Pressureless Sintering Process Research Articles

Investigation in Optimization Process Parameter of CaCu<sub>3</sub>Mn<sub>4</sub>O<sub>12</sub> via Sol-Gel Synthesis

Due to the disadvantageous of solid state in CaCu3Mn4O12 ceramic processing, the sol gel synthesis has been extensively studied to produce fine size of powder and better homogeneity at low processing temperature. This paper provides a summary to prove that the single phase of CaCu3Mn4O12 ceramic with high purity and characteristic is actually can be produced via sol gel synthesis at relative low temperature under pressureless sintering process. The stoichiometric CaCu3Mn4O12 powders were successfully prepared by the citric gel method with molar ratio of [citrates/metallic io; 1:2. The precursor gel formed was calcined and sintered at range 400 °C to 800 °C. A single-phase CaCu3Mn4O12 fairly well densified at relative low temperature under atmospheric sintering condition. Analysis has been done under XRD and FESEM. Result shown formation of CaCu3Mn4O12 started at 500 °C and was formed completely at 700 °C for 18h. FESEM results turned out that CaCu3Mn4O12 powder particle is submicron in size and highly agglomerates due to high calcinations temperature. Varying in sintering parameter is actually exhibit differences in phase formation, grain size and magnetic behavior.

Mechanical and dielectric properties of short carbon fiber reinforced Al2O3 composites with MgO additive

A series of short-carbon-fiber/Al2O3 composites with MgO as sintering additive were fabricated by pressureless sintering process. The effects of short carbon fiber (Csf) content on the mechanical, dielectric and microwave absorbing properties of the composite were investigated. The results show that the addition of MgO enhances the density, hardness and the flexural strength of the alumina ceramic. However, these mechanical properties of the Csf/Al2O3–MgO composite decrease with increasing Csf content. Both the real and imaginary parts of the complex permittivity increase with increasing Csf content in the frequency range of 8.2–12.4GHz, which is attributed to the increasing electron polarization and associated polarization relaxation, respectively. When the Csf content is 0.3wt%, the reflection loss less than −10dB and the minimum value of −27dB are obtained with the coating thickness being 1.4mm. The results indicate that the Csf/Al2O3 with MgO is an excellent candidate for microwave absorbing material with favorable mechanical property.

Study on Phase Stability during Pressure-Less Sintering Process of Slag α-Sialon Powder and Properties of As-Sintered Materials

The pressure-less sintering of slag α-Sialon powder, derived from slag wastes by self-propagating high-temperature synthesis technology, was carried out and the phase assemblages of slag α-Sialon sintered at different temperatures were studied. The results showed that the formation of β-Sialon phase did occur during the pressure-less sintering and was restrained to some extent via mass diffusion of Ca2+/Mg2+ added in embedded powder in samples. α-Sialon phase with two different unit cells was detected in samples sintered at 1700°C and above, and the formation mechanism was also discussed.

Effects of Chromite Additive on the Microstructure and Performance of Bauxite-based Fracturing Proppant

High-strength fracturing proppant was prepared by pressureless sintering process using bauxite(67wt% Al2O3)and chromite as raw materials. The influence of chromite content(0-5wt%) on phase composition, microstructure and breakage ratio were studied by XRD, SEM, EDS and the column compression method, respectively. The results indicate that the additive of chromite promotes the formation of solid solution of chrome-corundum(Al2-2x Cr2x O3,0x1)and rod-shape mullite phases, both of which are believed to strengthen the fracturing proppant. Additionally, liquid phase is formed by excessive ferrum located between the ceramic grains at relatively high sintering temperatures, which is important for accelerating the sintering densification. The sintering temperature decreases with the addition of chromite. The optimized proppant sample with 2wt% chromite doped, sintered at 1420 ℃ for 2 h shows a breakage ratio of only 1.8%under the pressure of 69 MPa. Furthermore, 2 wt% chromite doping decreases the sintering temperature by 60 ℃(from 1480 ℃ to 1420 ℃). This process enables the production of high-strength fracturing proppant from low-rank mineral materials and demonstrates a promising practical application.

