Quasi-stable temperature of the steady state of microwave heated hematite

Microwave heated materials often reach a quasi-stable temperature resulting in thermal runaway. To control steady state in microwave processing, it is important to predict the quasi-stable temperature of the steady state. We demonstrated that the microwave heating behavior of hematite varies significantly with its initial temperature. In microwave heating, hematite samples could not be heated from room temperature, whereas hematite samples preheated to 410 °C or higher was heated to a temperature of 1020 °C. The microwave heating behavior can be accurately predicted by considering the steady-state energy balance.     

Optimisation on the stability of CaO-doped partially stabilised zirconia by microwave heating

Partially stabilised zirconia has advantages for the applications in the metallurgical processes which have special requirements in corrosion resistance and high-temperature performance. In the present work, controllable microwave heating was used for the uniform thermal field and consequent microstructure improvement to further improve the stability of partially stabilised zirconia, which was 88.14% prepared by electric arc melting. Analyses including X-ray diffraction (XRD), Scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR) and Raman spectroscopy (Raman) were used to study the effect of temperature change on the phase composition and structure of the samples. After heating at temperatures of 900 °C, 1000 °C, 1100 °C, 1200 °C and 1300 °C for 1h, the stabilities of the heated product were 88.51%, 95.02%, 95.17%, 96.31% and 97.64%, respectively. From the phase transformations based on the experimental results, the discussion indicates that the martensitic transformation temperature of zirconia from m-ZrO₂ to t-ZrO₂ during the heating stage was reduced under the radiation of microwave energy.     

Experimental study on foam glass prepared by hydrothermal hot pressing-calcination technique using waste glass and fly ash

Foam glass is a lightweight and high-strength building and decoration material with superior performance in heat insulation, sound absorption, moisture resistance and fire protection. The use of waste glass powder and fly ash to prepare foam glass is one of the most important ways to utilize solid waste as a resource. In this study, waste glass powder and fly ash were used as raw materials to prepare foam glass by a hydrothermal hot pressing–calcination method. The effects of fly ash content (0 wt%, 10 wt%, 20 wt%, 30 wt%), heating rate (1 °C/min, 3 °C/min, 5 °C/min, 8 °C/min, 10 °C/min) and calcination temperature (600 °C, 700 °C, 750 °C, 800 °C, 850 °C, 900 °C) on the microscopic morphology, density, compressive strength, porosity and other properties of the foam glass samples were studied. Their microstructure and morphology were analyzed by thermogravimetric analysis–mass spectrometry, X-ray diffraction, scanning electron microscopy and Fourier transform infrared spectroscopy. At a fly ash content of 10 wt%, the heating rate was 5 °C/min, the calcination temperature was 800 °C, the foam glass density was 0.3 g/cm³, the compressive strength was 1.65 MPa, the total porosity was 75.5%, and the effective thermal conductivity was 0.206 W/m·K. The effective thermal conductivity models of the composite materials were used to verify the experimental data. The relationship between the thermal conductivity of foam glass materials and the related influencing factors was investigated.     

Microwave vs conventional porcelain firing: Macroscopic properties

This paper provides an overview of the macroscopic properties of porcelain tableware fired in a microwave furnace with six magnetrons (each with a nominal power of 900 W) operating at the frequency of 2.45 GHz. The dependence of firing temperature on physical properties such as shrinkage, water absorption, apparent porosity, bulk density, and impact resistance was analyzed. Emphasis is on the differences in the macroscopic properties of microwave and conventionally (gas and electric) fired porcelain. Batches were fired from room temperature up to above the optimum firing temperature (1380°C). Results show similar macroscopic properties for both firing methods, microwave heating required lower firing temperatures (between 1300°C and 1350°C), and shorter processing times (about 70 minutes). The main differences between microwave and electric firing methods occur in a temperature band of 300°C above the porcelain eutectic temperature (close to 1000°C).     

