Microstructure and Phase Stability of Yttria-Doped Tetragonal Zirconia Polycrystals Heat Treated in Nitrogen Atmosphere

The low-temperature degradation of zirconia (ZrO₂) that was doped with 3 mol% yttria (Y₂O₃) (3Y-TZP) was prevented by the heat treatment of sintered specimens in nitrogen. The heat treatment of sintered specimens resulted in a surface layer that was stabilized by nitrogen ions, whereas the interior was only slightly affected by the heat treatment. X-ray diffractometry and transmission electron microscopy analyses revealed that the stabilized surface layer consisted of cubic grains with tetragonal precipitates. Although the presence of the surface layer decreased the strength of the sintered 3Y-TZP, the strength of nitrified specimens was maintained when low-temperature annealing was applied.     

Relation between Chemical, Mechanical, and Electrical Properties of Nb2O5-Modified 95 Mol% PbZrO3-5 Mol% PbTiO3

The use of an Nb₂O₅-modified PZT (containing ∼95 mol% zirconate and ∼5 mol% titanate) in explosively driven electrical power supplies is described. A small but significant number of test specimens exhibit dielectric breakdown during explosive testing. A failure mechanism is proposed in which the electromechanically induced stresses result in fracture. Charged particles released during the fracture process initiate dielectric failure. The fracture toughness and mechanical strength of a given material are about the same for unpoled and poled material, but after pressure depoling, a given material typically exhibits a nominal 50% increase in toughness and strength. Varying the lead concentration from ∼5 at.% deficiency to ∼2 at.% excess results in a degradation in toughness and strength. An optimum niobium concentration is observed at ∼1.8 at.% in terms of mechanical properties and functional performance.     

Improving low temperature degradation of 3Y-TZP ceramics via high temperature carburizing

3Y-TZP ceramics are prepared by solid state method and surface carburization process, and the effect of surface carburization on its the low temperature degradation is studied. The conventional sintered samples completely lost its mechanical properties after aging for 15 h, while the failure time of the surface carburized samples are 300 h. In addition, the nuclear growth rate of the surface carburized samples (αd) and the nucleation rate (Nr) is lower than that of sintered samples, αd plays a dominant role in the degradation process at low temperature and is the key factor determining the aging rate. At the same time, it is found that carbon is dissolved in zirconia lattice in the form of electrically neutral atoms, which will not destroy the original charge balance and produce new oxygen vacancies when entering the interstitial site. More importantly, the precipitation rate of Y³⁺ from zirconia lattice is the key factor to determine the low-temperature phase transition of tetragonal-monoclinic(T-M). The treatment method of surface carburization has significantly improved the low-temperature degradation performance of 3Y-TZP ceramics, which provides a basis for the application of zirconia ceramics in low-temperature and humid environment.     

Strengthening of Alumina by Formation of a Mullite/Glass Layer on the Surface

A layer composed of mullite and silicate glass was caused to form on the surface of a high-purity alumina ceramic in order to enhance the strength of the material. The layer was formed by exposing the specimens above a bed of SiC platelets at 1400°C to a flowing H₂ atmosphere containing ∼0.1% H₂O. A reaction between the SiC platelets and the H₂O in the environment resulted in the generation of SiO gas. Some of the SiO gas subsequently reacted with ambient H₂O in the atmosphere, forming SiO₂O "smoke" which was deposited on, and reacted with, the alumina substrate. The strength of the ceramic was significantly improved by the reaction layer, which was found to be comprised of mullite and silicate glass. The increases in strength (about 60% above that of the material in the "as-polished" condition) was attributed to the blunting of surface cracks. A similar strengthening effect was observed in samples of the mate-rial which had been ground with a 220-grit diamond abra-sive wheel (as had all of the samples) but not polished.     

