Preparation of B<Subscript>4</Subscript>C-ZrB<Subscript>2</Subscript>-SiC eutectic ceramics by arc melting method

The B₄C-ZrB₂-SiC ternary composites with super hard and high toughness were obtained by arc melting in argon atmosphere. Microstructures were observed by SEM, and phase compositions were analyzed by XRD. The hardness and fracture toughness of ternary composites are 28 GPa and 4.5 MPa·m^1/2. The eutectic mole composition is 0.39B₄C-0.25ZrB₂-0.36SiC, and the eutectic lamellar microstructure is composed of B₄C matrix with the lamellar ZrB₂ and SiC grains.     

Directional solidification and growth characteristics of Al<Subscript>2</Subscript>O<Subscript>3</Subscript>/Er<Subscript>3</Subscript>Al<Subscript>5</Subscript>O<Subscript>12</Subscript>/ZrO<Subscript>2</Subscript> ternary eutectic ceramic by laser floating zone melting

Directionally solidified Al₂O₃/Er₃Al₅O₁₂/ZrO₂ ternary eutectic ceramic in situ composite rods with length of 110 mm have been fabricated by laser floating zone melting. The microstructural characteristics of steady growth zone, initial growth zone and solid/liquid interface are investigated under high temperature gradient. In the steady growth zone, the eutectic spacing (λ) is rapidly decreased as increasing the growth rate (V), and the corresponding relationship between growth rate and eutectic spacing is determined to be λ = 11.14 × V ^?1/2. The temperature gradient has been measured to be about 5.3 × 10³ K/cm. In the initial growth zone, the melting process and temperature distribution are recorded by infrared thermal imager, and several unstable complex microstructures are observed. In the quenched zone, the regular eutectics with minimum eutectic spacing of 200 nm are obtained. Moreover, the solid/liquid interface during solidification shows convex interface morphology and the interface height is gradually decreased as increasing the growth rate. The eutectic growth behaviors at the center and edge of the as-grown rod are compared and discussed.     

Phase relations in the LaB<Subscript>6</Subscript>-W<Subscript>2</Subscript>B<Subscript>5</Subscript> system

The LaB₆-W₂B₅ join in the ternary system La-B-W is shown to have a eutectic phase diagram with t _e= 2220°C and a eutectic composition of 30 mol % LaB₆ + 70 mol % W₂B₅. Data are presented on LaB₆-containing systems potentially attractive for designing mixed-phase ceramics.     

Microstructure and high-temperature strength of Al<Subscript>2</Subscript>O<Subscript>3</Subscript>/Er<Subscript>3</Subscript>Al<Subscript>5</Subscript>O<Subscript>12</Subscript>/ZrO<Subscript>2</Subscript> ternary melt growth composite

A new Al₂O₃/Er₃Al₅O₁₂(EAG)/ZrO₂ ternary MGC (Melt Growth Composite) with a novel microstructure has been fabricated by unidirectional solidification. This ternary MGC has a microstructure consisting of continuous networks of single-crystal Al₂O₃, single-crystal EAG and fine cubic-ZrO₂ phases without grain boundaries. The ternary MGC has also characteristic dimensions of the microstructure of around 2–4 m for EAG phases, around 2–4 m for Al₂O₃ phases reinforced with around 0.4–0.8 m cubic-ZrO₂ phases. No amorphous phases are formed at interfaces between phases in the ternary MGC. The ternary MGCs flexural strength at 1873 K is approximately 700 MPa, more than twice the 330 MPa of the Al₂O₃/EAG binary MGC. The fracture manner of the Al₂O₃/EAG/ZrO₂ ternary MGC at 1873 K shows the same intergranular fracture as the Al₂O₃/EAG binary MGC, but is significantly different from the transgranular fracture of the sintered ceramic.     

