Conventional Sintered Samples Research Articles

Fabrication of hydrothermal ageing resistant of 3 mol % yttria‐stabilised tetragonal zirconia polycrystalline via microwave sintering

AbstractThe influence of microwave sintering on the densification, mechanical performances, microstructure evolution and hydrothermal ageing behaviour of pure 3 mol % yttria‐stabilised tetragonal zirconia polycrystalline (3Y‐TZP) ceramics was compared with conventional sintered samples. Green bodies were sintered via conventional pressure‐less and microwave sintering method between 1200 °C to 1400 °C with dwelling time and firing rate at 120 min, 10 °C/min and 1 min, 20 °C/min. Result showed that reduced processing temperature and holding time is possible with microwave sintering technique for fabricating good resistant zirconia sample with bulk density, Young's modulus, and Vicker's hardness that are comparable to samples sintered with conventional method. However, the microwave sintered samples suffered from hydrothermal ageing where their average grain size is above critical size. The enhancement of hydrothermal ageing resistance of the sintered samples is associated with the decreasing grain size of the sintered samples instead of sintering method.

Structure and electrical properties of cold-sintered strontium-doped potassium sodium niobate

Cold sintering is an attractive method for sintering ferroelectric ceramics at temperatures at or below 300 °C. While we can practically obtain bulk samples by introducing a transient liquid and applied pressure, the mechanisms of sintering and final functional properties of cold-sintered ceramics are far from being understood. Here, we investigate the influence of grain size and sintering parameters on the microstructure and ferroelectric properties of 0.5 % Sr-doped K0.5Na0.5NbO3 ceramics. By comparing with the conventionally-sintered samples, we find that cold-sintered ceramics have higher dielectric permittivity and lower dielectric losses, which we attribute to smaller grains and higher relative density. Using atomic-scale analysis, we point out the presence of numerous lattice dislocations that likely act as pinning sites and thus inhibit any significant polarization-switching behaviour. Due to the slim P–E loop and high dielectric breakdown fields of the cold-sintered ceramics we consider its potential for energy storage applications.

Improved dielectric properties of Ba0.5Sr0.5TiO3–ZnAl2O4 composite ceramics using double sintering

Ba0.5Sr0.5TiO3–ZnAl2O4 composite ceramics were prepared by double sintering and conventional sintering. The results show that the double sintering can effectively reduce the ion diffusion between Ba0.5Sr0.5TiO3 and ZnAl2O4 phases. The double sintered samples exhibit higher density and more uniform grain size distribution than the conventional sintered samples. The dielectric permittivity of double sintered samples is lower than that of conventional sintered samples. Impedance spectrum analysis shows that the oxygen vacancy content and grain boundary resistance of the double sintered samples is lower than that of the conventional sintered samples, which indicates that the Q value of the double sintered samples is higher than that of the conventional sintered samples. The optimum dielectric tunability and Q value of double sintered 60 wt%Ba0.5Sr0.5TiO3-40 wt%ZnAl2O4 sample are 23.4% at 30 kV/cm and 276 at 2.257 GHz, respectively. Therefore, double sintering is a strategy that can effectively adjust the dielectric tunability and Q value of BST-ZA composite ceramics.

A comparative study of microstructure and electrical properties of (Al, Nb) co-doped TiO2 ceramics by different sintering methods

(Al, Nb) co-doped TiO2 colossal dielectric ceramics were effectively made using conventional sintering (CS), flash sintering (FS) and two-step flash sintering (TSFS), respectively. The structure and electrical characteristics of ceramic samples were thoroughly investigated. FS and TSFS can decrease the processing temperature by 20% and shorten the sintering time by 30 times compared with CS. All sintered samples were pure rutile TiO2 structure. The TSFS samples with finer grain size had higher permittivity than the CS samples. With increasing sintering temperature, the permittivity of the co-doped TiO2 ceramic increased first and then decreased. When the furnace temperature was 1080 °C, the TSFS ceramic sample showed the best overall performance: the dielectric constant (ε’) of 1.4 × 105, the dielectric loss (tanδ) of 0.57 at 1 kHz, and the nonlinear coefficient of 5.8. XPS and IS spectra indicated that the excellent dielectric properties are attributed to the internal barrier layer capacitance model. Therefore, the TSFS method provides an ideal way to prepare co-doped TiO2 colossal dielectric ceramics.

