Effect of dopants and sintering temperature on microstructure and low temperature degradation of dental Y-TZP-zirconia

Accelerated ageing of dental TZP were investigated at 134 °C for 2 h under 2.3 bar water vapor pressure. The TZP blanks were sintered in the range from 1350 to 1580 °C. The average grain size of 1350 and 1400 °C sintered materials were <0.3 μm whereas higher sintering temperatures led to larger grain sizes. The grain size and dopants influence the stability of tetragonal phase of zirconia under LTD conditions. The Y-TZP with average grain sizes <0.3 μm did not reveal the martensitic tetragonal-monoclinic phase transformation after ageing, whereas zirconia with grain sizes larger 0.3 μm showed fractions of monoclinic phase. Alumina and Ceria stabilized grain size and Y-TZP against LTD. Y-TZP with low amounts of Fe₂O₃ (<0.15%) used for coloring did not show any detrimental effects under LTD conditions. As the Y-TZP ceramics with grain size larger than 0.3 μm are not stable under LTD conditions they are not recommended for long term use in moist environment.     

Two step sintering of a novel calcium magnesium silicate bioceramic: Sintering parameters and mechanical characterization

Two-step sintering (TSS) was applied to control the grain growth during sintering of a novel calcium magnesium silicate (Ca₃MgSi₂O₈ – Merwinite) bioceramic. Sol–gel derived nanopowders with the mean particle size of about 90 nm were sintered under different TSS regimes to investigate the effect of sintering parameters on densification behavior and grain growth suppression. Results showed that sintering of merwinite nanopowder under optimum TSS condition (T₁ = 1300 °C, T₂ = 1250 °C) yielded fully dense bodies with finest microstructure. Merwinite compacts held at T₂ = 1250 °C for 20 h had the average grain size of 633 nm while the relative density of about 98% was achieved. Mechanical testing was performed to investigate the effect of grain growth suppression on the hardness and fracture toughness. Comparison of mechanical data for samples sintered under two sintering regimes, including TSS and normal sintering (NS), showed TSS process resulted in significant enhancement of fracture toughness from 1.77 to 2.68 MPa m^1/2.     

Suppression of grain growth in sub-micrometer alumina via two-step sintering method

Two-step sintering (TSS) was applied to suppress the accelerated grain growth of sub-micron (～150 nm) alumina powder. The application of an optimum TSS regime led to a remarkable decrease of grain size down to ～500 nm; while the grain size of the full-dense structures produced by conventional sintering ranged between 1 and 2 μm. To find how important the temperatures at sintering steps might be, several TSS regimes were conducted. The results showed that the temperatures at both sintering steps play vital roles in densification and grain growth of the alumina compacts. Based on the results, the optimum regime consisted of heating the green bodies up to 1250 °C (first step) and then holding at 1150 °C for more than 60 h (second step). This yielded the finest microstructure with no deterioration of the densification. Heating at 1300 °C (first step) and then at 1200 °C (second step) was not a successful procedure. Lowering the temperature of the second step down to 1100 °C resulted in exhaustion of the densification at 88% -theoretical density. A nearly full-dense structure with an average grain size of 850 nm was obtained when the temperature of the second step was increased to 1150 °C. Empirical results show that not only the first step temperature has to be high enough to reach a structure containing unstable pores, but the second sintering temperature must also be high enough to activate the densification mechanism without grain growth. This means that a considerable densification at the first step does not imply enough second-step densification.     

Analysis of reactions during sintering of CuO-doped 3Y-TZP nano-powder composites

3Y-TZP (yttria-doped tetragonal zirconia) and CuO nano powders were prepared by co-precipitation and copper oxalate complexation–precipitation techniques, respectively. During sintering of powder compacts (8 mol% CuO-doped 3Y-TZP) of this two-phase system several solid-state reactions clearly influence densification behaviour. These reactions were analysed by several techniques like XPS, DSC/TGA and high-temperature XRD. A strong dissolution of CuO in the 3Y-TZP matrix occurs below 600 °C, resulting in significant enrichment of CuO in a 3Y-TZP grain-boundary layer with a thickness of several nanometres. This “transient” liquid phase strongly enhances densification. Around 860 °C a solid-state reaction between CuO and yttria as segregated to the 3Y-TZP grain boundaries occurs, forming Y₂Cu₂O₅. This solid-state reaction induces the formation of the thermodynamic stable monoclinic zirconia phase. The formation of this solid phase also retards densification. Using this knowledge of microstructural development during sintering it was possible to obtain a dense nano–nano composite with a grain size of only 120 nm after sintering at 960 °C.     