Development of Surface Compressive Stresses in Zirconia–Alumina Composites by an Ion-Exchange Process

A two-step ion-exchange technique was developed for introducing compressive stresses on the surface of ZrO₂–Al₂O₃ composites. In the first step, a thin layer (∼250 μm) of Na-β"-Al₂O₃ was formed on the surface of the composite by a vapor-phase process at ∼1400°C. In the second step, Na⁺ ions were replaced by K⁺ ions by a heat treatment at ∼385°C for 2 h in a molten KNO₃ bath. Replacement of sodium by potassium led to the creation of surface compressive stresses. The flexural strength and Weibull modulus of ZrO₂–Al₂O₃ composite were ∼915 MPa and 10, respectively, for the as-sintered samples. By contrast, the flexural strength and Weibull modulus were ∼1140 MPa and 26, respectively, for the ion-exchanged samples. A residual surface compressive stress of ∼480 MPa was measured by a strain-gauge technique in K⁺-ion-exchanged samples. The presence of surface compressive stresses also was confirmed using an indentation technique. The technique developed here can be used to introduce compressive stresses on components of virtually any shape.     

Strength of Silicon Nitride after Thermal Shock

A water-quenching technique was used to evaluate the thermal-shock strength behavior of silicon nitride (Si₃N₄) ceramics in an air atmosphere. When the tensile surface was shielded from air during the heating and soaking process, the quenched specimens showed a gradual decrease in strength at temperatures above 600°C. However, the specimens with the air-exposed surface exhibited a ∼16% and ∼29% increase in strength after quenching from 800° and 1000°C, respectively. This is because of the occurrence of surface oxidation, which may cause the healing of surface cracks and the generation of surface compressive stresses. As a result, some preoxidation of Si₃N₄ components before exposure to a thermal-shock environment is recommended in practical applications.     

Hydrothermal Growth of Vertical ZnO Nanorods

Vertically aligned, single crystalline ZnO nanorods with a high packing density and diameter of ∼60 nm have been successfully synthesized via a low-temperature hydrothermal route on glass substrates pre-deposited with a ZnO seeding layer. The seeding layer exhibits an epitaxial effect on the growth and alignment of the ZnO nanorods. This epitaxial effect can arise from two considerations, namely the crystalline orientation and surface roughness of the seeding layer, which can be controlled by the curing temperature. The ZnO seeding layer that was cured at 350°C exhibited a preferred (0002) crystalline orientation of wurtzite hexagonal structure and a low surface roughness. It was demonstrated to promote the vertical growth of ZnO nanorods. The ZnO nanorods grew in an almost linear relationship with hydrothermal time up to 8 h, but thereafter started to dissolve as the reaction time extended beyond 8 h, due to competition from the homogeneous nucleation of ZnO microparticles in the solution.     

High-Temperature Strength Behavior of Hot-Pressed Si3N4: Evidence for Subcritical Crack Growth

The high-temperature strength of commercial hot-pressed Si₃N₄ was obtained for (1) two materials with different impurity contents, (2) the weak and strong material directions, (3) air and Ar ambients, and (4) different stressing rates. Strength degradation occurred at a lower temperature for the less pure material; both material directions exhibit the same rate of strength degradation. The testing ambient did not affect strength. The strength at temperatures ∼1200°C depended strongly on stressing rate. The presence of rough, crack-shaped topographical features on the fracture surface and the observation of large cracks that formed during stressing are reported as evidence for subcritical crack growth at high temperatures. It is hypothesized that accelerated creep caused by grain-boundary sliding at preexisting crack fronts is the mechanism responsible for the observed subcritical crack growth.     

