Transparent mullite ceramic from single-phase gel by Spark Plasma Sintering

Monophasic mullite precursors with composition of 3Al₂O₃·2SiO₂ (3:2) were synthesized and then were sintered by Spark Plasma Sintering (SPS) to form transparent mullite ceramics. The precursor powders were calcined at 1100 °C for 2 h. The sintering was carried out by heating the sample to 1450 °C, holding for 10 min. The sintered body obtained a relative bulk density of above 97.5% and an infrared transmittance of 75–82% in wavelength of 2.5–4.3 μm without any additive. When the precursor powders were calcined at below 1100 °C, it was unfavorable for completely eliminating the residual OH, H₂O and organic compound. However, when calcined temperature was too high, it was unfavorable either for full densification due to the absence of viscous flow of amorphous phase. At the same calcined temperature, the transmittance of sintered body was decreased with the increase of the sintering temperature above 1450 °C owing to the elongated grain growth.     

Enhanced densification of thin tape cast Ceria-Gadolinium Oxide (CGO) layers by rheological optimization of slurries

Optimized CGO-based slurries are formulated and shaped into thin dense layers via a tape-casting process. The formulation is adjusted with respect to the rheological behaviour. The internal structure and flow properties of slurries are explored with the aim of identifying the required conditions to obtain thin dense CGO layers at reduced sintering temperatures (1200 °C). We demonstrate a correlation between the rheological properties of the slurries, the sintering behaviour and the microstructure of the resulting tapes. Remarkably, a dense CGO layer less than 20 µm thick is obtained with a non-congested slurry, having optimized ceramic loading and liquid-like behaviour.     

Sintered material from alkaline basaltic tuffs

The possibility to obtain sintered material from alkaline basaltic tuffs is demonstrated. The parent rock was milled for 10–15 min, the resulting powder was pressed at 100 MPa and the obtained samples were heat-treated in the range of 1000–1140 °C. The sintering behaviour and the phase formation were studied by pycnometry, dilatometry, DTA, XRD and SEM.The final material was obtained by sintering at 1100 °C and is characterized by zero water absorption, 8–9 vol.% closed porosity and a structure similar to a glass-ceramic. Due to high crystallization trend of used composition, phase formation takes place during the sintering and cooling steps; this leads to a crystallinity of ～60% and formation of different crystal phases (pyroxene, anorthite, spinel and hematite).Despite the low-cost production cycle the obtained material is characterized by high mechanical properties: bending strength of 100 MPa and Young modulus of 90 GPa.     

Effect of calcination temperature on the fabrication of transparent lutetium titanate by spark plasma sintering

Transparent lutetium titanate (Lu₂Ti₂O₇) bodies were fabricated by spark plasma sintering using Lu₂O₃ and TiO₂ powders calcined from 700 °C to 1200 °C. No solid-state reaction was identified after calcination at 700 °C, whereas single-phase Lu₂Ti₂O₇ powder was prepared at 1100 and 1200 °C. The calcination at 700 °C promoted densification at the early stages of sintering, whereas residual pores at grain boundaries resulted in Lu₂Ti₂O₇ bodies with low transparency. Low-density and opaque Lu₂Ti₂O₇ bodies formed owing to the coarsening of the powder calcined at 1200 °C. The Lu₂Ti₂O₇ body sintered using the powder calcined at the moderate temperature of 1100 °C had a density of 99.5% with the highest transmittances of 41% and 74% at wavelengths of 550 nm and 2000 nm, respectively.     

Microstructure and electrical properties of ZnO–Bi2O3-based varistor ceramics by different sintering processes

The effect of sintering processes, such as open sintering, sintering inside a closed crucible, and sintering within a powder bed, on the microstructure and V–I characteristics of ZnO–Bi₂O₃-based varistor ceramics was investigated at sintering temperatures in the range 1000–1200 °C. The results from the experiments showed that the microstructure and electrical properties of the samples varied according to the sintering method and temperature. Optimal values for the electrical characteristics of the varistor ceramics by different sintering processes were obtained when the sintering was conducted at 1100 °C. At the same sintering temperature, the different processes affected the properties differently. At 1000 °C, the samples sintered within a powdered bed showed better electrical properties than those subjected to the other two processes, while at 1100 or 1200 °C, the samples sintered in an open crucible exhibited the best electrical properties.     

