Liquid Phase Sintering Method Research Articles

Structure, non-stoichiometry, valence of ions, dielectric and magnetic properties of single-phase Bi0.9La0.1FeO3−δ multiferroics

Abstract The structure, microstructure, valence states, non-stoichiometry, dielectric and magnetic properties of lanthanum-modified multiferroics have been studied by X-ray diffraction, thermogravimetric, iodometric titration, SEM, XPS, dielectric spectroscopy and magnetic methods. Multiferroics of bismuth ferrite have been obtained by a rapid liquid phase sintering method under different pressures, P, for compacting stoichiometric mixture of precursors. On the basis of the experimental data, the molar formulas of real BiFeO3−δ and Bi0.9La0.1FeO3−δ structures have been determined. The real structure contains Bi3+, La3+, Fe3+, Fe2+ and O2− ions as well as cation V(c) and anion V(a) vacancies. The optimal temperature regimes of the rapid liquid phase sintering method for obtaining the single-phase Bi0.9La0.1FeO3−δ, whose composition corresponds to the concentration region of destruction of a spin cycloid, have been defined. It has been established that oxygen non-stoichiometry δ and concentration of Fe2+ strongly depend on the pressure P. The initial dielectric permittivity of the Bi0.9La0.1FeO3−δ multiferroics can be controlled and changed by the pressure P more than 5000 times. The correlations between the composition, structure, non-stoichiometry, dielectric and magnetic properties in the BiFeO3−δ and Bi0.9La0.1FeO3−δ have been established. In the pure BiFeO3−δ at room temperature, rhombohedral distortions of a hexagonal structure with indications of a ferromagnetic double exchange have been detected. The magnetic structure of the single-phase Bi0.9La0.1FeO3−δ is homogeneous with a large value of coercivity, HC ≥ 10 kOe, at room temperature. An analysis of the magnetic properties indicates the appearance of a weak ferromagnetism due to the destruction of the spin cycloid by lanthanum ions.

Dynamic oxidation protective behaviors and mechanisms of HfB2-20wt%SiC composite coating for carbon materials

HfB2-20%wtSiC composite coating was prepared by liquid phase sintering method. After modification of HfB2 phase, the initial oxidation consumptions of the SiC coated samples were delayed from 500 ℃ to 800 ℃. Due to the higher oxidation activity of HfB2, the sufficient generated B2O3 is capable of inhibiting oxidation consumption of carbon substrate in oxidation activation region (800 ℃-1000 ℃) and fastest oxidation region (1000 ℃-1280 ℃). The enhanced oxidation activity of SiC above 1000℃ leads to the increased generation of SiO2, inhibiting the evaporation of B2O3 through the formation of Hf-B-Si-O glass layer, improving its stability and oxidation resistance above 1000℃. The heterogeneous refractory Hf-Oxides embedded in Hf-B-Si-O glass layer play role of reinforcement phases, restricting generation and spread of cracks. The inerting effect of Hf-B-Si-O glass layer strengthened with the increase of thermogravimetric analysis (TG) recycle oxidation times, indicating promising oxidation inhibition potential of the coating in dynamic aerobic environment.

Dynamic oxidation protective ultrahigh temperature ceramic TaB2-20%wtSiC composite coating for carbon material

TaB2–20%wtSiC composite coating containing high content of TaB2 was synthesized by liquid phase sintering method to enhance the anti-oxidation property of the carbon materials. The coated sample exhibits a trend of sustained weight gain whose final value is about 1.35%, while there is no weight loss zone in weight loss rate curves, but a rapid weight gain region around 800 °C. The generated compound glass layer presents a structure of Ta-oxides/TaBSiO/SiO2 triple glass layer. The rough surface of top Ta-oxides layer increases the oxidation defensive area of the glass layer, while the self-sealing TaBSiO/SiO2 compound glass ceramics layer can arrest the microcracks, blocking the fast diffusion channel of oxygen.