聚铝硅氧烷对聚碳酸酯的阻燃作用

采用水解缩合法制备了聚铝硅氧烷,用极限氧指数、热失重分析、扫描电子显微镜、X射线光电子能谱研究了聚铝硅氧烷对聚碳酸酯（PC）的阻燃作用机理。结果表明,聚铝硅氧烷可明显提高PC的极限氧指数;在热降解过程中,聚铝硅氧烷可使PC的最大热降解速率降低,800 ℃残炭率显著提高,在PC中添加5 %（质量分数,下同）的聚铝硅氧烷可使PC的800 ℃残炭率提高44.1 %;在燃烧过程中,聚铝硅氧烷会迁移到PC表面,与PC的降解产物发生相互作用,促进残炭的形成,硅(Si)和铝(Al)积聚在材料表面形成富含Si、Al的绝缘炭层,抑制了材料的进一步降解,阻碍了热量和可燃性气体的传递,从而有效地改善了PC的阻燃性能。     

Sol–Gel Route to Nanocrystalline Lithium Metasilicate Particles

Crystalline lithium metasilicate (Li₂SiO₃) nanoparticles have been synthesized using a sol–gel process with tetraethylorthosilicate and lithium ethoxide as precursors. The particle size examined by using transmission electron microscopy and BET-specific surface area techniques is in the range 5–50 nm, depending on the temperature at which the material is calcined. The crystalline Li₂SiO₃ forms at ambient temperature (∼40°C), and it remains in this phase after calcination at temperatures up to 850°C. The BET-specific surface area is ∼110 m²/g for material calcined at temperatures below 500°C, decreasing to ∼29 and ∼0.7 m²/g following calcination at 700° and 850°C, respectively. Solid-state ²⁹Si NMR spectroscopy shows the presence of only Q ² structural units in the material. The lithium metasilicate is further characterized using differential scanning calorimetry and thermogravimetric analysis, and Fourier transform infrared spectroscopy.     

Acoustic Emission Studies of Alumina-13% Titania Free-Standing Forms during Four-Point Bend Tests

Free-standing alumina-13% titania samples were manufactured with a water-stabilized plasma spray gun to a thickness of ∼5 mm. A thin layer of aluminum was arc sprayed prior to depositing the thick coating and etched away using hydrochloric acid. The so-obtained free-standing plate was cut into four-point-bend-test specimens with dimensions of ∼5 mm × 5 mm × 50 mm. The as-sprayed material consisted of a supersaturated solid solution, where titania was frozen inside the alumina matrix. Heat treatment was performed at 1450°C for 24 h and then at 1100°C for another 24 h. After heat treatment, titania precipitates were observed. The major phases of the as-sprayed and heat-treated samples were γ- and α-alumina, respectively. The porosity was ∼10% for as-sprayed samples; this value was reduced to ∼3% after heat treatment. Four-point bend tests were performed on the as-sprayed and heat-treated specimens in cross-section and in-plane directions. An acoustic emission technique was used to examine the cracking during the tests in situ. Microcracking prior to failure was observed for as-sprayed samples that were tested in the cross-section direction. However, when tested in the in-plane direction, catastrophic failure with less evidence of microcracking occurred. For heat-treated specimens, microcracks were usually observed when tests were performed in either of the orientations. Energy and amplitude distributions for each testing condition were examined. These distributions changed after heat treatment; however, no significant differences were distinguished when tests were performed in the cross-section or in-plane directions.     

Nanosilver-Coated Porous SiC–Si3N4 Composite Using Microwave-Assisted Process

Using a novel microwave-assisted process, nano-Ag-coated continuous porous SiC–Si₃N₄ substrate was fabricated from a solution containing AgNO₃ salts and ethylene glycol. The detailed microstructure of the fabricated substrate was investigated depending on the amount of AgNO₃ salts in the starting solution and the microwave irradiation time. From a solution containing 0.4 g of AgNO₃ for 60 s irradiation time, the Ag nanoparticles, ∼25 nm in diameter, were homogeneously coated on the continuous porous SiC–Si₃N₄ matrix as well as on the surface of the Si₃N₄ whiskers. However, the Ag nanoparticles (∼15 nm) deposited from a solution containing 0.6 g of AgNO₃ for 60 s irradiation time showed maximum homogeneity and narrow size distribution. The components of Si, N, and Ag were homogeneously distributed on the deposited layer. The deposited Ag nanoparticles covered with a thin (∼2 nm), amorphous layer had nanocrystallinity and adhered well to the surface of the Si₃N₄ whiskers.     

