VC- and Cr3C2-doped WC–NbC–Co hardmetals

This study compares the microstructure and mechanical properties of plain and 0.9 or 3.6 wt% VC- or Cr₃C₂-doped WC–12 wt% Co hardmetals with 40 wt% NbC, prepared by pulsed electric current sintering (PECS) in the solid state for 4 min at 1240 °C and conventional pressureless liquid phase sintering (CS) for 1 h at 1420 °C. The addition of VC or Cr₃C₂ was found to inhibit grain growth of the residual WC grains, whereas the size of the solid solution (Nb,W,V/Cr)C grains was hardly influenced. The type of grain growth inhibitor and densification temperature however, strongly influenced the composition of the NbC solid solution formed, which was thermodynamically and experimentally assessed.     

Low temperature sintering and properties of CaO–B2O3–SiO2 system glass ceramics for LTCC applications

The P₂O₅ + ZnO, ZrO₂ + TiO₂, B₂O₃ and a low-melting-point CaO–B₂O₃–SiO₂ glass (LG) are selected as the sintering additives, and the effect of their additions on the microwave dielectric properties, mechanical properties and microstructures of CaO–B₂O₃–SiO₂ system glass ceramics is investigated. It is found that the sintering temperature of pure CBS glass is higher than 950 °C and the sintering range is about 10 °C. With the above additions, the glass ceramics can be sintered between 820 °C and 900 °C. The dielectric properties of the samples are dependent on the additions, densification and microstructures of sintered bodies. The major phases of this material are CaSiO₃, CaB₂O₄ and SiO₂. With 10 wt% B₂O₃ and LG glass additions, the CBS glass ceramics have better mechanical properties, but worse dielectric properties. The _r values of 6.51 and 7.07, the tan δ values of 0.0029 and 0.0019 at 10 GHz, are obtained for the CBS glass ceramics sintered at 860 °C with 2 wt% P₂O₅ + 2 wt% ZnO and 2 wt% ZrO₂ + 2 wt% TiO₂ additions, respectively. This material is suitable to be used as the LTCC material for the application in wireless communications.     

Effect of the synthesis route on the microstructure and the reducibility of LaCoO3

The effect of the synthesis route on the microstructure and reducibility of lanthanum cobaltates (LaCoO₃) perovskites was examined. Two synthesis methods were used: thermal decomposition of freeze-dried La–Co-citrates and the Pechini method. The crystal structure, morphology and defect structure of LaCoO₃ were characterized by XRD powder diffraction, TEM and SEM analyses and electron paramagnetic resonance spectroscopy. The reducibility was tested by thermal programmed reduction with hydrogen. The intermediate stage of reduction was determined by ex situ XRD experiments. LaCoO₃ powders obtained by the Pechini method were reduced relatively easier as LaCoO₃ obtained from freeze-dried citrates. The LaCoO₃ reduction yielded Co metal and La₂O₃ via the formation of oxygen deficient Brownmillerite-type La₃Co₃O₈ and La₂Co₂O₅ oxides. For LaCoO₃ obtained from freeze-dried citrates and annealed at higher temperatures, Co metal, in addition to oxygen deficient perovskites, was formed at the initial stage of the reduction. The different reducibility of LaCoO₃ obtained by the Pechini method and that from the freeze-dried citrates was discussed taking into account the formation of oxygen-deficient phases from the Brownmillerite and Ruddlesden–Popper series during the reduction.     

New dielectric material system of La(Mg1/2Ti1/2)O3–Ca0.6La0.8/3TiO3 for microwave applications

The microstructure and microwave dielectric properties of xLa(Mg_1/2Ti_1/2)O₃–(1 − x)Ca_0.6La_0.8/3TiO₃ ceramics system with ZnO additions (0.5 wt.%) investigated by the conventional solid-state route have been studied. Doping with ZnO (0.5 wt.%) can effectively promote the densification and the dielectric properties of xLa(Mg_1/2Ti_1/2)O₃–(1 − x)Ca_0.6La_0.8/3TiO₃ ceramics. 0.6La(Mg_1/2Ti_1/2)O₃–0.4Ca_0.6La_0.8/3TiO₃ ceramics with 0.5 wt.% ZnO addition possess a dielectric constant (_r) of 43.6, a Q × f value of 48,000 (at 8 GHz) and a temperature coefficient of resonant frequency (τ_f) of −1 ppm/°C sintering at 1475 °C. As the content of La(Mg_1/2Ti_1/2)O₃ increases, the highest Q × f value of 62,900 (GHz) for x = 0.8 is achieved at the sintering temperature 1475 °C. A parallel-coupled line band-pass filter is designed and simulated using the proposed dielectric to study its performance.     

