A thermal perspective of flash sintering: The effect of AC current ramp rate on microstructure evolution

In flash sintering experiments, the thermal history of the sample is key to understanding the mechanisms underlying densification rate and final properties. By combining robust temperature measurements with current-ramp-rate control, this study examined the effects of the thermal profile on the flash sintering of yttria-stabilized zirconia, with experiments ranging from a few seconds to several hours. The final density was maximized at slower heating rates, although processes slower than a certain threshold led to grain growth. The amount of grain growth observed was comparable to a similar conventional thermal process. The bulk electrical conductivity correlated with the maximum temperature and cooling rate. The only property that exhibited behavior that could not be attributed to solely the thermal profile was the grain boundary conductivity, which was consistently higher than conventional in flash sintered samples. These results suggest that, during flash sintering, athermal electric field effects are relegated to the grain boundary.     

Influence of geometry on thermal gradients and hotspot formation during flash sintering

Hotspot or preferential current path formation is a major problem halting the advance of flash sintering as an industry applicable densification method. In-situ infrared thermal imaging shows significant dependence of the surface temperature distribution on the sample geometry, causing significant changes in the onset parameters, the course of the flash event and sample quality. Current-ramped flash sintering is compared to conventional flash sintering experiments and reveals an effective reduction in temperature differences. The role of the setup design on the thermal losses and the gradients is highlighted.     

Microstructure and defect gradients in DC and AC flash sintered ZnO

In this study, the microstructure and defect characteristics of flash sintering under direct current (DC) and alternating current (AC) were investigated and compared for the ZnO system. DC flash sintering resulted in grain size and porosity gradients within the sample, whereas AC flash sintering produced a sample with homogeneous and fine grain size. Raman spectroscopy revealed asymmetric peaks correlated to the lattice disorder. Using the Breit-Wigner-Fano model, the asymmetric peak fitting revealed redistribution of defects within the DC flash sintered ZnO, while the AC flash sintered ZnO had a comparable defect concentration to conventionally sintered ZnO.     

Effect of oxygen partial pressure on temperature for onset of flash sintering 3YSZ

In this paper, it is demonstrated that in an oxygen-enriched environment, the oxygen partial pressure affects the onset temperature of flash sintering of 3 mol% yttria-stabilized zirconia (3YSZ). Flash sintering experiments were performed with oxygen partial pressures in the range of 0.25–1 atm. The results indicate that the onset temperature increased by increasing oxygen partial pressure. According to the plots of the conductivity as a function of temperature, the oxygen partial pressure might affect the onset temperature, by changing the conductivity in the pre-flash stage. Combined to the analysis of the law of mass action, it was established that electron conduction might also represent a critical parameter during the pre-flash stage of flash sintering, excluding the oxygen vacancy conduction.     

Promoting core/surface homogeneity during flash sintering of 3YSZ ceramic by current path management: experimental and modelling studies

During flash sintering (FS) of ceramics, the heat loss by surface radiation is the main cause of temperature gradient between core and surface, which induces inhomogeneity in microstructure. To solve this problem, the judicious designing of sample geometry and electrodes configuration is proposed. Experimental and simulation results show that the application of dogbone shape, forked electrodes, and lower cross-section aspect ratio effectively shifts the current path in 3YSZ samples from core to near-surface during FS, compared to bar-shape samples with a single electrode at each end. Consequently, the temperature distribution becomes more uniform throughout the 3YSZ sample, resulting in increase in relative density from 92.7 % to 99.7 % and improved core/surface homogeneity in microstructure. These optimizations enable 3YSZ ceramics to obtain significant increase in flexural strength from 1203 ± 17 MPa to 1501 ± 15 MPa. A multiphysics model is implemented and compared with experimental results, which reveals the underlying mechanisms of improved sample homogeneity.     