Joining Si3N4-Based Ceramics with Oxidation-Formed Surface Layers

A simple processing technique has been developed for joining Si₃N₄-based ceramics. Thin (<5 μm thick), amorphous, or partially crystalline SiO₂-based surface layers were formed, via low-temperature oxidation (at 1200°C), on the faces to be joined. Joining of the surface-coated pieces could then be performed in an inert environment at typical sintering/joining temperatures (i.e., 1700°C), with or without applied gas pressure, via a transient viscous/liquid phase. This method was most effective for Si₃N₄ ceramics with single oxide sintering additives when a thin (∼1 μm thick), highly smooth (RMS roughness <60 nm) SiO₂ layer was formed, and essentially 'pore-free' joints could be formed. However, the method was less suitable for a multi-additive SiAlON material under current experimental conditions, as relatively high roughness (RMS roughness >400 nm) oxide scales formed, leaving residual porosity at the joint interface.     

Design of a novel multilayer low-temperature protection composite based on phase change microcapsules

It is necessary to develop a novel low-temperature protective material that ensures the flexibility and warmth of operation in a short time under the low-temperature environment (−30 to −80°C). Hence, a novel multilayer low-temperature protection composite (MPC) was prepared based on phase change microcapsules (microPCMs). MicroPCMs containing n-octadecane with melamine-urea-formaldehyde shell were successfully synthesized through in situ polymerization. Then the microPCMs were finished on the basic fabric's surface (biocomponent spunbond-spunlace nonwoven material) by foam coating and the silicone rubber was covered on the outermost surface. On the basis of scanning electron microscope (SEM) micrographs, microPCMs had a relatively spherical shape and a smooth surface, in which the average particle size was about 42.77 μm. The cross section morphology showed that the MPC was consisted of three layers structure including the base fabric, the microPCMs layer, and the silicone rubber layer. At −50 °C, the low-temperature resistance time of the MPC was 727 s and the power consumption for maintaining a certain temperature for 10 and 20 min of the MPC were 350.86 and 1392.66 J, respectively. Compared with the basic fabric, which has the same thickness as the MPC, the low-temperature resistance time of the MPC was prolonged about 5 min and the power consumption of the MPC for 10 min decreased by 55%, and for 20 minutes decreased by 33%, respectively. The MPC could guarantee the low-temperature protection effect in short time. It could be applied as the potential materials in the area of low-temperature protection. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 47534.     

Hydrothermal Synthesis of Thin Films of Barium Titanate Ceramic Nano-Tubes at 200°C

A novel, low-temperature synthesis method for producing BaTiO₃ thin films patterned in the form of nano-tubes ("honeycomb") on Ti substrates is reported. In this two-step method, the Ti substrate is first anodized to produce a surface layer (∼200–300-nm thickness) of amorphous titanium oxide nano-tube (∼100-nm diameter) arrays. In the second step, the anodized substrate is subjected to hydrothermal treatment in aqueous Ba(OH)₂, where the nano-tube arrays serve as templates for their hydrothermal conversion to polycrystalline BaTiO₃ nano-tubes. This opens the possibility of tailoring the nano-tube arrays and of using various precursor solutions and their combinations in the hydrothermal bath, to produce ordered, patterned thin-film structures of various Ti-containing ceramics. These could find use not only in a variety of electronic device applications but also in biomedical applications, where patterned thin films are also desirable.     

Low-Temperature Phase Relationships in the System ZrO2-TiO2

The phase relationships in the system ZrO₂-TiO₂ near the compound ZrTiO₄ have been clarified through an experimental study involving the characterization of both single-crystal and powder specimens, the latter prepared through conventional solid-state reaction and also by low-temperature co-precipitation methods. Zr_{1+ x} Ti_{1- x} O₄ (1/10 > x >-1/6), having the α-PbO₂-type structure, is found to transform on cooling between ∼1100° and ∼1150°C. Below this temperature there is an unusual, continuous phase transition leading to the formation of the stable low-temperature phase ZrTi₂O₆. Low-level doping with Y₂O₃ was found to enhance apparent cation ordering in intermediate compositions in the temperature range just below the phase transition.     