Pulsed electric current sintered Fe3Al bonded WC composites

WC based composites with 5, 10 and 20 vol.% Fe₃Al binder were consolidated via pulsed electric current sintering (PECS) in the solid state for 4 min at 1200 °C under a pressure of 90 MPa. Microstructural analysis revealed a homogeneous Fe₃Al binder distribution, ultrafine WC grains and dispersed Al₂O₃ particle clusters. The WC-5 vol.% Fe₃Al composite combines an excellent Vickers hardness of 25.6 GPa with very high Young’s modulus of 693 GPa, a fracture toughness of 7.6 MPa m^1/2 and flexural strength of 1000 MPa. With increasing Fe₃Al binder content, the hardness and stiffness decreased linearly to 19.9 and 539 GPa, respectively with increasing binder content up to 20 vol.%, while the fracture toughness and flexural strength were hardly influenced by the binder content. Compared to WC–Co cemented carbides processed under exactly the same conditions, the WC–Fe₃Al composites exhibit a substantially higher hardness and Young’s modulus.     

On the wide range of mechanical properties of ZTA and ATZ based dental ceramic composites by varying the Al2O3 and ZrO2 content

In this work the influence of pressureless sintering on the Vickers hardness and fracture toughness of ZrO₂ reinforced with Al₂O₃ particles (ATZ) and Al₂O₃ reinforced with ZrO₂ particles (ZTA) has been investigated. The ceramic composites were produced by means of uniaxial compacting at 50 MPa and the green compacts were heated to 1250 °C using a heating rate of 10 °C min⁻¹, then to 1500 °C at 6 °C min⁻¹ and maintained at this temperature during 2 h. After sintering, relative density over 94%, hardness values between 9.5 and 21.9 GPa, and fracture toughness as high as 3.6 MPa m^1/2 were obtained. The presence of TZ-3Y particles on the grain boundaries suggests that they inhibit notably the alumina grain growth. The grain sizes of pure Al₂O₃ and TZ-3Y as well as Al₂O₃ and TZ-3Y in the 20 wt% Al₂O₃+80 wt% TZ-3Y composite were 1.27 ± 0.51 μm, 0.57 ± 0.12 μm, 0.65 ± 0.19 μm and 0.41 ± 0.14 μm, respectively. The 20 wt% Al₂O₃ + 80 wt% ZrO₂ + 3 mol% Y₂O₃ (TZ-3Y) composite showed a hardness of 16.05 GPa and the maximum fracture toughness (7.44 MPa m^1/2) with an average grain size of 0.53 ± 0.17 μm. On the other side, the submicron grain size and residual porosity seem to be responsible for the high hardness and fracture toughness obtained. The reported values were higher than those obtained by other authors and are in concordance with international standards that could be suitable for dental applications.     

Electrical Properties of Atmospheric Plasma-Sprayed La10(SiO4)6O3 Electrolyte Coatings

In this present work, La₁₀(SiO₄)₆O₃ as a promising electrolyte candidate for intermediate temperature solid oxide fuel cells (IT-SOFCs) has been synthesized and its electrical property was investigated as a function of temperature. In order to improve the density and oxide ion conductivity of La₁₀(SiO₄)₆O₃, the feedstock powder was prepared by sintering the oxide mixture powders at proper sintering temperatures and times. The hexagonal apatite-type ceramic coatings with a typical composition of La₁₀(SiO₄)₆O₃ were deposited by atmospheric plasma spraying (APS) with different hydrogen flow rates. With increasing hydrogen flow rate oxide ion conductivity successively decreases. The highest ionic conductivity of the dense composite electrolyte coatings reaches a value of 2.4 mS/cm at 900 °C in air, which is comparable to other apatite-type lanthanum silicate (ATLS) conductors.     

Improved high-Q microwave dielectric resonator using ZnO-doped La(Co1/2Ti1/2)O3 ceramics

The microwave dielectric properties and the microstructures of ZnO-doped La(Co_1/2Ti_1/2)O₃ ceramics prepared by conventional solid-state route have been studied. Doped with ZnO (up to 0.75 wt%) can effectively promote the densification of La(Co_1/2Ti_1/2)O₃ ceramics with low sintering temperature. At 1320 °C, La(Co_1/2Ti_1/2)O₃ ceramics with 0.75 wt% ZnO addition possesses a dielectric constant (_r) of 30.2, a Q × f value of 73,000 GHz (at 8 GHz) and a temperature coefficient of resonant frequency (τ_f) of −35 ppm/°C.     

