Physicochemical properties of La<Subscript>3</Subscript>Ga<Subscript>5.5</Subscript>Ta<Subscript>0.5</Subscript>O<Subscript>14</Subscript>

We report the optical and dielectric properties and microhardness of La₃Ga_5.5Ta_0.5O₁₄ lanthanum gallium tantalate (langatate) crystals. Analysis of the optical transmission spectra of the crystals in relation to their refined compositions indicates that the bands at 34000–35000 and 27000–28000 cm⁻¹ are due to lanthanum and oxygen vacancies, respectively, and that the band at 20000–21000 cm⁻¹ is responsible for the yellow (orange) coloration of the crystals. Their resistivity and microhardness decrease with increasing oxygen vacancy concentration.     

Micro void growth in NiSnP layer between (Cu,Ni)<Subscript>6</Subscript>Sn<Subscript>5</Subscript> intermetallic compound and Ni<Subscript>3</Subscript>P by higher reflow temperature and multiple reflow

This study examines the growth mechanism of micro void called “Kirkendall voids” within NiSnP nano-crystalline layer between (Cu,Ni)₆Sn₅ intermetallic compound (IMC) and Ni₃P formed during two double reflow processes. The micro voids in NiSnP layer formed at the first reflow grow faster under the elevated reflow temperature than under the standard lead-free reflow, during the second reflow process. Despite the diffusion barrier Ni(P), the inward diffusion flux of Sn from (Cu,Ni)₆Sn₅ into NiSnP layer is much slower than the outward flux of Sn from NiSnP layer into Ni₃P, consequently leaving voids as NiSnP thickness increases. Results show that the thermal activation energy through the elevated reflow temperature has a higher influence in micro void growth than the number of reflows for the inward and outward diffusion flux difference of Sn within NiSnP layer in electroless Ni(P)/immersion Au and SnAgCu reaction system.     

The role of interfaces and matrix void nucleation mechanism on the ductile fracture process of discontinuous fibre-reinforced composites

The role of fibre morphology, interface failure and void nucleation mechanisms within the matrix on the deformation and fracture behaviour of discontinuous fibre-reinforced composites was numerically investigated. The matrix was modelled using a constitutive relationship that accounts for strength degradation resulting from the nucleation and growth of voids. For the matrix, two materials exhibiting identical strength and ductility but having different void-nucleation mechanisms (stress-controlled and strain-controlled) were considered and fibres were assumed to be elastic. The debonding behaviour at the fibre interfaces was simulated in terms of a cohesive zone model which describes the decohesion by both normal and tangential separation. The results indicate that in the absence of interface failure, for a given fibre morphology the void nucleation in the matrix is the key controlling parameter of the composite strength and ductility, hence, of the fracture toughness. The weak interfacial behaviour between the fibres and the matrix can significantly increase the ductility without sacrificing strength for certain fibre morphology and for certain matrix void-nucleation mechanisms.     

Role of Ti diffusion on the formation of phases in the Al<Subscript>2</Subscript>O<Subscript>3</Subscript>–Al<Subscript>2</Subscript>O<Subscript>3</Subscript> brazed interface

Diffusivities of Ti, Cu, Al and Ag in the interface of Al₂O₃–Al₂O₃ braze joints using Ag–Cu–Ti active filler alloy, have been calculated by Matano–Boltzman method. The Matano plane has been identified for each elemental diffusion at various brazing temperatures. The diffusivities of Ag, Cu and Al are almost insignificant on formation of interface during brazing, whereas the diffusivity of Ti changes significantly with the brazing temperature and controls the formation of different reaction product in the interface. Presence of TiO and Ti₃Cu₃O phases in the interface has been confirmed by transmission electron microscopy (TEM).     