Luster Pottery from the Thirteenth Century to the Sixteenth Century: A Nanostructured Thin Metallic Film

Luster is a decorative metallic film that was applied on the surface of medieval glazed pottery. It can be obtained via the low-temperature (∼650°C), controlled reduction of copper and silver compounds. In this paper, we show that luster is a thin layered film (200–500 nm thick) that contains metallic spherical nanocrystals dispersed in a silicon-rich matrix and has a metal-free outermost glassy layer that is 10–20 nm thick. Silver nanocrystals seem to be separated from those of copper, forming aggregates 5–100 μm in diameter. This composite structure exhibits optical properties that are dependent on both the particle size and the matrix. Luster is indeed the first reproducible nanostructured thin metallic film that was made by humans.     

Nucieation and Growth of Cracks in Vitreous-Bonded Aluminum Oxide at Elevated Temperatures

The nucleation and growth of cracks was studied at elevated temperatures on a grade of vitreous-bonded aluminum oxide that contained ∼8 vol% glass at the grain boundaries. Cracks were observed to nucleate within the vitreous phase, close to the tensile surface of the flexural test specimens used in these experiments. Crack nucleation occurred at a strain of ∼0.08% to 0.12% which corresponded to a crack nucleation time of ∼35% of the time to failure by creep rupture. Once nucleated, cracks propagated along grain boundaries, as long as the stress for crack propagation was maintained. The crack velocity for cracks that were nucleated by the creep process was found to be linearly proportional to the apparent stress intensity factor, whereas for cracks that were nucleated by indentation, the crack velocity was proportional to the fourth power of the apparent stress intensity factor.     

High-Temperature Chemical Stability of Plasma-Sprayed Ca05Sr05Zr4P6O24 Coatings on Nicalon/SiC Ceramic Matrix Composite and Ni-Based Superalloy Substrates

The potential application of Ca05Sr05Zr4P6O24 (CS50) as a corrosion-resistant coating material for Si-based ceramics and as a thermal barrier coating material for Ni-based superalloys was explored. A ∼200 (xm thick CS50 coating was prepared by air plasma spray with commercially available powder. A Nicalon/SiC ceramic matrix composite and a Ni-based superalloy coated with a ∼200 (xm thick metallic bond coat layer were used as substrate materials. Both the powder and coating contained ZrP2O7 as an impurity phase, and the coating was highly porous as-deposited. The coating deposited on the Nicalon/SiC substrate was chemically stable upon exposure to air and Na2SO4/O2 atmospheres at 1000°C for 100 h. In contrast, the coating sprayed onto the superalloy substrate significantly reacted with the bond coat surface after similar oxidation in air.     

Effects of an Electroplated Platinum Interlayer on the Morphology and Phases of Chemically-Vapor-Deposited Alumina on Single-Crystal, Nickel-Based Superalloy

The feasibility of preparing a thin layer of α-Al₂O₃ on the surface of a single-crystal, Ni-based superalloy was examined using a chloride-based chemical vapor deposition (CVD) process previously developed for cutting tool applications. A coating directly deposited by this method on the alloy surface consisted of ∼1 μm α-Al₂O₃ crystals in a matrix of amorphous Al₂O₃. When the alloy surface was predeposited with an electroplated Pt layer, the coating was mostly α-Al₂O₃, but with the presence of fine microcracks on the coating surface. In comparison to the results observed for pure Pt substrate, the role of the Pt interlayer was apparently to promote the rapid formation of κ-Al₂O₃ nuclei, which subsequently transformed to α-Al₂O₃ during the CVD growth process.