Effect of tantalum content on the microstructure and mechanical behavior of cermets based on (TixTa1−x)(C0.5N0.5) solid solutions

Titanium–tantalum carbonitride, (Ti, Ta)(C, N), based cermets with different Ti and Ta contents were prepared using a mechanically induced self-sustaining reaction and then densified using a pressureless sintering process. Complete microstructural and mechanical characterizations were performed on the materials, which revealed that the size of the carbonitride ceramic particle was significantly reduced when the Ta content was increased. The flexural strength and fracture toughness were measured using the ball on three balls test and the indentation microfracture test, respectively. The strength profile was analyzed under the framework of Weibull theory. The change in the mechanical properties as a function of the Ta content was correlated with the normalized microstructural parameters, such as the binder mean free path. The decrease in toughness and flexural strength was attributed to the presence of intermetallic compounds in the binder phase, which was also corroborated by the nanoindentation tests.

Pressureless sintering of single-phase tantalum carbide and niobium carbide

Abstract This work presents the results of studies on the preparation of single-phase polycrystalline tantalum carbide and niobium carbide. It has been found that it is possible to obtain polycrystals with high density in the pressureless sintering process at temperatures up to 2000 °C and therefore relatively low temperatures such as for the compounds with one of the highest melting points; TaC – 3985 °C and NbC – 3600 °C. Only carbon as a sintering additive was used. The main role of carbon is to reduce of oxide contamination. It has been shown that the determination of the amount of carbon required to reduce oxide contamination is only possible through the experimental method.

Application of response surface methodology (RSM) for optimization of the sintering process of preparation calcia partially stabilized zirconia (CaO-PSZ) using natural baddeleyite

Response surface methodology (RSM) was successfully applied to process of preparation calcia partially stabilized zirconia (CaO-PSZ). Besides that, natural baddeleyite was used as starting materials instead of chemical pure zirconia. The pressureless sintering process was optimized by the application of RSM. The independent variables, which had been found as the most effective variables on the relative density and bending strength by screening experiments, were determined as holding time, sintering temperature and heating rate. Two quadratic models were developed through RSM in terms of related independent variables to describe the relative density and bending strength as the responses. Based on contour plots and variance analysis, optimum operational conditions for maximizing relative density and bending strength, at cooling rate of 3°C/min, were 1540°C of sintering temperature, 5h of holding time and heating rate of 3°C/min to obtain 98.57% for relative density and 165.72MPa for bending strength.

Study on the Microstructure and Morphology of Sintering Process of SiC-TiB<sub>2</sub> Composite

This research employs pressureless sintering process to study the impact of different amount of titanium diboride on the performance of silicon carbide-titanium diboride composite. Results show that: no new phase was produced, and the whole process is a solid-phases sintering process. In this experiment, it has the smallest relative density and loss on ignition and porosity, and highest degree of densification of the silicon carbide-titanium diboride composite ceramics sintered body when the titanium diboride content is 50g, and the quality of SiC and TiB2 ratio is 1:1.

Sintering behaviour of silicon nitride powders produced by carbothermal reduction and nitridation

In this study, the sintering behaviour of silicon nitride (Si3N4) powders (having in situ form sintering aids/self-sintering additives) produced directly by the carbothermal reduction and nitridation (CRN) process is reported. The sintering of as-synthesised α-phase Si3N4 powders was studied, and the results were compared with a commercial powder. The α-Si3N4 powders, as-received contains magnesium, yttrium or lithium–yttrium-based oxides that were shaped with cold isostatic pressing and tape casting techniques. The compacts and tape casted samples are then pressureless-sintered at 1650–1750°C for up to 2h. After sintering, the density and the amount of β-phase formation were examined in relation to the sintering temperature and time. The highest density value of 3.20gcm−3 was obtained after only 30min of pressureless sintering (at 1700°C) of Si3N4 powders produced by CRN from silica initially containing 5wt.% Y2O3. Silicon nitride powders produced by the CRN process performed similarly or even better than results from the pressureless sintering process compared with the commercial one.