텅스텐의 열충격 손상에 대한 초음파 특성 평가"

Tungsten is used as a high temperature component because it does not change its mechanical and physical properties even at high temperatures, and it is manufactured by sintering. However, it is hard to produce tungsten because of the high sintering temperature. One of objectives of this study is to lower the sintering temperature by using additives and pressure. The damage characteristics due to thermal shock on the tungsten produced by this liquid phase sintering method were evaluated using a nondestructive technique. By using the ultrasonic pulse receiver technique, the amplitude and attenuation coefficient of the ultrasonic wave were measured as the thermal shock cycle of the material was increased, and the relationship between the thermal shock cycle and the attenuation coefficient was analyzed. It has been found that the tungsten surface crack increases as the thermal shock cycle increases, and the attenuation coefficient increases linearly due to scattering of wave by increasing cracks.

Ultrahigh temperature ceramic HfB2-SiC coating by liquid phase sintering method to protect carbon materials from oxidation

To investigate the anti-oxidation modification behaviors and mechanisms of the pure HfB2 phase in dynamic aerobic environment, HfB2-SiC coating was prepared through liquid phase sintering method. The average particle size of the synthetic pure phase HfB2 powders is about 335 nm. The TG curve of pure HfB2 powders exhibits excellent anti-oxidation modification ability for Si-based coating. After the modification of HfB2 phase, the anti-oxidation ability of SiC coating achieved prominent improvement. The initiative mass loss temperature of SiC coating lagged by 41.8%, while the percentage of mass loss and rate in fastest mass-loss zone of the SiC coating lagged by 57.6% and 33.3%, respectively. The generated B2O3 is responsible for the obviously improved oxidation resistance below 1200 °C. It compensates the oxidation protection weaknesses of the Si-based ceramic coating in this temperature region. With the fluid of the SiO2 glass layer, the dispersed Hf-oxides were inlaid in it to form the compound Hf-O-Si glass layer. The Hf-oxides heterogeneous phases with ultra-high melting temperature are able to improve the stability of the SiO2 glass layer at ultra-high temperature. It improves the ability of microcracks restriction and reduces the erosion of oxygen to the carbon matrix.

MACROSTRUCTURE AND STRENGTH OF AL–ZN–SN COMPOSITE PRODUCED BY LIQUID PHASE SINTERING OF AL–ZN ALLOY AND PURE TIN POWDER MIXTURE

The paper studies the liquid phase sintering features of compacts made of Al–10Zn alloy and Grade PO 2 tin powder mixture as well as the effect of sintering modes on the structure and strength of the (Al–10Zn)–40Sn antifriction composite formed. The porosity of original raw compacts ranged from 5 to 18 %. They were sintered in a vacuum furnace at a residual pressure of gases lower than 10–2 MPa. Sintering temperature varied from 550 to 615 °С, where partial aluminum wetting with liquid tin was observed. Sample holding time at a given sintering temperature was 30–180 min. Structural studies have shown that the particle size of aluminum and tin phases increased with an increase in sintering temperature and holding time. Mechanical properties of sintered composites were determined by the compression test. Test samples were cut from the middle area of sintered compacts. The tests have shown that (Al–10Zn)–40Sn composite samples have high ductility. Moreover, these samples exhibit higher strength in comparison with Al–40Sn sintered composite with a pure aluminum matrix due to more intensive strain hardening of the matrix at high deformation levels. It was found that the composites obtained when sintering samples with a low initial porosity and subjected to pre-exposure at low temperature have the highest strength. Based on the reported results it can be concluded that the liquid-phase sintering method within the specified temperature range allows to obtain the (Al–10Zn)–40Sn composites with a continuous aluminum matrix to effectively prevent localized deformation in the soft Sn interlayers. The optimum sintering temperature should not exceed 600 °С.