Facile preparation of thermal insulation materials by microwave sintering of ferronickel slag and fly ash cenosphere

A novel approach for preparing thermal insulation materials by microwave sintering of ferronickel slag (FNS) in the presence of fly ash cenosphere (FAC) was proposed and evaluated. The study showed that during microwave radiation, the contact interface between FNS and FAC would preferentially form magnesium iron chromate spinel and magnesium iron aluminate spinel particles as hot spots by absorbing microwave vigorously, promoting decomposition and transformation of the raw materials into the thermal insulation phases, mainly cordierite and enstatite. After sintering at 900 °C by microwave for only 20 min with the addition of 25 wt% FAC, a thermal insulation material with thermal conductivity of 0.41 W/(m·K), bulk density of 1.46 g/cm³, compressive strength of 30.72 MPa, water absorption of 21.07%, and linear shrinkage of 7.06% was obtained. Compared with the conventional sintering method, the temperature was reduced by 300 °C, with the sintering time shortened by 6 times. This study represents a good example for clean and efficient value-added utilization of FNS, FAC and other relavent solid wastes.     

Current-ramp assisted sintering of 3YSZ: Electrochemical and microstructural comparison to flash and thermal sintering

Flash sintering features an unoptimized and uncontrolled rise in current density and sample conductivity. By using a controlled current-ramp technique with a predetermined ramp function, microstructure and electrochemical properties can be improved. This current-ramp method is investigated through use of test functions that follow square-root, linear, and parabolic time dependence with comparison to conventional flash sintering and thermal sintering. Steeper ramp functions during the sintering result in higher activation energy, suggesting a change in the vacancy concentration for both the bulk and grain boundary regions. Estimation of the grain boundary domain width suggests a grain size dependence of the unique space charge contribution to conduction independent of sintering method. Contrary to conventional wisdom, flash sintering can actually result in enhanced grain growth compared to controlled current-ramps and conventional sintering, implying that uncontrolled rise in current to a set cutoff may not be the optimal method for densification.     

Effect of titanium diboride on the homogeneity of boron carbide ceramic by flash spark plasma sintering

A novel method, namely flash spark plasma sintering (FSPS), combining flash sintering and electric field assisted sintering, was utilized to densify boron carbide/titanium diboride (B₄C/TiB₂) composites. Further, sintering homogeneity of the composites with different contents of TiB₂ was systematically investigated and theoretical model was built. Results indicated that addition of 50?wt% TiB2 led to the densification of B₄C/TiB₂ composite by up to 97.7% with regional range 1.9% at 1872?°C under pressure of 4?MPa in 60?s. The preferential pathway of TiB₂ network proves to disperse the central current and distribute thermal flow throughout the specimen possibly via tunneling, electronic field emission effect at first stage and lower-resistance composite pathway latter, contributing to the increased homogeneity.     

Grain growth kinetics of 3 mol. % yttria-stabilized zirconia during flash sintering

Grain growth kinetics of dense 3 mol. % yttria-stabilized zirconia (3YSZ) ceramics during both DC flash sintering and conventional annealing were investigated using the grain size as a marker of microstructure evolution. The results indicated faster grain growth under greater current density. In contrast to conventionally annealed specimen, the grain boundary mobility was enhanced by almost two orders of magnitude with the applied electric current, revealing that joule heating alone was not sufficient to account for the experimental results. Instead, activation energy for grain growth decreased significantly due to electro-sintering. Systematic characterization of graded microstructure further indicated that local oxygen vacancies and specimen temperature were responsible for a grain size transition. Based on electrochemical reaction involved in flash sintering, grain size reduction at the cathode was proposed to be attributed to the local rearrangement of lattice cations and generated oxygen ions.     

Effect of atmospheric conditions on the onset electric field of ZnO and Y2O3 ceramics flash sintering at room temperature

Flash sintering of zinc oxide (ZnO) ceramics can be induced at room temperature (25 °C) by electrical breakdown at high electric field strength. However, a strong discharge may degrade the sample. This study investigated the effects of atmospheric pressure and composition on the onset electric field for the flash sintering of ZnO. The experimental results show that flash sintering of ZnO under a low electric field at room temperature can be achieved by adjusting the atmospheric conditions. Compared with the onset electric field strength under normal atmospheric conditions, the value for flash sintering of ZnO at 20 kPa in a mixture of 20% air +80% argon (Ar) can be reduced by 82% to approximately 700 V/cm. This method also applies to yttrium oxide (Y₂O₃) for a low electric field in flash sintering at room temperature, and is the first report on flash sintering of Y₂O₃ at room temperature.     