Microstructure and Toughening Mechanism of ZrO<sub>2</sub> Ceramic

In order to reduce its costs and increase its strength and toughness, 3Y-ZrO2/Al2O3 nanocomposite ceramics is fabricated through pressureless sintering process. Microstructure is detected by SEM and composition is analyzed by EDS. The results show that some of ZrO2 combined with Al2O3 forms ZrO2+Al2O3 solid solutions, and the others form particles which are distributed on grain boundaries and matrixes with different shapes. Those ZrO2 particles locating in the boundaries enhance the ceramic’s toughness by two modes: fracturing and tearing, and which one to be occur depends on the location relationship between load and boundary interface. The composition transition, i.e. the solid solution consisting of ZrO2+Al2O3 has formed encircling ZrO2 particles, and it has a positive contribution to ceramics toughness through the combination of chemical bond.

Fabrication of Transparent Alumina by Rapid Vacuum Pressureless Sintering Technology

Transparent alumina with high transmission has been first successfully prepared by rapid vacuum pressureless sintering processing at lower sintering temperature and reduced soaking time, compared with conventional pressureless sintering. The alumina sintered at 1670°C for 5 min has a residual porosity of 0.002% and a real in‐line transmission of 64% for a wavelength of 1100 nm. We show that soaking time has an effect on the densification and transparency of alumina sintered at 1650°C. With increasing sintering temperature and soaking time, densification and grain growth occurred, and above 20 min, grain size increases slowly. The real in‐line transmission of alumina increased with the densification and decreased with pore size growth. The light transmission and absorption were greatly affected by the grain size and pore defects.

Fabrication of Graphene/Zirconia Nanocomposite by Mixing Graphite Oxide and Zirconia Nanopowders and Pressureless Sintering

Graphene has unique two-dimensional nanostructure with high specific surface areas, superior mechanical properties which has promising applications for the development of high performance nanocomposite materials. Most of previous work on graphene nanocomposites addresses on polymer matrix. In the present work, we have fabricated zirconia and graphene nanocomposites (ZrO2/GNS) by simple mechanical mixing and pressureless sintering process. Microstructural observations of the composite show the homogeneous and random distributions of graphene nanosheet in the zirconia matrix. Scanning electronic microscopy observes partially pulled out graphene nanosheets with well combined interface with the matrix in fractural surface, showing promising reinforcement effects.

Study on ZrB<sub>2</sub>-SiC Prepared by Field Assisted Sintering

Zirconium diboride and silicon carbide are thought to have a low intrinsic sinterability due to their strong covalent bonds, low bulk and grain boundary diffusivities. ZrB2-SiC ceramic composites were prepared by a field assisted pressureless sintering process in the present work. The densification behavior and the effect of sintering temperature on microstructure and properties of sintered samples were studied. Pellets were in-situ formed by dry uniaxial pressing in the graphite die at a pressure of 50MPa for 3min and then sintered at a sintering temperature ranged from 1650 °C to 1950 °C with fixed heating rate and holding time. The current, voltage, temperature and displacement data were all collected by the real-time acquisition system. The bulk densities were determined by Archimedes method and the microstructure of samples was characterized by SEM. The onset of some measurable shrinkage of the green body was recorded at around 1400 °C regardless of the sintering temperature and significant shrinkage took place at higher temperature of around 1600 °C. For the sample sintered at 1950 °C, no shrinkage occurred after ~2min holding time. The relative density increased significantly with increasing temperatures and samples could be densified to a relative density of more than 99% at 1950 °C by the field assisted sintering process without obvious grain growth.

