.     

(Cr2/3Ti1/3)3AlC2 and (Cr5/8Ti3/8)4AlC3: New MAX‐phase Compounds in Ti–Cr–Al–C System

Two new MAX compounds, (Cr_2/3Ti_1/3)₃AlC₂ and (Cr_5/8Ti_3/8)₄AlC₃, were successfully synthesized by hot‐pressing elemental powders at 1500°C for 1 h under 30 MPa in a flowing argon atmosphere. Their crystal structures were indentified and characterized by X‐ray diffraction and transmission electron microscopy analysis. (Cr_2/3Ti_1/3)₃AlC₂ and (Cr_5/8Ti_3/8)₄AlC₃ have the same crystal structures with the well‐characterized Ti₃AlC₂ and Ti₄AlN₃, respectively.     

Characteristics of Transparent Conducting W‐Doped SnO2 Thin Films Prepared by Using the Magnetron Sputtering Method

Tungsten‐doped SnO₂ (WTO) thin films with a given thickness of about 300 nm have been prepared by magnetron sputtering with a substrate temperature in the range 400°C–700°C. The effects of substrate temperature on the structural, optical, and electrical properties and of WTO thin films have been investigated. A texture transition from (1 1 0) to (2 1 1) crystallographic orientations has experimentally been found by X‐ray diffraction measurements as substrate temperature is raised. It was found that all thin films showed smooth surface with no cracks and high transparency (>85%) with the optical band gap ranging from 4.22 to 4.32 eV. The mobility varied from 12.89 to 22.48 cm²·(V·s)^?1 without reducing the achieved high carrier concentration of about 1.6 × 10²⁰ cm^?3. Such an increase in mobility is shown to be clearly associated with the development of (2 0 0) but concurrent degradation of (1 1 0) in WTO thin films.     

Evolution of Mineralogical Phases by 27Al and 29Si NMR in MK‐Ca(OH)2 System Cured at 60°C

The evolution of the metastable phases in metakaolin/Ca(OH)₂ systems cured at high temperatures, remains mostly unknown, newer techniques may now help to establish both the kinetic mechanism of the pozzolanic reaction and the thermodynamic stability of the main hydrated hexagonal phases: Stratlingite (C₂ASH₈) and tetra calcium aluminate hydrate (C₄AH₁₃). For this reason this work examines the kinetics of the pozzolanic reaction in the MK/Ca(OH)₂ system over 123 d at 60°C using nuclear magnetic resonance spectroscopy (²⁷Al and ²⁹Si NMR). The results obtained by ²⁷Al and ²⁹Si NMR show that during the first 30 h, the metastable phases C₂ASH₈ and C₄AH₁₃, coexist with the cubic phase (C₃ASH₆) obtained directly from the pozzolanic reaction. The gel C–S–H is clearly identified after 21 h of reaction, whereas at shorter times the C–S–H bands overlap those with the unreacted metakaolin ones. After 123 d of pozzolanic reaction, the first signs of the cubic phase are detected, a consequence of the conversion reaction of the metastable phases, and a phenomenon not previously identified.     

Magnetic Properties of Mn‐doped SnO2 Thin Films Prepared by the Sol–Gel Dip Coating Method for Dilute Magnetic Semiconductors

Manganese‐doped tin oxide (SnO₂:Mn) thin films were deposited on glass substrates by the sol–gel dip coating technique. The effect on structural, morphological, magnetic, electrical, and optical properties in the films with different Mn concentrations (0–5 mol%) were investigated. X‐ray diffraction patterns (XRD) showed the deterioration of crystallinity with increase in Mn‐doping concentration. Scanning electron microscopy (SEM) studies showed an inhibition of grain growth with an increase in Mn concentration. X ray photoelectron spectroscopy (XPS) revealed the presence of Sn⁴⁺ and Mn³⁺ in SnO₂: Mn films. SnO₂: Mn films show ferromagnetic and paramagnetic behavior. These SnO₂:Mn films acquire n‐type conductivity for 0–3 mol% (SnO₂ ‐ Sn_0.97Mn_0.03O_{2) ‐}doping concentration and p type for 5 mol% Mn‐doping concentration(Sn_0.95Mn_0.05O₂) in SnO₂ films. An average transmittance of > 75% (in UV‐Vis region) was observed for all the SnO₂:Mn films. Optical band gap energy of SnO₂: Mn films were found to vary in the range 3.55 to 3.71 eV with the increase in Mn‐doping concentration. Photoluminescence (PL) spectra of the films exhibited an increase in the emission intensity with increase in Mn‐doping concentration which may be due to structural defects or luminescent centers, such as nanocrystals and defects in the SnO₂. Such SnO₂:Mn films with structural, magnetic and optical properties can be used as dilute magnetic semiconductors.