Rapid microwave sintering of centimetric zirconia: Scalability and electromagnetic-thermal-fluid-dynamic simulation

Microwave sintering is a method presenting the following advantages for flash sintering: contactless/volumetric heating, and the possibility to control the heating cycle of the microwave power. In this study, the transition from a typical 100 K/min to an ultra-rapid heating rate of 500 K/min is studied. The heating homogeneity of the typical hybrid configuration using silicon carbide susceptors is tested up to the stability limit of the system. We show that zirconia specimens as thick as 10 mm can be heated and sintered up to 500 K/min heating rate at which thermal cracks appear. However, the centimetric size of the specimens seems to favor coarsening implying an important remaining porosity in the end. A comprehensive simulation including microwave heating and convection has allowed the determination of the heating regime transition during the flash process and the quantification of each specimen's cooling fluxes.     

Dewatering of fine coal slurries by selective heating with microwaves

Comparative experimental studies on the dewatering of a fine coal slurry sample containing ≈52% moisture by microwave heating vis-à-vis thermal heating are reported. For thermal heating, thermogravimetric analysis (TGA) using different masses and different heating rates were employed. Similar experiments were carried out for dewatering by microwave heating using a conventional oven operating at 2.45 GHz and 800 W power. These quantitative experiments show that microwave heating is an order of magnitude more efficient than thermal heating, although about 10% of the residual moisture, perhaps trapped in the capillaries of the coal particles, is not removed even by microwave heating. Based on these results, a bench scale dewatering unit was designed and tested in which coal slurry in kg quantities was fed on a conveyor belt. The efficiency of this unit for dewatering fine coal slurries was estimated to be over 80% of the theoretical limit. A cost estimate of about $3/ton for the power consumed for 10% reduction of moisture is made.     

Reduced leakage current of multiferroic BiFeO3 ceramics with microwave synthesis

Preparation of multiferroic BiFeO₃(BFO) is reported using microwave heating. The prepared sample is characterized using x-ray diffraction, scanning electron microscopy, differential scanning calorimetry and leakage current measurements. It is observed that the BFO can be prepared with microwave heating at a fast heating rate, consisting of more homogeneous microstructure and better electrical properties. Phase purity of the sample is confirmed from x-ray diffraction measurements. Uniform grain size distribution is observed for the sample prepared with microwave heating. More than an order of magnitude reduction in the leakage current is observed for the sample prepared with microwave heating as compared to conventional radiant heating.     

Ultra-fast firing: Effect of heating rate on sintering of 3YSZ,with and without an electric field

It has recently been reported that ceramics can be sintered in a few seconds with the aid of an electric field (“flash sintering”). This investigation tests the possibility that the accelerated sintering is a consequence of the rapid heating rate involved rather than a direct effect of the electric field on mass transport. The sintering of 3YSZ powder compacts at a temperature of ∼1300 °C was compared (i) in flash sintering, (ii) with rapid heating rates produced without the application of an electric field, and (iii) with conventional heating rates. The results show that rapid heating can accelerate sintering by over 2 orders of magnitude compared with heating to the same temperature at conventional rates, even without the application of an electric field. It is concluded that the rapid densification in flash sintering of 3YSZ is at least partly a consequence of the rapid heating involved. Possible explanations are discussed.     

Modelling pyrolysis with dynamic heating

In literature, the reaction kinetic of pyrolysis process is often determined and modelled under constant heating rates. In reality, the heating rate of an industrial pyrolysis process is difficult or often not necessary to be kept constant. The variation of heating rate at different reaction stages, termed “dynamic heating”, governs the pyrolysis performance such as production rate, energy consumption, product quality, etc. In this work, pyrolysis progress with dynamic heating is being studied. The rate and reaction heat of tyre pyrolysis at different heating rates are obtained experimentally. A transient model considering the effect of dynamic heating was then developed and compared with the conventional static heating model. Results show that a higher heating rate favours the production of volatiles and shifts the overall pyrolysis heat flow to more endothermic. The significance of the dynamic heating model was observed for processes with large feed size and/or with high heating rate.     