Comparison of ZrB2–ZrO2f ceramics prepared by hot pressing and pressureless sintering

Zirconium diboride based ultra-high temperature ceramics toughened by zirconia fiber (ZrB2–ZrO2f) were prepared by hot pressing and pressureless sintering, and the effect of two sintering techniques on the phase composition, microstructures and mechanical properties of ZrB2–ZrO2f ceramics were studied in detail. The densification behavior was investigated through the analysis of the density curves. The microstructures and mechanical properties of ZrB2–ZrO2f ceramics were analyzed and compared in order to research the influence of the two sintering techniques. Results indicated that the hot-pressing process was more suitable for preparing ZrB2–ZrO2f ceramics than pressureless sintering process. The comprehensive properties of ZrB2 plus 30vol.% ZrO2f ceramics obtained at temperature 1950°C by hot pressing for 2h were optimal, the flexural strength and fracture toughness reached 633MPa and 5.6MPa·m1/2, respectively. The higher flexural strength was attributed to the smaller size of grains and higher relative density, furthermore, the toughening mechanisms were fiber debonding, fiber pull-out, crack deflection and transformation toughening.

Fabrication and mechanical properties of [formula omitted] functionally graded material by electrophoretic deposition

This study describes the synthesis of Al2O3/SiC/ZrO2 functionally graded material (FGM) in bio-implants (artificial joints) by electrophoretic deposition (EPD). A suitable suspension that was based on 2-butanone was applied for the EPD of Al2O3/SiC/ZrO2, and a pressureless sintering process was applied as a presintering. Hot isostatic pressing (HIP) was used to densify the deposit, with beneficial mechanical properties after 2 h at 1800 °C in Ar atmosphere. The maximum hardness in the outer layer (90 vol.% Al2O3+10 vol.% SiC) and maximum fracture toughness in the core layer (75 vol.% Al2O3+10 vol.% SiC + 15 vol.% ZrO2) composite were 20.8±0.3 GPa and 8±0.1MPam1/2, respectively.The results, when compared with results from Al2O3/ZrO2 FGM, showed that SiC increased the compressive stresses in the outer layers, while the inner layers were under a residual tensile stress.

Thermal conductivity of (Hf1−xZrx)6Ta2O17 (x = 0, 0.1, 0.3 and 0.5) ceramics

(Hf1−xZrx)6Ta2O17 (x=0, 0.1, 0.3 and 0.5) ceramic powders were synthesized by conventional solid-state method. The samples of these ceramics were prepared by pressureless sintering process at 1600°C for 8h in air. X-ray diffraction characterization indicated that all samples were single phase with orthorhombic structure. The thermal diffusivities of the samples from 200°C to 1000°C were characterized by laser flash diffusivity measurement. The thermal conductivities of (Hf1−xZrx)6Ta2O17 decreased with the increase of zirconium content x. Thermal conductivity of (Hf0.5Zr0.5)6Ta2O17 was about 2.75W/mK which was comparable with that of dense partially yttria-stabilized zirconia (∼2.5W/mK).

Inverse core–rim microstructure in (Ti,Ta)(C,N)-based cermets developed by a mechanically induced self-sustaining reaction

Cermets with a nominal composition (Ti0.8Ta0.2C0.5N0.5— 20wt.% Co) were synthesised by a mechanically induced self-sustaining reaction (MSR) process from stoichiometric elemental powder blends. The MSR allowed the production of a complex (Ti,Ta)(C,N) solid solution, which was the raw material used for the sintering process. The pressureless sintering process was performed at temperatures between 1400°C and 1600°C in an inert atmosphere. The microstructural characterisation showed a complex microstructure composed of a ceramic phase with an unusual inverse core–rim structure and a Ti–Ta–Co intermetallic phase that acted as the binder.

