Texturing of hydrothermally synthesized BaTiO3 in a strong magnetic field by slip casting

Barium titanate powder was processed by slip casting in a rotating strong magnetic field of 9.4 T. The orientation factor of the sintered compact was analyzed by the X-ray diffraction technique and the microstructure (grain-size) was analyzed by scanning electron microscope. The hydrothermally prepared barium titanate was used as matrix material and the molten-salt synthesized barium titanate, with a larger particle-size, was used as template for the templated grain-growth process. Addition of large template particles was observed to increase the orientation factor of the sintered cast (5 vol% loading). Template particles acted as starting grains for the abnormal grain-growth process and the average grain-size was increased after sintering. Increasing the solid loading (15 vol%) resulted in a similar orientation factor with a decrease of the average grain-size by more than half. However, addition of templates to the 15 vol% cast had a negative effect on the orientation factor. The impingement of growing particles was stated as the primary cause of particle misorientation resulting in a low orientation factor after sintering. Different heating conditions were tested and it was determined that a slow heating rate gave the highest orientation factor, the smallest average grain-size and the highest relative density.     

Effect of starch addition on microstructure and properties of highly porous alumina ceramics

Porous alumina ceramics with ultra-high porosity were prepared through combining the gel-casting process with the pore-forming agent technique. Porosity and pore size distribution of the sintered bulks were evaluated with and without adding starch, respectively. In particular, the influences of starch addition on the properties, including thermal conductivity and compressive strength were studied. It was found that the incorporation of starch increased the nominal solid loading in the suspension and subsequently promoted the particle packing efficiency. The porosity is raised with increasing starch content from 0 to 30 vol%, which brings the decrease in thermal conductivity, whereas the compressive strength isn't seriously degraded. The further higher starch addition (40 vol%), however, would deteriorate the performance of the alumina porous ceramics. It is believed that the appropriate starch amount (lower than 30 vol%), working as a pore-forming agent, suppresses the driving force of densification without affecting the connections of neighboring grains while excessive starch amount would lead to the collapse of the porous structure.     

Effect of temperature and heating rate on the sintering performance of SiC-Al2O3-Dy2O3 and SiC-Al2O3-Yb2O3 systems

Samples of SiC+10 vol%(Al₂O₃+Dy₂O₃) and SiC+10 vol%(Al₂O₃+Yb₂O₃) mixtures were obtained by cold isostatic pressing and sintered for one hour in a dilatometer at 1800 °C and 1900 °C, applying heating rates of 10, 20 and 30 °C/min. The results of the complete sintering cycle indicated that the heating rates do not significantly influence the shrinkage, but that temperature and total sintering time may be relevant factors. The compacts sintered at 1900 °C shrank on average 9% more than those sintered at 1800 °C, and it was found that the sintering time can be reduced by 40–50% at faster heating rates. The maximum shrinkage rates occurred at temperatures lower than those of the sintering thresholds for the two mixtures, two temperatures and three heating rates. It was also found that after formation of the liquid, the mechanisms of particle rearrangement and solution-precipitation were not as fast as reported in the literature, even at high heating rates, for example 30 °C/min, but they are responsible for much of the shrinkage occurring throughout the sintering cycle.     

Particle size effect of starting SiC on processing,microstructures and mechanical properties of liquid phase-sintered SiC

Dense SiC (97.3–99.2% relative density) of 1.1–3.5 μm average grain size was prepared by the combination of colloidal processing of bimodal SiC particles with sintering additives (Al₂O₃ plus Y₂O₃, 2–4 vol%) and subsequent hot-pressing at 1900–1950 °C. The fracture toughness of SiC was sensitive to the grain boundary thickness which was controlled by grain size and amount of oxide additives. A maximum fracture toughness (6.2 MPa m^1/2) was measured at 20 nm of grain boundary thickness. The mixing of 30 nm SiC (25 vol%) with 800 nm SiC (75 vol%) was effective to reduce the flaw size of fracture origin, in addition to a high fracture toughness, leading to the increase of flexural strength. However, the processing of a mixture of 30 nm SiC (25 vol%)–330 nm SiC (75 vol%) provided too small grains (1.1 μm average grain size), resultant thin grain boundaries (12 nm), decreased fracture toughness, and relatively large defect of fracture origin, resulting in the decreased strength.     