The influence of surface heating on the birefringence distribution in injection molded parts

In order to reduce the frozen-in orientation, which is usually present in injection molded products, the effect of high-performance mold surface heaters was investigated. The heaters are capable of changing their surface temperature by 70°C within a few tenths of a second typically, which minimizes their effect on the cooling time. The effects of the heating time, the instant of heating, and the heating power on the birefringence distribution of a polystyrene resin were studied. Reductions of the birefringence peak by a factor 4 to 7 were observed. The birefringence is removed most effectively by heating briefly before and during the injection stage. A heating pulse of about one second and with a power density of 20 W/cm² then seems to be sufficient. A minimum power density of 10 W/cm² is needed for the relaxation to occur in this specific system.     

State-of-the-art plants for drying and high-temperature heating of ladles

The Stal’proekt Institute has developed state-of-the-art plants for drying and high-temperature heating of steel-teeming ladles. Two new plants for drying tundish ladle lining have been designed for machine No. 6 for continuous billet casting at the Oskol’skii Electrometallurgical Works. Plants for drying and heating 30-ton teeming ladles to a temperature of 800°C and for high-temperature heating of ladle lining (1200°C) have been designed for a currently constructed metallurgical works. All these plants recover flue gas heat and use it for heating air supplied for combustion, thus decreasing the consumption of fuel. The process of drying and heating of lining is analyzed using a mathematical model developed at the institute. The plants are equipped with automatic control systems controlling the temperature schedule of drying and heating the lining. The specified plants ensure cost-effective operation in an automatic mode __________ Translated from Novye Ogneupory, No. 10, pp. 21–25, October, 2006.     

Measurements of the melting point of graphite and the properties of liquid carbon (a review for 1963-2003)

Scientific literature on the melting temperature of graphite and its properties in melting is reviewed, beginning with the study by Bundy in 1963 and proceeding up to 2003. Data obtained by Pirani in 1930 that has been quoted in some recent publications is also considered. Successive experimental data and theoretical predictions on the melting point of carbon are summarized. The history of carbon studies, starting in 1963, is given, covering both laser and electrical heating of the graphite. The main divergence in the experimental results is in the value of the true melting temperature of graphite in the range of 4000 or 5000 K.The paper first describes laser heating. Pulsed laser heating of graphite usually shows the absence of a melting temperature plateau in the heating of a low-density graphite specimen (only a deflection point is observed on the increasing signal of the pyrometer). Carbon vapour, as a result of graphite sublimation, usually plays a leading role in the temperature measurements near the melting point under slow heating.The volume electrical heating of graphite is then discussed. Several electrical pulse investigations are listed: measurements of different properties; heating of low density graphite; and very slow pulse heating up to steady-state by alternating current. A separate section shows the data on spectral emissivity investigations, which are required in graphite temperature measurements.Reliable experimental data for the graphite melting point are presented: enthalpy of solid state under melting (10.5 kJ/g); enthalpy of liquid state under melting (20.5 kJ/g); heat of graphite melting (10 kJ/g); liquid-carbon resistivity (730 μΩ cm) near the melting point at a density of 1.8 g/cm³ under high pressures (several GPa); estimation of expansion (70%) during melting at 100 MPa pressure; and melting temperature T_m = 4800 ± 100 K at a pressure 10-100 MPa. Most of these data are obtained by electrical fast heating (1-5 μs), that are supported by the data of carefully executed laser-pulse heating.     

Sintering of a clay from Burkina Faso by dilatometry Influence of the applied load and the pre-sintering heating rate

The sintering of a pottery clay from Burkina Faso was studied as a function of the heating rate, at 3 or 10°C/min. The experimental method used was loading dilatometry in isothermal conditions at 1120°C. In these conditions, we found that the densification rate of the material is low, but tend to a limiting value after 2 h at 1120°C, depending on the pre-sintering heating rate and the load used. The relationship between the pre-sintering heating rate and the densification rate indicated the existence of a weakly organised material at higher heating rates. Nevertheless, higher values of shrinkage were observed when the temperature increased continuously. It is, therefore, proposed that the material is subject to a preferential solid state diffusion mechanism at face to face of the remaining kaolinite layers at high temperatures. This mechanism is favoured by higher heating rates, mainly in the temperature range corresponding to the structural reorganisation of the metakaolin phase.     