In situ studies on defect formation dynamics in flash-sintered TiO2.

Flash-sintered (FS) ceramics have shown promising mechanical deformability at room temperature compared to conventional sintered ceramics. One major contributing factor to plasticity is high-density defects, such as dislocations, stacking faults and point defects, resulted presumably from the high electrical field during flash sintering. However, such direct experiemtnal evidence for defect formation and evolution under the electric field remains lacking. Here we performed in situ biasing experiments in FS and conventionally sintered (CS) polycrystalline TiO2 in a transmission electron microscope (TEM) to compare the defect evolution dynamics. In situ TEM studies revealed the coalescence of point defects under the electrical field in both FS and CS TiO2 and the subsequent formation of stacking faults, which are often referred to as Wadsley defects. Surprisingly, under the electrical field, the average fault growth rate in the FS samples is 10 times as much as that in the CS TiO2. Furthermore, the Magnéli phase, a 3D oxygen-deficient phase formed by the aggregation of Wadsley defects, is observed in the FS samples, but not in the CS samples. The present study provides new insights into defect dynamics in FS ceramics.

Thermal-assisted cold sintering study of Al2O3 ceramics: Enabled with a soluble γ-Al2O3 intermediate phase

Thermal-assisted cold sintering process (TA-CSP) has been applied to fabricate high dense α-Al2O3 ceramics with submicron grain sizes. The α-Al2O3 (80 wt%) and γ-Al2O3 (20 wt%) powders are firstly mixed and then cold sintered at 300 °C to produce a green bulk with a relatively high density of ∼ 86.9 %, and then later a second heat treatment (800–1350 °C) is applied to finally fabricate (∼ 98 % dense) α-Al2O3 ceramics with grain sizes of 720 nm. A microstructural analysis with XRD and TEM suggests that the TA-CSP samples not only complete the final densification but also drive a phase transition of γ-Al2O3 to α-Al2O3. To put into perspective the Hardness and Young's modulus of TA-CSP samples reach ∼ 14 GPa and ∼ 335 GPa, respectively, which is comparable to conventional sintered samples processed at higher temperatures of 1500–1700 °C. Therefore, it is feasible to utilize TA-CSP to prepare α-Al2O3 ceramics with small grain sizes at low sintering temperatures.

Effects of sintering techniques and parameters on microstructure and mechanical properties of Al-15Si-2,5Cu-0.5Mg compacts and Al-15Si-2,5Cu-0.5Mg/B4C composites

In this study, Al-Si compacts and Al-Si/B4C composites were produced from hypereutectic Al-Si alloy powders by cold pressing/conventional sintering and cold pressing/microwave sintering techniques. The effects of sintering temperature, sintering time and B4C particle addition on microstructural properties, density, hardness and transverse rupture strength were investigated. The phases and intermetallic compounds formed in the microstructure were determined by X-ray diffraction. Densities of the samples were measured by Archimedes’ technique. Macro hardness measurements were carried out with Brinell hardness test. Three-point bending tests were applied to the samples according to ASTM-B528–16. Elemental Al regions, master alloy (Al-Si-Cu-Mg) regions, primary Si particles and Cu- and Mg-rich secondary phases were determined in the microstructure. It was determined that the Cu- and Mg-rich secondary phases are θ(CuAl2), β(Mg2Si), γ(Al2CuMg) phases. An increase in porosity was determined in conventional sintered samples compared to green Al-Si specimens. In addition, it was observed that the amount of porosity increases, grain size and primary Si grains coarse with increasing sintering temperature in conventional sintering. The change in the amount of pores of the samples produced by microwave sintering without holding time at the sintering temperature is not as obvious as in conventional sintering. Similar microstructures were obtained in the samples at increasing sintering temperatures in microwave sintering. In the samples produced by conventional sintering, the hardness value decreased with increasing sintering temperature. Addition of 5 wt% B4C caused an increase in hardness at all sintering temperatures. The addition of 10 and 15 wt% B4C caused a decrease in hardness. In microwave sintering, the general trend in hardness change was a decrease with increasing B4C particle addition and increasing sintering time. Transverse rupture strength values close to and similar to each other in conventional sintered samples were obtained at different sintering temperatures. Transverse rupture strength values increased with the addition of 5 wt% B4C particles, and decreased with the addition of 10 and 15 wt% B4C particles.

