In situ study on microwave sintering of ZTA ceramic: Effect of ZrO2 content on densification,hardness, and toughness

Better understanding of the effect of multimode‐microwave sintering of zirconia‐toughened alumina (ZTA) was investigated. A comparative dilatometric analysis was conducted between conventional and microwave heating processes, to clarify the influence of zirconia on the densification of ZTA under electromagnetic field. The thermal gradient on sample measurements indicates the change to the microwave volumetric heating is improved by zirconia which adsorbs microwave energy better, thus acting as a susceptor. The most beneficial effect on microstructure, toughness, and hardness were observed at the optimal zirconia content of 10 vol%. The results with both microwave and conventional sintering illustrate the strengthening effect on the composite by zirconia. Of special interest, multimode microwave sintering creates a finer homogeneous microstructure, with resulting hardness and toughening comparable to those obtained for conventional sintering, as well as improved densification, and at lower cost.     

Shrinkage rate control during a flash state by current-ramping for 3 mol% Y2O3-doped ZrO2 polycrystals

3 mol% Y₂O₃-doped ZrO₂ green compacts with rectangular shapes were sintered by maintaining the shrinkage rates at constant values under alternating electric fields by ramping the electric current during flash states. Green compacts were furnace-heated under 40-100 V_rms/cm until current hits an initial current limit of 100 mA_rms. After then, current-ramping was started to keep the shrinkage rates constant by increasing the limit current value using a programmable power supply operating in a current control mode. Highly densified 3 mol% Y₂O₃-doped ZrO₂ polycrystals with a density of 6.05 g/cm³ as a bulk density and a grain size of about 0.4 μm were obtained at a furnace temperature of about 930℃, 50 V_rms/cm with 1000 Hz and shrinkage rate of about 120 μm/min (0.8%/min against initial lengths of green compacts). The Vickers hardness and indentation fracture toughness of the compact exhibit similar values to those obtained from thermally sintered compacts.     

Microstructure development and optical properties of Fe:ZnSe transparent ceramics sintered by spark plasma sintering

Fe:ZnSe transparent ceramics were prepared by spark plasma sintering. Fe:ZnSe powders synthesized via co-precipitation yielded well-dispersed particles with an average particle size of 550 nm. These powders were in the cubic phase Fe:ZnSe, indicating the successful substitution of Fe²⁺ for Zn²⁺. The highest relative density, 99.4%, was obtained by increasing the pressure and sintering time. The effects of sintering temperature, pressure, and time on the microstructure of SPS prepared ceramics were presented by micrographs. With increasing sintering temperature, from 600°C to 900°C, the average grain size increased from < 1 to 10 μm. The intergranular fracture indicated no neck formation in the sintering process. High pressure was essential for the densification process. The average grain size deceased from approximately 10 to 5 μm when the pressure was increased. Increasing the sintering time from 10 to 120 minutes lead to a change in the microstructure, from inter- to transgranular fracture, and eliminated the micropores. The as-prepared Fe:ZnSe ceramics were composed of single-phased cubic ZnSe. The sample sintered at 900°C under a pressure of 90 MPa for 120 minutes yielded a transmittance of approximately 60% at 1.4 μm and 68% at 7.5 μm and had residual micropores as its main scattering source. There was a strong characteristic absorption peak of Fe²⁺ ions at around 3 μm, which was red-shifted compared to Fe:ZnS transparent ceramics. Fe:ZnSe transparent ceramics have a reddish-brown color and it could be a promising mid-infrared laser material.     

Thermal and electrical properties of additive-free rapidly hot-pressed SiC ceramics

The thermal and electrical properties of newly developed additive free SiC ceramics processed at a temperature as low as 1850 °C (RHP0) and SiC ceramics with 0.79 vol.% Y₂O₃-Sc₂O₃ additives (RHP79) were investigated and compared with those of the chemically vapor-deposited SiC (CVD-SiC) reference material. The additive free RHP0 showed a very high thermal conductivity, as high as 164 Wm⁻¹ K⁻¹, and a low electrical resistivity of 1.2 × 10⁻¹ Ω cm at room temperature (RT), which are the highest thermal conductivity and the lowest electrical resistivity yet seen in sintered SiC ceramics processed at ≤1900 °C. The thermal conductivity and electrical resistivity values of RHP79 were 117 Wm⁻¹ K⁻¹ and 9.5 × 10⁻² Ω cm, respectively. The thermal and electrical conductivities of CVD-SiC parallel to the direction of growth were ∼324 Wm⁻¹ K⁻¹ and ∼5 × 10⁻⁴Ω⁻¹ cm⁻¹ at RT, respectively.     