Sintering behaviour of a ZnO waste powder obtained as by-product from brass smelting

The effect of two sintering methods (conventional sintering and two-step sintering) and of alumina addition on the sintering behaviour of a ZnO-rich waste powder (ZnO > 95 wt%), a by-product from brass smelting industry, was studied aiming to improve the sintered density and grain size. Both conventional sintering and two-step sintering methods did not lead to fully dense powder compacts, as densification was conditioned by abnormal grain growth and the particle size of the ZnO-rich residue. When two-step sintering was used the grain growth was reduced comparatively to conventional sintering method. The highest relative sintered density (about 90%) was achieved when samples of ZnO waste and samples of ZnO waste with 2 wt% added Al₂O₃ were processed by two-step sintering and corresponded to a mean grain size of around 18 µm and 7 µm, respectively. XRD and SEM results indicated that alumina addition helped to inhibit grain growth due to the formation of gahnite spinel (ZnAl₂O₄) precipitates in the grain boundaries of zincite (ZnO) grains.     

Spark plasma sintering of ultra-porous γ-Al2O3

Nanocrystalline gamma alumina (γ-Al₂O₃) powder with a crystallite size of ~10 nm was synthesized by oxidation of high purity aluminium plate in a humid atmosphere followed by annealing in air. Spark plasma sintering (SPS) at different sintering parameters (temperature, dwell time, heating rate, pressure) were studied for this highly porous γ-Al₂O₃ in correlation with the evolution in microstructure and density of the ceramics. SPS sintering cycles using different heating rates were carried out at 1050–1550 °C with dwell times of 3 min and 20 min under uniaxial pressure of 80 MPa. Alumina sintered at 1550 °C for 20 min reached 99% of the theoretical density and average grain size of 8.5 µm. Significant grain growth was observed in ceramics sintered at temperatures above 1250 °C.     

Sintering behavior of spark plasma sintered alumina with graphene nanoplatelet reinforcement

Graphene nanoplatelet (GNP) reinforced alumina is synthesized by spark plasma sintering (SPS) using process conditions of 1100–1500 °C, 3–10 min dwell time, and 45–90 MPa in order to investigate the effects of GNP on sintering behavior. High volume fractions of GNP (5–15 vol%) are utilized in order to accentuate effects of GNPs. GNP effects on sintering behavior are assessed by evaluating microstructural evolution, grain growth kinetics, and microhardness. The addition of GNPs is found to suppress grain growth by a grain wrapping mechanism resulting in a 10% increase in activation energy when GNP content is increased beyond 5 vol %. Grain growth suppression partially mitigates a decrease in hardness due to the introduction of the soft GNP phase. Evidence of GNPs serving as a sintering aid are seen at short sintering times (3 min), while densification and grain size are observed to level off with extended sintering time (10 min). The application of higher pressures enhances densification, which enables GNPs to more effectively wrap around grains resulting in enhanced grain growth suppression.     

Spark plasma sintering of combustion-synthesized β-SiAlON powders

In this paper, the sintering behavior of β-Si_6−zAl_zO_zN_8−z (z=1) powder prepared by combustion synthesis (CS) was studied using spark plasma sintering (SPS). The CSed powder was ball milled for various durations from 0.5 to 20 h and was then sintered at different temperatures with heating rates varying from 30 °C/min to 200 °C/min. The effects of ball milling, sintering temperature, and heating rate on sinterability, final microstructure, and mechanical property were investigated. A long period of ball milling reduced the particle size and subsequently accelerated the sintering process. However, the fine powder was easily agglomerated to form secondary particles, which accordingly decreased the densification of the SPS product. The high sintering temperature accelerated the densification process, whereas the high heating rate reduced the grain growth and increased the relative density of the sintered product.     