Effect of heating rate on properties of transparent aluminum oxynitride sintered by spark plasma sintering

High heating rates ranging from 50 to 250°C/min are selected to rapidly sinter transparent aluminum oxynitride (AlON) ceramics by spark plasma sintering (SPS) at 1600°C under 60 MPa using a bimodal AlON powder synthesized by the carbothermal reduction and nitridation method. With 1 minute holding time before cooling, all the specimens show high density and high transparence. The maximum transmittance is up to 74.5%-80.6%, where the maximum transmittance is positively correlated with the heating rate. Further analysis reveals that faster heating rates enable the decrease in the amount of the AlON phase decomposed into the α-Al₂O₃ and AlN phases during heating. These α-Al₂O₃ and AlN phases have to be converted back to AlON at the final stage of sintering, which indicates that a decrease in the amount of the α-Al₂O₃ and AlN phases via the boosted heating leads to the higher transmittance of the AlON ceramics. The high heating rates and short holding duration of the SPS utilized in this study result in the fine grain size of the obtained ceramics (1-6 μm) compared to that of the AlON ceramics fabricated by the conventional sintering method. This effect of high heating rates is confirmed by the coupled densification-grain growth modeling. In turn, the obtained AlON specimens exhibit a Vickers hardness of 15.87-16.62 GPa.     

热负荷分配比例对抽凝-背压供热机组能耗影响

抽凝-背压供热模式是实现能量梯级利用、降低火力发电煤耗的有效途径,研究不同室外温度下供热凝汽器与尖峰加热器热负荷分配比例对机组能耗的影响,确定最佳热负荷分配比例,是抽凝-背压供热机组节能降耗的核心问题之一。本文利用热网变工况模型及Ebsilon软件仿真,以某310MW抽凝-背压供热机组为研究对象,分析了供热期不同温度下供热凝汽器与尖峰加热器热负荷分配比例不同时机组的发电功率及煤耗。结果表明：对于抽凝-背压热电联产机组,并非供热凝汽器热负荷比例越高而发电功率越高,供热期不同阶段,机组发电功率随供热凝汽器热负荷变化呈现不同规律;相同室外温度下,供热凝汽器与尖峰加热器热负荷分配比例对机组能耗影响很大,凝汽器热负荷比例不同时,其极差最小值和最大值分别为2.02g/（kW·h）和5.50g/（kW·h）。     

Microwave irradiated and thermally heated olive stone activated carbon for nickel adsorption from synthetic wastewater: A comparative study

In attempt to compare the removal efficiency and yield of the activated carbon prepared using the conventional and microwave‐assisted heating is the focus of this work. Toward this olive stone (a biomass precursor) is activated using the popular activating agent potassium hydroxide. The process optimization exercise is carried out by using the standard full factorial statistical design of experiments (response surface methodology). The activated carbons prepared under the optimized conditions are compared based on the adsorption capacity and yield. The adsorption capacity was found higher using microwave heating as compared with conventional heating. The microwave heating requires significantly lesser holding time as compared to conventional heating method to produce activated carbon of comparable quality, with higher yield. The BET surface area of carbon using microwave heating is significantly higher than the conventional heating. Although the mesopore surface area of carbon is not vary significantly, the activation time, power, and nitrogen gas consumption are significantly lower than the conventional heating rendering that the activation process via microwave is more economical than that via conventional heating. The adsorption isotherm data fitted the Langmuir isotherm well and the monolayer adsorption capacity was found to be 12.0 and 8.42 mg/g for microwave and thermally heated activated carbon, respectively. Regeneration studies showed that microwave‐irradiated and thermally heated olive stone could be used several times by desorption with an HCl reagent. Both carbons can be used for the efficient removal of Ni²⁺ (>99%) from contaminated wastewater. © 2013 American Institute of Chemical Engineers AIChE J, 60: 237–250, 2014     

An ultraviolet phosphor from submicrometer-sized particles of gadolonium-doped yttrium oxide prepared by heating of precursors in a polymer solution