Low temperature synthesis of pure phase TaB2 powders and its oxidation protection modification behaviors for Si-based ceramic coating in dynamic oxidation environments

To reveal the generation mechanisms of the Ta-Si-O glass ceramics layer in dynamic oxidation environments, a 40 wt% TaB2-SiC coating was prepared by liquid phase sintering method. To obtain pure phase TaB2 powders at lower temperature (1500 °C), excessive B2O3 powders were added in raw materials to eliminate the TaC byproduct phase. The hexagonal pure phase TaB2 powders own average particle size of about 386 nm. During the TGA dynamic oxidation tests, after the modification of 40 wt% TaB2, the initial weight loss temperature of the sample delayed by about 48%, while the weight loss percentage and rate in fastest weight loss zone decreased by about 61% and 53%, respectively. During oxidation, the generated Ta-oxides were peeled and carried away by the formed fluid SiO2 glass layer to form “Ta-oxides halation” at first, which results the dissolution of Ta-oxides in the SiO2 glass, thus forming the Ta-Si-O glass ceramics with dendritic structure. With the spread of the SiO2 glass layer and growth of the Ta-Si-O dendrite, the Ta-Si-O glass ceramics gradually cover on the surface of the SiO2 glass layer, forming the structure of Ta-Si-O/SiO2 double glass layer that is capable of sealing and arresting of microcracks.

Preparation of TaB2-SiC oxidation protective coating for carbon materials by liquid phase sintering

To control the microstructure and amounts of TaB2 phase in the TaB2-SiC coating, a novel liquid phase sintering method was developed on the basis of in-situ reaction method to prepare the TaB2-SiC coating, which includes synthesis of TaB2 powders and further preparation of TaB2-SiC coating. With Ta2O5, B2O3 and C employed as raw materials, hexagonal TaB2 powders were prepared by carbothermal reduction method at 1500 °C, whose mean particle size is 491 nm. The TaB2, SiC, C powders, and the low melting point phases Si and silica sol were used to prepare the TaB2-SiC coating by liquid phase sintering at 2373 K. The thickness of the coating is about 350 µm. Compared with the SiC coating, the weight loss of the samples modified by TaB2 decreased from 17.7% to 11.8%, and the average weight loss rate of the fastest weightloss zone reduced from −6 × 10−3 mg cm−2 s−1 to −5 × 10−3 mg cm−2 s−1. During oxidation, the Ta-oxides would gradually dissolve in the silicate glass to form Ta-Si-O glass ceramics with dendritic structure, which significantly improved the toughness and stability of the glass layer. The Ta-Si-O glass ceramics possesses the ability of sealing and arresting the microcracks, which can enhance the oxidation protective ability of the coating.

Microstructure and Mechanical Properties of SiC-Materials Sintered in the Liquid Phase with the Addition of a Finely Dispersed Agent

The free liquid phase sintering method was applied to obtain dense (relative density 99.5%) materials based on silicon carbide. A eutectic mixture of MgO, Y2O3 and Al2O3 of the composition corresponding to the coexisting garnet – spinel phase curve was used as the sintering agent. In the preparation of the charge powders, the method of chemical deposition from salt solutions was used, that produced eutectic composition oxides uniformly distributed on the silicon carbide powder particles. The materials produced show good mechanical properties: elastic strain energy 410 GPa, flexural strength 680 MPa, KIc = 6.0 MPa·m1/2, HV 21.5 GPa, the values of which are close to the corresponding indices for the hot-pressed carbide-silicon materials.

Preparation and characterization of macroporous SiC ceramic membrane for treatment of waste water

Porous SiC based materials present high mechanical, chemical and thermal robustness and thus have been largely applied to water-filtration technologies. In this study, circular disc shaped SiC microfiltration membranes were prepared by dry pressing of commercially available SiC powder with yttria and alumina as additives followed by a low-cost oxide bonding technique. The membranes fabricated were characterized using standard characterization techniques like Scanning Electron Microscopy (SEM), Powder X-ray Diffraction (PXRD), porosity and pore size distribution analysis and compared with the membrane prepared by liquid phase sintering route from the same powder composition. Finally, water permeation studies were carried out in a standard membrane module and clean water flux was determined. These membranes were found well suited for treatment of oily waste water and grey water. The membrane prepared by oxide bonding method effectively removed ~ 89–93% of COD, ~ 77–86% of oil/grease and 88.4–92% of TSS from kitchen waste water and the removal efficiency are better compared to the membrane prepared by liquid phase sintering method. The effects of corrosions on the membranes were investigated in strong acid and alkali solution at 90 °C. The membranes prepared by oxide bonding method showed better corrosion resistance with retention of mechanical strength.