Densification and biocompatibility of sintering 3.0 mol% yttria-tetragonal ZrO2 polycrystal ceramics with x wt% Fe2O3 and 5.0 wt% mica powders additive

The densification and biocompatibility of sintered 3.0 mol% yttria-tetragonal zirconia polycrystal (3Y-TZP) ceramics, with X wt% Fe₂O₃ and 5.0 wt% mica powders (denoted by 3Y-TZP: X-5.0 wt% mica) have been studied. When the pellets of 3Y-TZP: X-5.0 wt% mica were sintered at 1300 °C for 1 h, the relative shrinkage increases from 19.20–19.43% with the X increased from 0.3 to 1.0. The relative shrinkage of pellets containing 1.0 wt% Fe₂O₃ (X=1.0) increased from 19.43–19.59% when sintering temperatures were raised from 1300 °C to 1450 °C. X-ray diffraction results show that the pellets of 3Y-TZP: X-5.0 wt% mica sintered at 1400 °C for 1 h only contained single phase of tetragonal ZrO₂ (t-ZrO₂). When the sintering temperature was higher than 1400 °C, the Vickers microhardness was greatest in the pellets with X=0.5. Within pellets with the same Fe₂O₃ content, the dominant wavelength (λ_d) was only slightly different for pellets sintered at 1300 °C and those sintered at 1450 °C. The results of the materials were evaluated in vitro cytotoxicity tests reveals that the powders and sintered pellets are safe materials. The oral mucosa irritation tests did not find erythema or histopathological change including normal epithelium, and was free from leucocyte infiltration, vascular congestion and oedema.     

Sintering behaviour and microstructure of 3Y-TZP + 8 mol% CuO nano-powder composite

Nanocrystalline 3Y-TZP and copper-oxide powders were prepared by co-precipitation of metal chlorides and copper oxalate complexation–precipitation, respectively. A significant enhancement in sintering activity of 3Y-TZP nano-powders, without presence of liquid phase, was achieved by addition of 8 mol% CuO nano-powder, resulting in an extremely fast densification between 750 and 900 °C. This enhancement in sintering activity was explained by an increase in grain-boundary mobility as caused by dissolution of CuO in the 3Y-TZP matrix. The nano-powder composite was densified to 96% by pressureless sintering at 1130 °C for 1 h. Considerable tetragonal to monoclinic phase transformation of the zirconia phase was observed by high temperature XRD analysis. This zirconia phase transformation is discussed in terms of reactions between CuO and yttria as segregated to the 3Y-TZP grain boundaries.     

Ageing of dental zirconia ceramics

An in vitro study was designed to monitor the t–m transformation of biomedical grade 3Y-TZP ceramics produced from two ready-to-press granulated powders of the same nominal chemical composition, but differing in their specific surface area. Disc-shaped specimens were formed by uni-axial dry-pressing and sintering in air for 2 h in the temperature range 1400–1550 °C, resulting in a mean grain size of 0.26–0.57 μm. The sintered specimens were subjected to accelerated ageing in deionized water at 134 °C for up to 48 h and verified in terms of the amount of the transformed monoclinic fraction, the thickness of the transformed layer, and the bi-axial flexural strength. The sintering-temperature-dependent transformability during ageing was confirmed, whereby the role of the starting-powder characteristics was more pronounced at lower sintering temperatures and was almost negligible at higher sintering temperatures.     