Nucleation behavior and microstructure of Al<Subscript>2</Subscript>O<Subscript>3</Subscript>-poor LAS glass–ceramics

Influence of MgO and K₂O on the nucleation behavior of Al₂O₃-poor LAS (Li₂O–Al₂O₃–SiO₂) base glasses was investigated by thermal analysis and, the effect on microstructure and surface topography of glass–ceramics was also examined by SEM, AFM and TEM. According to results of thermal analysis, the exothermic peak temperature of the glass showed a decrease with increase of nucleation temperature to nucleation time of 6 h. But some glasses nucleated for 9 h showed nucleation rate-like curve with maximum point. The dependence of reciprocal value of the exothermic peak temperature on the nucleation temperature indicated that an introduction of MgO might accelerate the nucleation of the base glass and thus result in rough surface topography of glass–ceramics. On the other hand, in the case of glass–ceramics containing K₂O the main crystalline phase was lithium metasilicate and they showed fine microstructure resulting in smooth surface topography. TEM micrographs of as-quenched and nucleated glasses showed no trace of phase separation affecting nucleation or final microstructure.     

Diffusion bonding of Ti<Subscript>3</Subscript>AlC<Subscript>2</Subscript> ceramic via a Si interlayer

Based on the structure characteristic of Ti₃AlC₂ and the easy formation of Ti₃Al_{1 − x} Si _x C₂ solid solution, a Si interlayer was selected to join Ti₃AlC₂ layered ceramic by diffusion bonding method. Joining was performed at 1,300–1,400 °C for 120 min under 5 MPa load in an Ar atmosphere. The phase composition and interface microstructure of the joints were investigated by XRD, SEM and EPMA. The results revealed that Ti₃Al(Si)C₂ solid solution formed at the interface. The mechanism of bonding is attributed to silicon diffusing inward the Ti₃AlC₂. The strength of joints was evaluated by a 3-point bending test. The jointed specimens exhibit a high flexural strength of 285 ± 11 MPa, which is about 80% of that of the Ti₃AlC₂; and retain this strength up to 1,000 °C. The high mechanical performance of the joints indicates that diffusion bonding via a Si interlayer is effective to bond Ti₃AlC₂ ceramic.     

Hydrogen-induced diffusion phase transformations in Nd<Subscript>2</Subscript>Fe<Subscript>14</Subscript>B and Sm<Subscript>2</Subscript>Fe<Subscript>17</Subscript> magnetically hard alloys

The Nd₂Fe₁₄B and Sm₂Fe₁₇ magnetically hard alloys become thermodynamically unstable under the action of hydrogen and suffer hydrogen-induced direct and inverse phase transformations at elevated temperatures. The kinetics of these transformations is investigated. It is shown that they are diffusion-controlled and develop according to the mechanism of nucleation and growth. Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 44, No. 5, pp. 105–111, September–October, 2008.     

Development of Fe<Subscript>3</Subscript>C,SiC and Al<Subscript>4</Subscript>C<Subscript>3</Subscript> compounds during mechanical alloying

Mechanical alloying process, as a solid-state technique, is a very useful method for fabrication of high melting point compounds like metal carbides and nitrides, which additionally have nanocrystalline structure with improved properties. In this work the development of several carbides including iron, aluminium and silicon carbides by the mechanical alloying process and the effect of subsequent heat treatment were investigated. Mixtures of elemental powders of Fe–C, Si–C and Al–C were mechanically alloyed, nominally at room temperature using a laboratory planetary ball mill. Structural changes of samples were studied by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The results showed that the aluminium carbide (Al₄C₃) could not be synthesized by mechanical alloying process alone, even after long milling times. A suitable subsequent heat treatment was required to allow Al–C reaction to take place kinetically. In contrast mechanical alloying of Fe–C as well as Si–C systems directly led to the formation of Fe₃C and SiC carbides after sufficient milling time. In all cases the end product had a nanosized structure.     

Preparation and thermomechanical properties of stir cast Al-2Mg-11TiO<Subscript>2</Subscript> (rutile) composite

Al-2Mg-11TiO₂ composite was successfully prepared by the conventional vortex method. The macrostructural observation revealed columnar structure with rutile particles being distributed throughout the matrix in the form of agglomerates. Microstructural observation showed the presence of micro voids in the particle-enriched zone. Electrical resistivity measurement showed a phase transformation at 360°C, which was consistent during DSC studies due to the precipitation of TiAl₃ phase. As-cast composite was both hot rolled and cold rolled successfully to 50 and 40% reduction, respectively. The mechanical properties of the thermomechanically-worked composite were studied. From fractographic analysis, it was clear that the crack had nucleated at the particle/matrix interface and propagated through the matrix by microvoid coalescence. Ultimate tensile strength of cold worked composite was found to be better than the hot worked material.     