Intrusion features of a high-speed striker of a porous tungsten-based alloy with a strengthening filler in a steel barrier

The complex problem of increasing the penetrating power of strikers based on highly porous tungsten composites is considered by improving their strengthening properties by alloying the hardening components under high-speed collision conditions. Using the method of liquid-phase sintering, we fabricated samples of strikers based on a porous WNiFeCo alloy (tungsten + nickel + iron + cobalt), alloyed with tungsten carbide with cobalt (WCCo8) and titanium-tungsten carbide (TiWC). Dynamic tests of the strikers from the developed alloys were carried out at the collision velocity with a steel barrier of the order of 2800 m/s. The penetration depth of the striker based on a porous WNiFeCo alloy doped with tungsten carbides is 30% higher than the penetration depth of a striker of a monolithic WNiFe-90 alloy (tungsten + nickel + iron with a tungsten content of 90%).

Fast initializing solid state electrochemical carbon dioxide sensor fabricated by a tape casting technique using yttria stabilized zirconia and sodium beta alumina heterojunction

Solid-state bi-electrolyte carbon dioxide sensors with an yttira stabilized zirconia (YSZ) and sodium beta alumina (NBA) heterojunction were fabricated using the tape casting method to measure CO2 concentrations in the air. The enhanced interface between NBA, expressed by (1+x) Na2O·11 A12O3 and YSZ was made by the liquid phase sintering method while varying the ratio of Na2O/Al2O3 from x=0 to x=0.6. YSZ/Pt electrode was used as an air reference, maintaining a constant oxygen concentration of 21%, and Na2CO3 was used as a CO2 sensing electrode. The responses of the sensors to various concentrations of CO2 were evaluated at temperatures ranging from 550°C to 650°C. The electric potential difference of the sensors exhibited Nernstian behavior with sensitivity greater than 99% of the theoretical value at 650°C. The activity of Na2O at the bi-electrolyte junction remained constant, showing invariance with the CO2 concentrations tested. The time for the initial stabilization was as short as 66s.

SINTERING AS A METHOD OF PRODUCING HARD AL–SN COMPOSITES WITH A HIGH SECOND PHASE CONTENT

The structure and mechanical properties of Al–Sn composites produced by vacuum liquid-phase sintering of a mixture of aluminum (ASD-4) and tin (PО2) powders were studied. Sintering of raw briquettes with a porosity of ~15 % was carried out at a temperature of 570–620°C and a holding time of 0,5 to 2,0 hours. The tin concentration in briquettes was increased by step of 10 wt.% and reached 50 wt.%. It was found that the liquid-phase sintering method makes it possible to produce composites with a high second phase content and a continuous Al-matrix capable to prevent localization of deformation in layers of soft Sn-phase under external loading. The optimal composite sintering mode corresponds to a holding time of 1 hour at a temperature of 600 °C. The increase of tin proportion leads to a decrease of the aluminum matrix binding, wherein the matrix remains continuous when the tin content does not exceed 50 wt.% (27 vol.%). Evaluation of mechanical properties of sintered materials was carried out by the compression test. The strength of produced sintered composite materials (CM) is described by an ideal mixture formula: σ КМ = σ Al f Al + σ Sn f Sn , where σ Sn is a constant, because tin is not hardened, and σ Al value is determined by the compression curve of pure aluminum.

