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.     

Preparation and microwave dielectric properties of low-loss MgZrNb2O8 ceramics

A novel low-loss microwave dielectric material MgZrNb₂O₈ was reported for the first time. Single-phase MgZrNb₂O₈ was prepared by a conventional mixed-oxide route and sintered in the temperature range of 1280–1360 °C. The microstructure and microwave dielectric properties were investigated systematically. The X-ray diffraction results showed that all samples exhibit a single wolframite structure. When the sintering temperature was lower than 1340 °C, the Q×f value mainly depended on the relative density. However, when the sintering temperature was above 1340 °C, the Q×f value mainly relied on the grain morphology in addition to the density. The MgZrNb₂O₈ ceramic sintered at 1340 °C for 4 h exhibited excellent microwave dielectric of ε_r=26, Q×f=120,816 GHz (where f=6.85 GHz), and τ_f=?50.2 ppm/°C. These results demonstrate that MgZrNb₂O₈ could be a promising candidate material for the application of highly selective microwave ceramic resonators and filters.     

Structural,electrical, and magnetic properties of Zn substituted magnesium ferrite

Zinc substituted magnesium (Mg–Zn) ferrites with the general formula Mg_1−xZn_xFe₂O₄ (x=0.00, 0.25, 0.50, 0.75, and 1.00) were prepared using the solution combustion route. The dried powder after calcination (700 °C for 2 h) was compacted and sintered at 1050 °C for 3 h. The structural, morphological, dielectric and magnetic properties of the sintered ferrites were studied using X-ray diffraction (XRD), scanning electron microscopy (SEM), impedance spectroscopy, and vibration sample magnetometry (VSM). The XRD analysis of sintered samples confirmed that the expected spinel cubic phase was formed for all samples. The crystallite sizes evaluated using Scherre's formula were found to be in the range of 47–80 nm. SEM analysis showed homogeneous grains with a polyhedral structure. The electrical conductivity increased with increasing frequency, which is normal dielectric behavior for such materials. The dielectric constant, dielectric loss tangent, and AC conductivity were found to be lowest for x=0.50. The VSM results showed that the zinc concentration had a significant influence on the saturation magnetization and coercivity.     

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.     

Structure and microwave dielectric properties of Ba1−xSrxMg2V2O8 ceramics

The Ba_1−xSr_xMg₂V₂O₈ (0≤x≤0.4) microwave dielectric ceramics were fabricated by a standard solid-state reaction method. The formation of a continuous solid solution within the whole composition range was identified. The ceramic samples could be well densified in the temperature range of 885–975 °C in air for 4 h. The permittivity ε_r was found to increase with increasing ionic polarizabilities. The Q×f values were believed to be closely related with packing fraction and grain refinement. The Sr²⁺ substitution contributed to a monotonous increase of the A-site bond valence, such that the τ_f value experienced a considerable variation from negative to positive values. The optimum microwave dielectric properties of an ε_r of 13.3, a high Qxf of 86,640 GHz (9.6 Hz) and a near-zero τ_f of −6 ppm/°C could be yielded in the x=0.15 sample when sintered at 915 °C for 4 h.     

Comparison between microwave and conventional calcination techniques in regard to reactivity and morphology of co-precipitated BaTiO3 powder,and the electrical and energy storage properties of the sintered samples

Barium titanate (BaTiO₃) nanopowders were synthesized by an aqueous co-precipitation method followed by calcination. Either 2.45 GHz microwaves or conventional heating was used in order to investigate the impact of these techniques on the synthesis time, microstructure, and electrical properties of the materials. The heating temperatures ranged from 620 °C to 810 °C. X-ray diffraction (XRD) revealed pure BaTiO₃ formation by microwave heating in a noticeably shorter time (five minutes) compared to conventional heating (3 h). Field emission scanning electron microscopy (FESEM) results confirmed that the microwave process led to nanocube formation, whereas in the conventional procedure, the particles tended to form spherical shapes. To evaluate the electrical properties, the samples heated at 620 °C were conventionally sintered at 1280 °C, 1330 °C, and 1380 °C. Higher dielectric, piezoelectric, and ferroelectric properties and more energy-saving efficiency (ε_r=1012, tan δ=0.035 d₃₃=85 pC/N, p_r=6.2 µC/cm² and η=48% respectively) were achieved in the microwave-heated BaTiO₃ sintered at 1380 °C compared to the conventionally heated BaTiO₃ (ε_r=824, tan δ=0.030 d₃₃=75 pC/N, p_r=5 µC/cm² and η=27%) demonstrating that microwave calcination substantially affects the final electrical properties.     