Damping characteristics of pure aluminum: A comparison of microwave and conventional sintering

The current research focuses on how heating mode in powder metallurgy affects the damping properties of pure aluminum. The aluminum powder was compacted in a hydraulic press measuring 40 x 12 x 1.5mm3 and heated in a muffle furnace (conventional) and a microwave sintering furnace. The damping measurements were conducted on the samples using a dynamic mechanical analyzer under dual cantilever mode at various vibrating frequencies of 0.1, 1 and 10Hz from room temperature (RT) to 150°C at constant strain. Results demonstrated that the microwave sintered samples exhibit high storage modulus and high damping capacities compared to conventional sintered samples. The mechanisms that support this behavior are investigated and presented.

A Comparative Study on Crack-Healing Ability of Al2O3/SiC Structural Ceramic Composites Synthesized by Microwave Sintering and Conventional Electrical Sintering

This study was conducted to assess and compare the crack-healing ability of conventional electrical sintered and microwave sintered Al2O3/x wt. % SiC (x = 5, 10, 15, and 20) structural ceramic composites. The crack-healing ability of both conventional electrical sintered and microwave sintered specimens was studied by introducing a crack of ∼100 µm length by Vickers’s indentation and conducting a heat treatment at 1200°C for dwell time of 1 h in air. The flexural or bending strength of sintered, cracked, and crack-healed specimens was determined by three-point bending test, and the phase variations by X-ray diffraction and SEM micrographs before and after crack-healing of both the sintering methods were studied and compared. The results show that almost all the specimens recovered their strength after crack-healing, but the strength of microwave sintered Al2O3/SiC structural ceramic composites has been shown to be better than that of conventional electrical sintered Al2O3/SiC structural ceramic composites. The microwave sintered crack-healed Al2O3/10 wt. % SiC specimen shows higher flexural strength of 794 MPa, which was 105% when compared with conventional electrical sintered Al2O3/10 wt. % SiC and crack-healed Al2O3/10 wt. % SiC specimen. It was found by X-ray diffractogram that before crack-healing, all the conventional electrical sintered samples have SiO2 phase which reduce the crack-healing ability and microwave sintered samples with 15 and 20 wt. % SiC show lesser SiO2 phase and 5 and 10 wt. % SiC samples have no SiO2 phase before crack-healing. However, after crack-healing treatment, all the samples have distinct SiO2 phase along with Al2O3 and SiC phases. Microwave sintered Al2O3/10 wt. % SiC specimen cracks were fully healed which was evident in SEM micrographs.

Development of a processing map for safe flash sintering of gadolinium‐doped ceria

AbstractFlash sintering was discovered in 2010, where a dog‐bone‐shaped zirconia sample was sintered at a furnace temperature of 850°C in <5 s by applying electric fields of ~100 V cm−1 directly to the specimen. Since its discovery, it has been successfully applied to several if not all oxides and even ceramics of complex compositions. Among several processing parameters in flash sintering, the electrical parameters, i.e., electric field and electric current, were found to influence the onset temperature for flash and the degree of densification respectively. In this work, we have systematically investigated the influence of the electrical parameters on the onset temperature, densification behavior, and microstructure of the flash sintered samples. In particular, we focus on the development of a processing map that delineates the safe and fail regions for flash sintering over a wide range of applied current densities and electric fields. As a proof of concept, gadolinium‐doped ceria (GDC) is shown as an example for developing of such a processing map for flash sintering, which can also be transferred to different materials systems. Localization of current coupled with hot spot formation and crack formation is identified as two distinct failure modes in flash sintering. The grain size distribution across the current localized and nominal regions of the specimen was analyzed. The specimens show exaggerated grain growth near the positive electrode (anode). The region adjacent to the negative electrodes (cathode) showed retarded densification with large concentration of isolated pores. The electrical conductivity of the flash sintered and conventional sintered samples shows identical electrical conductivity. This quantitative analysis indicates that similar sintering quality of the GDC can be achieved by flash sintering at temperature as low as 680°C.