The microstructural origin of rapid densification in 3YSZ during ultra-fast firing with or without an electric field

Recent research has shown that very rapid heating of 3YSZ powder compacts (ultra-fast firing), whether by passing an electric current through the sample (flash sintering) or by using external heat sources, causes a great acceleration of densification rate for a given relative density and temperature. Here, the microstructural evolution of 3YSZ is studied using four sintering methods with widely differing heating rates, produced with or without electric fields. The microstructural development depended greatly on thermal history. Most significantly, slow, conventional heating resulted in pores much larger than the grain size, whereas most pores were smaller than the grain size with the rapid heating methods, whether the heating involved an electric field or not. The smaller pore size clearly provides a major contribution to the acceleration of densification following rapid heating. In contrast, grain growth was not suppressed by rapid heating but was suppressed by an electric field.     

Evolution of thermal insulation of plasma-sprayed thermal barrier coating systems with exposure to high temperature

The thermal insulation potential of plasma-sprayed yttria-stabilized zirconia thermal barrier coatings is generally assessed via the evaluation of the ceramic layer. However, ageing of the complete system leads to microstructural transformations that may also play a role in the heat transport properties. This study thus investigated the microstructure-heat insulation relationships of different TBC systems in their as-deposited state and when aged under various conditions, through the systematic analysis of both microstructure and thermal diffusivity. The latter was measured from room temperature up to 1100 °C using the laser-flash technique, while the porous microstructure was assessed using image analysis. The different coatings exhibited relatively similar thermal diffusivity values that were shown to be mostly influenced by the thin porosities in contrast to larger defects. The thermal insulation of the TBC systems after exposure to high temperature was shown to be stable despite the microstructural variations introduced by cracks, oxidation and chemical degradations.     

Review of flash sintering: materials,mechanisms and modelling

Flash sintering (FS) is an energy efficient sintering technique involving electrical Joule heating, which allows very rapid densification (<60?s) of particulate materials. Since the first publication on flash-sintered zirconia (3YSZ) in 2010, it has been intensively researched and applied to a wide range of materials. Going back more than a century ago, we have found a close similarity between FS of oxides and Nernst glowers developed in 1897. This review provides a comprehensive overview of FS and is based on a literature survey consisting of 88 papers and seven patents. It correlates processing parameters (i.e. electric field magnitude, current density, waveforms (AC, DC) and frequency, furnace temperature, electrode materials/configuration, externally applied pressure and sintering atmosphere) with microstructures and densification mechanisms. Theorised mechanisms driving the rapid densification are substantiated by modelling work, advanced in situ analysis techniques and by established theories applied to electric current assisted/activated sintering techniques. The possibility of applying FS to a wider range of materials and its implementation in industrial scale processes are discussed.     

Investigation of temperature approximation methods during flash sintering of ZnO

The lattice expansion in ZnO, using in-situ X ray diffraction, has been investigated during flash sintering with varying current densities. While current flow through the specimen enhances the kinetics of sintering for ZnO, the temperature is not high enough to claim thermal runaway or localized melting. Unlike the case of yttria stabilized zirconia [1,2], experimental temperature approximations predict comparable specimen temperature to conventional sintering temperature of ZnO. Microstructural analysis supports the findings of the in-situ temperature approximations. In comparison with black body radiation, a gap between theoretical value and measured value was found due to flaws in the theoretical model. In addition, a new type of flash sintering was introduced, with current ramp, to avoid the power spike which has been the source of much debate about the transition from voltage to current control. The advantage of this method is in the controlled sintering kinetics thus avoiding the channeling found in dielectric materials [3].     

