Microwave-assisted sintering of dental porcelains

An investigation was made of hybrid microwave-assisted sintering of dental porcelains, using five commercial ceramic frits employed in the production of dental porcelains. The powders were characterized, transformed into prismatic test specimens, and subjected to conventional and microwave sintering. Microwave sintering was performed at a frequency of 2.45 GHz, using a susceptor material and in the absence of vacuum. The apparent density and apparent porosity of the sintered samples were characterized based on the Archimedes principle. They were also analyzed by X-ray diffraction (XRD) and scanning electron microscopy (SEM), and their flexural strength and microhardness were determined by the Vickers method. The powders, which showed a broad particle size distribution with a high fraction of particles of dimensions larger than 30 μm, were composed of amorphous phase and leucite particles. Microwave sintering yielded ceramic bodies whose apparent porosity (t-test, p<0.05) was the same or very similar to that of the conventionally sintered samples, while the apparent density (t-test, p<0.05) of most of the microwave sintered samples was the same or slightly lower. Although the microwave sintered samples showed larger average pore sizes (t-test, p<0.05), four of the five samples used in this study showed the same flexural strength (t-test, p<0.05) and all the ceramics under study showed the same surface microhardness (t-test, p<0.05).     

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.     

Glass–ceramic frits for porcelain stoneware bodies: Effects on sintering,phase composition and technological properties

In the present work, the effects of glass–ceramic frits (10 wt.%) added to a porcelain stoneware body in replacement of non-plastic raw materials were evaluated simulating the tile-making process. Each glass–ceramic frit plays its own peculiar effect on the compositional properties and only some precursors behave as real glass–ceramic materials. The positive influence of glass–ceramic precursors in promoting the sintering stands out when temperature onset densification and sintering rate are considered: both of them are improved with respect to the reference body. The presence of glass–ceramic frits allows to preserve good technological properties, complying with the latest requirements of the industrial practice.     

Effect of firing temperature and atmosphere on sintering of ceramics made from Bayer process bauxite residue

Bauxite residue, the principal waste from the Bayer process, was dried, pressed and studied for its thermal and sintering behaviour under different atmospheres, up to 1100 °C. For sintering in air and N₂, shrinkage begins at 800 °C and ranges from 2.6% to 13.9%, after firing at 1000–1100 °C. Bulk density varies from 1.7 to 2.3 g/cm³ whereas water absorption from 31.5% to 17.7%. The main crystalline phases identified on firing in air were hematite (Fe₂O₃), gehlenite (Ca₂Al₂SiO₇) and perovskite (CaTiO₃) whereas magnetite (Fe₃O₄) was also found on firing in N₂. Microstructures are characterised by irregularly shaped, <20 μm Feret diameter, pores in a ceramic matrix with interconnected porosity. The average pore size is greater in samples fired in N₂. On sintering in 4%H₂/Ar, shrinkage begins at 710 °C. After firing at 1100 °C, shrinkage is 20.1% and water absorption 1%. The main crystalline phases are magnetite, wustite (FeO), gehlenite and perovskite. Microstructures are characterised by a compact heterogeneous matrix, with isolated <15 μm Feret diameter, closed pores. The grains have reacted with the adjacent phase and their shape is rounded with no sharp facets. Increased sintering temperature results in improved physical properties for all atmospheres tested and in higher average pore size when sintering takes place in air and N₂. The use of magnetite-reducing sintering conditions can potentially assist in the production of a variety of ceramic compositions containing bauxite residue.     

The effect of the wet-milling process on sintering temperature and the amount of additive of SiAlON ceramics

In this study, nano-sized SiAlON powders were produced by wet milling at elevated speeds as a top-to-bottom process. Before the milling process, different milling times and mediums were performed for the determination of the most efficient milling system. The milled powders were characterized by BET and X-ray diffraction (XRD) measurements and the results were compared to standard samples. The standard powders were produced using a conventional process (the ball to powder ratio was 1:1.5, at 300 rpm, for 1.5 h) having a few hundred nanometer particle size. The nano powders were milled using a wet-milling process in an optimum medium so that the particle size was decreased down to ≈70 nm. The samples, produced from the nano powders, were densified at 150 °C lower degrees than the sintering temperature of samples which were produced by a conventional method (185 nm). However, the phase transformation of α → β-SiAlON was also observed related to the amount of additives. This transformation affected the mechanical properties of the SiAlON ceramic. The results were discussed using the relationship between density, phase composition, microstructure and mechanical properties.     