The Effect of Microwave on the Crystallization Behavior of CMAS System Glass-Ceramics.

The microwave sintering of glass-ceramics, non-thermal microwave effect, and crystal growth mechanism remain important challenges in materials science. In this study, we focus on developing approaches to affect crystal growth in the glass network of glass-ceramics by microwave heating, rather than performing a single study on the crystal structure and type. Raman spectroscopy is used to detect the structure of the glass network. We demonstrated that the non-thermal microwave effect promoted the diffusion of metal ions, which promoted the aggregation and precipitation of metal ions in the glass network to form crystals. The samples produced by microwave heating contain more non-bridging oxygen bonds than conventional sintered samples; therefore, the non-thermal microwave effect has a depolymerization effect on the glass network of the sample. Under the influence of microwave field, many metal ions precipitate, which precipitates many crystal nuclei. In addition, many active metal ions are captured during the crystal nucleus growth, which shortens the sintering process of glass-ceramics.

Spark plasma sintering of SnO2 based varistors

In this research, for the first time, SnO2-based varistors were fabricated via spark plasma sintering technique (SPS) and the microstructure and electrical properties of these varistors were investigated. Furthermore, the effect of post-annealing temperature in oxygen atmosphere on electrical properties of the SPSed samples was studied. The SPS process was performed at the sintering temperatures of 600, 650, and 700 ᵒC for 15 min with a maximum pressure of 90 MPa under vacuum condition. The SPSed sample which was sintered at 650 ᵒC possessed maximum density of 98% and the ultra-fine-grained microstructure with the mean grain size of 380 nm. Surprisingly, all SPSed samples exhibited Ohmic behavior with very low electric resistances. After post-annealing in oxygen atmosphere, Ohmic to non-Ohmic transition was observed in SPSed samples. The oxygen deficiency during the SPS process was responsible for the Ohmic characteristic of SPSed samples. Post-annealing of SPSed samples in the oxygen atmosphere resulted in Schottky barriers formation through oxygen adsorption at grain boundaries. The sample post-annealed at 1050 ᵒC presented the best non-Ohmic parameters including high breakdown electric field of 4500 V/cm and the nonlinear coefficient of 13. These electrical parameters are comparable with the conventional-sintered samples which was sintered at 1300 ᵒC and its breakdown electric field and nonlinear coefficient were equal to 900 V/cm and 8, respectfully.

Effects of Sintering Method and BaTiO3 Dopant on the Microstructure and Electric Properties of Bi (Fe0.9Al0.05Yb0.05) O3-Based Ceramics

The (1 − x) Bi (Fe0.9Al0.05Yb0.05) O3–xBaTiO3 (BFAYO-x BTO) ceramics were synthesized by microwave sintering (MWS) and conventional sintering (CS) methods. The crystal structure, surface morphology, dielectric and ferroelectric properties were investigated. XRD patterns show that BTO tetragonal and BFO rhombohedral phases coexist in both MWS and CS samples, and that the diffraction peaks of MWS samples shift to lower angles due to the larger ionic radius of Ba2+ compared with Bi3+. The lattice constant of BFAYO-x BTO ceramics increases with the increase of BTO content. It was found that the average grain size of the MWS ceramic is much smaller than that of the CS ceramic. Dielectric measurements reveal that the dielectric loss and dielectric constant increase with BTO content increasing. The introduction of BTO into BFAYO-based ceramic contribute to the enhancement of the dielectric relaxation behavior. The P–E hysteresis loops demonstrate that the remnant polarization (Pr) and the coercive field (EC) of the MWS samples are smaller than those of the CS samples. Pr and EC first increase, then decrease and then increase again with the increase of BTO content. The maximum and minimum values of the remnant polarization were obtained at x = 0.25 and 0.3, respectively.