Phase transformation on hydroxyapatite decomposition

The collapse of sintered hydroxyapatite (HA) has been attributed to HA decomposition; however, the detailed variations in microstructure are still unclear. Two phase transformation routes of HA decomposition during sintering were identified by transmission electron microscopy in this study. In the first route, HA is transformed to tetracalcium phosphate and needle-like β-tricalcium phosphate which is subsequently converted to α-tricalcium phosphate (α-TCP) above 1100 °C. In the second route, HA is transformed directly to α-TCP and calcium oxide at 1400 °C, accompanied by nanopore formation. In the second route, the α-TCP grew with a preferred orientation to form stripe-like grains. Further holding at 1400 °C for 4 h resulted in recrystallization; i.e., equi-axial grains formed within a stripe-like grain. Nanopore defects dispersed in the α-TCP grains are the main factor for the low density and decreased mechanical strength of the sintered bulk.     

Improvement of the mechanical properties of SiC reticulated porous ceramics with optimized three-layered struts for porous media combustion

Silicon carbide reticulated porous ceramics (SiC RPCs) with three-layered struts were fabricated by polymer replica method, followed by infiltrating alumina slurries containing silicon (slurry-Si) and andalusite (slurry-An), respectively. The effects of composition of infiltration slurries on the strut structure, mechanical properties and thermal shock resistance of SiC RPCs were investigated. The results showed that the SiC RPCs infiltrated with slurry-Si and slurry-An exhibited better mechanical properties and thermal shock resistance in comparison with those of alumina slurry infiltration, even obtained the considerable strength at 1300 °C. In slurry-Si, silicon was oxidized into SiO₂ in the temperature range from 1300 °C to 1400 °C and it reacted with Al₂O₃ into mullite phase at 1450 °C. Meantime, the addition of silicon in slurry-Si could reduce SiC oxidation of SiC RPCs during firing process in contrast with alumina slurry. With regard to slurry-An, andalusite started to transform into mullite phase at 1300 °C and the secondary mullitization occurred at 1450 °C. The enhanced mechanical properties and thermal shock resistance of SiC RPCs infiltrated alumina slurries containing silicon and andalusite were attributed to the optimized microstructure and the triangular zone (inner layer of strut) with mullite bonded corundum via reaction sintering. In addition, the generation of residual compressive stress together with better interlocked needle-like mullite led to the crack-deflection in SiC skeleton, thus improving the thermal shock resistance of obtained SiC RPCs.     

Influence of injection temperature and pressure on the properties of alumina parts fabricated by low pressure injection molding (LPIM)

The quality of ceramics parts made by powder injection molding (PIM) method is influenced by a range of factors such as powder and binder characteristics, rheological behavior of feedstock, molding parameters and debinding and sintering conditions. In this study, to optimize the molding parameters, the effect of injection temperature and pressure on the properties of alumina ceramics in the LPIM process were thoroughly studied. Experimental tests were conducted on alumina feedstock with 60 vol% powder. Injection molding was carried out at temperatures and pressures of 70–100 °C and 0.1–0.6 MPa respectively. Results showed that increase in injection temperature and pressure and the resulting increase in flow rate leads to the formation of void which impairs the properties of molded parts. The SEM studies showed that injection at temperature of 100 °C results in evaporation of binder components. From the processing point of view, the temperature of 80 °C and pressure of 0.6 MPa seems to be the most suitable condition for injection molding. In addition, the effects of sintering conditions (temperature and time) on the microstructure and mechanical properties are discussed. The best final properties were found using injection molding under the above stated conditions, thermal debinding and sintering at 1700 °C during 3 h.     