Gadolonium-doped yttrium oxide (Y₂O₃:Gd) was synthesized by simple heating of precursors in a polymer solution. This material is potentially useful as an ultraviolet source, since ultraviolet light is emitted when electron transition between energy states in Gd ions occurs. The grain sizes of the particles were found to be sub-micron down to several tens of nanometers. Optimum conditions for producing highly crystalline material with small grain and crystal sizes was investigated by varying the parameters for the synthesis, such as heating temperature, heating time, and dopant concentration. A heating temperature at 800 °C and a heating time of 30 min was optimum, i.e., appreciably high crystallinity and small grain sizes were produced. The particles produce ultraviolet light, peaking at 315 nm, and the intensity of the light depends on the dopant concentration. The maximum intensity was achieved at a dopant concentration of 5 to 10% at./at.     

Experimental investigation of infrared rapid surface heating for injection molding

A low cost and practical infrared rapid surface heating system for injection molding is designed and investigated. The system was designed to assemble on the mold and a control system was used to operate the motion of the lamp holder. Four infrared halogen lamps (1 kW each) were used as the radiative source to heat the surface of mold insert. The temperature increase is verified on the mold plate with a thermal video system. Two types of specular reflectors combined with different bulb configurations were applied to study the heating ability of radiation heating. A modified spiral flow mold was used to test the enhancing filling ability of the rapid surface heating system. Three resins, PP, PMMA and PC were molded in the spiral flow injection molding experiments. If spherical reflector and centralized lamp configuration are used, the temperature at the center of the mold surface is the highest. The temperature of mold center surface is raised from 83°C to 188°C with 15 s of infrared heating. Because the surface temperature of the mold insert is higher than the glass transition temperature of resins before filling, the flow distance of resins in the modified spiral flow mold will be increased. The location effect of the infrared surface heating system on a thin‐long cavity was studied to demonstrate the possibility of using smaller infrared heating area on a large mold surface. A microprobe cavity also demonstrated that with the assistance of infrared heating technology the formability of a microprobe can be greatly improved. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 102: 3704–3713, 2006     

Lignin-based carbon fibers: Oxidative thermostabilization of kraft lignin

The thermostabilization of lignin fibers used as precursors for carbon fibers was studied at temperatures up to 340 °C at various heating rates in the presence of air. The glass transition temperature (T_g) of the thermally treated lignin varied inversely with hydrogen content and was found to be independent of heating rate or oxidation temperature. A continuous heating transformation (CHT) diagram was constructed from kinetic data and used to predict the optimum heating rate for thermostabilization; a heating rate of 0.06 °C/min or lower was required in order to maintain T_g > T during thermostabilization. Elemental and mass analyses show that carbon and hydrogen content decrease during air oxidation at constant heating rates. The hydrogen loss is sigmoidal, which is consistent with autocatalytic processes. A net increase in oxygen occurs up to 200-250 °C; at higher temperatures, oxygen is lost. Spectroscopic analyses revealed the oxidation of susceptible groups within the lignin macromolecule to ketones, phenols and possibly carboxylic acids in the early stage of the reaction; the later stage involving the loss of CO₂ and water and the formation of anhydrides and possibly esters. Slower heating rates favored oxygen gain and, consequently, higher glass transition temperatures (T_g) as opposed to faster heating rates.     

The effect of undrained heating on a fluid-saturated hardened cement paste

The effect of undrained heating on volume change and induced pore pressure increase is an important point to properly understand the behaviour and evaluate the integrity of an oil well cement sheath submitted to rapid temperature changes. This thermal pressurization of the pore fluid is due to the discrepancy between the thermal expansion coefficients of the pore fluid and of the solid matrix. The equations governing the undrained thermo-hydro-mechanical response of a porous material are presented and the effect of undrained heating is studied experimentally for a saturated hardened cement paste. The measured value of the thermal pressurization coefficient is equal to 0.6 MPa/°C. The drained and undrained thermal expansion coefficients of the hardened cement paste are also measured in the heating tests. The anomalous thermal behaviour of cement paste pore fluid is back analysed from the results of the undrained heating test.     