Microstructure and properties of SiC-whisker-reinforced Si<Subscript>3</Subscript>N<Subscript>4</Subscript> ceramics with calcium aluminate additions

Silicon-nitride-based ceramics containing Al₂O₃-CaO sintering aids and reinforced with silicon carbide whiskers have been prepared by hot pressing at 1650°C in a nitrogen atmosphere, and their microstructure, phase composition, and mechanical properties have been studied. The results indicate that the Si₃N₄ ceramic containing 15 wt % calcium aluminate additions and 10 wt % SiC fibers has a dense microstructure with a uniform distribution of skeletal and dendritic silicon carbide crystals. The observed variations in the morphology of the crystals are tentatively attributed to the secondary crystallization of silicon carbide from the eutectic calcium aluminate melt during cooling.     

Phase relations in the LaB<Subscript>6</Subscript>-MoB<Subscript>2</Subscript> system

LaB₆-MoB_2.2 alloys have been prepared at 2600°C in high-purity helium in a vacuum induction furnace. As found by microstructural analysis and x-ray diffraction, the components of this system form a eutectic. The microstructure of the eutectic has been studied. MoB_2.2 is shown to undergo a transformation during cooling, with the precipitation of submicron-sized Mo₂B₅ and α-MoB particles. For this reason, its microhardness increases with an increase in the melting temperature of the alloys because the diffusion rate rises with temperature.     

Phase equilibria in the systems AgInSe<Subscript>2</Subscript>-HgIn<Subscript>2</Subscript>Se<Subscript>4</Subscript> and AgInSe<Subscript>2</Subscript>-HgSe

The equilibrium phase diagrams along the AgInSe₂-HgIn₂Se₄ and AgInSe₂-HgSe joins of the ternary system Ag₂Se-HgSe-In₂Se₃ have been constructed using X-ray diffraction and differential thermal analysis. Both joins are pseudobinary, with eutectic phase diagrams (type V in Roseboom’s classification). The eutectics are located at ≃30 mol % HgIn₂Se₄ (melting point of 1000 K) and ≃54 mol % HgSe (993 K), respectively. Both systems have considerable terminal solid-solution ranges.     

Elastic properties of Na<Subscript>2</Subscript>O-ZnO-ZnF<Subscript>2</Subscript>-B<Subscript>2</Subscript>O<Subscript>3</Subscript> oxyfluoride glasses

Elastic properties of Na₂O-ZnO-ZnF₂-B₂O₃ oxyfluoride glasses with different ZnF₂ concentrations have been investigated using ultrasonic velocity measurements at room temperature, at a frequency of 10 MHz. Glasses prepared by melt quenching method were suitably polished for the ultrasonic velocity measurements using pulse-echo superposition method. Various elastic moduli have been calculated and their compositional dependence has been examined. The compositional dependence of elastic moduli with the concentration of ZnF₂ shows decrease in the moduli initially, with further increase in ZnF₂ the moduli sharply increases and then again tend to decrease when ZnF₂ concentration is 20 mol%. The values of Poisson’s ratio lie in the range of 0·24–0·30, which is typical to covalent bonded network. The variation of θ _D with ZnF₂ indicates complex behaviour of the glass network. The results have been analysed in view of the modified borate glass network. Addition of ZnF₂ into the pure glass seems to influence the borate network by replacement of B-O-B linkages with B-O-Zn.     

Influence of CuO and B<Subscript>2</Subscript>O<Subscript>3</Subscript> on sintering and dielectric properties of tungsten bronze type microwave ceramics: a case study in Ba<Subscript>4</Subscript>Nd<Subscript>9.3</Subscript>Ti<Subscript>18</Subscript>O<Subscript>54</Subscript>

The effect of CuO and B₂O₃ co-doping on the sintering behavior, microstructure and microwave dielectric properties of tungsten bronze type Ba₄Nd_9.3Ti₁₈O₅₄ (BNT) ceramics has been investigated by means of a traditional solid-state mixed oxide route. On the one hand, it was indicated that the mixture of CuO and B₂O₃ is an effective sintering aid for BNT matrix compositions owing to the existence of a low-temperature eutectic reaction. On the other hand, it was found that the addition of CuO and B₂O₃ has an obvious effect on microwave dielectric properties of BNT ceramics, depending on the amount of sintering aids, the sample density and microstructure. The liquid phases from sintering aids can promote densification, but simultaneously induce grain growth which tends to decrease the sintering driving force. BNT ceramics doped with 3 wt% CuO–B₂O₃ mixture can be well sintered at 950°C for 4 h and still exhibit relatively good microwave dielectric properties.     