Microstructure and mechanical properties of the spark plasma sintered TaC/SiC composites: Effects of sintering temperatures

TaC/SiC composites with 20 vol.% SiC addition were densified by spark plasma sintering at 1600–1900 °C for 5 min under 40 MPa. Effects of sintering temperatures on the densification, microstructures and mechanical properties of composites were investigated. The results showed the materials achieved >98% of theoretical density at a temperature as low as 1600 °C. While the TaC grains grew slightly with the sintering temperature increasing, the SiC particles in materials decreased in size. Equiaxed to elongated grain morphology transformation was observed in the SiC phase in the 1900 °C material to obtain a higher flexural strength and fracture toughness of 715 MPa and 6.7 MPa m^1/2, respectively. Lattice enlargement of the TaC phase in the 1900 °C material suggested possible Si diffusion into TaC grains. Ta was also detected in SiC grains by energy dispersive spectroscopy. Glassy pockets present at multi-grain junctions explained the enhanced densification.     

Sintering behavior of waste serpentine from abdasht chromite mines Abdasht chromite mines and kaolin blends

The aim of this study is investigating on sintering behavior of Abdasht waste serpentine and kaolin blends. According to this, three formulations of dry milled waste serpentine with 25%, 50% and 75% high grade kaolin were wet milled. The slurries were then dried, sieved and uniaxially pressed at 100 MPa and fired for 2 h soaking time at temperatures between 1100 and 1400 °C. Sintered samples were investigated by bulk density, apparent porosity, water absorption, linear shrinkage and phase changes with raising temperatures in order to characterize their sintering process. It was revealed that all samples were starting to melt at 1350 °C and the sintering was completed for all specimens at 1300 °C. The only phases of fully sintered samples were cordierite and enstatite. Cordierite concentration, however, increased with enhancing kaolin percentage in composition. The results of this study can introduce Abdasht waste serpentine as magnesium silicate source into the ceramic industries and may help to solve environmental problems caused by several million tones wastes in Abdasht chromite mines.     

Research of phase transformation induced biodegradable properties on hydroxyapatite and tricalcium phosphate based bioceramic

Biodegradable calcium phosphate composites consisting of tricalcium phosphate (α-TCP) and hydroxyapatite (HA) were prepared using a two-step sintering method. The ratio of α-TCP/HA was controlled by modulating the sintering temperature. The initial calcination process at 800 °C causes HA dehydroxylation and induces the early transformation of HA into α-TCP in subsequent sintering processes. At the optimum sintering temperature of 1300 °C, the material is comprised of a moderate ratio of α-TCP to HA (3:7) and possesses a hardness of 5.0 GPa. The high temperature phase transformation from HA to α-TCP accompanied by bonded water loss, which results in the formation of nano-pores within the α-TCP matrix, hardly deteriorated the mechanical strength of the composite. This pore-containing structure also provided a convincing evidence for the origin of the high degradability of α-TCP in a biological environment.     

Effect of sintering temperature on microstructure and mechanical properties of zirconia-toughened alumina machinable dental ceramics

This study investigated the effect of sintering temperature on the microstructure and mechanical properties of dental zirconia-toughened alumina (ZTA) machinable ceramics. Six groups of gelcast ZTA ceramic samples sintered at temperatures between 1100 °C and 1450 °C were prepared. The microstructure was investigated by mercury intrusion porosimetry (MIP), X-ray diffraction (XRD), and scanning electron microscopy (SEM) techniques. The mechanical properties were characterized by flexural strength, fracture toughness, Vickers hardness, and machinability. Overall, with increasing temperature, the relative density, flexural strength, fracture toughness, and Vickers hardness values increased and more tetragonal ZrO₂ transformed into monoclinic ZrO₂; on the other hand, the porosity and pore size decreased. Significantly lower brittleness indexes were observed in groups sintered below 1300 °C, and the lowest values were observed at 1200 °C. The highest flexural strength and fracture toughness of ceramics reached 348.27 MPa and 5.23 MPa m^1/2 when sintered at 1450 °C, respectively. By considering the various properties of gelcast ZTA that varied with the sintering temperature, the optimal temperature for excellent machinability was determined to be approximately 1200–1250 °C, and in this range, a low brittleness index and moderate strength of 0.74–1.19 µm^−1/2 and 46.89–120.15 MPa, respectively, were realized.     

