Further Increase In Sintering Temperature Research Articles

Non-Arrhenius behavior of grain growth in (K, Na)NbO3-based ceramics and its effect on piezoelectric properties

A study was performed of the grain growth behavior of ordinary (K, Na)NbO3–based ceramics, including (K, Na)NbO3–Bi0.5Na0.5TiO3 and (K, Na)NbO3–Bi0.5Na0·5ZrO3 systems. These ceramics were sintered at a series of temperatures. Via the observation of surface microstructures and the analysis of Arrhenius graphs, the grain growth of both ceramics was found to experience three stages with increasing sintering temperature. In the first stage, which was in a relatively low temperature range, increasing the sintering temperature resulted in a gradual increase in grain size, complying with the characteristics of thermal activation. As the sintering temperature rose to a higher temperature range, the grain growth entered the second stage, where increasing the sintering temperature caused the grain size to decrease, showing an anti-thermal grain growth behavior. In the third stage, the ceramics resumed the behavior of thermally activated grain growth with the further increase of sintering temperature. The reasons were discussed for the non-Arrhenius grain growth behavior of (K, Na)NbO3–based ceramics. In addition, the effect of non-Arrhenius grain growth behavior on piezoelectric properties was studied as well.

Optimization of sintering conditions for improved microstructural and mechanical properties of dense Ce0.8Gd0.2O2-δ-FeCo2O4 oxygen transport membranes

Ce0.8Gd0.2O2-δ-FeCo2O4 composite is an excellent oxygen transport membrane material with good chemical stability for applications in oxygen separation and membrane reactors. To improve microstructural and mechanical properties, sintering profiles for Ce0.8Gd0.2O2-δ-FeCo2O4 composites were optimized. Different sintering temperatures are selected based on our study of phase interactions among the initial powder mixtures using high-temperature X-ray diffraction. The results reveal that the phase interaction at ∼1050 ℃ accelerates densification process, and a further increase of sintering temperature to 1200 ℃ contributes to the homogenization of the pore distribution. A higher density and an improved homogeneity of pore distribution result in enhanced mechanical strength. However, the density decreases once the sintering temperature reaches 1350 ℃. Hence, the optimal sintering temperature considering both microstructural and mechanical properties appears to be 1200 ℃. Sintering at this temperature results in a microstructure with a density exceeding 99 % with only small surface defects and a high average flexural strength of approximately 266 MPa.

Influence of Sintering Temperature on the Structure and Properties of Powder Iron Aluminide Fe3Al

The paper examines the impact of heating temperature and isothermal holding time during sintering in a vacuum on the phase composition, structure, and mechanical properties of Fe3Al iron aluminide synthesized from a mixture of iron and aluminum powders. Dilatometric studies have shown the complexity of occurring changes in the density of the billets during sintering. First, the porosity increases from 15% in the initial billet to 45% during sintering at 950°C, and after decreases to 5% at a sintering temperature of 1450°C. The synthesis of intermetallic compounds at powder mixture heating was examined by the X-ray diffraction method. It was shown that up to 30% of intermetallic compound Fe2Al5 is formed during an hour-long isothermal holding at 600°C. Increasing the holding time to 3 hours or temperature to 950°C results in the formation and gradual growth of FeAl (B2) aluminide content. With a further increase of sintering temperature up to 1450°C, the amount of A2 phase becomes bigger, and concentration of iron and aluminum in it approaches to stoichiometric ratio Fe3Al. The mechanical properties of iron aluminide intensify with increasing sintering temperature, especially rapidly after sintering at 1450°C, when planar pores close and perfect interparticle contacts form.

Concurrent sinter-crystallization and microwave dielectric characterization of CaO-MgO-TiO2-SiO2 glass-ceramics

ABSTRACT The present work aims to clarify sinter-crystallization behavior and microwave dielectric features of CaO-MgO-TiO2-SiO2 glass-ceramics fabricated via one-step sintering procedure at 800–950°C for 4 h. In this regard, starting glasses were obtained from a conventional melt quenching technique, then subjected to the sinter-crystallization heat treatment. The crystallization behavior of sintered glass-ceramics was studied using X-ray diffraction (XRD) and electron backscatter diffraction (EBSD). On the basis of the obtained results, diopside (CaMgSi2O6) precipitated as the main crystalline phase in all sintered glass-ceramics; however titanite (CaTiSiO5) was found as the secondary crystalline phase in glass-ceramics containing high TiO2 content. The degrees of crystallization of the glass-ceramics prepared procedures were also characterized. Optimized glass-ceramics sintered at 800°C for 4 h showed the maximum relative density of 99%. Further increase of sintering temperature led to gradual decrease of density. The most promising microwave dielectric properties of the sintered glass-ceramics were as εr = 6.8–9.4 and Q × f = 3,735–41,359 GHz.