Effect of Zr substitution on structural, magnetic, and optical properties of Bi0.9Dy0.1Fe1−xZrxO3 multiferroic ceramics prepared by rapid liquid phase sintering method

Zr substituted Bi0.9Dy0.1Fe1−xZrxO3 (x=0.03, 0.06 and 0.10) multiferroic ceramics were synthesized by rapid liquid phase sintering technique to improve its multiferroic properties. Rietveld structural refinement of XRD patterns and Raman spectra revealed a partial structural phase transition from rhombohedral (R3c) to biphasic structure (R3c+P4mm) on codoping. The substitution of larger ionic radii and higher valence Zr4+ ions at Fe-site leads to decrease in the grain size as a result of charge compensation at Fe site. The weak ferromagnetic behavior were observed in all samples along with maximum Mr value of 0.159emu/g for x=0.03 concentration, which is also endorsed by second order Raman modes. The distortion in FeO6 octahedra due to Zr substitution leads to splitting of electronic bands of 3.2eV into multiplets, which in turn reduced the optical band gap value in the range of 2.06–2.10eV for all samples.

Crystal structure refinement, ferroelectric and ferromagnetic properties of Ho3+ modified BiFeO3 multiferroic material

Multiferroic Bi1-xHoxFeO3 (x = 0, 0.05, 0.1) ceramics have been prepared by rapid liquid phase sintering method. The effect of Ho3+ doping on the crystal structure, dielectric, ferroelectric properties, TN and TM of BiFeO3 ceramics is studied. The result shows that all the peaks for Bi1-xHoxFeO3 samples can be indexed according to the crystal structure of pure BiFeO3 by XRD and the grain size from 1 to 5 μm is observed for BiFeO3 samples with Ho3+ doped. The dielectric behavior of Bi1-xHoxFeO3 ceramics varies with frequency and temperature, which might be understood in terms of an oxygen vacancy, the displacement of Fe3+ ions and lattice phase transition. There is a perfect dielectric hysteresis phenomenon in the ɛr-V curves of Bi1-xHoxFeO3 samples at bias voltage with 10 V. Under the action of the bias voltage, more and more dipoles are“frozen”in the bias voltage direction and quit polarization gradually, resulting in lower dielectric constant. The unsaturated P-E hysteresis loop of Bi1-xHoxFeO3 (x = 0.05, 0.1) samples is obtained with a large 2Pr of 3.08 μC/cm2. The Pr, Mr of Bi0.9Ho0.1FeO3 sample is nearly 23, 35 times as large as that of BiFeO3, respectively. It can be inferred that Ho3+ doping in BiFeO3 ceramics is proved to be an effective way to modulate the ferroelectric and the magnetic properties. It shows that the TN of BiFeO3 changes slightly from 644 K to 638 K and the TM of Bi1-xHoxFeO3 will reduce from 878 K to 860 K with increasing Ho3+ content. It can be attributed to the Fe3+–O2+–Fe3+ super-exchange strength and the relative stability of the magnetic structure.

Rare earth and transition metal doped BiFeO3 ceramics: structural, magnetic and dielectric characterization

Here we studied the effect of rare earth (Y) and transition metal (Co, Ni) substitution on the structure, magnetic and dielectric properties of the BiFeO3 (BFO) multiferroic ceramic. The samples were synthesized by modified rapid liquid phase sintering method. The phase purity and structural characteristics of the ceramics were investigated by X-ray diffraction and scanning electron microscope. The results revealed the uniform micro-structure with reduced grain size due to the rare earth doping in the BFO. Compared to the pure BFO, enhanced coercivity and resistivity were observed for the co-doped ceramics with Y and Co. The improvement of temperature stability of magnetism and electrical conductivity was found for ceramic co-doped with Y+3 and Ni+2 ions. The possible origin of the improvement in these properties has been discussed on the basis of the size effect and nature of the dopants.