Effect of structural, thermal and flow parameters on steam reforming of methane in a catalytic microreactor

Steam reforming of methane in microchannels, embedded in a monolith is numerically modelled. Horizontal heating layers at equal intervals within the monolith are maintained at constant temperature. The channels are coated internally with catalyst to enhance gas–solid heterogeneous reaction. The numerical method combines the analytical solution for heat transfer through a fin, extended to a stack of fins, and the reactive flow of gases through an iterative procedure. The method offers a tool for quick design of a micro-structure, without considering detailed CFD-based model. In addition, the method can be suitably modified to address thermal management in electronic chip.The temperature within a stack between two heating layers drops near the centre of the stack, in case of an endothermic reaction. This drop, signifying the deviation from isothermal behaviour is found more near the heating layer, and tapers off near the centre of the stack. When the feed temperature is significantly less than the temperature of the heating layer, the portion of the reactor, away from the heating layer remains at a substantially lower temperature, particularly when the number of channels between two heating layers is large. Accordingly, the conversions in the individual channels at the outlet are affected. If the channel wall becomes thicker, the drop in fluid temperature away from the heating layer is more. The increase in feed velocity leads to larger drop in temperature and overall conversion. The decrease in thermal conductivity and the increase in number of channels between two heating layers enhance the temperature drop. None of these functionalities appears to be linear.     

Drying Enhancement of Clay Slab by Microwave Heating

The effect of internal heating by microwave on the drying behavior of a slab was studied. A wet sample of kaolin pressed into a slab was subjected in microwave irradiation of 2.45 GHz. The absorption of microwave energy into a wet slab can be expressed by a function of the moisture content and the pathway length, which is a similar form to Lambert-Beer's law. The drying behavior was compared among three modes: microwave irradiation, hot air heating and radiation heating in an oven. Microwave heating with a constant power resulted in breaking the sample when the internal temperature achieves at 373 K. However, if the power was controlled to maintain the temperature less than the boiling point of water, the drying succeeded without any crack generation until the completion with a significantly faster drying rate than in convective heating or in the oven. It is also noted that the transient behavior of the temperature is quite different from the conventional drying.     

Flash microwave sintering of zirconia by multiple susceptors cascade strategy

In the field of flash sintering, microwave energy represents an interesting way to densify ceramics complex shapes, thanks to a contactless volumetric heating. Attaining a fast and homogeneous heating is a critical parameter and hybrid heating, using silicon carbide susceptors, is generally used. In this study, an original multiple susceptors cascade strategy is developed, using both SiC and 3D-printed ZrO₂ susceptors. This novel configuration follows perfectly the flash heating scheme, even for high heating rates up to 1000 K.min^-1 and leads to a high stability of the “flash” hybrid heating. Flash microwave sintering produced dense (97 % relative density) microstructures within 45 s. Based on comprehensive multiphysics simulations of the overall process, in-situ dilatometry measurements, kinetics method analysis and microstructural characterizations, this work highlights the sintering behavior of zirconia and the temperature distribution during flash microwave sintering.     

Rapid synthesis of thin SAPO-34 membranes using microwave heating

SAPO-34 membranes were prepared by microwave (MW) heating method using a colloidal solution containing tetraethylammonium hydroxide as a template. SAPO-34 in the form of seed and membranes were investigated for their properties such as morphology, pore characteristic, crystallinity and thickness, using characterization method of scanning electron microscope (SEM), X-ray diffraction (XRD), high resolution transmission electron microscopy (HRTEM), FT-IR and nitrogen adsorption-desorption. SAPO-34 membrane was also prepared using conventional hydrothermal heating and studied for its comparison with those formed by MW heating. SAPO-34 membrane containing homogenous SAPO-34 crystals with average size of ~0.7 μm was formed during MW heating. Compare to the conventional hydrothermal heating, MW heating facilitates formation of SAPO-34 crystals with narrower size distribution due to the highly uniform volumetric heating provided by microwave heating. MW heating was able to produce thinner SAPO-34 membrane (1–2 μm) where as hydrothermal heating formed thicker SAPO-34 membrane (~3.6–5.5 μm). The synthesis time for membrane formation was significantly shortened from 24 h for conventional hydrothermal heating to 2 h for microwave heating at 200 °C.