Effect of iron oxide coloring agent on the sintering behavior of dental yttria-stabilized zirconia

To explore the use of yttria-stabilized zirconia (YSZ) for applications in dentistry, the effect of iron oxide coloring agent on the sintering behavior of YSZ is investigated. Through the use of a small amount of iron nitrate, the color of YSZ can be tailored. The iron nitrate starts to decompose to result in iron oxide, then to dissolve into zirconia grains before the shrinkage is even started. The iron solutes enhance the sintering activity of zirconia in terms of the temperatures at the start of shrinkage and at the maximum shrinkage rate. However, the size of zirconia grains is also increased along with Fe content. More monoclinic phase is found in the specimens with higher Fe content. The formation of m-phase is detrimental to both hardness and toughness of zirconia, limiting the amount of coloring agent can be added.     

The sintering kinetics of porcelain bodies made from waste glass and fly ash

Porcelain is a material produced from kaoline, quartz and potassium-feldspar. Recently, research of new materials, for example non-hazardous wastes, that are able to replace traditional fluxing agents without changing the process or quality of the final products has been realized. The aim of this work is to study the possibility of the use of glass powder waste and fly ash together for manufacturing porcelain. Instead of quartz, fly ash was used at the selected porcelain composition. The waste glass was added partially and fully in replacement of potassium-feldspar. Samples were fired in an electric furnace with a heating rate of 10 °C/min at 1100, 1150 and 1200 °C for a period of 1, 2, 3 and 5 h. The sintered samples were characterised by XRD (X-ray diffraction) and SEM (scanning electron microscopy). Sintering activation energies were determined based on the bulk density result. At 10, 15, 20 and 25 wt.% glass waste addition, the apparent activation energies were calculated to be 145, 113.5, 70.4 and 53.74 kJ/mol, respectively. It was found that the sintering activation energy decreased with increasing waste glass addition.     

Effect of chromium additive on sintering and oxidation behavior of HfB2-SiC ceramics

The effect of chromium admixture on the processes in the HfB₂-SiC ceramic powder system during its pressureless sintering at 1600?°C was studied. It was shown that an increase in chromium content from 0% to 15.5% in the HfB₂-SiC ceramic powder mixture leads to a continuous increase in its relative density up to 90%. A transient liquid phase Cr-Si-C-B is formed at 1600?°C, and it promotes intense sintering of HfB₂ and SiC powders. The oxidation resistance of HfB₂-SiC-Cr ceramics was studied in static air at 1000–1500?°C. It was shown that the oxidation resistance is greatly improved due to a decrease in the porosity of the sintered ceramic system because of chromium additive. The presence of chromium oxide in the formed surface glassy layer can also lead to the increase in the oxidation resistance. These results suggest that chromium can be considered as a promising sintering additive for HfB₂-SiC and similar systems.     

Effects of sintering temperature and holding time on porosity and shrinkage of glass tubes

The influences of sintering temperature and holding time on porosity and shrinkage of glass tubes have been studied by optical microscope. It is evident that there exists three stages for the sintering process of glass. At the first stage, both increasing temperature and prolonging the holding time contribute to lowering the porosity and to intensifying the shrinkage greatly. At the second stage, the glass further densifies and the voids among particles become smaller and less. Finally, at the third stage the shrinkage rate almost keeps unchanged to sintering temperature and holding time.     

Effect of sintering temperature on mechanical properties of magnesia partially stabilized zirconia refractory

The optimized sintering conditions for a 3.5 wt% magnesia partially stabilized zirconia (Mg-PSZ) refractory were proposed in our recent research. The influence of the sintering temperature on the development of phase composition, microstructure, densification, thermal expansion and mechanical strength was studied in detail by X-ray diffraction (XRD), scanning electron microscope (SEM), He-pycnometer, high temperature dilatometry and three-point bending test. The samples sintered at 1670 °C had the highest bend strength, the maximum densification, the lowest thermal expansion coefficient (CTE), a homogeneous microstructure and a linear change in thermal expansion.     

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.     