Effect of consolidation parameters on structural,microstructural and electrical properties of magnesium titanate ceramics

The aim of this study is to explore the influence of the processing route on the structural and physical properties of bulk MgTiO₃ ceramics. Commercially available MgO and TiO₂ powders were mechanically activated in a planetary ball mill. Green bodies were formed by an isostatic pressure of 300 MPa. The sintering of these samples was done either by the Two-Step Sintering (TSS) approach or by conventional pressureless sintering followed by Hot Isostatic Pressing (post-HIPing). The first set of compacts was sintered by TSS in air at 1300 °C for 30 min and the next step was performed at 1200 °C for 20 h. The density of the obtained samples after the two-step sintering reached almost 90% of the theoretical density (%TD). The second set of compacts was sintered at 1400 °C for 30 min in air. The samples without open porosity were post-sintered by the HIP at 1200 °C for 2 h in an argon atmosphere at a pressure of 200 MPa. The density significantly increased up to 96%TD. The differences between samples prepared by these two techniques were also analyzed by XRD and SEM. The lattice vibration spectra were obtained using Raman spectroscopy and they indicate a high degree of lattice disorder, as well as high values of the oxygen vacancy concentration. Electrical characteristic were established in the frequency range from 10 kHz to 10 GHz. The choice of the processing route had considerable influence on structural and physical properties of MgTiO₃ ceramics.     

Dielectric and magnetic properties of Nickel ferrite ceramics using crystalline powders derived from DL alanine fuel in sol–gel auto-combustion

Dielectric and magnetic properties of NiFe₂O₄ ceramics prepared with powders using DL-alanine fuel in the sol–gel auto combustion technique are studied. DL-alanine fuel yields crystalline as-burnt powders, and when used for ceramic processing yields varying microstructure at different sintering temperatures. The dielectric properties are influenced by the resulting microstructure and the magnetic properties show slight change in saturation magnetization M_s (~44 – 46 emu/g). The coercive fields, dielectric losses and dispersion are reduced considerably at higher sintering temperatures (1200–1300 °C). The influence of changing microstructure is analyzed through dielectric response, complex impedance analysis and electrical modulus spectroscopy in the frequency range (10⁻²–10⁷ Hz) to understand the interactions from the grain and grain boundary phases. Sintering at 1200 °C, is found to be optimum, yields lower losses & reduced dielectric dispersion, and high resistivity (3.4×10⁸ Ω cm).     

A comparative study of the magnetic and microwave properties of Al3+ and In3+ substituted Mg–Mn ferrites

The effect of substitution of diamagnetic Al³⁺ and In³⁺ ions for partial Fe³⁺ ions in a spinel lattice on the magnetic and microwave properties of magnesium–manganese (Mg–Mn) ferrites has been studied. Three kinds of Mg–Mn based ferrites with compositions of Mg_0.9Mn_0.1Fe₂O₄, Mg_0.9Mn_0.1Al_0.1Fe_1.9O₄, and Mg_0.9Mn_0.1In_0.1Fe_1.9O₄ were prepared by the solid-state reaction route. Each mixture of high-purity starting materials (oxide powders) in stoichiometric amounts was calcined at 1100 °C for 4 h, and the debinded green compacts were sintered at 1350 °C for 4 h. XRD examination confirmed that the sintered ferrite samples had a single-phase cubic spinel structure. The incorporation of Al³⁺ or In³⁺ ions in place of Fe³⁺ ions in Mg–Mn ferrites increased the average particle size, decreased the Curie temperature, and resulted in a broader resonance linewidth as compared to un-substituted Mg–Mn ferrites in the X-band. In this study, the In³⁺ substituted Mg–Mn ferrites exhibited the highest saturation magnetization of 35.7 emu/g, the lowest coercivity of 4.1 Oe, and the highest Q×f value of 1050 GHz at a frequency of 6.5 GHz.     