Processing of CaLa2S4 infrared transparent ceramics: A comparative study of HP and FAST/SPS techniques

The densification of CaLa₂S₄ (CLS) powders prepared by combustion method was investigated by the use of Field-Assisted Sintering Technique (FAST) and Hot Pressing (HP). CLS powders were sintered using FAST at 1000°C at different pressures and heating rates and sintered by HP under 120 MPa from 800°C to 1100°C for 6 hours with a heating rate of 10°C/min. Comparison of both techniques was further realized by use of the same conditions of pressure, dwell time, and heating rate. Complementary techniques (XRD, SEM-EDS, density measurements, FTIR spectroscopy) were employed to correlate the sintering processes/parameters to the microstructural/compositional developments and optical transmission of the ceramics. Both sintering techniques produce ceramics with submicrometer grain size and relative density of about 99%. Nevertheless, HP is more suitable to densify CLS ceramics without fragmentation and also reach higher transmission than FAST. Transmission of 40%–45% was measured out of a possible maximum of 69% based on the Fresnel losses in the 8-14 μm window when HP is applied at 1000°C for 6 hours under 120 MPa. In both techniques, ceramics undergo reduction issues that originate from graphitic sintering atmosphere.     

Enhanced high-temperature strength in textured (Ti1/3Zr1/3Hf1/3)B2 medium-entropy ceramics via strong magnetic field

This study prepared textured (Ti_1/3Zr_1/3Hf_1/3)B₂ medium-entropy ceramics for the first time that maintain enhanced flexural strength up to 1800°C using single-phase (Ti_1/3Zr_1/3Hf_1/3)B₂ powders, slip casting under a strong magnetic field, and hot-pressed sintering methods. Effects of WC additive and strong magnetic field direction on the phase compositions, orientation degree, microstructure evolution, and high-temperature flexural strength of (Ti_1/3Zr_1/3Hf_1/3)B₂ were investigated. (Ti_1/3Zr_1/3Hf_1/3)B₂ grain grows along the a,b-axes, resulting in a platelet-like morphology. Pressure parallel and perpendicular to the magnetic field direction can promote the orientation degree and hinder the texture structure formation, respectively. Reaction products of W(B,C) and (Ti,Zr,Hf)C between (Ti_1/3Zr_1/3Hf_1/3)B₂ and WC additive can efficiently refine the (Ti_1/3Zr_1/3Hf_1/3)B₂ grain size and promote grain orientation. (Ti_1/3Zr_1/3Hf_1/3)B₂ ceramics doped with 5 vol.% WC yielded a Lotgering orientation factor of 0.74 through slip casting under a strong magnetic field (12 T) and hot-pressed sintering at 1900°C. Furthermore, cleaning the boundary by W(B,C) and introducing texture can enhance the grain-boundary strength and improve its high-temperature flexural strength. The four-point flexural strength of textured (Ti_1/3Zr_1/3Hf_1/3)B₂-5 vol.% WC ceramics was 770 ± 59 MPa at 1600°C and 638 ± 117 MPa at 1800°C.     

Anisotropic properties of textured h-BN matrix ceramics prepared using 3Y2O3-5Al2O3(-4MgO) as sintering additives

Textured hexagonal boron nitride (h-BN) matrix composite ceramics were prepared by hot pressing using 3Y₂O₃-5Al₂O₃ (mole ratio of 3:5) and 3Y₂O₃-5Al₂O₃-4MgO (mole ratio of 3:5:4) as liquid phase sintering additives, respectively. During the sintering process with liquid phase environments, platelike h-BN grains were rotated to be perpendicular to the sintering pressure, forming the preferred orientation with the c-axis parallel to the sintering pressure. Both h-BN matrix ceramic specimens show significant texture microstructures and anisotropic mechanical and thermal properties. The h-BN matrix ceramics prepared with 3Y₂O₃-5Al₂O₃-4MgO possess higher texture degree and better mechanical properties. While the anisotropy of thermal conductivities of that prepared with 3Y₂O₃-5Al₂O₃ is more significant. The phase compositions and degree of grain orientation are the key factors that affect their anisotropic properties.     

