Studies on ionic conductivity of stabilized zirconia ceramics (8YSZ) densified through conventional and non-conventional sintering methodologies

Densification studies of 8 mol% yttria stabilized zirconia ceramics were carried out by employing the sintering techniques of conventional ramp and hold (CRH), spark plasma sintering (SPS), microwave sintering (MWS) and two-stage sintering (TSS). Sintering parameters were optimized for the above techniques to achieve a sintered density of >99% TD. Microstructure evaluation and grain size analysis indicated substantial variation in grain sizes, ranging from 4.67 μm to 1.16 μm, based on the sintering methodologies employed. Further, sample was also sintered by SPS technique at 1425 °C and grains were intentionally grown to 8.8 μm in order to elucidate the effect of grain size on the ionic conductivity. Impedance spectroscopy was used to determine the grain and grain boundary conductivities of the above specimens in the temperature range of RT to 800 °C. Highest conductivity of 0.134 S/cm was exhibited by SPS sample having an average grain size of 1.16 μm and a decrease in conductivity to 0.104 S/cm was observed for SPS sample with a grain size of 8.8 μm. Ionic conductivity of all other samples sintered vide the techniques of TSS, CRH and MWS samples was found to be ∼0.09 S/cm. Highest conductivity irrespective of the grain size of SPS sintered samples, can be attributed to the low densification temperature of 1325 °C as compared to other sintering techniques which necessitated high temperatures of ∼1500 °C. The exposure to high temperatures while sintering with TSS, CRH and MWS resulted into yttria segregation leading to the depletion of yttria content in fully stabilized zirconia stoichiometry as evidenced by Energy Dispersive Spectroscopy (EDS) studies.     

Amorphous carbon nanostructures from chlorination of ferrocene

The chlorination of ferrocene at different temperature conditions yields several carbon nanostructures, which were studied by means of transmission and scanning electron microscopies. Amorphous carbon nanotubes (α-CNTs) up to 10 μm long with thick walls and ∼15 nm of internal diameter were observed in a sample treated at 200 °C during 30 min. They consisted on ∼90% of carbon, while the remaining 10% consists on iron and chlorine. At this temperature, amorphous carbon bags and open-ended branches were also found. When chlorinating ferrocene at the same temperature but with longer reaction time (180 min), no α-CNTs were formed. At higher temperature (300 °C, 30 min), amorphous carbon bags were found, with lower content of residual chlorine and iron, and presenting thinner walls. In the sample treated at even higher temperature (900 °C, 30 min) the carbon nanobags (wall thickness ∼12 nm) were almost spherical and more graphitic, and without impurities.     

Improvement of microstructural properties of 3Y-TZP materials by conventional and non-conventional sintering techniques

3 mol% Y₂O₃-stabilized zirconia nanopowders were fabricated using various sintering techniques; conventional sintering (CS) and non-conventional sintering such as microwave (MW) and pulsed electric current-assisted-sintering (PECS) at 1300 °C and 1400 °C. A considerable difference in the densification behaviour between conventional and non-conventional sintered specimens was observed. The MW materials attain a bulk density 99.4% theoretical density (t.d.) at 1300 °C, while the CS materials attain only 92.5% t.d. and PECS 98.7% t.d. Detailed microstructural evaluation indicated that a low temperature densification leading to finer grain sizes (135 nm) could be achieved by PECS followed by MW with an average sintered grain size of 188 nm and CS 225 nm. It is believed that the high heating rate and effective particle packing are responsible for the improvements in these properties.     