In vitro bioactivity, molecular structure and mechanical properties of zirconia-carbonated hydroxyapatite nanobiocomposites sintered at different temperatures

Zirconia-carbonated hydroxyapatite (ZrO2-CHA) nanocomposites powders, with various CHA contents, have been prepared using high energy ball milling. In order to investigate the influence of their exposure to different firing temperatures; namely 800, 1000, 1200, 1400 and 1500 °C on their molecular structure and microstructure, X-ray diffraction (XRD) technique, Fourier transform infrared (FTIR) spectroscopy and scanning electron microscopy (SEM) equipped with energy dispersive spectroscopy (EDS), respectively were employed. Additionally, their mechanical properties were also measured. Then, the in vitro bioactivity of the sintered samples, at different temperatures, was examined by FTIR spectroscopy and SEM after their soaking in simulated body fluid for 10 days. The obtained results pointed out that the elevation of sintering temperatures to 1000 °C caused fair densification and good mechanical properties irrespective to the partial degenartion of CHA and the formation of tricalcium phosphate (TCP) and calcium zirconate (CaZrO3) phases. However, further increase of sintering temperature led to dramatic decreases in these properties and then, they were significantly enhanced by the increasing of sintering temperature up to 1500 °C. The formation of the abovementioned phases negatively affected the bioactivity of these nanocomposites. Calculated molecular electrostatic potential confirmed the reactivity of TCP and CaZrO3 phases.

Microstructure and electrical properties of Pb(Zr0.5Ti0.5)O3-Pb(Zn1/3Nb2/3)O3-Pb(Ni1/3Nb2/3)O3+xS3Ti2O7 ceramics

0.83Pb(Zr0.5Ti0.5)O3− 0.11Pb(Zn1/3Nb2/3)O3− 0.06Pb(Ni1/3Nb2/3)O3 (PZNNT) ceramics with different content of plate-like Sr3Ti2O7 compound was prepared. Effect of Sr3Ti2O7 amount and sintering temperature on the microstructures and piezoelectric properties of PZNNT ceramics were investigated. The results revealed that the content of tetragonal phase at morphotropic phase boundary (MPB) and grain size decreased significantly by increasing Sr3Ti2O7 content. In addition, with further increase of sintering temperature for better density, the content of tetragonal phase and electric properties of PZNNT-5 wt%Sr3Ti2O7 ceramics gradually increased. The optimal piezoelectric and dielectric properties of PZNNT-5 wt%Sr3Ti2O7 ceramics sintered at 1040 °C for 2 h was d33= 572 pC/N, d33×g33= 17,630 × 10–15 m2/N and kp= 0.57, which is potential candidate materials used for piezoelectric energy harvesting.

Crystal Structure, Microstructure, Dielectric and Piezoelectric Properties of Lead-Free KNN Ceramics Fabricated via Combustion Method

The combustion method has been developed to prepare lead-free K0.5Na0.5NbO3: KNN ceramics. Urea and glycine were used as a fuel to reduce the reaction temperatures. The effects of fuel type, calcination and sintering conditions on phase formation, morphology evolution, density, and electrical properties were investigated. A pure orthorhombic perovskite structure was obtained from KNN powders calcined at 700°C using glycine as a fuel. The average particle size increased approximately from 0.54 to 0.64 μm as the sintering temperature increased from 650 to 700°C. For the sintered ceramics, pure perovskite was found in the KNN samples sintered below 1150°C. The microstructure of the KNN ceramics exhibited a square or rectangular shape and the average grain size increased with increasing sintering temperatures. The transition temperatures slightly decreased with increasing sintering temperatures. The density and dielectric constant of the ceramics tended to increase with increasing temperature up to 1050°C, where it reached a value of 94.6% and 5839, respectively, and then decreased with further increase of sintering temperature. The piezoelectric d33 value of the most-dense ceramics was 114 pC/N.