Effect of Nd-doping on structural,thermal and electrochemical properties of LaFe0.5Cr0.5O3 perovskites

The structural, thermal and electrochemical properties of the perovskite-type compound La_1−xNd_xFe_0.5Cr_0.5O₃ (x=0.10, 0.15, 0.20) are investigated by X-ray diffraction, thermal expansion, thermal diffusion, thermal conductivity and impedance spectroscopy measurements. Rietveld refinement shows that the compounds crystallize with orthorhombic symmetry in the space group Pbnm. The average thermal expansion coefficient decreases as the content of Nd increases. The average coefficient of thermal expansion in the temperature range of 30–850 °C is 10.12×10⁻⁶, 9.48×10⁻⁶ and 7.51×10⁻⁶ °C⁻¹ for samples with x=0.1, 0.15 and 0.2, respectively. Thermogravimetric analyses show small weight gain at high temperatures which correspond to filling up of oxygen vacancies as well as the valence change of the transition metals. The electrical conductivity measured by four-probe method shows that the conductivity increases with the content of Nd; the electrical conductivity at 520 °C is about 4.71×10⁻³, 6.59×10⁻³ and 9.62×10⁻³ S cm⁻¹ for samples with x=0.10, 0.15 and 0.20, respectively. The thermal diffusivity of the samples decreases monotonically as temperature increases. At 600 °C, the thermal diffusivity is 0.00425, 0.00455 and 0.00485 cm² s⁻¹ for samples with x=0.10, 0.15 and 0.20, respectively. Impedance measurements in symmetrical cell arrangement in air reveal that the polarization resistance decreases from 55 Ω cm⁻² to 22.5 Ω cm⁻² for increasing temperature from 800 °C to 900 °C, respectively.     

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.     

Improving the electrical properties of niobium-doped titania sputtering targets by sintering in oxygen-deficient atmospheres

Niobium-doped titania (TNO) film can be used as a transparent conductive oxide (TCO) film due to its excellent conductivity and visible transparency. The performances of TNO sputtering targets are thus critical issues in optimizing sputtered films. This study clarifies the influences of inert and reducing atmospheres on the microstructure, densification, crystal structure, and electrical properties of TNO sputtering targets. The results indicate that a sintering atmosphere of 90% Ar–10% H₂ can result in a lower sintered density, larger grain size, and lower resistivity than can an atmosphere of Ar, followed by one of air. Sintering in 90% Ar–10% H₂ or Ar obviously decreases the resistivity of TiO₂, from >10⁸ Ω cm to <10⁻¹ Ω cm, and the TNO target, from >10¹ Ω cm to <10⁻¹ Ω cm. The resistivity of TNO target sintered at 1200 °C in 90% Ar–10% H₂ is as low as 1.8×10⁻² Ω cm.     

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.     

Microstructural evolution,mechanical and thermal properties of LaB6 embedded in Si-B-C-N prepared by spark plasma sintering

Si-B-C-N monoliths with 5 wt% LaB₆ additives were prepared by spark plasma sintering at 1250–2000 °C and 50 MPa using a mechanically alloyed mixture of graphite, c-Si, h-BN and LaB₆ powders as the starting materials. Microstructural evolution, mechanical and thermal properties of the as-prepared La/Si-B-C-N monoliths were investigated. The densification of the ceramics starts at 1160° and ends at 1800 °C with the formation of La-containing compounds coupled with SiC and BN(C) phases. La-containing BN(C) grains develop into a lamellar structure at 1900 °C offering improved fracture toughness and decreased Vickers hardness, flexural strength and elastic modulus. The formation of lamellar BN(C) is also responsible for a high thermal expansion coefficient of 4.2×10⁻⁶ /°C.     