Room temperature multiferroic and magnetodielectric properties in Sm and Sc co-doped BiFeO3 ceramics

In this work, Sm and Sc co-doped Bi1−xSmxFe1−yScyO3 (x = 0.00–0.20; y = 0.03) ceramics are fabricated by a rapid liquid phase sintering method, in order to develop single-phase multiferroics with large magnetization and polarization. X-ray diffraction and Raman spectroscopic studies reveal that the ceramics are single-phase with a structural transition from rhombohedral to orthorhombic structures near x = 0.15. Electric and magnetic measurement results indicate that the transition significantly enhances the multiferroic properties, which stems from the Sm/Sc doping induced collapse of space-modulated spin structure and internal structural distortion. At an optimized composition of Bi0.85Sm0.15Fe0.97Sc0.03O3 (x = 0.15), a remanent polarization of 16.5 μC cm−2, a magnetization 0.2020 emu g−1, and a magnetodielectric effect of 0.46% can be obtained. These results clearly demonstrate a potential application for Sm/Sc doped BiFeO3 ceramics in the field of multiferroic devices.

Structural, dielectric, vibrational and magnetic properties of Sm doped BiFeO3 multiferroic ceramics prepared by a rapid liquid phase sintering method

Rare earth Sm substituted Bi1−xSmxFeO3 with x=0, 0.025, 0.05, 0.075 and 0.10 polycrystalline ceramics were synthesized by a rapid liquid phase sintering method. The effect of varying composition of Sm substitution on the structural, dielectric, vibrational, optical and magnetic properties of doped BiFeO3 (BFO) ceramics have been investigated. X-ray diffraction patterns of the synthesized rare earth substituted multiferroic ceramics showed the pure phase formation with distorted rhombohedral structure with space group R3c. Good agreement between the observed and calculated diffraction patterns of Sm doped BFO ceramics in Rietveld refinement analysis of the X-ray diffraction patterns and Raman spectroscopy also confirmed the distorted rhombohedral perovskite structure with R3c symmetry. Dielectric measurements showed improved dielectric properties and magnetoelectric coupling around Néel temperature in all the doped samples. FTIR analysis establishes O–Fe–O and Fe–O stretching vibrations in BiFeO3 and Sm-doped BiFeO3. Photoluminescence (PL) spectra showed visible range emissions in modified BiFeO3 ceramics. The magnetic hysteresis measurements at room temperature and 5K showed the increase in the magnetization with the increase in doping concentration of Sm which is due to the structural distortion and partial destruction of spin cycloid caused by Sm doping in BFO ceramics.

The development of BiFeO3-based ceramics

The multiferroic properties of BiFeO3-based ceramics were improved through optimizing their sintering method and doping with certain rare earth elements in pure BiFeO3. Some methods, especially liquid-phase sintering method has largely decreased the densities of oxygen vacancies and Fe2+ in BiFeO3-based ceramics, and thus their resistivity became high enough to measure the saturated polarization and the large piezoelectric d 33 coefficient under the high electric field of >150 kV/cm. Besides, multiferroic properties were improved through the rare earth elements doping in pure BiFeO3. Magnetization commonly increases with the proportional increase of Nd, La, Sm and Dy contents up to ~30%, while ferroelectric phase can transform to paraelectric phase at a certain proportion. An improved magnetoelectric coupling was often observed at ferroelectric phase with a relatively large proportion. Besides, an enhanced piezoelectric coefficient is expected in BiFeO3-based ceramics with morphotropic phase boundaries as they are already observed in thin epitaxial BiFeO3 films.

Structure and Tribotechnical Properties of Al-Sn Alloys Prepared by the Method of Liquid-Phase Sintering

The structure and tribotechnical properties of sintered Al-Sn alloys were studied. It was pointed that the sintering of pure metals powders allows save the skeleton structure of aluminium matrix at the enlarging concentration of tin up to 50% that twice higher its content in commercial alloys this system. It was established that the high Sn content in sintered alloys leads to their well wear resistance under the dry friction conditions, but it not has an influence on the value of friction coefficient.