Low-temperature sintering of (K,Na)NbO3-based lead-free piezoceramics with addition of LiF

Dense Li/Ta-codoped KNN-based piezoceramics with d₃₃^* up to 375 pm/V were successfully fabricated by conventional sintering at a temperature as low as 900 °C by using LiF as a sintering additive. The reduction of densification temperature up to 200 °C was realized by a transient liquid phase sintering mechanism, consequently no grain boundary phase was observed in the sintered samples. It was found that the addition of LiF could further shift down the tetragonal–orthorhombic transition point (T_T-O), indicating that a small amount of Li⁺ could diffuse into the A-site of KNN matrix. The introduction of LiF enhanced the linearity of strain curves of the ceramics, which is unambiguously in favor for the actuator application. The present work reveals that low-temperature sintered LiF-doped KNN-based piezoceramics demonstrates promising potential in multilayer-structured actuator applications.     

Preparation of c-axis textured SiC ceramics by a strong magnetic field of 6 T assisted gel-casting process

C-axis textured SiC ceramics were prepared by a strong magnetic field of 6 T assisted gel-casting and subsequent pressureless sintering. The optimal suspension parameters for gel-casting were determined by analyzing the influences of pH value and dispersant content on the stability and dispersibility of suspensions. The effect of sintering conditions on the texture development and properties of SiC ceramics was discussed. It was found that the increasing sintering temperature or holding time promoted the densification process of SiC ceramics. The c-axis of SiC grain was aligned parallel to the magnetic field by applying a strong magnetic field of 6 T. The degree of texture of SiC ceramics showed a slightly increasing trend with the increase of sintering temperature or holding time. When the samples were sintered at 1950 °C for 4 h or 6 h, the large elongated grains were formed in the samples, leading to the extremely evident anisotropic microstructure on different planes. Textured SiC ceramics exhibited the anisotropic bending strength.     

Enhanced dielectric and piezoelectric properties in Li/Sb-modified (Na,K)NbO3 ceramics by optimizing sintering temperature

In this paper, lead-free (Na_0.474K_0.474Li_0.052)(Nb_0.948Sb_0.052)O₃ ceramics were synthesized by a conventional solid-state reaction route. The effects of sintering temperature on the crystal structure, microstructure, densification, dielectric properties, and ferroelectric properties of the KNNLS ceramics were addressed. X-ray diffraction patterns and Raman spectrum indicated a transition from orthorhombic to tetragonal phase during the sintering temperature region. This transition is attributed to the migration of Li between the matrix grain and grain boundary. Scanning electron microscopy study revealed increased grain size and enhanced densification with increasing sintering temperature. The density of the ceramics sintered at 1080 °C reached a maximum value of 4.22 g/cm³. KNNLS ceramics sintered at an optimum temperature of 1080 °C exhibited high piezoelectric properties, that is 242 pC/N for d₃₃, 0.42 for k_p and 18.2 μC/cm² for P_r.     

Densification and mechanical properties of hot-pressed ZrN ceramics doped with Zr or Ti

The densification of hot-pressed ZrN ceramics doped with Zr or Ti have been investigated at 1500–1700 °C. It is shown that either Zr or Ti additive can facilitate the densification process. ZrN with 20 mol% Zr or Ti (named ZNZ and ZNT) sintered at 1700 °C can achieve above 98% relative densities whereas densification temperature up to 2000 °C is necessary for pure ZrN. The densification improvements are attributed to solid solution of Zr or Ti into ZrN to form non-stoichiometric ZrN_1?x or (Zr, Ti)N_1?x. The microstructures and mechanical properties of ZNZ and ZNT samples have been examined. Large grain size and flat fracture surface existed in ZNT sample sintered at 1700 °C, which lead to poor toughness as low as 2.3 MPa m^1/2. On the contrary, the fracture toughness of ZNZ sample sintered at 1700 °C was up to 5.9 MPa m^1/2, attributed to fine and uniform grain size distribution.     