Production of TiB2 by SHS and HCl leaching at different temperatures: Characterization and investigation of sintering behavior by SPS

Sub-micron sized TiB₂ ceramic powders were prepared via self-propagating high-temperature synthesis (SHS) followed by HCl leaching at different temperatures. Purified powders obtained using optimum process parameters were consolidated by field assisted sintering technology/spark plasma sintering (FAST/SPS) technique. Phase and microstructural analyses of both the powder and sintered samples were carried out by X-ray diffractometer (XRD) and scanning electron microscope (SEM). The chemical analyses and particle size measurements of the specimen were conducted by inductively coupled plasma-mass spectrometry (ICP-MS) and dynamic light scattering (DLS) techniques. The final properties of the sintered sample were determined in terms of density and microhardness. The effects of different HCl leaching temperatures on the formation, microstructure, particle size, purity and sintering behavior of the SHS-produced TiB₂ powders were investigated. The SHS reaction of TiO₂-B₂O₃-Mg powders as a starting mixture yielded MgO, Mg₃(BO₃)₂ and Mg beside the desired phase TiB₂. All three magnesium containing by-products were completely removed by performing hot HCl leaching. TiB₂ powders after SHS reaction and leaching with 9.3 M HCl for 30 min at 80 °C revealed a minimum purity of 98.4% and a homogenous particle size distribution with an average particle size of 536 nm. In the ultimate SPS experiment which was conducted at 1500 °C for 5 min under a pressure of 50 MPa, a relative density of 94.9% and a micro-hardness value of 24.56 GPa were achieved.     

Particle size effects in flash sintering

The study of 3 mol% yttria stabilized zirconia (3YSZ) with different particle sizes provides new insights into flash sintering. Four powders, all with the same crystallite size but various particle size were investigated: described as nominally 1 μm (D80 = 0.51 μm, meaning 80 vol% has a size less than 0.51 μm), 2 μm (D80 = 0.90 μm), 5 μm (D80 = 2.11 μm) and 10 μm (D80 = 3.09 μm). While the furnace temperature for flash sintering, at a field of 100 V cm^?1, increased from 920 °C to 1040 °C with particle size, the specimen temperature in all instances remained at ～1200 °C. The quantum increase in density decreased with larger particles. The grain size distribution of conventionally and flash sintered specimens remained similar, with some evidence of a preponderance of nanograins in the flash sintered specimens. Joule heating was well below the temperatures that would have been required for sintering in a few seconds. An explanation based upon the nucleation of Frenkel pairs is proposed.     

Sintering properties of hydroxyapatite powders prepared using different methods

In the present work, the sintering behaviour of HA particles prepared via the wet precipitation method (HAp) and wet mechanochemical technique (HAwm) was investigated. The sintering behaviour of a commercial HA powder (HAc) was also studied for comparison purpose. All the three powders were characterised in terms of particle size, Ca/P ratio and crystal size. Green samples were prepared and sintered in air at temperatures ranging from 1000 °C to 1400 °C. The sintered bodies were studied in terms of the phase stability, relative density, Young's modulus, Vickers hardness, fracture toughness and grain size. The results indicated that HAwm samples suffered phase decomposition while the HAp and HAc sintered samples showed no phase disruption throughout the temperature range employed. The HAp samples exhibited the overall best densification and properties when compared to the HAc and HAwm samples. Furthermore, the results showed that mechanical properties of sintered samples were governed by both the bulk density and the grain size.     

Characterization of IT-SOFC non-symmetrical anode sintered through conventional furnace and microwave

This work investigated the mechanical and electrical properties of NiO–SDC/SDC anode sintered by two different methods: in a microwave at about 1200 °C for 1 h and in a conventional furnace at 1200 °C with a holding time of 1 h (total sintering time of 21 h). Nano-powders Sm_0.2Ce_0.8O_1.9 (SDC) and NiO were mixed using a high-energy ball mill, followed by the co-pressing technique at a compaction pressure of 400 MPa. No binder was used between the layers. The electrical behaviors of all sintered samples were studied using electrochemical impedance spectra in the frequency range of 0.01–10⁵ Hz under 97% H₂–3% H₂O, an amplitude of 10 mV, and at high temperature range of 600–800 °C. Results indicate that the non-symmetrical NiO–SDC/SDC anode achieved through microwave sintering has finer grain size and higher electrochemical performance. However, hardness and Young׳s modulus increased in the samples sintered through a conventional furnace.     