Processing of mesocarbon microbeads to high-performance materials: Part I. Studies towards the sintering mechanism

Owing to their exceptional properties, carbon-carbon composites have a variety of important applications. One attractive approach for the efficient and low-cost production of such materials is to utilize the unique sintering ability of mesocarbon microbeads (MCMB). However, the mechanism of MCMB sintering is not fully understood. In this work, detailed studies are made towards this goal, using dilatometric, thermogravimetric, and mass spectrometry techniques along with microstructural analysis. It is shown by independent measurements that significant changes in pycnometric density, mass loss, and shrinkage all occur in the same temperature range (800-1200 K). Based on the obtained results, a new explanation is suggested for the high sinterability of the investigated material, which includes two main stages: (i) neck formation between particles by a viscous phase non-densifying sintering mechanism (<800 K); (ii) rapid sample shrinkage due to crystallographic transformations leading to changes of theoretical particle density in the temperature range 800-1200 K.     

Development and characterisation of triaxial electrical porcelains from Ugandan ceramic minerals

Ten formulations of triaxial porcelain composed from 30–60% clay, 20–45% feldspar and 20–25% sand, were prepared from raw materials sourced from Ugandan deposits. Specimens were made using the plastic extrusion method and characterized in terms of constituent oxide composition, flexural strength, fracture toughness, dielectric strength, microstructure and phase properties using ICP-AES analyses, 4-point load strength test, Vicker's indentation, FEG-SEM and powder-XRD analyses, respectively. XRD studies revealed that the crystalline phases are mullite and quartz and their intensity is almost identical for all samples fired at 1250 °C but there is a decrease in quartz content as temperature is increased. Samples with 20% sand content resulted in higher density, modulus of rupture and fracture toughness compared to those containing 25% sand. The major factor influencing bending strength was found to be porosity in samples as opposed to crystallinity. A sample with 67.3% SiO₂, 20.2% Al₂O₃, 3.4% K₂O and 6.3% others exhibited best properties.     

Effect of sintering regimes on the microstructure and magnetic properties of LiTiZn ferrite ceramics

In present work, the influence of sintering regimes on the microstructure, saturation magnetization, density and porosity, the grain size, the Curie point, and the temperature dependence of the initial permeability of LiTiZn ferrite ceramics was investigated. Ceramics was prepared by a standard ceramic technique. The formation of a single-phase cubic spinel structure was confirmed by XRD analysis. The Curie point was determined from both the temperature dependences of the initial permeability and the method of thermogravimetric measurements in a magnetic field. Density/porosity and the grain size, the Curie point and magnetization are sensitive to the sintering regime. The initial permeability of ferrite decreases with sintering temperature (in the range of 1010–1150?°С) and grain size increasing that contradicts the generally accepted Globus and Smith-Wijn theories. A possible reason of such behavior is the formation of intragranular pores growing with the increase in the sintering temperature and inhibiting the domain wall motion inside the grain. These results correspond to the porosity of the investigated ferrite ceramic samples, which grows with sintering temperature increasing.The non-stoichiometry arising due to evaporation of lithium and zinc oxides at temperature above 1010?°C affects the initial permeability. In this work, a qualitative assessment of the defective state of ferrite samples obtained under various sintering regimes was given.     

Effects of the sintering atmosphere on Nb-based dielectrics

The effects of a forming atmosphere on the stability, the sintering and the dielectric properties of Ba₅Nb₄O₁₅, BaNb₂O₆, ZnNb₂O₆ and Zn₃Nb₂O₈ ceramics were investigated, because of the primary importance of the sintering atmosphere in relation to copper sintering. These Nb-based materials were sintered in air and in Ar/H₂10%. Zn-containing samples are very sensitive to the reductive atmosphere. ZnO volatilises at 800–850 °C and the resulting compound does not exhibit the expected properties. BaNb₂O₆ and Ba₅Nb₄O₁₅ are more stable in term of relative weight loss. Nevertheless, the phase analysis reveals a modification of the BaNb₂O₆ phase, what induces the degradation of the dielectric property stability versus temperature. The properties of Ba₅Nb₄O₁₅ are not modified by a sintering in reductive atmosphere. A relative permittivity of 38.8, a permittivity temperature coefficient of −150 ppm °C⁻¹ and an insulating resistivity of 10^10.9 Ω cm were obtained for this latter.     

Sintering of a clay from Burkina Faso by dilatometry Influence of the applied load and the pre-sintering heating rate

The sintering of a pottery clay from Burkina Faso was studied as a function of the heating rate, at 3 or 10°C/min. The experimental method used was loading dilatometry in isothermal conditions at 1120°C. In these conditions, we found that the densification rate of the material is low, but tend to a limiting value after 2 h at 1120°C, depending on the pre-sintering heating rate and the load used. The relationship between the pre-sintering heating rate and the densification rate indicated the existence of a weakly organised material at higher heating rates. Nevertheless, higher values of shrinkage were observed when the temperature increased continuously. It is, therefore, proposed that the material is subject to a preferential solid state diffusion mechanism at face to face of the remaining kaolinite layers at high temperatures. This mechanism is favoured by higher heating rates, mainly in the temperature range corresponding to the structural reorganisation of the metakaolin phase.     