Chemical and microstructural characterisation of HfO2-Y2O3 ceramics with high amount of Y2O3 (33, 40 and 50 mol. %) manufactured using spark plasma sintering

Hafnia based ceramics are potential promising candidates to be used as thermal barrier coatings (TBC) for applications in the field of propulsion. In this study, Spark Plasma Sintering (SPS) of fully stabilised hafnia with yttrium oxide (yttria) was investigated to provide a better understanding of the effect of manufacturing parameters, on the crystallography, chemistry and microstructure of the material. Several hafnia powders, containing different amounts of yttria (33 mol. %, 40 mol. % or 50 mol. %), were sintered by SPS at different temperature levels ranging from 1600 °C to 1850 °C. On these materials, X-ray diffraction patterns associated with scanning electron micrographs have highlighted the influence of both the sintering temperature and the amount of yttria on the final composition, the lattice parameter and the microstructure of hafnia-based materials. In the end, it is established that, for all quantities of yttrium employed, the main phase is Y₂Hf₂O₇ with very high densification levels.     

Structural,magnetic and cryogenic magneto-caloric properties in RE2FeCrO6 (RE = Er and Tm) compounds

In this work, polycrystalline Er₂FeCrO₆ (EFCO) and Tm₂FeCrO₆ (TFCO) oxides were fabricated via conventional sol-gel method, and studied with respect of crystal structure together with cryogenic magnetic and magneto-caloric properties. Both oxides are confirmed to exhibit B-site disordered hexagonal perovskite-type crystal structure. Two magnetic transitions around 11.7 and 5.7 K for EFCO, whereas only one transition around 10.5 K for TFCO, have been observed. Both oxides exhibit considerable cryogenic reversible magneto-caloric effects. The values of magnetic entropy change peak and relative cooling power (refrigerant capacity) with 0–5 T magnetic field change reach 11.95 J/kg-K and 215.8 (169.8) J/kg for EFCO, and 4.78 J/kg-K and 123.6 (97.8) J/kg for TFCO, respectively, indicating the present Er₂FeCrO₆ oxide is also considerable for cryogenic magnetic cooling application.     

Structural,morphological and magnetic parameters investigation of multiferroic (1-x)Bi2Fe4O9- xCoFe2O4 nanocomposite ceramics

(1-x)Bi₂Fe₄O₉- xCoFe₂O₄ (0.0≤ x ≤1.0) multiferroic nanocomposites were prepared by wet chemical procedures combining reverse chemical co-precipitation and Pechini-type sol–gel techniques followed by mechanical blending process. The XRD and SAED results showed that the diffraction patterns are perfectly indexed to the constituent phases present in composite samples. The crystallite sizes of the constituent phases were 35.4 and 39.4 nm for cobalt and bismuth ferrites, respectively. The characteristic peaks in FT-IR spectra confirmed formation and purity of all specimens. FESEM micrographs revealed the uniform phase distribution with the mean grain size of approximately 40 and 230 nm for CoFe₂O₄ and Bi₂Fe₄O₉, respectively. TEM micrograph indicated suitable distribution in the as-prepared composite sample. The VSM results revealed that saturation and remnant magnetization increase by increasing CFO content in composites. Based on the results obtained from M-H curves, magnetic properties of composites did not originate only from linear combination of parent phases. The recorded coercivity values of all nanocomposites, for example 1443 Oe for x = 0.4 were higher than those of each parent phase i.e. 708 Oe for CoFe₂O₄ and 149 Oe for Bi₂Fe₄O_9, showing a noticeable improvement in magnetic properties.     

Structure,magnetic properties and cryogenic magneto-caloric effect (MCE) in RE2FeAlO6 (RE = Gd,Dy, Ho) oxides

In this study, we present the crystal structure, magnetic properties, and cryogenic magneto-caloric effect (MCE) of RE₂FeAlO₆ (RE = Gd, Dy, Ho) oxides. The XRD refinement analysis suggests that all the RE₂FeAlO₆ oxides are crystallized in B-site disordered orthorhombic structure. The RE₂FeAlO₆ oxides exhibit large MCEs around T_C. The peak magnetic entropy change (-ΔS_M) and refrigeration capacity (RC) are 25.9 J/(kgK) and 240.1 J/kg for Gd₂FeAlO₆, 10.7 J/(kgK) and 274.9 J/kg for Dy₂FeAlO₆, 9.6 J/(kgK) and 249.6 J/kg for Ho₂FeAlO₆ under ΔH of 0–70 kOe, respectively. Notably, Gd₂FeAlO₆ exhibits promising magneto-caloric performance and therefore is a favorable candidate for cryogenic magnetic refrigeration.     