Effect of Sintering Temperature on the Microstructure, Electrical, and Magnetic Properties of Bi 0 . 8 5 Eu 0 . 1 5 FeO 3 Ceramics

Polycrystalline Bi 0.85Eu 0.15FeO 3 ceramics were synthesized by solid-state reaction method with rapid liquid phase sintering process at various sintering temperatures. The dependence of structural, microstructural, electrical, and magnetic properties on sintering temperature was systematically investigated. X-ray diffraction measurements reveal that single perovskite phase is developed in Bi 0.85Eu 0.15FeO 3 ceramics sintered at 850 and 870∘ C, while secondary phases can be detected in the samples sintered at 890∘ C due to the volatilization of Bi 3+ ions, and the crystallinity increases with increasing sintering temperature from 850 to 890 °C. The scanning electron microscopy investigation has suggested that the grain size increases with increasing sintering temperature from 855 to 890∘ C; while the pore size decreases with increasing sintering temperature from 850 to 870∘ C and then increases with a further increase of sintering temperature. The electrical and magnetic measurements show that the leakage current, dielectric constant, dielectric loss, and magnetic properties are strongly dependent on the sintering temperature. The Bi 0.85Eu 0.15FeO 3 ceramics sintered at 870∘ C have the lower leakage current, higher dielectric constant, and lower dielectric loss. The room temperature magnetization increases with increasing sintering temperature from 850 to 890 °C. The possible reason for all the above observations was discussed.

Effect of Sintering Conditions on the Phase Formation and Microstructure of Bi-2223/Ag/Ni Composite-Sheathed Tapes

Effect of sintering conditions in the first heat treatment on the phase and microstructure evolution of Bi-2223/Ag/Ni composite-sheathed tapes fabricated by powder-in-tube method was studied. Samples were sintered at different temperatures for different time in an atmosphere of 14.5% O2. The results showed that this higher O2 atmosphere improved the content of Bi-2223 phase. XRD and SEM results showed that Bi-2223 content increased with the extension of the dwelling time. Meanwhile, the sintering temperature had significant influence on the Bi-2223 content and secondary phase dimensions as well. With the increase of the sintering temperature, the Bi-2223 content reached to a maximum at 834. With further increase of sintering temperature, the Bi-2223 phase was decomposed to Bi-2212 and others secondary phases.

Spark plasma and microwave sintering of Al6061 and Al2124 alloys

Despite the importance of aluminum alloys as candidate materials for applications in aerospace and automotive industries, very little work has been published on spark plasma and microwave processing of aluminum alloys. In the present work, the possibility was explored to process Al2124 and Al6061 alloys by spark plasma and microwave sintering techniques, and the microstructures and properties were compared. The alloys were sintered for 20 min at 400, 450, and 500°C. It is found that compared to microwave sintering, spark plasma sintering is an effective way to obtain homogenous, dense, and hard alloys. Fully dense (100%) Al6061 and Al2124 alloys were obtained by spark plasma sintering for 20 min at 450 and 500°C, respectively. Maximum relative densities were achieved for Al6061 (92.52%) and Al2124 (93.52%) alloys by microwave sintering at 500°C for 20 min. The Vickers microhardness of spark plasma sintered samples increases with the increase of sintering temperature from 400 to 500°C, and reaches the values of Hv 70.16 and Hv 117.10 for Al6061 and Al2124 alloys, respectively. For microwave sintered samples, the microhardness increases with the increase of sintering temperature from 400 to 450°C, and then decreases with the further increase of sintering temperature to 500°C.

Microstructure and magnetic properties of NdFeB magnet prepared by spark plasma sintering

Nd 15Dy 1.2Fe balAl 0.8B 6 magnet was prepared by spark plasma sintering (SPS) technique. Sintering between 810 and 830 °C and consequent tempering result in relatively high magnetic properties and sintering density, which are comparable to the conventionally sintered ones. Further increase of sintering temperature gains a slight increase in sintering density and decrease in magnetic properties. Grain size of NdFeB prepared by SPS is fine and uniform, which is much smaller than that of conventionally sintered ones. Continuous thin film of the Nd-rich phase is formed along the grain boundary and the density of magnet is increased through tempering, which is the main reason for improving the magnetic properties. Furthermore, TEM observations indicate a conversion of crystal structure of the Nd-rich phase at triple junction site after tempering.