Thermally induced crystallization of mechanosynthesized chlorapatite–titania composite nanopowders

The thermally induced crystallization of mechanosynthesized chlorapatite–titania composite nanopowder was investigated. Firstly, the composite nanopowder was produced after 5 h of milling. The mechanosynthesized powder was annealed in the range 900–1300 °C for 1 h. The 5 h milled sample showed the crystallite size and lattice strain of about 23±1 nm and 0.0107±0.00054, respectively. During the subsequent annealing, crystallization of the milled sample at 900 °C and severe decomposition of nanocomposite at 1300 °C were detected. Accordingly, the fraction of crystalline phase reached a maximum around 96±4% at 900 °C and then declined to 80±4% at 1300 °C. The lattice strain decreased drastically to about 0.0002±0.00001 at 1300 °C, while the crystallite size increased significantly to around 277±14 nm. Based on the obtained data, the unit cell volume of CAp went down during the thermal treatment due to the ion exchange reaction between chlorapatite and titania. According to the electron microscopic observations, the morphological features of composite nanopowders were influenced strongly by the annealing temperature. The 5 h milled sample was composed of spheroidal particles with an average size of about 35 and 190 nm before and after annealing at 900 °C, respectively. At 1100 °C, the coalescence of nanospheres was dominant. Finally, a porous composite structure comprised of coarse grains with an average size of about 1 μm along with finer grains with a mean size of around 100 nm was formed at 1300 °C.     

Oxidation of ZrC-30 vol% SiC composite in air from low to ultrahigh temperature

Oxidation of ZrC-30 vol.% SiC is investigated in air using furnace and oxyacetylene torch. The microstructure and phase composition of oxide scales are analyzed via SEM, XRD, and Raman. At 800 and 1100 °C, SiC is embedded in the porous and cracked ZrO₂ scales, which have a single-layer structure and are almost non-protective. At 1300 and 1500 °C, the protective effect of oxide scales is enhanced by the formed SiO₂. The scales consist of two subscales, outer and inner layers, during oxidation at 1300 °C for ≥1 h, and 1500 °C for ≥15 min. The growth kinetics of both layers is analyzed. At ∼1700 °C, a new layer is observed between the outer and inner layers, which should contain less carbon than the inner layer. At ∼2100 °C, the oxide scale is porous and contains many big holes. This scale shows a single-layer structure, which mainly consists of ZrO₂.     

Recession behavior of Lu2SiO5 under a high speed steam jet at high temperatures

Study of recession behavior of Lu₂SiO₅ bulk was performed in high speed steam jet with a velocity of ∼50 m/s temperature range between 1300 and 1500 °C for 100 h. X-ray results showed that no phase change was observed for all samples after steam exposure. Detailed scanning electron microscopy examinations showed some cracks formation was observed on the bulk surface for the samples of 1400 and 1500 °C. Also, porous structure was formed on the bulk surface for the samples of 1300 and 1400 °C. As for 1500 °C sample, the porous structure disappeared after exposure test. The high magnification images of 1300 °C sample showed the depletion of grain boundary glassy phase. However, for 1400 °C sample, boundary phase was formed again, and the grain growth can be identified for the sample of 1500 °C. The recession mechanism can be explained by a mass transfer of evaporated species from the bulk surface and the weight loss rate measured can be expressed by Arrhenius plot.     

Processing of dense nanostructured HAP ceramics by sintering and hot pressing

A nanosized HAP powder was sintered and hot pressed, in order to obtain dense HAP ceramics. In a first series of experiments, the powder was isostatically pressed into uniform green compacts and sintered at temperatures ranging from 1000 °C to 1200 °C in air atmosphere for different times. In a second series, the isostatically pressed green compacts were hot pressed in argon atmosphere at 900 °C, 950 °C and 1000 °C. The SEM micrograph of the sample sintered at 1200 °C for 2 h showed a uniform 3 μm mean grain size dense microstructure. In the case of hot pressed HAP compacts, full dense, translucent nanostructures were obtained having mean grain size below 100 nm and improved mechanical properties. With the grain size decreasing from 3 μm to 50 nm, the fracture toughness of pure HAP ceramics increased from 0.28 MPa m^1/2 to 1.52 MPa m^1/2.     