Wear resistant boron carbide compacts produced by pressureless sintering

Boron carbide compacts were produced by pressureless sintering at 2200 °C/2 h and 2250 °C/2 h in Ar atmosphere, using a starting powder with a particle size smaller than 3 µm. Effects of carbon addition (3.5 wt%) and methanol washing of the starting powder were investigated on the densification, Vickers hardness, and micro-abrasive wear resistance of the samples. The removal of oxide phases by methanol washing allowed the production, with no sintering additive, of highly densified (93.6% of theoretical density), hard (25.4 GPa), and highly wear resistant (wear coefficient =2.9×10^–14 m³/N.m) boron carbide compacts sintered at 2250 °C. This optimized combination of properties was a consequence of a reduced grain growth without the deleterious effects associated to the carbon addition. Methanol washing of the starting powder is a simple and general approach to produce, without additives, high quality, wear resistant boron carbide compacts by pressureless sintering.     

Diamond-like carbon sintered compacts formed by spark plasma sintering

A spark plasma sintering (SPS) method was utilized for the novel production of diamond-like carbon (DLC) compacts. Two amorphous carbon powders with different particle sizes (45 μm and 24 nm diameter) were employed as starting materials for the sintering experiments. The carbon powders were sintered using a SPS system at various sintering temperatures and holding times. The structural properties of the sintered compacts were evaluated using X-ray diffraction (XRD) analysis and high-resolution transmission electron microscopy (HRTEM). Disk-shaped compacts were obtained by sintering the powder with a particle diameter of 45 μm, although the compacts were very brittle and easily broken. However, sintering of the 24 nm diameter powder particles at temperatures of 1473 to 1573 K with a holding time of 300 s led to the successful production of sintered compacts without breakage. Reflection peaks related to graphite structure were observed in XRD patterns of the compacts sintered from the 24 nm diameter particles. HRTEM analysis revealed that the compacts sintered at 1473 K with a holding time of 300 s had an amorphous structure and consisted of 34% sp³ carbon bonding. Evaluation of the structural properties indicated that sintered compacts with DLC structure could be created by the SPS method with 24 nm diameter amorphous carbon particles.     

Bimodal distribution of filler on viscosity and thermal expansion of glass composites

This paper investigates the bimodal oxide filler system to study the viscous behavior and thermal expansion properties of glass composites. Zinc oxide and cordierite, which are two types of filler, with different average diameters (10 μm and 1 μm, respectively), were considered in a Bi2O3 containing glass with various volume fractions (up to 40 vol%). The experimental results for the composites with the bimodal filler distribution show a reduced viscosity. The viscosity increased from fine particles to coarse particles with an increase in the volume fraction of the composite. Both viscosity and coefficient of thermal expansion (CTE) decreased significantly in the composite with the cordierite filler. The CTE is determined from the volume fraction with respect to particle size and distribution. On the other hand, viscosity is dependent on the particle distribution, particle size, and volume fraction of the composite.     

Grain size effect on the structural and dielectric properties of Pb0.85La0.15TiO3 ferroelectric ceramic compound

This paper presents a study of the influence of particle size on the structural and dielectric properties of Pb_0.85La_0.15TiO₃ (PLT15) ferroelectric ceramic samples. The samples were prepared with average grain size of 1.69 ± 0.08 μm and 146 ± 8 nm using, respectively, conventional and spark plasma sintering techniques. A decrease in the tetragonality degree as the crystallite size decreased was explained by an internal stress caused by the existence of a large amount of grain boundaries. The local structure exhibited no significant modification and the dielectric measurements showed a diffuse phase transition and a reduction in the permittivity magnitude at T_m as the average grain size decreased. The nanostructured ceramic sample prepared at a relatively lower temperature and sintering time presented a dielectric constant value of approximately 2000 at room temperature.     