Opacifier embedded and fiber reinforced alumina-based aerogel composites for ultra-high temperature thermal insulation

A novel method was developed to uniformly disperse sub-micron TiO₂ opacifier into fiber reinforcements using agar and silica as binders via freeze drying. TiO₂ opacifier/ fiber/ alumina-based aerogel ternary (TFA) composites with high strength and excellent high-temperature thermal insulation were successfully prepared by sol-gel route, impregnation process and supercritical fluid drying. The microstructure, mechanical and thermal insulation properties of TFA composites were investigated comprehensively. The results show that the mechanical property of TFA composites can be significantly enhanced by mullite fiber felt and the incorporation of SiO₂ binder. The effect of TiO₂ opacifier on the high-temperature thermal conductivity was studied by adjusting the content of TiO₂ from 0 to 15 wt%. The obtained TFA composites exhibit high Young's modulus of up to 3.58 MPa and low high-temperature thermal conductivities of 0.129 W/m·K at 800 °C and 0.168 W/m·K at 1000 °C, respectively. The mechanism of heat transfer in TFA composites at high-temperature was also analyzed.     

Plasma-sprayed nanostructured YSZ thermal barrier coatings: Thermal insulation capability and adhesion strength

Adhesion strength and thermal insulation of nanostructured Yttria Stabilized Zirconia (YSZ) thermal barrier coatings (TBC) were investigated and compared with those of conventional YSZ TBCs. A Nickel based superalloy (IN-738LC) was used as the substrate with NiCrAlY bond coat, and nanostructured and conventional YSZ top coats were applied by using air plasma spray (APS). The adhesion strength of coatings was evaluated according to ASTM C633-01, and their thermal insulation capability was evaluated using a specially designed test setup at an electrical furnace. The results revealed the nanostructured YSZ coating to have a bimodal microstructure consisting of nanosized particles and microcolumnar grains. The bimodal microstructure of nanostructured coatings prevented crack propagation by splat boundaries and unmelted particles, thereby improving the bonding strength. Also, due to the presence of nano-zones in the microstructure of nano TBCs, coatings exhibited superior thermal insulation capability.     

Sintering effect on microstructural evolution and mechanical properties of spark plasma sintered Ti matrix composites reinforced by reduced graphene oxides

Ti matrix composites reinforced with 0.6?wt% reduced graphene oxide (rGO) sheets were fabricated using spark plasma sintering (SPS) technology at different sintering temperatures from 800?°C to 1100?°C. Effects of SPS sintering temperature on microstructural evolution and mechanical properties of rGO/Ti composites were studied. Results showed that with an increase in the sintering temperature, the relative density and densification of the composites were improved. The Ti grains were apparently refined owing to the presence of rGO. The optimum sintering temperature was found to be 1000?°C with a duration of 5?min under a pressure of 45?MPa in vacuum, and the structure of rGO was retained. At the same time, the reaction between Ti matrix and rGO at such high sintering temperatures resulted in uniform distribution of micro/nano TiC particle inside the rGO/Ti composites. The sintered rGO/Ti composites exhibited the best mechanical properties at the sintering temperature of 1000?°C, obtaining the values of micro-hardness, ultimate tensile strength, 0.2% yield strength of 224 HV, 535?MPa and 446?MPa, respectively. These are much higher than the composites sintered at the temperature of 900?°C. The fracture mode of the composites was found to change from a predominate trans-granular mode at low sintering temperatures to a ductile fracture mode with quasi-cleavage at higher temperatures, which is consistent with the theoretical calculations.     

Monodisperse hollow TiO2 spheres for thermal insulation materials: Template-free synthesis,characterization and properties

Hollow TiO₂ spheres were easily fabricated through a template-free solvothermal route only using tetrabutyl titanate (TBT) as raw material and absolute ethanol as solvent. The morphology of hollow TiO₂ spheres was successfully controlled by adjusting the reaction time and reaction temperature. The formation process of hollow TiO₂ spheres includes three steps based on the experimental results: the hydrolysis of TBT to produce Ti(OH)₄ clusters in water formed from an ethanol etherification reaction under high temperature and high pressure, the assembly of Ti(OH)₄ clusters into solid spheres, and the transformation of solid TiO₂ spheres into hollow TiO₂ spheres. Furthermore, the as-prepared hollow TiO₂ spheres exhibited good thermal insulation performance including heat barrier and heat reflection.