Polypropylene/SiO<Subscript>2</Subscript> nanocomposites filled with different nanosilicas: thermal and mechanical properties,morphology and interphase characterization

Untreated and surface-treated SiO₂ nanoparticles with different alkyl chain length (described as C0, 3C1, C8 and C16 according to the number of carbon atoms) on particle surface were used as fillers for isotactic polypropylene (iPP). The iPP/SiO₂ composites containing 2.3 vol% of nanoparticles were prepared by melt blending and injection moulding. The dispersion quality of nanoparticles in matrix was examined using scanning electron microscopy (SEM). The crystallization behaviour of iPP was examined using differential scanning calorimetry (DSC). The mechanical properties of all samples were characterized by tensile test, compact tension (CT) test and dynamic mechanical thermal analysis (DMTA). The particle–matrix interphase behaviour was also examined and discussed. SEM images show that different silicas show different dispersion quality in matrix due to different hydrophobicity. The crystallinity and spherulite size of matrix are overall decreased in composites. The tensile properties of iPP/SiO₂ composites show clear relationship with alkyl chain length on particle surface, i.e. increasing alkyl chain length leads to decreased tensile modulus but increased tensile yield strength and strain, indicating increased interfacial interactions with increased alkyl chain length. The 3C1-composite shows the highest fracture toughness with an improvement by 9% compared to neat iPP, whereas the other composites show decreased values of fracture toughness.     

The influence of the cooling rate on bulk metallic glass formation in Mg<Subscript>80</Subscript>Cu<Subscript>15</Subscript>Y<Subscript>5</Subscript> and Mg<Subscript>80</Subscript>Cu<Subscript>10</Subscript>Y<Subscript>10</Subscript>

The Mg–Cu–Y system is known to be one of the best glass formers among the various existing magnesium alloys. The compositions chosen for the current study were Mg₈₀Cu₁₅Y₅ and Mg₈₀Cu₁₀Y₁₀. Different casting processes yielded four different microstructures that were analyzed by means of X-ray diffraction, scanning electron microscopy, high resolution scanning electron microscopy, and energy-dispersive X-ray spectroscopy chemical analysis. The different casting procedures were gravity castings of 3 mm diameter specimens into a copper mold held at different temperatures (cooled to −195 °C with the aid of liquid nitrogen, held at room temperature and heated up to 300 °C) and melt-spinning. Detailed microstructure study was then performed on the melt-spun specimen using transmission electron microscopy and high resolution transmission electron microscopy. The above-mentioned investigation revealed a crystalline rather than amorphous structure. The observed microstructure could not be explained on the basis of current models referring to the frequency of nucleation events.     

Mechanical properties of bovine hydroxyapatite (BHA) composites doped with SiO<Subscript>2</Subscript>, MgO,Al<Subscript>2</Subscript>O<Subscript>3</Subscript>, and ZrO<Subscript>2</Subscript>

Biologically derived hydroxyapatite from calcinated (at 850 °C) bovine bones (BHA) was doped with 5 wt% and 10 wt% of SiO₂, MgO, Al₂O₃ and ZrO₂ (stabilized with 8% Y₂O₃). The aim was to improve the sintering ability and the mechanical properties (compression strength and hardness) of the resultant BHA-composites. Cylindrical samples were sintered at several temperatures between 1,000 and 1,300 °C for 4 h in air. The experimental results showed that sintering generally occurs at 1,200 °C. The BHA–MgO composites showed the best sintering performance. In the BHA–SiO₂ composites, extended formation of glassy phase occurred at 1,300 °C, resulting in structural degradation of the resultant samples. No sound reinforcement was achieved in the case of doping with Al₂O₃ and zirconia probably due to the big gap between the optimum sintering temperatures of BHA and these two oxides.     