Studies on the fabrication of tool steel components with micro-features by PIM

Fabrication of structures with fine features has been a challenge and has required sophisticated equipment. In the current work, the use of a cost-effective method, PIM (powder injection molding) was evaluated to fabricate structures with fine features such as sharp edges with dimension in microns. The selected component consisted of micro features of two steps having a cross-section dimension of 0.6×0.07 mm and 0.6×0.06 mm. In order to achieve good mechanical properties coupled with wear resistance, an M2 tool steel material was selected. The mould tip region with two-step micro features was fabricated using into multi-mould pieces to ensure the edge sharpness of the two-step micro features in the mould cavity and to create an air vent to release the entrapped gas at the moulding stage. The moulded samples were first debound and then sintered at different temperatures. The results of actual sintering at 1,244, 1,252, 1,260, and 1,280°C were analyzed, since transition to liquid phase sintering occurs in this region. Visual observation, microstructural analysis and hardness distribution were carried out on the samples sintering at different temperatures. Incomplete sintering occurred at 1,244°C while at 1,252°C distortion of the samples was identified. Sintering at 1,260°C yielded parts with good integrity and strength and a further increase of sintering temperature deteriorated the characteristics. Five samples were chosen randomly from three batches of 100 pieces for a pilot production scale. Their dimensional flatness was measured and the result showed that flatness less than ±0.3% can be achieved under optimized sintering temperature of 1,260°C ± 0.3°C. This means that the M2 tool steel component with micro features can be possibly produced by PIM process in mass production.

Sintering, microstructure and conductivity of La 2NiO 4+ δ ceramic

Microstructure and electrical conducting properties of La 2NiO 4+ δ ceramic were investigated in the sintering temperature range 1200–1400 °C. The results demonstrate that the microstructure and electrical conducting properties of La 2NiO 4+ δ ceramic are sensitive to sintering temperature. Compared with a progressive densification development with sintering temperature from 1200 to 1300 °C along with an insignificant change in grain size, there is an exaggerated grain growth in the specimens sintered at higher temperatures. Increasing sintering temperature from 1200 to 1300 °C resulted in an enhancement of electrical conducting properties. Further increase of sintering temperature exceeding 1300 °C reduced the electrical conducting properties. A close relation between the microstructure and electrical conducting properties was suggested for La 2NiO 4+ δ ceramic. With respect to the electrical conducting properties, the preferred sintering temperature of La 2NiO 4+ δ ceramic was ascertained to be 1300 °C. The specimen sintered at 1300 °C exhibits a generally uniform microstructure together with electrical conductivities of 76–95 S cm −1 at 600–800 °C.

Alumina ceramics fabricated from bimodal alumina with additives

Superfine α-Al 2O 3 synthesized by a precipitation method and commercial α-Al 2O 3 were used to prepare bimodal alumina. Densification of bimodal alumina with MgO, SiO 2 as additives was investigated from 1400 to 1550 °C. The addition of superfine α-Al 2O 3 is not effective to enhance the densification at the sintering temperature of 1400 °C. With further increase of the sintering temperature, the densities of alumina ceramics from bimodal alumina with different contents of superfine α-Al 2O 3 are similar in values. The microstructure evolution, as well as mechanical properties of the alumina ceramics was also studied. For the 1450 °C sintered Al 2O 3 ceramics, fine grains located in the skeletal structure of the coarse grains and the increasing fine α-Al 2O 3 led to fine-grain structure. Spinel and mullite were observed in the fine-grain regions. For the 1500 °C sintered Al 2O 3 ceramic, there was significant grain growth. Mullite, spinel and MAS glass formed in the triple-grain junctions. Both 1450 and 1500 °C sintered Al 2O 3 ceramics have no liquid phase in grain boundaries. The addition of superfine α-Al 2O 3 can distinctly enhance the bending strength of alumina ceramics. The best mechanical properties were gained when the temperature was 1450 °C and then deteriorated with further increase of sintering temperature.

A correlation between intrinsic coercivity–electrical conductivity–thermal treatment in a Nd 11.9MM 2.9Fe 73.9Co 3.3Ni 1.1B 6.9-type magnet

In this work, the effects of sintering temperature, cooling rate after sintering and post-sintering annealing treatments on the microstructure and magnetic properties of a Nd 11.9MM 2.9Fe 73.9Co 3.3Ni 1.1B 6.9-type magnet have been investigated. It was found that the best magnetic properties were only obtained at a narrow range of sintering temperature at 1333–1353 K. Porosity was found to be minimized with increasing sintering temperature up to 1333 K. However, some grain growth occurred by increasing the sintering temperature from 1333 to 1353 K. Further increase of sintering temperature resulted in a significant decrease in the magnetic properties due to appearance of α-iron in the microstructure of the magnets and/or grain growth. The dependence of intrinsic coercivity ( H cJ) on the cooling rate (after sintering) and post-sintering annealing treatments has been related to electrical conductivity (EC) of the magnets; thus an increase in the EC resulted in a significant improvement in the H cJ values. This could be due to reduction of residual stresses within the specimens during slow cooling after sintering/post-sintering annealing treatments. The compatibility between the variations of the H cJ and EC with the cooling rates (after sintering) or a post-sintering annealing treatment at around 873 K confirms this mater.