Sintered glass-ceramics from Municipal Solid Waste-incinerator fly ashes—part I: the influence of the heating rate on the sinter-crystallisation

A glass composition, made by mixing 70% of MSW ashes and 30% of waste from feldspar production, was studied. The batch was melted at 1350 °C and the melt was quenched in water. The chemical stability of the glass was investigated by TCLP leaching test. DTA experiment, at 10 °C/min heating rate in the 20–1200 °C range, showed that the glass has a high crystallisation trend with a gehlenite (2CaO.Al₂O₃.SiO₂) formation at 935 °C. The percentage of crystal phase formed as a function of the heat treatment was measured by density variation and XRD. In order to obtain sintered glass ceramics, the frit was heat treated in Al₂O₃ moulds at 950, 1000 and 1050 °C by 2 and 20 °C/min heating rates. At high heating rate predominantly surface crystallisation occurred with fibre-like crystals growing perpendicularly from the surface; at low heating rate bulk crystallisation took place. At low rate, the sintering was inhibited by the crystallisation process. Non porous sintered samples were obtained by a 30 °C/min heating rate and a 40 min isothermal step at 1120 °C, near the liquidus temperature.     

Enhanced microwave-assisted sintering of X7R ceramic dielectrics for use in multilayer capacitors

A hybrid furnace, allowing the simultaneous application of microwave (2.45 GHz) and radiant energy, has been used to investigate the sintering of a commercial X7R powder commonly used in multilayer capacitors. Samples were processed at temperatures in the range 1060–1120 °C. Enhanced sintering has been observed in the form of accelerated densification when a microwave field is applied. At 1090 °C for example, 99.4% dense material was obtained by microwave-assisted heating but only 96.9% density was reached by conventional heating. The relative permittivity of the microwave sintered material was ～2200, typically 10%, higher than for conventionally sintering. The Curie temperature was lower by 5 °C in the microwave heated samples.     

Melter Feed Reactions at T ≤ 700°C for Nuclear Waste Vitrification

To understand feed‐to‐glass conversion for the vitrification of nuclear waste, we investigated batch reactions and phase transitions in a simulated nuclear waste glass melter feed heated at 5 K/min up to 700°C using optical microscopy, scanning electron microscopy with energy‐dispersive X‐ray spectroscopy, and X‐ray diffraction. To determine the content and composition of leachable phases, we performed leaching tests; the leachates were analyzed by inductively coupled plasma atomic emission spectroscopy. By 400°C, gibbsite and sodium borates lost water and converted to amorphous phase, whereas other metallic hydroxides dehydrated to oxides. Between 400°C and 700°C, carbonates decomposed before 500°C; amorphous aluminum oxide and calcium oxide reacted with the sodium borate and formed the more durable amorphous borate phase along with intermediate crystalline products; above 500°C, quartz began to dissolve, and hematite started to convert to trevorite.     

Changes of vitamin E content in rice bran with different heat treatment

Oil yield from unheated rice bran was 17.6% whereas that of microwave‐heated rice bran increased to up to 18.4%. Content and composition of vitamin E in rice bran oil were affected by microwave heating. Especially, contents of α‐tocopherol, α‐tocotrienol, and γ‐tocotrienol as well as total vitamin E were significantly (P <0.05) increased when the rice bran was subjected to microwave heating for up to 30 s. When rice bran was heated in an electric roaster up to 20 min at 170 °C, 5 min at 180 °C, and 3 min at 190 °C, the total vitamin E content in rice bran oil increased significantly (P <0.05) followed by a considerable decline beyond those time points. Microwave heating was more effective for an increase in the vitamin E content than electric roaster heating. However, longer heating with both microwave and electric roaster caused a significant degradation of vitamin E resulting in a decreased content of total vitamin E.     

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.     