Synthesis and ion conductivity of (Bi 2O 3) 0.75(Dy 2O 3) 0.25 ceramics with grain sizes from the nano to the micro scale

Using (Bi 2O 3) 0.75(Dy 2O 3) 0.25 nano-powder synthesized by reverse titration co-precipitation method as raw material, dense ceramics were sintered by both Spark Plasma Sintering (SPS) and pressureless sintering. According to the predominance area diagram of Bi–O binary system, the sintering conditions under SPS were optimized. (Bi 2O 3) 0.75(Dy 2O 3) 0.25 ceramics with relative density higher than 95% and an average grain size of 20 nm were sintered in only 10 min up to 500 °C. During the pressureless sintering process, the grain growth behavior of (Bi 2O 3) 0.75(Dy 2O 3) 0.25 followed a parabolic trend, expressed as D 2 − D 0 2 = Kt, and the apparent activation energy of grain growth was found to be 284 kJ mol − 1 . Dense (Bi 2O 3) 0.75(Dy 2O 3) 0.25 ceramics with different grain sizes were obtained, and the effect of grain size on ion conductivity was investigated by impedance spectroscopy. It was shown that the total ion conductivity was not affected by the grain size down to 100 nm, however lower conductivity was measured for the sample with the smallest grain size (20 nm). But, although only the δ phase was evidenced by X-ray diffraction for this sample, a closer inspection by Raman spectroscopy revealed traces of α-Bi 2O 3.

Investigation of Phase Composition and Microstructure of the FeAl/Al<sub>2</sub>O<sub>3</sub> Composites Fabricated by Mechanical Alloying Process and Pressureless Sintering Process

The FeAl/Al2O3 composites were fabricated by pressureless sintering process. The FeAl intermetallics compounds powders were fabricated by mechanical alloying and heat treatment process. The FeAl intermetallics compounds powders and Al2O3 powders were mixed and the FeAl/Al2O3 composite powders were prepared. The FeAl/Al2O3 composites bulks were fabricated by pressureless sintering process at 1600°C for 2h. The phase composition and microstructure of FeAl intermetallics compounds powders produced by mechanical alloying and heat treatment were investigated. The phase composition and microstructure of the FeAl/Al2O3 composites sintered bulks were investigated. The XRD patterns results showed that the Fe-Al intermetallics compounds powders were fabricated by mechanical alloying for 60h. The FeAl intermetallics compounds powders were fabricated by heat treatment at 800°C, 900°C and 1000°C. The microstructure showed that the mean particles size of the FeAl intermetallics compounds powders produced by mechanical alloying and heat treatment process was rather fine and about 4-5μm. The XRD patterns results showed that there existed the FeAl phase and Al2O3 phase in sintered composites. The FeAl/Al2O3 composites bulks exhibited the homogenous and compact microstructure. The mean particles size of FeAl was about 4-5μm and the mean particles size of Al2O3 was about 5-10μm. The density and relative density of the FeAl/Al2O3 composites increased gradually with the increase of FeAl content.

Preparation and thermophysical properties of LaMgAl 11O 19–Yb 3Al 5O 12 ceramic composites

LaMgAl 11O 19–Yb 3Al 5O 12 ceramic composites were prepared by pressureless sintering process at 1700 °C for 10 h in air. The microstructure and thermophysical properties of the composites were characterized by X-ray diffraction, scanning electron microscopy, high-temperature dilatometer and laser flash diffusivity measurements. LaMgAl 11O 19–Yb 3Al 5O 12 ceramic composites are composed of magnetoplumbite and garnet structures. LaMgAl 11O 19–Yb 3Al 5O 12 ceramic composites exhibit typical linear increase in thermal expansion with the increase of temperature. The measured thermal diffusivity gradually decreases with increasing temperature. Thermal conductivity of LaMgAl 11O 19–Yb 3Al 5O 12 ceramic composites is in the range of 2.6–3.9 W·m −1·K −1 from room temperature to 1200 °C.