Thermal-assisted cold sintering study of a lithium electrolyte: Li13.9Sr0.1Zn(GeO4)4

The Thermal-Assisted Cold Sintering Process (TA-CSP) was applied to Li13.9Sr0.1Zn(GeO4)4 (LSZG), a lithium ion conducting electrolyte. The powders were both decomposed and densified between 120 °C and 200 °C under a cold sintering process with an uniaxial pressure of 300 MPa for 1 h and a water transient liquid phase. Then these glassy phases can be recrystallized with a second thermal annealing process with temperatures at 800 °C under a 5-h hold in air. The annealed CSP sample has smaller grain size than those processed with much higher temperature conventional sintering. The electrical properties of the cold sintered LSZG were characterized after the recrystallization process with electrochemical impedance spectroscopy over a temperature range between 25 °C and 400 °C. The data suggests that the lithium ion conductivity is 3.0 × 10−2 Scm−1 at 400 °C; this property is competitive with the conventional sintered samples processed at 1150 °C, and also the best performance reported in the literature. So, the CSP process followed by a recrystallization shows much low temperature advantage over the conventional sintering of the LSZG system.

Effects of Sintering Method and BiAlO3 Dopant on Dielectric Relaxation and Energy Storage Properties of BaTiO3–BiYbO3 Ceramics

(0.9 – x)BaTiO3–0.1BiYbO3–xBiAlO3 (x = 0, 0.02, 0.04, and 0.08) lead‐free ferroelectric ceramics are synthesized using conventional sintering (CS) and microwave sintering (MWS) methods. The microstructure, dielectric and ferroelectric properties, relaxation behavior, and energy storage properties are investigated systematically. X‐ray diffraction (XRD) patterns show a single perovskite phase for all samples. Scanning electron microscope (SEM) images indicate that MWS method can impede the grain growth, whereas the introduction of BiAlO3 can lead to the increase in the grain size. Dielectric measurements reveal that an obvious dielectric relaxation behavior can be strengthened by MWS, and the improvement of the dielectric constant (εr) of ceramics is realized by MWS. Incorporated with BiAlO3 contributes to enhancement of the frequency and temperature stabilities of the dielectric constant for BaTiO3–BiYbO3 ceramics. The slim hysteresis loops are observed, and the maximum polarization of the sample sintered by MWS is higher than that sintered by CS. The MWS samples possess a higher energy density which is about 2 times larger than that of CS samples, and the largest energy storage density ≈0.86 J cm−3 and energy storage efficiency ≈97.44% are, respectively, obtained at x = 0 and 0.04 for MWS ceramics.

Experimental investigations into mechanical and thermal properties of rapid manufactured copper parts

In the present paper, mechanical and thermal properties of rapidly manufactured copper parts were studied. The combination of three-dimensional printing and ultrasonic assisted pressureless sintering was used to fabricate copper parts. First, the ultimate tensile strength and thermal conductivity were compared between ultrasonic assisted and conventional pressureless sintered samples. The homogenously mixing of particles and local heat generation by ultrasonic vibrations promoted the sintering driving process and resulted in better mechanical and thermal properties. Furthermore, response surface methodology was adopted for the comprehensive study of the ultrasonic sintering parameters (sintering temperature, heating rate, and soaking time with ultrasonic vibrations) on ultimate tensile strength and thermal conductivity of the fabricated sample. Analysis of variance was performed to identify the significant factors and interactions. The image processing method was used to identify the surface porosity at different parameter levels to analyse the experimental results. High ultimate tensile strength was obtained at high sintering temperature, long soaking time, and slow heating rate with low surface porosity. After 60 min of soaking time, no significant effect was observed on the thermal conductivity of the fabricated sample. The significant interactions revealed less effect of soaking time at low sintering temperatures for ultimate tensile strength and less effect of heating rate at low sintering temperatures for thermal conductivity. Multi-objective optimization was carried out to identify parameters for maximum ultimate tensile strength and maximum thermal conductivity.

Microwave sintering and fracture behavior of zirconia ceramics

Yttria and Lanthana co-stabilized zirconia ceramics were successfully prepared by microwave hybrid sintering with a multi-mode cavity at 2.45 GHz. Microhardness, bending strength, fracture toughness, and relative density were tested. The fracture behavior of 3Y–ZrO2 ceramics was studied after sintering. The results showed the average toughness values of 1550 °C microwave sintered and conventional sintered samples were 7.68 MPa m1/2 and 4.31 MPa m1/2 respectively. The average grain size of microwave sintered samples was 500 nm which was smaller than that (700 nm) of conventional sintered samples. The microstructure of fracture samples showed that the intergra1nular fractures were main features for microwave sintered samples, whereas the numerous transgranular fractures were observed in conventional sintered samples. The XRD results revealed that content of monoclinic phase gradually increase with the increasing of microwave sintering temperature in 3Y–ZrO2 samples. It was proposed that the grain boundary diffusion and induced phase transformation worked together weakening the grain boundary bonding strength under the effect of microwave field.