Global insight into microwave stoneware firing: Macro and microstructural changes

Industrial competition and environmental concerns lead to the exploration of alternative and energy-efficient technologies for ceramic materials processing. The main objective of this work was to present microwave heating as a viable option for stoneware processing. Stoneware functional properties are presented and discussed, with emphasis on impact strength, water absorption, porosity, and color. Microstructure analyses show that microwave- and gas-fired samples have higher densifications than electrically fired samples. A relevant finding for processing conditions is that microwave firing requires temperatures approximately 100°C lower than those required by conventional firing. Microwave-fired samples’ rupture energies are approximately twice (0.57 ± 0.06 (J)) those of the reference samples (0.26 ± 0.03 (J)), and their water absorptions are approximately one-half (1.5% at 1170°C and 0.8% at 1190°C) of those of the reference samples (2.0%), whereas the water absorption of electrically fired samples at 1180°C has been estimated to be 7.5%. Color analysis also evidences a shift to lower microwave firing temperatures, what is attributed to the enhanced transformations promoted by microwave heating when comparing with the transformations promoted by conventional (gas or electric) heating.     

Heating parameter optimization and optical properties of Nd:YAG transparent ceramics prepared by microwave sintering

Nd-doped YAG transparent ceramics were prepared by microwave sintering. In this paper, the green bodies from high-purity commercial powders were sintered from 900 °C to 1750 °C for different lengths of time (0.5–2 h) by microwave heating. By optimizing the microwave heating parameters (the heating rate at different stages of microwave sintering, sintering temperature and holding time), the microstructures and optical properties of transparent ceramics can be effectively improved. The phase transformation, densification process and optical properties of Nd:YAG transparent ceramics were discussed. The liquid phases strongly absorb microwave radiation and affect the sintering results of samples during microwave sintering. The highest in-line transmittances of Nd:YAG transparent ceramic fabricated at 1750 °C for 2 h were 76.5% at 400 nm and 80.6% at 1064 nm. The fluorescence emission spectra and lifetime depending on different heating conditions were also discussed.     

Evolution of thermograph parameters during the oxidation of extra virgin olive oil

The modulated differential scanning calorimetry (M‐DSC) was used as a rapid and effective method to characterize the olive oil at different levels of oxidation. Thermograph parameters have been related to oxidative degradation of the triglycerides. In this study, their relation to the characteristic off‐flavor compounds, correlated to the oxidative degradation of the oil, was also investigated. Extra virgin olive oil samples were subjected to the following oxidation treatments: a) purged with air using glass washing bottles at two flow rate values, b) heated in a conventional oven at two area/oil mass ratios, and c) heated in a microwave oven also at two area/oil mass ratios. Samples were withdrawn and analyzed at predetermined intervals. Flavor and off‐flavor compounds were isolated using a dynamic thermal stripping apparatus and transferred into a gas chromatograph by using a thermal desorption unit. All oil samples were analyzed by M‐DSC during cooling from 25 °C to ?60 °C at 7 °C/min, and heating back to 40 °C at 10 °C/min. High correlation values were obtained between various M‐DSC thermograph parameters and certain volatile compounds. Results showed that M‐DSC could be used as a simple method to indicate compositional changes in olive oil during oxidation.     

Microstructural characterization and crystallization behaviour of (1 − x)TeO2–xWO3 (x = 0.15, 0.25, 0.3 mol) glasses

DTA, XRD and SEM investigations were conducted on the (1 − x)TeO₂–xWO₃ glasses (where x = 0.15, 0.25 and 0.3). Whereas the 0.75TeO₂–0.25WO₃ and 0.7TeO₂–0.3WO₃ glasses show no exothermic peaks, an indication of no crystallization in their glassy matrices, two crystallization peaks were observed on the DTA plot of the 0.85TeO₂–0.15WO₃ glass. On the basis of the XRD measurements of the 0.85TeO₂–0.15WO₃ glass samples heated to 510 °C and 550 °C (above the peak crystallization temperatures), α-TeO₂ (paratellurite), γ-TeO₂ and WO₃ phases were detected in the sample heated to 510 °C and the α-TeO₂ and WO₃ phases were present in the sample heated to 550 °C. SEM micrographs taken from the 0.85TeO₂–0.15WO₃ glass heated to 510 °C showed that centrosymmetrical crystals were formed as a result of surface crystallization and were between 3 μm and 15 μm in width and 12 μm and 30 μm in length. On the other hand, SEM investigations of the 0.85TeO₂–0.15WO₃ glass heated to 550 °C revealed the evidence of bulk massive crystallization resulting in lamellar crystals between 1 μm and 3 μm in width and 5 μm and 30 μm in length. DTA analyses were carried out at different heating rates and the Avrami constants for the 0.85TeO₂–0.15WO₃ glass heated to 510 °C and 550 °C were calculated as 1.2 and 3.9, respectively. Using the modified Kissinger equation, activation energies for crystallization were determined as 265.5 kJ/mol and 258.6 kJ/mol for the 0.85TeO₂–0.15WO₃ glass heated to 510 °C and 550 °C, respectively.     