Ti<Subscript>3</Subscript>SiC<Subscript>2</Subscript>/TiC composites prepared by PDS

Synthesis of composite materials with improved mechanical properties is considered. Pulse discharge sintering (PDS) technique was utilized for consolidation and synthesis of double phase Ti₃SiC₂/TiC composites from the initial powders TiH₂/SiC/TiC. Scanning electron microscopy with energy-dispersive spectrometry (SEM with EDS) and X-ray diffractometry (XRD) were exploited for the analysis of the microstructure and composition of the sintered specimens. Mechanical tests showed high bending and compression strength and low Vickers hardness of Ti₃SiC₂-rich specimens. The reasons of this behaviour are in the features of the textured microstructure of Ti₃SiC₂ phase.     

Phase diagram of the W<Subscript>2</Subscript>B<Subscript>5</Subscript>-ZrB<Subscript>2</Subscript> system

The T-x phase diagram of the W₂B₅-ZrB₂ join in the ternary system Zr-W-B is mapped out. W₂B₅ and ZrB₂ form a eutectic at 20 mol % ZrB₂ with a melting point of 2180°C.Translated from Neorganicheskie Materialy, Vol. 41, No. 3, 2005, pp. 290–292.Original Russian Text Copyright © 2005 by Ordanyan, Boldin, Suvorov, Smirnov.This revised version was published online in April 2005 with a corrected cover date.This revised version was published online in April 2005 with a corrected cover date.     

Microstructure and phase transformations in a Ni<Subscript>50</Subscript>Mn<Subscript>29</Subscript>Ga<Subscript>16</Subscript>Gd<Subscript>5</Subscript> alloy with a high transformation temperature

A Heusler Ni₅₀Mn₂₉Ga₁₆Gd₅ alloy with a high transformation temperature has been obtained by substituting 5 at% Gd for Ga in a ternary Ni₅₀Mn₂₉Ga₂₁ ferromagnetic shape memory alloy. The microstructure and phase transformations in the Ni₅₀Mn₂₉Ga₁₆Gd₅ alloy have been investigated by scanning electron microscopy, transmission electron microscopy, X-ray diffraction and differential scanning calorimetry. It is shown that the microstructure of the Ni₅₀Mn₂₉Ga₁₆Gd₅ alloy consists of matrix and hexagonal Gd (Ni,Mn)₄Ga phase, which indicates a eutectic structure composed of these two phases. One-step thermoelastic martensitic transformation occurs in this quaternary alloy. Ni₅₀Mn₂₉Ga₁₆Gd₅ alloy exhibits a martensite transformation start temperature up to 524 K, approximately 200 K higher than that of Ni₅₀Mn₂₉Ga₂₁ alloy. At room temperature, non-modulated martensite with twin substructure is observed in Ni₅₀Mn₂₉Ga₁₆Gd₅ alloy.     

Development of hybrid Mg/Al<Subscript>2</Subscript>O<Subscript>3</Subscript> composites with improved properties using microwave assisted rapid sintering route

In the present study, hybrid magnesium based composites reinforced with an equivalent of 5 vol.% of micron and nano-sized Al₂O₃ particulates were synthesized using powder metallurgy technique incorporating an innovative microwave assisted rapid sintering technique. Microstructural characterization revealed near equiaxed grain morphology and the presence of minimal porosity in all the samples. Mechanical characterization studies revealed that the coupled addition of micron and nano-sized particulate reinforcements in magnesium matrix leads to a significant increase in hardness, elastic modulus, 0.2% yield strength, ultimate tensile strength and a decrease in ductility when compared to pure magnesium. Tensile testing results further revealed an increase in elastic modulus and ductility with no apparent change in the 0.2% yield strength and ultimate tensile strength of the hybrid composites upon the addition of nano-sized alumina particulates from 0.5 to 0.75 volume percent. With an increase in nano-sized alumina particulates from 0.75 to 1%, the overall mechanical properties of the hybrid composites were enhanced with an increase being observed in the elastic modulus, 0.2% yield strength and ductility of the composites. An attempt is made in this study to investigate the feasibility of the processing methodology and to study the effects of the addition of particulate reinforcements of different sizes on the microstructure, physical and mechanical properties of magnesium.     