Crystallization processes,compressibility, sinterability and mechanical properties of La-monazite-type ceramics

Lanthanide orthophosphate ceramics with monazite structure gained broad interest for several industrial applications. The crystallization processes, compressibility and sinterability of monazite-type lanthanum orthophosphate powder hydrothermally synthesized at 200 °C as well as mechanical properties of the sintered compacts were investigated. Based on a combination of thermo- and surface area analyses, X-ray diffraction as well as scanning electron microscopy studies it was found that the crystallization process occurs at ∼500 °C and the final crystallization of LaPO₄ monoclinic phase takes place at 1400 °C. The sintered pellets are characterized by a density of 98% of theoretical density, a Vickers hardness of 5.7 ± 0.1 GPa and fracture toughness of 1.4 ± 0.1 MPa m^0.5.     

Enhancement of the interfacial polarization resistance of La0.6Sr0.4Co0.2Fe0.8O3-δ cathode by microwave-assisted combustion method

Thermogravimetry, phase formation, microstructural evolution, specific surface area, and electrical properties of La_0.6Sr_0.4Co_0.2Fe_0.8O_3−δ (LSCF) cathode were studied as functions of its preparation technique. The pure perovskite LSCF cathode powder was synthesized through glycine–nitrate process (GNP) using microwave heating technique. Compared with conventional heating technique, microwave heating allows the rapid combustion to occur simultaneously between the nitrates and glycine in a controllable manner. The resulting powder is a single-phase nanocrystallite with a mean particle size of 113 nm and a high specific surface area of 12.2 m²/g, after calcination at 800 °C. Impedance analysis indicates that microwave heating has significantly reduced the polarization resistance of LSCF cathode. The area specific resistance (ASR) value of 0.059 and 0.097 Ω cm² at 800 °C and 750 °C, respectively, were observed. These values were twofold lower than the corresponding ASR of the cathode (0.133 and 0.259 Ω cm² at 800 °C and 750 °C, respectively) prepared through conventional heating. Results suggest that the microwave heating GNP strongly contributes to the enhancement of the LSCF cathode performance for intermediate temperature solid oxide fuel cells.     

Properties of NiFe2O4 ceramics from powders obtained by auto-combustion synthesis with different fuels

Nickel ferrite (NiFe₂O₄) powders derived by auto-combustion synthesis using three different fuels (citric acid, glycine and dl-alanine) have been characterized. The sintering behavior of ceramics using these powders has been compared. Oxygen balance (OB) setting for the chemical reaction is found to regulate the combustion reaction rate. A rapid reaction rate and a high flame temperature are achieved with dl alanine fuel yielding single phase NiFe₂O₄ powder in the as-burnt stage, whereas powders derived with citric acid and glycine fuels show poor crystallinity and necessitate post-annealing. The powder particles are largely agglomerated with a non-uniform distribution in shape and size, and the average particle size is estimated in the range ~ 54–71 nm. Powders derived from dl-alanine fuel show better phase purity, smaller crystallite size, larger surface area and superior sintering behavior. Additional Raman modes discerned for dl-alanine derived powder support a 1:1 ordering of Ni²⁺ and Fe³⁺ at the octahedral sites relating to microscopic tetragonal P4₁22 symmetry expected theoretically for the formation of NiFe₂O₄ with inverse spinel structure. Microstructure of sintered ceramics depends on the precursor powders that are used and sintering at 1200 °C is found to be optimum. Citric acid and glycine derived powders yield high saturation magnetization (M_s~47–49 emu/g), but poor dielectric properties, whereas dl-alanine derived powders yield ceramics with high resistivity (~3.4×10⁸ Ω cm), low dielectric loss (tan δ~0.003 at 1 MHz) and high magnetization (46 emu/g). Dielectric dispersion and impedance analysis show good correlation with the changes in the ceramic microstructure.     