Microstructure and properties of SiCf/SiC composite joints with CaO–Y2O3–Al2O3–SiO2 interlayer

Using CaO, Y₂O₃, Al₂O₃, and SiO₂ micron-powders as raw materials, CaO–Y₂O₃–Al₂O₃–SiO₂ (CYAS) glass was prepared using water cooling method. The coefficient of thermal expansion (CTE) of CYAS glass was found to be 4.3 × 10^?6/K, which was similar to that of SiC_f/SiC composites. The glass transition temperature of CYAS glass was determined to be 723.1 °C. With the increase of temperature, CYAS glass powder exhibited crystallization and sintering behaviors. Below 1300 °C, yttrium disilicate, mullite and cristobalite crystals gradually precipitated out. However, above 1300 °C, the crystals started diminishing, eventually disappearing after heat treatment at 1400 °C. CYAS glass powder was used to join SiC_f/SiC composites. The results showed that the joint gradually densified as brazing temperature increased, while the phase in the interlayer was consistent with that of glass powder heated at the same temperature. The holding time had little effect on phase composition of the joint, while longer holding time was more beneficial to the elimination of residual bubbles in the interlayer and promoted the infiltration of glass solder into SiC_f/SiC composites. The joint brazed at 1400 °C/30 min was dense and defect-free with the highest shear strength of about 57.1 MPa.     

Effect of Microwave Processing on the Crystallization and Energy Density of BaO‐Na2O‐Nb2O5‐SiO2‐B2O3 Glass‐Ceramics

Barium sodium niobate (BNN) glass‐ceramics were successfully synthesized through a controlled crystallization method, using both a conventional and a microwave hybrid heating process. The dielectric properties of glass‐ceramics devitrified at different temperatures and conditions were measured. It was found that the dielectric constant increased with higher crystallization temperature, from 750°C to 1000°C, and that growth of the crystalline phase above 900°C was essential to enhancing the relative permittivity and overall energy storage properties of the material. The highest energy storage was found for materials crystallized conventionally at 1000°C with a discharge energy density of 0.13 J/cm³ at a maximum field of 100 kV/cm. Rapid microwave heating was found to not give significant enhancement in dielectric properties, and coarsening of the ferroelectric crystals was found to be critical for higher energy storage.     

Near‐zero thermal expansion transparent lithium aluminosilicate glass‐ceramic by microwave hybrid heat treatment

The material of choice for space applications which demand very high dimensional stability is lithium aluminosilicate (LAS) based Ultra Low thermal Expansion Glass‐Ceramic (ULEGC). Generally, the controlled crystallization process recommended for the processing of transparent ULEGC involves a long soaking duration to achieve the required crystal number density. This paper brings out the process optimization procedure adopted for realizing transparent and nanocrystalline ULEGC from conventionally processed LAS glass using microwave‐assisted (hybrid) crystallization. The experimental strategy involves two stages (i) identification of the optimum crystallization temperature (T_c) under a microwave field (ii) optimization of a microwave‐assisted crystallization process to achieve near zero Coefficient of Thermal Expansion (CTE).. Optimum heat‐treatment schedules for nucleation and crystallization under a microwave environment were found to be 720°C/24 hours and 775°C/0.3 hours, respectively. The optimized heat‐treatment condition revealed the efficacy of the microwave hybrid heating, by producing nanocrystalline (35‐50 nm) and transparent (>82%) ULEGC having a thermal expansion of ?0.03 × 10^?6 K (0°C to 50°C).     