Sinterability,microstructure and compressive strength of porous glass-ceramics from metallurgical silicon slag and waste glass

Porous glass-ceramics have been prepared by the direct sintering of powder mixtures of metallurgical silicon slag and waste glass. The thermal behavior of silicon slag was examined by differential thermal analysis and thermogravimetry to clarify the foaming mechanism of porous glass-ceramics. The mass loss of silicon slag below 700 °C was attributed to the oxidation of amorphous carbon from residual metallurgical coke in the silicon slag, and the mass gain above 800 °C to the passive oxidation of silicon carbide. The porosity of sintered glass-ceramics was characterized in terms of the apparent density and pore size. By simply adjusting the content of waste glass and sintering parameters (i.e. temperature, time and heating rate), the apparent density changed from 0.4 g/cm³ to 0.5 g/cm³, and the pore size from 0.7 mm to 1.4 mm. In addition to the existing crystalline phases in the silicon slag, the gehlenite phase appeared in the sintered glass-ceramics. The compressive strength of porous glass-ceramics firstly increased and then decreased with the sintering temperature, reaching a maximal value of 1.8 MPa at 750 °C. The mechanical strength was primarily influenced by the crystallinity of glass-ceramics and the interfaces between the crystalline phases and the glassy matrix. These sintered porous glass-ceramics exhibit superior properties such as light-weight, heat-insulation and sound-absorption, and could found their potential applications in the construction decoration.     

Porous ceramic monoliths based on diatomite

Porous silica ceramics were obtained at low forming pressure and low sintering temperature by using diatomaceous earth as a silica source and boric acid as an inexpensive additive. The starting raw material, diatomite from surface coal mine Kolubara, Serbia, was purified from organic and inorganic impurities by using heat and chemical treatment. Boric acid was used as binding and sintering aid up to 2 wt%. Powder was compacted by using different pressures of 40, 60 and 80 MPa. The pressed samples were sintered at 850, 1000, 1150, and 1300 °C for 4 h in air. A relatively high porosity in the range of 60–70% is obtained for the samples pressed at 40, 60 and 80 MPa and sintered at 1000 °C. Median pore size diameters are in the range of macroporous up to 2 μm in the samples sintered at 1150 and 1300 °C. X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy, scaning electron microscopy (SEM) and mercury porosimetry measurements were employed to characterize the phases, functional groups, microstructure and pore size distribution of the obtained samples. In addition, measurements of densities and open porosities by immersion technique, according to Archimedes principle, were used. The relations between mechanical properties (Young modulus, Poisson ratio, and compressive strength) versus content of boric acid in the investigated samples were studied and disscussed.     

Effect of atmosphere and sintering time on the microstructure and mechanical properties at high temperatures of α-SiC sintered with liquid phase Y2O3–Al2O3

The influence that the atmosphere (N₂ or Ar) and sintering time have on microstructure evolution in liquid-phase-sintered α-SiC (LPS-α-SiC) and on its mechanical properties at high temperature was investigated. The microstructure of the samples sintered in N₂ was equiaxed with a grain size of 0.70 μm and a density of 98% of the theoretical value regardless of the sintering time. In contrast, samples sintered in Ar had an elongated-grain microstructure with a density decreasing from 99 to 95% and a grain size increasing from 0.64 to 1.61 μm as the sintering time increased from 1 to 7 h. The mechanical behaviour at 1450 °C showed the samples sintered in nitrogen to be brittle and fail at very low strains, with a fracture stress increasing from 400 to 800 MPa as the sintering time increased. In contrast, the samples sintered in Ar were quasi-ductile with increasing strain to failure as the sintering time increased, and a fracture stress strongly linked to the form and size of the grains. These differences in the mechanical properties of the two materials are discussed in the text. During mechanical tests, a loss of intergranular phase takes place in a region, between 50 and 150 μm thick, close to the surface of the samples—the effect being more important in the samples sintered in Ar.