Fabrication of toughened B4C composites with high electrical conductivity using MAX phase as a novel sintering aid

MAX phase Ti₃AlC₂ was chosen as a novel sintering aid to prepare electrically conductive B₄C composites with high strength and toughness. Dense B₄C composites can be obtained at a hot-pressing temperature as low as 1850 °C with 15 vol% Ti₃AlC₂. The enhanced sinterability was mainly ascribed to the in situ reactions between B₄C and Ti₃AlC₂ as well as the liquid phase decomposed from Ti₃AlC₂. Both the Vickers hardness and fracture toughness increase with increasing Ti₃AlC₂ amount, and high hardness and toughness values of 28.5 GPa and 7.02 MPa m^−1/2 respectively were achieved for B₄C composites sintered with 20 vol% Ti₃AlC₂ at 1900 °C. Crack deflection by homogenously distributed TiB₂ particles was identified as the main toughening mechanism. Besides, B₄C composites sintered with Ti₃AlC₂ show significantly improved electrical conductivity due to the percolation of highly conductive TiB₂ phase, which could enhance the machinability of B₄C composites largely by allowing electrical discharge machining.     

Dielectric characteristics of donor-acceptor modified BaTiO3 ceramics

Sm/Mn codoped BaTiO₃ ceramics were investigated for their microstructure and dielectric characteristics. The powders were prepared by the conventional solid state procedure. The concentration of Sm₂O₃ as a donor dopant has been kept from 0.1 up to 5.0 at%. The content of MnO₂ as acceptor was kept constant at 0.05 at% Mn in all samples. The specimens were sintered at 1290 °C, 1320 °C and 1350 °C in an air atmosphere for two hours.A mainly uniform and homogeneous microstructure with average grain size ranging from 0.3 µm to 2.0 µm was observed in low doped samples. In highly doped samples, apart from the fine grained matrix, the appearance of local area with secondary abnormal grains was observed.The dielectric properties were investigated as a function of frequency and temperature. The low doped samples exhibit the high value of dielectric permittivity at room temperature and the greatest change at the Curie temperature. The highest value of dielectric constant (ε_r=6800) was measured for 0.1Sm/BaTiO₃ samples sintered at 1350 °C. A nearly flat permittivity-temperature response and lower values of ε_r were obtained in specimens with 2.0 and 5.0 at% additive content. The dielectric constant increases with the increase of sintering temperature. The dissipation factor ranged from 0.01 to 0.22 and decreases with the increase of sintering temperature. The Curie constant (C), Curie-Weiss temperature (T₀) and critical exponent of nonlinearity (γ ) were calculated using a Curie-Weiss and modified Curie-Weiss law. The highest value of Curie constant (C=9.06·10⁵ K) was measured in 0.1 at% doped samples. The Curie constant decreased with increasing dopant content. The γ values, ranging from 1.001 to 1.58, point out the sharp phase transition in low doped samples, and the diffuse phase transition in heavily doped BaTiO₃ samples.     

Signature of multiferroicity and pyroelectricity close to room temperature in BaFe12O19 hexaferrite

In this paper, we have reported the signature of multiferroicity and pyroelectricity in BaFe₁₂O₁₉ hexaferrite close to room temperature. The BaFe₁₂O₁₉ hexaferrite samples are synthesized by co-precipitation method at different sintering temperature ranging from 800 to 1200 °C and study their structural, ferroelectric, magnetic, magnetoelectric and pyroelectric properties. X-ray Diffraction patterns show the pure phase formation for all samples. Morphological changes are examined through the scanning electron microscope. The maximum ferroelectric polarization (0.66 μC/cm²) is observed for the sample sintered at 1200 °C, however maximum magnetic polarization 74 emu/g is observed for sample sintered at 1000 °C. Magneto-electric coupling measurements are also performed through dynamic method and average magneto-electric coupling coefficient (~ 7.05 × 10⁻⁷ mV/cm Oe²) is observed at room temperature for the sample sintered at 1200 °C. Furthermore, maximum pyroelectric constant (147 × 10⁻¹³C/cm² °C) is observed at 75 °C for BaFe₁₂O₁₉ samples sintered at 1200 °C. The observation of both multiferroicity and pyroelectricity close to room temperature in BaFe₁₂O₁₉ hexaferrite is interesting and useful for multifunctional devices.