Effects of PbO and SrO contents on crystallization and dielectric properties of PbO–SrO–Na2O–Nb2O5–SiO2 glass–ceramics system

PbO–SrO–Na₂O–Nb₂O₅–SiO₂ glass–ceramics were prepared via roll-quenching followed by controlled crystallization from 700 °C to 900 °C. The effects of PbO and SrO contents on crystallization and dielectric properties were investigated. The results show that Pb₂Nb₂O₇, Sr₂Nb₂O₇ and their solid solutions crystallize at 700 °C, NaNbO₃ is the primary phase at 800 °C, Pb₂Nb₂O₇ disappears and PbNb₂O₆ forms at 900 °C. The dielectric properties of the glass–ceramics formed through controlled crystallization has a strong dependence on the phase compositions that were developed during heat treatment. The highest dielectric constants (∼600) are found in samples with 6.0 mol% SrO annealed at 900 °C for 3 h. The dielectric–temperature characteristics of the samples show stability over the range from −60 °C to 180 °C, except the sample without SrO heated at 900 °C.     

Sintering behaviour of nano-crystalline γ-Al2O3 powder without additives at 2-7 GPa

Al₂O₃ ceramics were fabricated without additives under high pressure (2-7 GPa) at different temperatures (600-1200 °C) using nanocrystalline alumina powder with metastable γ-Al₂O₃ phase as the starting material.It was shown that high pressure increases the nucleation rate while reducing the growth rate of the transformed α phase so that its grain size decreases and nano-scale grains in the sintered structure can be achieved.On the other hand the sintered samples at 7 GPa and high temperature (1000 °C) have shown micron-scale large grain sizes compared to those sintered at lower pressures, for the same temperature and sintering time. This could be attributed to the higher input energy in the system at high pressure and high temperature conditions, thereby reaching the final stage in sintering more quickly.In this work, the best combination of grain size (∼200 nm) and density (98.0% TD) was obtained under the sintering condition of 1000 °C at 7 GPa with a holding time of 1 min.Thus for high pressure/high temperature conditions, the sintering time should be reduced to prevent grain growth.     

Effect of sintering temperature on ferroelectric properties of 0.94(K0.5Na0.5)NbO3–0.06LNbO3 system

Lead free 0.94(K_0.5Na_0.5NbO₃)–0.06(LiNbO₃) (KNN–LN) system was synthesized by conventional solid state reaction route (CSSRR). The KNN–LN system was calcined at 850 °C for 6 h for the formation of single perovskite phase whereas the sintering was done at 1050 °C, 1080 °C and 1100 °C for 4 h, respectively. The KNN–LN samples sintered at 1080 °C show better properties: room temperature (RT) dielectric constant (?_r) ∼936, dielectric loss (tan δ) ∼0.016 at 1 MHz, a relatively high bulk density (ρ) ∼4.385 g/cm³, which is 97.5% of the theoretical density (TD ∼ 4.51), remnant polarization (P_r) ∼6.4 μC/cm² and coercive field (E_c) ∼9.6 kV/cm have been observed.     

Ultra-fast densification of boron carbide ceramics under high heating rate and high pressure

Boron carbide ceramics were obtained in 2 min by a method based on self-propagating high-temperature synthesis plus quick pressing (SHS/QP). The samples were densified to 98% of theoretical density under a large mechanical pressure (120 MPa) and a fast heating rate (2300 °C/min). The microstructure and mechanical properties were studied. The sample obtained at this heating rate presents an average grain size of 3 μm and a hardness of 34 ± 0.2 GPa.     

Two-step sintering of fine alumina–zirconia ceramics

This work investigates the feasibility to the fabrication of high density of fine alumina–5 wt.% zirconia ceramics by two-step sintering process. First step is carried out by constant-heating-rate (CHR) sintering in order to obtain an initial high density and a second step is held at a lower temperature by isothermal sintering aiming to increase the density without obvious grain growth. Experiments are conducted to determine the appropriate temperatures for each step. The temperature range between 1400 and 1450 °C is effective for the first step sintering (T₁) due to its highest densification rate. The isothermal sintering is then carried out at 1350–1400 °C (T₂) for various hours in order to avoid the surface diffusion and improve the density at the same time. The content of zirconia provides a pinning effect to the grain growth of alumina. A high ceramic density over 99% with small alumina size controlled in submicron level (0.62–0.88 μm) is achieved.     