The Development of Microstructure and Ferroelectric Properties of Bi<Subscript>4</Subscript>Ti<Subscript>3.96</Subscript>Nb<Subscript>0.04</Subscript>O<Subscript>12</Subscript> Thin Films

Bi₄Ti_3.96Nb_0.04O₁₂ thin films were successfully deposited on Pt(111)/Ti/SiO₂/Si(100) substrates by a sol–gel method and rapid thermal annealing. The effects of Nb-substitution and annealing temperature (500–800°C) on the microstructure and ferroelectric properties of bismuth titanate thin films were investigated. X-ray diffraction analysis reveals that the intensities of (117) peaks are relatively broad and weak at annealing temperatures smaller than 700°C. With the increase of annealing temperature from 500°C to 800°C, the grain size of Bi₄Ti_3.96Nb_0.04O₁₂ thin films increases. The Bi₄Ti_3.96Nb_0.04O₁₂ thin films annealed at 700°C exhibit the highest remanent polarization (2P _r), 36 μC/cm² and lowest coercive field (2E _c), 110 kV/cm. The improved ferroelectric properties can be attributed to the substitution of Nb⁵⁺ to Ti⁴⁺ in Bi₄Ti₃O₁₂ assisting the elimination of defects such as oxygen vacancy and vacancy complexes.     

Preparation,crystal structure,and electrical conductivity of the solid electrolyte Zr<Subscript>0.86</Subscript>Sc<Subscript>0.12</Subscript>Y<Subscript>0.02</Subscript>O<Subscript>1.93</Subscript>

The solid electrolyte Zr_0.88Sc_0.12Y_0.02O_1.93 for reduced-temperature SOFCs has been characterized by Rietveld X-ray powder diffraction analysis and conductivity measurements in the temperature range 295–970 K. Gas-tight nanostructured ceramic composites consisting of cubic, rhombohedral, and monoclinic phases have been produced by reaction sintering of mechanochemically prepared powders. The oxygen ion conductivity of the ceramic prepared by sintering at 1630 K, with a relative density of 94%, is three times lower than that of ceramics fabricated from DKKK Zr_0.89Sc_0.1Ce_0.01O_1.95 powder, but raising the sintering temperature to 1670 K increases the density of the ceramic to 99%, and its conductivity reaches the level of the DKKK ceramics. The core-shell ceramic nanocomposite obtained in this study possesses high mechanical strength and a reduced activation energy for grain-boundary conduction.     

Detailed Evolution Mechanism of Interfacial Void Morphology in Diffusion Bonding

Similar diffusion bonding of 1Cr11Ni2W2MoV stainless steel was conducted at different bonding temperatures. The interface characteristics and mechanical properties of joints were examined, and the evolution of interfacial void morphology was analyzed in detail. The results showed that there were four typical interfacial void shapes generating sequentially: the large scraggly voids, penny-shaped voids, ellipse voids and rounded voids. The variation of interfacial void shape was dominated by surface diffusion, while the reduction of void volume was ascribed to the combined effects of plastic flow of materials around voids,interface diffusion and volume diffusion. Owing to the elimination of void from the bonding interface,the sound joint obtained could exhibit nearly full interfacial contact, and present excellent mechanical properties, in which the microhardness and shear strength of joint matched those of base material.     

The relationship between TiB<Subscript>2</Subscript> volume fraction and fatigue crack growth behavior in the in situ TiB<Subscript>2</Subscript>/A356 composites

The relationship between TiB₂ volume fraction and fatigue crack growth behavior in the A356 alloy matrix composites reinforced with 3, 5.6, and 7.8 vol% in situ TiB₂ particles has been investigated. The mechanisms of crack propagation in the TiB₂/A356 composites were also discussed. The results show that the 3 vol% TiB₂/A356 composite has nearly the same crack growth behavior as the matrix alloy, while the 5.6 vol% TiB₂/A356 composite exhibits a little bit faster crack growth rate. The 7.8 vol% TiB₂/A356 composite presents the lowest resistance to crack growth, indicating that the crack growth is accelerated by increasing TiB₂ volume fraction. Fractographies reveal that an increase in TiB₂ volume fraction results in a change from the formation of striation and slip to the failure of voids nucleation, growth, and coalescence. Cracks tend to propagate within the matrix and avoid eutectic silicon and TiB₂ particles in the intermediate ΔK region, while prefer to propagate along interfaces of eutectic silicon and TiB₂ particles and link the fractured eutectic silicon particles in the near fractured ΔK region. Furthermore, the propensity for the separation of TiB₂ increases with the increase in TiB₂ volume fraction. The massive voids caused by fractured eutectic silicon and separated TiB₂ particles propagate and coalesce, and then accelerates the crack growth in TiB₂/A356 composites.     