A low-cost lightweight microwave absorber: Silicon carbide synthesized from tissue

Recently, microwave absorbing materials have been widely used with the development of electromagnetic wave technology such as 5G communication. It is urgent to develop low-cost electromagnetic wave absorbing materials to meet the increasing civil market demand. Herein, we report a novel and simple strategy to synthesize lightweight silicon carbide microwave absorber by calcining tissue and glass microspheres at high temperature. The results show that higher synthesis temperature can improve the crystallinity of SiC, generate more whiskers and reduce the impurity content. This in turn increases the dielectric loss of the material. The products synthesized at 1600 °C have excellent microwave absorption properties. At the thickness of 1.3 mm, it achieves a reflection loss of ?32.5 dB and an effective absorption bandwidth of 4.1 GHz. This study broadens the way for the potential reuse of waste paper and glass, and provides a useful reference for the preparation of low-cost microwave absorbing materials.     

A novel method for production of activated carbon from waste tea by chemical activation with microwave energy

This study presents the production of activated carbon from waste tea. Activated carbons were prepared by phosphoric acid activation with and without microwave treatment and carbonisation of the waste tea under nitrogen atmosphere at various temperatures and different phosphoric acid/precursor impregnation ratios. The surface properties of the activated carbons were investigated by elemental analysis, BET surface area, SEM, FTIR. Prior to heat treatment conducted in a furnace, the mixture of the waste tea and H₃PO₄ was treated with microwave heating. The maximum BET surface area was 1157 m²/g for the sample treated with microwave energy and then carbonised at 350 °C. In case of application of conventional method, the BET surface area of the resultant material was 928.8 m²/g using the same precursor and conditions. According to the Dubinin–Radushkevich (DR) method the micropore surface area for the sample treated with microwave energy was higher than the sample obtained from the conventional method. Results show that microwave heating reasonably influenced the micropore surface area of the samples as well as the BET surface area.The samples activated were also characterised in terms of the cumulative pore and micropore volumes according to the BJH, DR and t-methods, respectively.     

3D-printed strontium-doped BG-CaSiO3-HA composite scaffolds promote critical bone defect repair by improving mechanical strength and increasing osteogenic activity

The release of silicon and calcium elements contained in silicon-based materials promotes the formation of bone. For bioactive glass prepared by the sol-gel method, water-soluble binders are usually added when preparing 3D printed scaffolds. However, the obtained scaffolds are prone to collapse when exposed to water and have low strength. At the same time, the binder needs to be removed for clinical applications, so the 3D printed scaffolds need to be sintered. Under high temperature, bioactive glass scaffolds will be transformed into composite scaffolds composed of bioglass, CaSiO₃ and hydroxyapatite, while different sintering temperatures will form different crystal types of CaSiO₃. In this study, SrBG-βCS-HA and BG-βCS-HA were obtained at a heating rate of 5 °C/min to 1100 °C and at the same rate to room temperature. SrBG-αCS-HA and BG-αCS-HA were obtained at a heating rate of 2 °C/min to 1200 °C and at the same rate to room temperature. In vitro and in vivo experiments verified that the presence of strontium in the obtained scaffolds after sintering further enhanced the osteogenic properties of the scaffolds. SrBG-αCaSiO₃-HA and SrBG-βCaSiO₃-HA were found to have relatively better osteogenic properties. The results show that SrBG-CaSiO₃-HA 3D printing scaffolds have excellent clinical application potential.     

Microwave Processing for Improved Ionic Conductivity in Li2O–Al2O3–TiO2–P2O5 Glass‐Ceramics

Li_1.4Al_0.4Ti_1.6(PO₄)₃ (LATP) was synthesized using a glass‐ceramics approach through crystallization in a conventional box furnace and a modified microwave furnace. The microstructure of samples that were microwave processed at 1000°C showed a larger average grain size (0.87 μm) when compared with the grain size of conventionally processed samples (0.30 μm) at the same temperature. Microwave processing led to significant enhancement of the conductivity when compared with conventional processing for all crystallization temperatures investigated. The highest total conductivity achieved was of glass microwave processed at 1000°C, with a conductivity of 5.33 × 10^?4 S/cm. This conductivity was five times higher than that of LATP crystallized conventionally at the same temperature.