Effect of CeO2 on the sintering behaviour of zirconia–alumina composite

Precursor for a composite containing equimolar proportion of ZrO₂ and Al₂O₃ having higher reactivity has been synthesized by the wet interaction of inorganic salts in aqueous phase. Sintering of the compact body indicated that the densification rate increased from 1500 °C. The role of CeO₂ as a dopant was positive with respect to densification and retention of t-ZrO₂.     

Effect of TiO2 additions on densification and mechanical properties of new mulltifunction resistant porcelains using economic raw materials

The aim of this work is to determine the effect of TiO₂ on sintering and mechanical proprieties of new multifunction resistant (MFR) porcelain prepared from local abundant raw materials. Based on a preliminary work, the new selected composition was 30 wt% kaolins (20 wt% kaolin halloysite type + 10 wt% kaolin Tamazart), 45 wt% k-feldspar and 25 wt% quartz and containing different contents of TiO₂ (3, 5 and 8 wt%). The sintering temperatures of mixtures were between 1140 and 1260 °C. Subsequently, the obtained phases in the elaborated samples were investigated by X-ray diffraction and Fourier transform infrared spectroscopy analyses, Raman spectroscopy and SEM analysis. The optimum sintering conditions gave a higher bulk density (2.47 g.cm⁻³) and excellent mechanical properties: The three point flexural strength (3PFS), Vickers micro-hardness (VMH) and apparent porosity (P_A) of porcelains sintered at 1160 °C were 238 MPa, 12.3 GPa and 2%, respectively. This obtained 3PFS value is drastically higher than that achieved for conventional porcelains (ranged between 60 and 80 MPa). Moreover, these two best 3PFS (238 MPa) and VMH (12.3 GPa) values achieved for this new MFR porcelains were considerably higher when compared to those values (3PFS=218 MPa and VMH=6.5 GPa) obtained by others for porcelain −30% ZrO₂ composite, even though their mixtures were hot pressed in vacuum at 970 °C for 2 min. Besides, the maximum value achieved for the new MFR porcelains is nearby that of the flexural strength of porcelain containing 5 wt% TiO₂ and 30 wt % alumina (about 240 MPa). In other words, the presence of 30 wt % alumina in their product well confirm the benefic effect of the used raw materials (saving 30 wt % alumina) on porcelain strengthening.     

Synthesis and densification behaviour of magnesium aluminate spinel: Effect of Dy2O3

Magnesium aluminate spinels have been developed by reaction sintering of calcined alumina and calcined magnesia and its densification behaviour was studied in presence of Dy₂O₃. Green bar made from stoichiometric spinel composition with and without Dy₂O₃ were subjected to dilatometric study, densification study and microstructural evaluation by SEM. It was found that Dy₂O₃ additive does not have significant effect on the spinelisation but favours the densification of the spinel. Microstructure of sintered spinel without any additive is non-uniform with some exaggerated grain growth. Dy₂O₃ prevents the exaggerated grain growth and thereby helps in the densification process.     

Effects of microwave sintering on the properties of porous hydroxyapatite scaffolds

Microwave sintering was used to process porous hydroxyapatite scaffolds fabricated by the extrusion deposition technique. The effects of microwave sintering on the microstructure, phase composition, degradation, compressive strength and biological properties of the scaffolds were investigated. After rapid sintering, scaffolds with controlled structure, high densification and fine grains were obtained. A significant increase in mechanical strength was observed relative to conventional sintering. The scaffolds (55–60% porosity) microwave sintered at 1200 °C for 30 min exhibited the highest average compressive strength (45.57 MPa). The degradation was determined by immersing the scaffolds in physiological saline and monitoring the Ca²⁺concentration. The results indicated that the microwave-sintered scaffolds possessed higher solubility than conventionally sintered scaffolds, as it released more Ca²⁺ at the same temperature. Furthermore, an in vitro MC3T3-E1 cell culturing study showed significant cell adhesion, distribution, and proliferation in the microwave-sintered scaffolds. These results confirm that microwave sintering has a positive effect on the properties of porous hydroxyapatite scaffolds for bone tissue engineering applications.