Strengthening mechanism of nano-Al<Subscript>2</Subscript>O<Subscript>3</Subscript> particles reinforced Sn3.5Ag0.5Cu lead-free solder

To improve the properties of the eutectic Sn3.5Ag0.5Cu lead-free solder, various amounts of mixed nano-Al₂O₃ particles were added. The microstructure, thermal analysis, density, thermal expansion coefficient (CTE), and mechanical behavior were studied. The results of differential scanning calorimetry (DSC) indicate that the melting point of the composite solder doped with nano-Al₂O₃ particles is slightly higher that of the Sn3.5Ag0.5Cu lead-free solder and has a eutectic peak. The Sn3.5Ag0.5Cu composite solders exhibited lower density values and thermal expansion coefficient (CTE) values than did the unreinforced solder matrix. Compared to solder without the addition of nano-Al₂O₃ particles, the formation of dendritic β-Sn grains, the Ag₃Sn phase average size, and the spacing of lamellae decreased significantly in the composite solder matrix. The mechanical properties also improved with increasing weight percentages of nano-Al₂O₃ particles. However, the ductility of the Sn3.5Ag0.5Cu composite solder decreased. For the addition of 1 wt% nano-Al₂O₃ particles, microporosity was observed both at and along the grain boundary regions, coupled with the presence of second-phase particles (i.e. nano-Al₂O₃ and Ag₃Sn).     

Design of Si<Subscript>3</Subscript>N<Subscript>4</Subscript>-based ceramic laminates by the residual stresses

Ceramic laminates with strong interfaces between layers are considered a very promising material for different engineering applications because of the potential for increasing fracture toughness by designing high residual compressive and low residual tensile stresses in separate layers. In this work, Si₃N₄/Si₃N₄-TiN ceramic laminates with strong interfaces were manufactured by rolling and hot pressing techniques. The investigation of their mechanical properties has shown that the increase in apparent fracture toughness can be achieved for the Si₃N₄/Si₃N₄-20 wt.%TiN composite, while further increase of TiN content in the layers with residual tensile stresses lead to a formation of multiple cracks, and as a result, a significant decrease in the mechanical performance of the composites. Micro-Raman spectroscopy was used to measure the frequency shift across the Si₃N₄/Si₃N₄-20 wt.%TiN laminate. These preliminary Raman results can be useful for further analysis of residual stress distribution in the laminate.     

Vickers hardness measurements and some physical properties of Pr<Subscript>2</Subscript>O<Subscript>3</Subscript> doped Bi-2212 superconductors

This study deals with the effect of Pr₂O₃ addition on the structural, superconducting and mechanical properties of Bi-2212 superconductor by means of X-ray diffraction analysis (XRD), scanning electron microscopy (SEM), electron dispersive X-ray (EDX), Vickers microhardness and resistivity measurements. The samples studied are prepared using the standard solid-state reaction method. Onset transition temperatures (T _c ^onset) of the samples are estimated from the dc resistivity measurements. Furthermore, the phase ratio and lattice parameters a and c are determined from XRD patterns when the microstructure, surface morphology and element composition analyses of the samples are investigated by SEM and EDX measurements, respectively. Additionally, vickers microhardness, elastic modulus, yield strength and fracture toughness values of the samples are deduced from microhardness measurements. It is found that T _c ^onset values of the samples increase from 87 to 97 K with the Pr₂O₃ addition. According to the refinement of cell parameters done by considering the structural modulation, the doping is confirmed by both an increase of the lattice parameter a and a decrease of the cell parameter c of the samples in comparison with that of the pure sample. As for SEM measurements, it is obtained that the surface morphology and grain connectivity degrade with the increase of the Pr₂O₃ addition. Moreover, EDX images show that the elements used for the preparation of samples distribute homogeneously and the Pr atoms enter into the crystal structure by replacing Sr atoms. To sum up, the Pr₂O₃ addition is found to suppress the mechanical, microstructural and superconducting properties of the Bi-2212 superconductor.     