Facile preparation of soluble and conductive polyaniline in the presence of lignosulfonate and a constant magnetic field (0.4 T)

A convenient and economical method for preparing soluble conductive polyaniline nanoparticles is introduced in this article. Polyaniline (PANI) was prepared by utilizing a renewable resource calcium lignosulfonate (LS) as dopant in the presence (PANI‐LS‐M) and absence (PANI‐LS‐A) of a constant magnetic field (MF). Their structures and properties were systematically studied. Compared with PANI‐LS‐A, the PANI‐LS‐M exhibited a much higher conductivity, solubility, and thermo stability, which was due to the effective doping of the LS caused by the stretching and orientation effects of the MF. The conductivity of the PANI‐LS‐M (20.2 S/cm), which was prepared in the presence of a MF (0.4 T) with a polymerization yield of as much as 96%, was increased by four times compared with that of the PANI‐LS‐A (4.9 S/cm). Meanwhile, the solubility of the PANI‐LS‐M was two times higher than that of the PANI‐LS‐A in solvents such as NMP, dimethyl sulfoxide, dimethyl formamide, formic acid, and acetidin. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 40467.     

Cryogenic magnetic properties in the pyrochlore RE2TiMnO7 (RE = Dy and Ho) compounds

Pyrochlore RE₂TiMnO₇ (RE = Dy and Ho) compounds were synthesized using a ceramic method, and their crystal structures, microstructures, and cryogenic magnetic properties were determined. Dy₂TiMnO₇ and Ho₂TiMnO₇ compounds belong to the Fd-3m space group with a cubic pyrochlore structure and show a considerable reversible magnetocaloric effect (MCE) in the vicinity of the second-order ferromagnetic-paramagnetic (FM-PM) phase transition at the Curie temperature (T_C) of 7.6 and 7.5?K, respectively. The obtained maximal values of magnetic entropy change (-ΔS_M^max) for Dy₂TiMnO₇ and Ho₂TiMnO₇ are 12.50 and 13.95?J/kg K under a magnetic field change (ΔH) of 0–7?T, respectively. Correspondingly, the relative cooling power (RCP) values reach 530.3 and 573.2?J/kg, respectively.     

Cis and trans radicals generated in helical poly(propargyl acetate)s prepared using a [Rh(norbornadiene)Cl]2 catalyst

The poly(propargyl acetate) (A) having a helical cis-transoid structure was stereospecifically prepared using the Rh complex catalyst, [Rh(norbornadiene)Cl]₂, in MeOH or NEt₃ solvent at 0 and 40 °C in moderate yield. Electron spin resonance (ESR) analysis of the polymer revealed the formation of the cis (B) and trans (C) radicals which were produced through the thermal rotational scission of the helical cis CC bonds in the main-chain during the polymerization. The spatial and geometrical structure was successfully deduced using the two analogues’ polymers in which either methyl or methylene group is deuterated, by the aide of computer simulation of the observed ESR spectra together with the calculation of spin density of the two radicals.     

Methanol oxidation on carbon-supported Pt-Ru and TiO2 (Pt-Ru/TiO2/C) electrocatalyst prepared using polygonal barrel-sputtering method

Methanol oxidation performance of a carbon-supported Pt-Ru alloy catalyst used at the direct methanol fuel cell (DMFC) anode is improved by adding TiO₂. However, the methanol oxidation performance of the electrocatalyst described above must be enhanced further to realize practical application in DMFCs. In this study, we used our original surface-modifying technique termed the “polygonal barrel-sputtering method” to prepare a carbon-supported Pt-Ru and TiO₂ (Pt-Ru/TiO₂/C) electrocatalyst offering higher methanol oxidation performance. The obtained results show that the methanol oxidation performance of the prepared Pt-Ru/TiO₂/C is superior to that using wet process as the TiO₂ deposition method. Furthermore, for our sputtering method, the peak current of methanol oxidation on the Pt-Ru/TiO₂/C is enhanced by increasing the TiO₂ deposited amount up to 2.8 wt.%. These results suggest that a Pt-Ru/TiO₂ interface area is increased using the polygonal barrel-sputtering method, providing the high methanol oxidation performance of Pt-Ru/TiO₂/C.