Sintering,microstructure and conductivity of La2NiO4+δ ceramic

Microstructure and electrical conducting properties of La₂NiO_4+δ ceramic were investigated in the sintering temperature range 1200–1400 °C. The results demonstrate that the microstructure and electrical conducting properties of La₂NiO_4+δ ceramic are sensitive to sintering temperature. Compared with a progressive densification development with sintering temperature from 1200 to 1300 °C along with an insignificant change in grain size, there is an exaggerated grain growth in the specimens sintered at higher temperatures. Increasing sintering temperature from 1200 to 1300 °C resulted in an enhancement of electrical conducting properties. Further increase of sintering temperature exceeding 1300 °C reduced the electrical conducting properties. A close relation between the microstructure and electrical conducting properties was suggested for La₂NiO_4+δ ceramic. With respect to the electrical conducting properties, the preferred sintering temperature of La₂NiO_4+δ ceramic was ascertained to be 1300 °C. The specimen sintered at 1300 °C exhibits a generally uniform microstructure together with electrical conductivities of 76–95 S cm⁻¹ at 600–800 °C.     

Fabrication of YAG transparent ceramics by two-step sintering

Yttrium aluminum garnet (YAG) nanopowders with mean particle size of about 50 nm synthesized by a modified co-precipitation method were used to sinter bulk YAG ceramic by two-step sintering method. Full densification was achieved by heating the sample up to 1800 °C followed by holding at 1550 °C for 10 h. Transparent YAG ceramics were obtained by suppressing grain-boundary migration while promoting grain-boundary diffusion during the two-step sintering process. The microstructure of the YAG ceramic is homogeneous without abnormal grain growth and the transmittance of the sintered sample is 43%.     

Synthesis of nanocrystalline 8 mol% yttria stabilized zirconia by the oleate complex route

Nanocrystalline 8 mol% yttria stabilized zirconia (YSZ) powder has been synthesized by the oleate complex route. Oleate complexes of zirconium and yttrium were formed in the hexane rich layer by the reaction of sodium oleate with zirconyl chloride and yttrium chloride at the interface of the two ternary solutions in water–ethanol–hexane system. The zirconyl oletae and yttrium oleate complexes on heating decomposed to oxide through the formation of carbonate intermediates. The powder obtained by calcination at 600 °C for 2 h was cubic YSZ with surface area of 42 m²/g. The YSZ powder contained primary particles of ∼300 nm size and the primary particles were aggregate of crystallites of 5–10 nm. The compacts prepared from the YSZ powder were sintered to ∼99% TD (theoretical density) at 1400 °C. The sintered YSZ had a low average grain size of 0.73 μm.     

Effect of YF3 on the phase stability and sinterability of hydroxyapatite—Partially stabilized zirconia composites

Pure hydroxyapatite (HA), HA and partially stabilized zirconia composites (PSZ) with YF₃ and HA–PSZ composite containing 5 wt% PSZ without YF₃ were sintered in air at 900 °C, 1100 °C and 1300 °C for 1 h. The reactions and transformation of the phases in the composites were determined by X-ray diffraction. All the composites with or without YF₃ showed desirable thermal stability below 1300 °C and besides various amounts of CaZrO₃, any amount of tri-calcium phosphate (TCP) was not observed. Above 1100 °C, composites with YF₃ showed higher thermal stability than the composites without YF₃. On the other hand, pure HA started to decompose and TCP was observed at 1300 °C. Composites with YF₃ showed improved thermal stability than the composite containing 5 wt% PSZ without YF₃ and pure HA at lower sintering temperatures such as 900 °C and 1100 °C. However, it was observed that the increasing amount of YF₃ addition caused negative effect on the thermal stability of the composites. 5ZHA composites with YF₃ showed the highest relative density among all of the composites with or without YF₃.     