Structural and electrical properties of Ba(Fe<Subscript>1/3</Subscript>Nb<Subscript>1/3</Subscript>Ta<Subscript>1/3</Subscript>)O<Subscript>3</Subscript> perovskite

Ba(Fe_1/3Nb_1/3Ta_1/3)O₃ (BFNT) perovskite compound (phase purity>99%) was synthesized by conventional ceramic preparation method. XRD, microstructure, impedance spectroscopy and ac conductivity properties were analyzed in this study. BFNT compound has a cubic crystal structure, having grain size of 0.31 μm. This compound has shown normal ferroelectric behaviour but not obeying Curie-Weiss law. The impedance and electrical studies have been performed as a function of frequency and temperature. Impedance as a function of frequency revealed single relaxation process. The impedance spectroscopic plots exhibit the major response due to grains with partial contribution from the grain boundary and negligible electrode effect. Complex impedance plot showed data points lying on a single semicircle, implying the response originated from a single capacitive element corresponding to the bulk grains. Also, the centre of semicircle lies below the real axis indicating non-Debye type relaxation. Relaxation time was calculated from Z″_max of Cole–Cole plots. It is observed that conduction is due to hopping of charge carriers. Activation energies were computed from the Arrhenius plots of the sample.     

Influence of the concentration of Sb<Subscript>2</Subscript>O<Subscript>3</Subscript> on the electrical properties of SnO<Subscript>2</Subscript> varistors

Varistors are electronic materials with nonohmic behavior. In traditional SnO₂ varistors, CoO acts as a densifying agent, Nb₂O₅ increases the electrical conductivity of SnO₂ grains, and Cr₂O₃ produces a more uniform microstructure and acts as an oxygen retaining agent at the grain boundaries. The present work involved a systematic study of the substitution of Nb₂O₅ for Sb₂O₃ in the composition of a ternary varistor system. The compositions were prepared by conventional wet ceramic processing using deionized water, and the resulting slips were dried by spray-drying. Pellets were produced under a pressure of 330 MPa and sintered at 1,350 °C for 2 h. Similar to the behavior of Nb₂O₅, increasing the concentration of Sb₂O₃ reduced the nonlinear behavior of the ceramic and its breakdown electric field while increasing its leakage current. The samples’ microstructure showed greater porosity, suggesting that higher concentrations of Sb₂O₃ reduce the sintering rate, probably in response to the higher concentration of tin vacancies in the structure.     

Study of Fe<Subscript>3</Subscript>O<Subscript>4</Subscript> Nano-Magnetic Ferrofluid by Atomic Force Microscope

The magnetic ferrofluid is a special intelligent material and its many properties can be controlled by external magnetic field. This paper introduces the preparation, performance and applications of Fe₃O₄ nano-ferrofluid, principally observed by the microstructure characterization by Atomic Force Microscopy (AFM), and we measured the particle size and morphology of nano-ferrofluid. The magnetic property was analyzed from the microstructure characterization, and the analysis results show that the size of the Fe₃O₄ nano-particle is about 5 nm; the magnetic property is closely related to the chain microstructure and is influenced by the nano-particle distribution in the Fe₃O₄ magnetic ferrofluid.     

Crystallization and thermally induced surface relief effects in a Mg<Subscript>65</Subscript>Cu<Subscript>25</Subscript>Y<Subscript>10</Subscript> bulk metallic glass

The crystallization behaviour of Mg₆₅Cu₂₅Y₁₀ bulk metallic glass (BMG) under different reheating conditions was investigated. X-ray diffraction spectrometery (XRD), differential scanning calorimetery (DSC), scanning electron microscopy (SEM) and atomic force microscopy (AFM) were employed to examine the crystallization of different samples and the surface relief generated on as-polished surfaces during heat treatment. Different phase constituents were found in samples that experienced different reheating stages. It is proposed that both the reheating temperature and holding time have a significant effect on the phase constituents. The BMG was found to generate surface corrugations of amplitude 1–2 μm during annealing above its crystallization temperature. Such thermally induced surface relief effects are probably a result of the development of surface stresses generated by volumetric changes associated with crystallization of the residual amorphous phase.