Effects of Y2O3 addition on the phase evolution and thermophysical properties of lanthanum zirconate

Lanthanum zirconate (La₂Zr₂O₇, LZ) powders with the addition of various Y₂O₃ contents for potential thermal barrier coatings (TBCs) application were synthesized by solid-state reaction. The structure evolution, sintering-resistance and thermophysical properties of the synthesized powders and sintered ceramics were systematically studied. X-ray diffraction (XRD) results indicate that LZ containing 3–12 wt.% Y₂O₃ mainly keeps a pyrochlore-type structure, and two new phases of LaYO₃ and Y_0.18Zr_0.82O_1.91 are also detected. Raman spectra confirm that the higher the Y₂O₃ content, the easier is the formation of LaYO₃. With the increase of Y₂O₃ content, the linear thermal expansion coefficients (TEC) of different La₂O₃–ZrO₂–Y₂O₃ ceramics may gradually decrease. Additionally, LZ ceramics doped with 3 wt.% Y₂O₃ have the lowest relative density and the highest sintering-resistance among all samples, implying that it is the best candidate for TBCs among those samples in this investigation.     

Structures, electrical and thermal expansion properties of Sr-doped La2Mo2O9 oxide-ion conductors

The oxide-ion conductors (La_1−xSr_x)₂Mo₂O_9−δ (x = 0.01–0.08) were prepared by means of a conventional solid-state reaction. The effects of Sr doping for La site on the structures, electrical and thermal expansion properties of the oxide-ion conductor La₂Mo₂O₉ were investigated using X-ray diffraction, direct current four-probe method, thermal dilatometer and scanning electron microscopy, respectively. The results show that the lattice constants were first decreased, then increased, and decreased again with the increase of Sr doping content. The solid solubility of Sr in (La_1−xSr_x)₂Mo₂O_9−δ is x = 0.07. The sinterability of samples is markedly improved with the increase of Sr doping content. The sintered density of sample x > 0.07 is higher than 96% of its theoretical density. When x > 0.02, doping Sr in La₂Mo₂O₉ can inhibit the excessive growth of grains, thus increases the sintered density of samples. The structural transition temperature shifts to the low side with the increase of Sr doping content, and the phase transition is completely suppressed when the doping content reaches 0.07. The conductivity of sample increases with the increase of Sr doping content. The conductivity of sample x = 0.07 reaches a maximum of 0.078 S/cm and 0.101 S/cm at 800 °C and 850 °C, respectively. In this study, it was demonstrated that doping 7 mol% Sr for La site not only can completely suppress the structural phase transition in La₂Mo₂O₉, but also can effectively enhance electrical conductivity of samples at higher temperature.     

Hydrogen diffusion studies in Zr-based Laves phase AB2 alloys

The diffusion of hydrogen in the tetrahedral interstitials of bulk spherical Laves phase ZrMn_0.85Cr_0.1V_0.05Fe_0.5Ni_0.5 and ZrMn_0.85Cr_0.1V_0.05Fe_0.5Ni_0.5 + 1 wt% B alloys has been studied in the α-phase (solid-solution) region over the temperature range 450–650 °C, for hydrogen pressures up to 100 mbar using Sieverts-type apparatus. The diffusion constants have been determined from the gas–solid reaction, where the gas pressure dependence on time has been measured at fixed temperature. The results have been discussed on the basis of Fick's law of diffusion. The dependence of diffusion constant on alloy composition and initial pressure has been evaluated. Activation energy has been obtained from the temperature dependence of diffusion using Arrhenius relation.     

Effect of MoSi2/rare earth composite particles on microstructure and mechanical properties of molybdenum

MoSi₂/La₂O₃ and MoSi₂/Y₂O₃ composite particles were prepared by mechanical milling and doped into molybdenum by solid-solid method, respectively. Rods with a diameter of 17 mm were made by pressing and sintering. The effects of different composite particles on microstructures and strength of the as-sintered molybdenum were investigated. Results show that the MoSi₂/La₂O₃ and MoSi₂/Y₂O₃ composite particles transformed to La₂O₃/Mo₅Si₃ and Y₂O₃/Mo₅Si₃ composite particles due to the in situ reaction between Mo and MoSi₂ during sintering process. Mo₅Si₃/La₂O₃ and Mo₅Si₃/Y₂O₃ composite particles can reduce the grain size and improve both strength and toughness of sintered molybdenum significantly. Mo₅Si₃/Y₂O₃ composite particles contribute more to the strength, while the effect of Mo₅Si₃/La₂O₃ on toughness is greater than that of Mo₅Si₃/Y₂O₃.     