Fabrication of Sn doped Ba(Mg1/3Ta2/3)O3 transparent ceramics by a solid state reaction method

Transparent disordered BMT ceramics were obtained by solid state reaction. Sn⁴⁺ ions were incorporated to make the B site of the perovskite structure disordered. The stoichiometric powder mixture with and without Sn doping was calcinated at 1300 °C, respectively and they were both characterized. After dry pressing, the pellets with Sn doping were sintered at 1600 °C in oxygen atmosphere for 4 h. The grain size of the transparent ceramics is around 12 μm. No pores were detected in or among the grains. The inline transmittance of the material is 66% at 580 nm. The refractive index is 2.09 at 1600 nm.     

Densification of additive-free polycrystalline β-SiC by spark-plasma sintering

Additive-free β-SiC nanopowders were densified by spark-plasma sintering in the temperature range 1650-2200 °C, holding time 1-30 min, and pressure 50-150 MPa. The starting nanopowder was further refined and mechanically activated by ball milling in order to promote densification. In a detailed parametric study, temperature, holding time, and/or pressure were varied to investigate their effect on the densification and grain growth during sintering. High densifications were obtained for temperatures greater than 1800 °C. Full densification (98.0 ± 0.6%) was reached at a sintering temperature of 2100 °C. Depending on the sintering conditions, nanostructured materials can be produced with grain sizes between 90 and 100 nm, and microstructured materials with grain sizes between 0.22 and 2.39 μm. Pressure has a major effect on the grain size, it being necessary to use a pressure of 150 MPa to obtain nanostructured materials.     

OH and COOH functionalized single walled carbon nanotubes-reinforced alumina ceramic nanocomposites

Alumina ceramics reinforced with 1 wt.% single-walled carbon nanotube (SWCNT) were fabricated via spark plasma sintering (SPS) of composite powders containing carboxyl (COOH) or hydroxyl (OH) group functionalized single-walled carbon nanotubes. The samples were SPS’ed at 1600 °C under 50 MPa pressure for holding time of 5 min and at a heating rate of 4 °C/s. The effects of CNT addition having different surface functional groups on microstructure, conductivity, density and hardness were reported. It was shown that nanotube addition decreased the grain sizeof alumina from 3.17 μm to 2.11 μm for COOH-SWCNT reinforcement and to 2.28 μm for COOH-SWCNT reinforcement. The hardness values of the composites are similar for all samples but there is 4.5 and 7.5 times increase in electrical conductivity with respect to monolithic alumina for COOH-SWCNT and OH-SWCNT, respectively. It was also shown by TEM and FEG SEM observations that transgranular fracture behaviour of alumina was changed to mostly intergranular fracture mode by the addition of both types of CNTs which may be due to location of CNTs along the grain boundaries. A significant grain size reduction in alumina is considered toresult fromthe suppressing effect of CNTs during sintering.     

Densification and microstructure evolution of Cr,Nd:YAG transparent ceramics for self-Q-switched laser

Transparent 0.1 at.% Cr, 1.0 at.% Nd:YAG ceramics were fabricated by solid-state reaction and vacuum sintering using commercial Y₂O₃, α-Al₂O₃, Cr₂O₃ and Nd₂O₃ as raw materials. CaO and tetraethoxysilane (TEOS) were used as charge compensator and sintering aid, respectively. The powders were mixed in ethanol and doped with TEOS, dried and pressed. Pressed samples were sintered from 1450 to 1800 °C for 10 h. The relative density increased from 68.8% to 99.4% at the sintering temperature from 1450 to 1700 °C. Grain size increased with increase of sintering temperature and obvious grain growth occurred between 1650 and1700 °C. For the Cr,Nd:YAG ceramics sintered at 1750 and 1800 °C for 10 h, nearly pore-free microstructures with average particle size of ∼10 μm were obtained. The optical transmittance of the 1800 °C sintered sample was ∼70% in the infrared wavelength.