Influence of processing route on microstructure and mechanical properties of MgAl2O4 spinel

This paper reports on process dependant microstructural and mechanical properties of MgAl₂O₄ spinel (MAS) ceramics. Two MAS powders with different chemical compositions were synthesized by solid-state reaction of alumina and calcined caustic magnesia at 1400 °C for 1 h. The surface of the as obtained MAS powders was passivated against hydrolysis by coating it with H₃PO₄ and Al(H₂PO₄)₃ species dissolved in ethanol at 80 °C for 24 h. The as protected powders could then be dispersed in aqueous solutions of tetramethylammonium hydroxide (TMAH) and Duramax D-3005 as dispersing agents to obtain stable slurries with 45 vol.% solids loading. The stable aqueous MAS slurries were consolidated by slip casting (SC), gelcasting (GC), hydrolysis assisted solidification (HAS) and hydrolysis induced aqueous gelcasting (GCHAS) routes, fully dried and then sintered for 1 h at 1650 °C. For comparison purposes, dense MAS ceramics were also prepared following a conventional dry-powder pressing (DP) and temperature induced gelation (TIG) routes. All the sintered MAS ceramics were thoroughly characterized for bulk density, apparent porosity, water absorption capacity, SEM microstructure, XRD phase, hardness, 3-point bend strength, and percentage of shrinkage to evaluate the suitability of the processing routes for fabricating defect free components with near-net shape. Among the various techniques employed, the GCHAS was found to be best for fabricating near-net shape MAS ceramics.     

Hot pressing of bimodal alumina powders with magnesium aluminosilicate (MAS) addition

High quality alumina ceramics were fabricated by hot-pressed sintering using bimodal alumina with superfine component as raw material and magnesium aluminosilicate (MAS) glass as sintering aid. Densification behavior, microstructure evolution and mechanical properties of alumina were investigated from 1300 °C to 1450 °C. The bimodal alumina powders were sintered to 99.8% of the theoretical value at 1400 °C and a comparative dense microstructure with a few plate-like abnormal grains was observed. With increase of sintering temperature up to 1450 °C, many fine matrix grains were consumed and quite a few abnormal grains impinged upon each other. For the alumina ceramics hot-pressed from bimodal alumina with 30 wt.% superfine component, optimal mechanical properties were obtained at 1400 °C. The bending strength and fracture toughness were 522 MPa and 5.0 MPa m^1/2, respectively.     

Comparison among different sintering routes for preparing alumina-YAG nanocomposites

Al₂O₃-YAG (50 vol.%) nanocomposite powders were prepared by wet-chemical synthesis and characterized by DTA-TG, XRD and TEM analyses. Amorphous powders were pre-heated at different temperatures (namely 600 °C, 800 °C, 900 °C and 1215 °C) and the influence of this thermal treatment on sintering behavior, final microstructure and density was investigated. The best performing sample was that pre-calcined at 900 °C, which yields dense bodies with a micronic/slightly sub-micronic microstructure after sintering at 1600 °C. A pre-treatment step to induce controlled crystallisation of the amorphous powder as well as a fast sintering procedure for green compacts, were also performed as a comparison.Finally, the previously stated thermal pre-treatment of the amorphous product was coupled to an extensive mechanical activation performed by wet planetary/ball milling. This procedure was highly effective in lowering the densification temperature, so that fully dense Al₂O₃-YAG composites, with a mean grain size smaller than 200 nm, were obtained by sintering in the temperature range 1370–1420 °C.     