Mechanical properties improvement related to the isothermal holding time in Si3N4 ceramics sintered with an alternative additive

In the present work, the influence of the isothermal holding time on the physical (relative density and mass loss), chemical (α–β transformation and intergranular phase crystallization) and mechanical (hardness and fracture toughness) properties of Si₃N₄ ceramics with Al₂O₃ and CTR₂O₃ as additives has been studied. CTR₂O₃ is a natural rare earth oxide mixture, produced at DEMAR-FAENQUIL from the mineral xenotime, consisting mainly of Y₂O₃, Yb₂O₃, Er₂O₃ and Dy₂O₃. The increase in hardness and fracture toughness with increasing duration of isothermal sintering is discussed in regard of densification, α–β Si₃N₄ phase transformation and microstructure. The microstructural variations were decisive for the increase of fracture toughness, because larger grains (>4 μm) with higher aspect ratios (>6) developed during increased sinter periods, enhancing crack deflection and crack-bridging mechanism. In this way longer isothermal holding times contribute to the improvement of the physical and mechanical properties of silicon nitride based ceramics.     

Formation and properties of BaxFe3−xO4 with spinel structure by mechanochemical reaction of α-Fe2O3 and BaCO3

Magnetic Ba_xFe_3−xO₄ (x  0.23) with spinel structure was fabricated by ball milling of mixture of BaCO₃ and nonmagnetic α-Fe₂O₃ powders, and the molar ratio of BaCO₃ and α-Fe₂O₃ is 1:6. In the milling process, a mechanochemical reaction took place between BaCO₃ and α-Fe₂O₃, and Ba cation incorporated into α-Fe₂O₃ with rhombohedral structure to form a α-(Fe,Ba)₂O₃ solid solution. The Ba content in the α-(Fe,Ba)₂O₃ increased with increasing milling time, when the Ba content exceeded a limited solubility, the α-(Fe,Ba)₂O₃ transformed into a phase of Ba_xFe_3−xO₄ with spinel structure, where the Ba cation occupied an octahedral site or tetrahedral site. The product obtained in the balling process was different from that prepared in the annealing process at atmospheric pressure, which was BaFe₂O₄ with orthorhombic structure. Accompanying the crystal structure transition from α-(Fe,Ba)₂O₃ to Ba_xFe_3−xO₄, the magnetic properties also changed from nonmagnetism into ferromagnetism. The saturation magnetization was 53.3 emu/g and coercivity was 113.7 Oe. The mechanism of transitions of the crystal structure was discussed in the present work.     

B2O3 modified SiC–MoSi2 oxidation resistant coating for carbon/carbon composites by a two-step pack cementation

To protect carbon/carbon (C/C) composites against oxidation, a B₂O₃ modified SiC–MoSi₂ coating was prepared by a two-step pack cementation. The microstructure and the oxidation resistant property of the coating were studied. The results show that, the as-received coating is a dense structure, and is composed of α-SiC, β-SiC and MoSi₂. The B₂O₃ modified SiC–MoSi₂ coating has excellent oxidation resistant property, and can protect C/C composites from oxidation at 1773 K in air for more than 242 h. The failure of the coating was considered to arise from the existence of the penetration cracks in the coating during the slow cooling from 1873 to 673 K.     

Preparation and properties of sintered molybdenum doped with La2O3/MoSi2

Sintered Mo with the addition of La₂O₃/MoSi₂ was prepared via the process of solid–solid doping + powder metallurgy. X-ray diffraction experiment, hardness test, three-point bending test and high-temperature tensile test were carried out to characterize the samples. The XRD pattern of a typical sample shows that the sintered Mo was mainly composed of Mo, La₂O₃ and Mo₅Si₃. Mo₅Si₃ was probably formed through the reaction between MoSi₂ and the Mo matrix. Densities and fracture toughnesses of both doped Mo and pure Mo were measured and contrasted. Sintered Mo with the addition of 0.2 wt% La₂O₃/MoSi₂ has the highest toughness, while more addition of La₂O₃/MoSi₂ has smaller effect on improving toughness or even embrittles Mo. The results of three-point bending test and high-temperature tensile test show that the bending strength and high-temperature tensile strength of doped Mo are both higher than those of pure Mo. The formation of Mo₅Si₃ improves the high-temperature strength. The La₂O₃/Mo₅Si₃ dispersed in the Mo matrix refined the grains, and thus strengthened the Mo matrix by dispersion strengthening and grain refinement.     