Sintering,microstructure and hardness of Y-TZP- 64S bioglass ceramics

3 mol% yttria-partially stabilized zirconia (Y-TZP) powder and a sol-gel derived CaO- P₂O₅- SiO₂ (64S) bioglass, were used to produce Y-TZP- 64S slip cast compacts. The compacts with 10.5 and 19.9 vol% 64S were sintered at different temperatures up to 1500 °C using 5 and 10 °C/min heating/cooling rates. The densification behaviour, crystalline phase formation and zirconia grain growth were investigated as a function of sintering temperature and 64S glass content. Ca₃(PO₄)₂ along with SiO₂ as a major phase were obtained from thermal decomposition of the 64S glass at 950–1500 °C. Both 64S additions, 10.5 and 19.9 vol%, promoted the sintering process at a lower temperature with respect to Y-TZP (1500 °C); the SiO₂ phase markedly increased the Y-TZP solid state sintering rate at the intermediate stage. The rapidly cooling at 10 °C/min inhibited the t-m transformation of Y-TZP and markedly reduced that of Y-TZP- 64S at 1300–1500 °C. Sintered Y-TZP with 10.5 vol% 64S, nearly fully densified at 1300–1400 °C, was constituted by polygonal ZrSiO₄ particles and elongated Ca₂P₂O₇ particles uniformly distributed in the tetragonal zirconia fine grain matrix. This ceramic exhibited similar hardness to that of Y-TZP sintered at 1500 °C; the in situ formation of calcium phosphate will have the potential to improve the Y-TZP biological properties without significantly affecting its hardness.     

Manipulating microstructure and mechanical properties of CuO doped 3Y-TZP nano-ceramics using spark-plasma sintering

Nano-powder composites of 3Y-TZP doped with 8 mol% CuO were processed by spark-plasma sintering (SPS). A 96% dense composite ceramic with an average grain size of 70 nm was obtained by applying the SPS process at 1100 °C and 100 MPa for 1 min. In contrast to normal, pressureless, sintering during SPS reactions between CuO and 3Y-TZP were suppressed, the CuO phase was reduced to metallic Cu, while the 3Y-TZP phase remained almost purely tetragonal. Annealing after SPS results in grain growth and tetragonal to monoclinic zirconia phase transformation. The grain size and monoclinic zirconia phase content are strongly dependent on the annealing temperature. By combining the processing techniques studied in this work, including traditional pressureless sintering, properties of the composite ceramic can be tuned via manipulation of microstructure. Tuning the mechanical properties of dense 8 mol% CuO doped 3Y-TZP composite ceramic by utilising different processing techniques is given as an example.     

Effect of sintering temperature on structural and dielectric properties of Bi and Li co-substituted barium titanate ceramic

Bi and Li co-substituted barium titanate, Ba_0.98(Bi,Li)_0.02TiO₃ ceramic samples were sintered at different temperatures using conventional solid state sintering technique. X-Ray Diffraction patterns confirm tetragonal phase in all the sintered samples. Microstructure analysis using Scanning Electron Microscopy (SEM) reveals increasing grain sizes with an increase in sintering temperature. Dielectric spectroscopy performed in the range of 40 Hz to 2 MHz at room temperature shows that the dielectric constant increases with increasing sintering temperature, reaching a maximum of value of 1200 at 40 Hz where the dielectric loss observed was less than 0.02 for samples sintered at 1300 °C. Temperature dependant impedance spectroscopy data in the range of 30–300 °C was used to measure AC conductivity. The activation energy of grains was deduced through Arrhenius plots. Loss tangent at different frequencies for 1300 °C sintered samples was less than 0.1 over the entire temperature range. The high dielectric constant with a low dielectric loss at elevated temperatures make Ba_0.98(Bi,Li)_0.02TiO₃ samples suitable for Multi-Layer Ceramic Capacitors (MLCC)s used in high-temperature applications.     

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.     

Dielectric characterization of microwave sintered lead zirconate titanate ceramics

Highly reactive lead zirconate titanate powders (PZT) with different compositions were successfully synthesized by the oxidant-peroxo method (OPM) and used to prepare dense ceramic samples with composition near to the morphotropic phase boundary (MPB) sintered at 1000 °C for 2 h using a tubular conventional oven and a commercial microwave system. Crystalline phases were identified in the powder and ceramic samples by X-ray powder diffraction and FT-Raman spectroscopy at room temperature. The fractured surface of the ceramic sample showed a high degree of densification with fairly uniform grain sizes. Dielectric constants measured in the range from 30 to 500 °C at different frequencies (1, 10 and 100 kHz) indicated a normal ferroelectric behavior regardless of the sintering method. Samples sintered by a microwave radiation (MW) method and composition near to the MPB region showed a maximum dielectric constant of 17.911 and an anomalous high Curie temperature of 465 °C.     