Fabrication and mechanical properties of WC particulate reinforced Cu<Subscript>47</Subscript>Ti<Subscript>33</Subscript>Zr<Subscript>11</Subscript>Ni<Subscript>6</Subscript>Sn<Subscript>2</Subscript>Si<Subscript>1</Subscript> bulk metallic glass matrix composites

The mechanical behavior of the WC particulate (WC_p) reinforced Cu₄₇Ti₃₃Zr₁₁Ni₆Sn₂Si₁ bulk metallic glass (BMG) matrix composites has been examined. The mechanical properties are improved with increasing WC_p content up to 20 wt%. The ultimate compression strength and plastic strain of the composite containing 20 wt% WC_p are 2.4 GPa and 2.4%, while those of the monolithic BMG are 1.6 GPa and ∼0%, respectively. The multiple shear band formation and crack deflections through WC particles have been identified as the main mechanism for the improved toughness.     

Electrical properties of pulsed laser deposited Bi<Subscript>2</Subscript>Zn<Subscript>2/3</Subscript>Nb<Subscript>4/3</Subscript>O<Subscript>7</Subscript> thin films for high K gate dielectric application

Metal Insulator Semiconductor (MIS) capacitors with monoclinic bismuth zinc niobate pyrocholre having the composition Bi2Zn2_/3Nb_4/3O₇ (m-BZN) dielectric layer were fabricated and characterized. Capacitance voltage (C–V) and current voltage measurements were utilized to obtain the dielectric properties, leakage current density and interface quality. The results shows that the obtained m-BZN thin films presents a high dielectric constant in between 30 and 70, a good interface quality with silicon and a leakage current density of 10 μA/cm² for a field strength of 100 kV/cm which is acceptable for high performance logic circuits. The equilent oxide thickness for the films annealed at 200 °C was 10 nm. These results suggest that m-BZN thin films can be potentially integrated as gate dielectric materials in CMOS technology.     

Micromechanical characteristics of flux-grown SmAlO<Subscript>3</Subscript> single crystal

Results on mechanical characterization of flux-grown samarium aluminate (SmAlO₃) crystals in the load range from 0.098 to 0.98 N are reported. The variation of microhardness with applied load is found to be nonlinear and is explained by using the Hays–Kendall law. The applicability of the Hays–Kendall law leads to the load-independent value of hardness. The indentations are associated with cracks only at higher loads (over 0.686 N) and the cracks developed are Palmqvist’s in nature. The hardness results and the indentation-induced cracking yield the value of fracture toughness (K_c ), brittleness index (B_i ), and yield strength (σ_Y ) for SmAlO₃ crystal.     

Dielectric relaxation in double perovskite oxide,Ho<Subscript>2</Subscript>CdTiO<Subscript>6</Subscript>

A new double perovskite oxide holmium cadmium titanate, Ho₂CdTiO₆ (HCT), prepared by solid state reaction technique is investigated by impedance spectroscopy in a temperature range 50–400°C and a frequency range 75 Hz–1 MHz. The crystal structure has been determined by powder X-ray diffraction which shows monoclinic phase at room temperature. An analysis of complex permittivity with frequency was carried out assuming a distribution of relaxation times as confirmed by Cole–Cole plot. The frequency dependent electrical data are analysed in the framework of conductivity and electric modulus formalisms. The frequencies corresponding to the maxima of the imaginary electric modulus at various temperatures are found to obey an Arrhenius law with an activation energy of 0·13 eV. The scaling behaviour of imaginary part of electric modulus suggests that the relaxation describes the same mechanism at various temperatures. Nyquist plots are drawn to identify an equivalent circuit and to know the bulk and interface contributions.