Ethanol-assistant solution combustion method to prepare La2O2S:Yb,Pr nanometer phosphor

La₂O₂S:Yb,Pr nanometer phosphor have been prepared by a novel ethanol-assistant solution combustion (EASC) method in ethanol–water solution medium via a relatively mild and slow combustion process, using ethanol as preigniting fuel and inexpensive thioacetamide as sulfur-containing organic fuel. Investigation results show: ethanol preigniting serves the role of ignition combustion decomposition reaction between rare earth nitrate and organic fuel and promotes the formation of uniform and less agglomerative rare earth oxysulfide nanocrystals. Single phase Ln₂O₂S:Yb,Pr nanocrystals are obtained with appropriate ethanol/water ratio and further annealing at 1000 °C for 2 h in reduced atmosphere. The La₂O₂S:Yb,Pr nanocrystals exhibit an intense green emission assigned to the Pr³⁺ ions ³P₀ → ³H₄ transition and a broad near infrared (NIR) emission under 980 nm pump. The defects residing in La₂O₂S:Yb,Pr nanocrystals is the probable origins for the NIR emission. The annealing process makes the green emission improve and the NIR emission weaken. After annealing treatment at 1000 °C, the upconversion luminescence intensity is close to that of the bulk sample prepared by a solid-state reaction method.     

Dielectric dispersion in Li2O–MoO3–B2O3 glass system doped with V2O5

Li₂O–MoO₃–B₂O₃ glasses containing different amounts of V₂O₅, ranging from 0 to 1.5 mol%, were prepared. The dielectric properties (viz., constant ′, loss tan δ, AC conductivity σ_ac over a wide range of frequency and temperature) have been studied as a function of the concentration of vanadium ions. The variation of AC conductivity with the concentration of V₂O₅ passes through a maximum at 0.8 mol% V₂O₅. In the high-temperature region, the AC conduction seems to be connected with the mixed conduction, viz., electronic and ionic conduction. The dielectric relaxation effects exhibited by these glasses have been analyzed quantitatively by pseudo Cole–Cole plot method and the spreading of relaxation times has been established. Further analysis of these results has been carried out with the aid of the data on ESR, IR and optical absorption spectra.     

A new ternary compound YbZn2As2 with mixed valency of Yb

A new compound with chemical formula YbZn₂As₂ and ‘anti’-La₂O₃-type crystal structure (space group P3m1) has been synthesized for the first time. The trigonal lattice constants of the compound are a=0.4157 and c=0.6954 nm. In the temperature range 77–500 K, the magnetic susceptibility of YbZn₂As₂ follows the Curie–Weiss law, indicating antiferromagnetic interactions of the Yb ions and yielding a Curie temperature θ=−52.8 K and an effective magnetic moment μ_eff=2.35 μ_B (the Bohr magneton) per Yb ion. This means that a part of the Yb ions has valency 3+, instead of 2+ for all the Yb ions, as would be expected from their formal oxidation number, i.e. YbZn₂As₂ is a compound with mixed valency of Yb. YbZn₂As₂ exhibits p-type conductivity with a room temperature electrical resistivity of 0.15 Ω cm which decreases when lowering temperature and reaches a practically constant value of 0.08 Ω cm below 20 K.     

Effect of rare earth La2O3 on the microstructure and mechanical properties of TiC/W composites

In this study, La₂O₃ was investigated as an additive to TiC/W composites. The composites were prepared by vacuum hot pressing, and the microstructure and mechanical properties of the composites were investigated. Experimental results show that the grain size of the TiC/W composites is reduced by TiC particles. When 0.5 wt.% La₂O₃ is added to the composites, the grain size is reduced further. According to TEM analysis, La₂O₃ can alleviate the aggregation of TiC particles. With La₂O₃ addition, the relative density of the TiC/W composites can be improved from 95.1% to 96.5%. The hardness and elastic modulus of the TiC/W + 0.5 wt.% La₂O₃ composite are little improved, but the flexural strength and the fracture toughness increase to 796 MPa and 10.07 MPa·m^1/2 respectively, which are higher than those of the TiC/W composites.