Microwave sintering and grain growth behavior of nano-grained BaTiO3 materials

In this paper, we investigated the effect of microwave sintering parameters on the development of the microstructure of nano-grained BaTiO₃ materials co-doped with Y and Mg species. It is observed that the materials can not only be sintered densely at a lower temperature (1150 °C) and a shorter soaking time (20 min), but also the grain growth can be suppressed by 2.45 GHz microwave heating process. However, the grain growth exhibits a unique tendency in some processing conditions such as microwave sintering for longer intervals (≧60 min) or at higher temperatures (1200 °C). The grain growth behavior after densification was investigated in terms of the phenomenological kinetics, and the activation energy for grain growth using microwave sintering (59.4 kJ/mol) is considerably less than that of the conventionally sintered ones (96.0 kJ/mol), which indicates that microwave sintering process can accelerate the densification rate of the BaTiO₃ materials comparing with the conventional sintering process.     

Densification of SiC by SPS-effects of time,temperature and pressure

Temperature, holding time and conditions of pressure application, three of the most important spark plasma sintering (SPS) parameters, have been reviewed to assess their effect on the densification and grain growth kinetics of a pure commercially available submicrometer-sized silicon carbide powder. Experiments were performed in the 1750–1850 °C temperature range with holding time from 1 to 10 min. Two pressure setups were used: one with pressure (75 MPa) applied at 1000 °C and the other with ultimate pressure applied at sintering temperature. Experimental data highlighted the fact that temperature and holding time have a different impact on grain growth and densification. Diffusion and migration mechanisms that promote grain growth were found to be strongly dependent on temperature, the latter being linked to pulsed current intensity. Conditions of pressure application suggest that the ultimate pressure applied at higher temperature increases densification by keeping small surface contact between particles.     

Rationalizing the role of magnesia and titania on sintering of α-alumina

The rationalization of selection of sintering additives for α-alumina was investigated using two oxides (MgO and TiO₂) to discern their individual roles. Using both dynamic heating study in a thermomechanical analyzer and static heat treatment, the precise role of each oxide was established. Grain growth trajectory of different doped samples sintered at 1700 °C revealed that MgO neither significantly affected densification nor facilitated grain growth upto 1700 °C. MgO reacted with alumina to form spinel prior to the densification process. Thus it could not generate further extrinsic defects in corundum lattice during sintering, which usually facilitate densification. In contrast, TiO₂ significantly enhanced the densification and promoted grain growth in α-alumina. At 1700 °C, the average grain size of titania doped samples were 7.7x larger than undoped ones and 10x larger than magnesia dopes samples. The sintered grains developed higher aspect ratio when TiO₂ was used which may be ascribed to preferred growth of the 012 and 024 planes of corundum. The nearly perfect junction of grain boundaries meeting at ~120° indicates absence of liquid phase and that the entire sintering process most probably took place in solid state for both MgO and TiO₂ doped samples.     

Synthesis and characterization of dense and fine nickel ferrite ceramics through two-step sintering

Two-step sintering (TSS) has been employed in the current study to suppress the accelerated grain growth of NiFe₂O₄ nanopowder compacts in the final sintering stage. Experiments are conducted to determine the appropriate temperatures for each step. The temperature range from 1200 °C to 1300 °C is effective for the first-step sintering (T₁) due to its highest densification rate. The second-step sintering temperature (T₂) should be within the kinetic window, where grain boundary diffusion is maintained but grain boundary migration is suppressed. The grain sizes of high density (≥98% theoretical density) NiFe₂O₄ compacts produced by TSS are smaller than 700 nm, while that of those formed by CS are over 2.5 μm. The evidence indicates that the saturation magnetization of nearly full NiFe₂O₄ ceramics is independent of grain size and likewise high, with the corresponding values of approximately 54 emu/g. The Vickers hardness and fracture toughness both increase with the decrease of grain size and porosity.