EELS characterisation of β-tricalcium phosphate and hydroxyapatite

Hydroxyapatite (HA) and β-tricalcium phosphate (β-TCP) are of great interest due to their potential application as bone-replacement materials. In particular, composites made of a mixture of these Ca-phosphates revealed improved mechanical properties; however, the reason for this improvement is unknown. Future development and properties enhancement of such bioceramics is linked to the possibility to characterise their particular microstructure. In this context, the ability to quickly identify individual grains of HA and β-TCP within these composites will allow acquiring information about the phase distributions and the phase-boundary microstructure. The aim of the present study is, therefore, to demonstrate that electron energy-loss spectroscopy (EELS) can be successfully employed to differentiate between individual grains of HA and β-TCP. In particular, the analysis of the near-edge structure of the oxygen K-ionisation edge allows detection of a characteristic signal at ca. 536 eV that can be employed as an identification tool for HA. EELS investigations were performed first on as-received and calcined (1000 °C) HA and β-TCP powders and subsequently on pure bulk HA and β-TCP samples sintered at 1250 °C. Finally, this method was successfully applied to a HA/β-TCP (50/50 wt.%) composite sintered at 1250 °C.     

Aqueous tape casting process for SiC

The condition for the preparation of stable SiC slurries by aqueous tape casting was identified. To acquire stable uniform slurries, the influences of dispersant, solid loading, sintering additives, binder and plasticizer on the rheological properties and viscosity were investigated. The conditions for preparing stable SiC slurries were studied and optimized. After tape casting and drying, the green SiC sheets showed smooth surface and homogeneous microstructure. The SiC ceramic can be densified to 98.89% after hot-pressing at 1850 °C (at 25 MPa in Ar for 30 min). The flexural strength, hardness, and toughness are 779.5 ± 39.2 MPa, 21.51 ± 0.70 GPa and 5.54 ± 0.26 MPa m^1/2, respectively. SEM shows a fine microstructure with few pores in the sintered samples. The fracture surface exhibited predominantly intergranular fracture type.     

Manufacture of a non-stoichiometric LSM cathode SOFC material by aqueous tape casting

This work deals with the manufacture of non-stoichiometric strontium-doped lanthanum manganite (LSM) as a cathode material by aqueous colloidal processing. This requires some knowledge of the processability and sinterability of this material. The stability of aqueous suspensions of a fine non-stoichiometric LSM powder was studied by measuring the zeta potential as a function of pH and deflocculant content. Concentrated suspensions were prepared to solids loadings as high as 50 vol.%. The best dispersing conditions and the influence of binders and tape casting performance were determined by means of rheological measurements. LSM cathode tapes were characterized in the green state and after sintering at 1500 °C/2 h, leading to high density compacts. Maximum sintering rate is achieved at 1350 °C. Once the sintering behavior is known a porous material can be easily designed using a sintering temperature compatible with the other components of the semi-cell.     

Influence of sintering temperature on microstructure and mechanical properties of WC-8Ni cemented carbide produced by vacuum sintering

The WC-8Ni powder was prepared by the ball milling method, then consolidated via a vacuum sintering technique. The influence of sintering temperature varying from 1375 °C up to 1500 °C on microstructure and mechanical properties of WC-8Ni cemented carbide was investigated. The best mechanical properties of the samples have been achieved at sintering temperature of 1450 °C. At which the relative density, hardness and fracture toughness (K_IC) of the samples are 99.81%, 13.23 GPa and 24.22 MPa m^1/2, respectively. The effect of η-phase identified by Murakami etching method and XRD technique on the mechanical properties was also discussed.     

On the effects of LiF on the synthesis and reactive sintering of gahnite (ZnAl2O4)

The effects of LiF on the synthesis and reactive sintering of polycrystalline gahnite (zinc aluminate spinel, ZnAl₂O₄) were studied using XRD, high-temperature simultaneous thermal analysis and a spark plasma sintering (SPS) apparatus. It was demonstrated that the LiF reduces the onset of synthesis by about 200 °C and plays an important role in the densification process. SPS consolidation of a LiF-doped ZnO-Al₂O₃ mixture under an applied pressure of 150 MPa and at a sintering temperature of 1100 °C for 20 min generated fully dense gahnite with adequate transparency and mechanical properties.