Microwave Sintering of Yttria-Stabilised Zirconia

3mol% yttria-stabilized tetragonal zirconia polycrystal (3Y-TZP) ceramic is widely used as engineering material owing to its great strength. Recently, 3Y-TZP ceramic is receiving great response in dental restoration due to its bio-compatible properties and aesthetic appearance. To achieve high bulk density, high sintering temperature (> 1500 °C), low firing rate (10 °C/min) coupled with long dwelling period (2 hours) are required in conventional sintering (CS) to fabricate 3Y-TZP ceramic which resulted in high energy consumption. Therefore, non-conventional sintering technique with low energy consumption such as microwave (MW) sintering is worth to explore as almost 100 % of electromagnetic energy is converted into heat largely within the sample itself. The commercial available 3Y-TZP powder was MW fired from 1200 °C to 1400 °C. The 3Y-TZP ceramics fabricated via MW sintering was compared with those CS samples by assessing the densification and mechanical properties of the ceramics. It was revealed that the total sintering time was reduced by 75 % for microwave-sintered 3Y-TZP ceramics to achieve relative density of ∼ 98 %, Young’s modulus of ∼ 205 GPa, and Vicker’s hardness of 14.15 GPa at low sintering temperature (1200 °C) as compared to CS ceramic that required temperature above 1250 °C. However, the properties of both MW and CS-sintered ceramics are comparable when the sintering temperature exceeds 1250 °C.

Enhanced dielectric and piezoelectric properties in microwave sintered ( $$\hbox {Ba}_{0.997}\hbox {Nd}_{0.003})\hbox {TiO}_{3}$$ Ba 0.997 Nd 0.003 ) TiO 3 ceramic when compared to conventional sintered ceramics

Dielectric, conductivity and piezoelectric properties have been studied on (\(\hbox {Ba}_{0.997}\hbox {Nd}_{0.003})\hbox {TiO}_{3}\) ceramic samples prepared through microwave sintered (MWS) and conventional sintered (CS) routes and the results are presented in this paper. The room temperature dielectric constant at 10 kHz for CS and MWS samples are 1245 and 5250 respectively. Room temperature dielectric constant in MWS sample was almost four times higher than that of the CS sample. The value of \(K_{\mathrm{t}}\) is found to be 0.998 and 0.997; whereas the value of \(d_{33}\) is \(7.72 \hbox { nm V}^{-1}\) (573 K) and \(444.66 \hbox { nm V}^{-1}\) (573 K) for CS and MWS samples, respectively. In the present study almost 57 times enhancement in piezoelectric charge constant (\(d_{33}\)) is observed for the MWS \(\hbox {Ba}_{0.997}\hbox {Nd}_{0.003}\hbox {TiO}_{3}\) ceramic when compared to the CS ceramic.

Effects of sintering conditions on the microstructure and mechanical properties of SiC prepared using powders recovered from kerf loss sludge

The effects of sintering conditions on the microstructure and mechanical properties of the sintered SiC prepared using the SiC powder recovered from the kerf loss sludge were investigated. The recovered SiC powders were consolidated by spark plasma sintering (SPS) and conventional sintering methods. The effects of sintering temperature, time and methods (SPS and conventional sintering) on the phase, grain size and density of SiC were systematically studied. The Vickers hardness of spark plasma-sintered (SPSed) samples was higher than that of conventional sintered samples due to small grain size. When holding time was increased from 10 to 30 min, the grain size and relative density of SPSed samples were also increased, which lead to the almost constant Vickers hardness by competing effects of grain size and relative density. When holding time was over 30 min, no appreciable change of the relative density and grain size were observed, which can lead to similar values of Vickers hardness. SPS process can be used to make SiC with high density and hardness at relatively low temperature compared with the conventional sintering process.