Field-assisted Sintering Process Research Articles

Magnetic field-assisted sintering of nickel-doped silver microwire to improve densification and conductivity

In this paper, a doping and field-assisted sintering process is provided to accelerate the densification of silver microwires. Silver microwire doped with a small amount of nickel nanoparticles sintered in a magnetic field can efficiently improve the densification and conductivity of wires. The silver microwire doped with nickel mass fraction of 5 wt% was sintered in a magnetic field with the intensity of 15 mT achieved the maximum shrinkage. Compared with the wire made of pure silver nanoparticles sintered without magnetic field, the shrinkage rate increased by 67.7%, and the resistivity reduced by 15.5%. We reasonably analyzed the wire densification mechanism by scanning the surface and internal structure of the microwire.

Thermal Wave Effect and Sintering Activation Energy at the Initial Stage of Field Assisted Sintering Process for Non-conductive Al2O3 Powders

The effect of thermal wave at the initial stage for non-conductive Al2O3 powders compact in field assisted sintering technique (FAST) was investigated. The Lord and Shulman type generalized thermoselastic theory was introduced to describe the influence of thermal-mechanical interaction, as well as the heat transport and thermal focusing caused by thermal wave propagation. The expression of vacancy concentration difference of the particles was deduced by considering transient thermal stress. Subsequently, the relationship between activation energy and vacancy concentration difference was obtained. The mechanism of surface diffusion, volume diffusion, simultaneous surface and volume diffusion was analyzed. The numerical simulations indicate that low sintering temperature can obtain high local temperature by the superposition effect of thermal wave. Vacancy concentration differences were improved during FAST compared with hot-pressure and pressureless sintering, thereby decreasing the sintering time. By contrast, the activation energy declined with the decrease of vacancy concentration difference in the neck growth process.

Dynamic assessment of direct-current mobility in field-assisted sintered oxide dispersion-strengthened V-4Cr-4Ti alloys

BackgroundVanadium alloy is one of the potential candidate material for structural applications in a commercial fusion reactor. Extended survival of a structural material has a direct consequence on the net energy produced in a fusion reaction, it is important to develop ultra-functional materials with tailored microstructures, to meet the harsh fusion environments. Microstructure of material, indeed depend upon the thermodynamics and kinetics of material processing.MethodsAiming to meet the harsh fusion conditions, we have developed oxide dispersion strengthened V-4Cr-4Ti alloys by high energy ball milling and field assisted sintering technique. Possible microstructural, morphological aftermaths observed in ball milled yttria dispersed V-4Cr-4Ti powders is explored.Results and conclusionElectron microscopy and laser particle analysis acknowledge that yttria addition aids powder agglomeration during ball milling. Ball milled powder was then consolidated (to a relative density of ~100%) using field assisted sintering technique, under optimal sintering conditions. Densification profile has implied that heterogeneous powder characteristic (apparent particle size and shape of powder) tends to impede the direct-current conductivity across the powder particle during various stages of field assisted sintering. In order to understand the kinetics of the field assisted sintering process on the starting powders, a new method was developed to compute the activation energy required for the direct-current conductivity across the individual powder particles. Relatively higher activation energy (for direct-current conductivity) is required for sintering yttria dispersed V-4Cr-4Ti powder than its V-4Cr-4Ti counterpart.Graphical Quantitative dynamic sintering kinetics analysis of FAST processed vanadium alloys

Thermal stability and field assisted sintering of cerium-doped YSZ ceramic nanoparticles obtained via a hydrothermal process

Owing to its extraordinary range of properties, yttria-doped zirconia holds a unique place among the ceramic oxide systems. To improve the properties for some specific custom design applications, co-doping with other rare earth oxides such as ceria is needed. The aim of this paper is to identify the correlations between the phase composition evolution with increasing thermal treatment temperature in order to establish the thermal stability in connection with the ceria content and how does it influence the yttria-stabilised zirconia microstructure. The ZrO 2 –3Y 2 O 3 – n CeO 2 ( n = 3, 6 and 9 wt.%) samples were obtained by a hydrothermal process and submitted to a thermal treatment up to 1600 °C. Intensive characterization was performed via X-ray powder diffraction and EDX analysis. It was found that up to 400 °C, a monophasic structure was formed. At higher temperatures tetragonal zirconia is formed as a major phase with the presence of secondary monoclinic and cubic phases, depending on the Ce content and thermal treatment temperature. Sintered compacts with densities up to 99.5% from the theoretical density were obtained starting from the 6%CeO 2 –3%Y 2 O 3 –ZrO 2 -nanostructured powders using a special field-assisted (FAST) sintering process. With increasing CeO 2 content to 9% only, tetragonal zirconia with 6–9 nm crystallite sizes is formed during the FAST sintering process.

Liquid phase flash sintering in magnesia silicate glass-containing alumina

Magnesia silicate glass-containing alumina was flash sintered using an E-Field in the 500–1500V/cm range. The addition of glass allows to reduce the current needed for densification and improves the shrinkage obtained during field-assisted sintering process. This behaviour is related to the different sintering mechanisms involved in the two materials, i.e. solid state sintering for pure alumina and liquid phase sintering for glass-containing alumina.The estimated activation energy for conduction during flash sintering is compatible with ionic diffusion in silicate melt. Moreover, evidence of magnesium diffusion toward the cathode is recorded. The estimated sample temperature is in almost all cases lower than 1355°C, which is the temperature at which the first liquid is formed in the ternary system MgO-SiO2-Al2O3. Finally, it is shown that the application of an E-Field accounts for efficient liquid phase sintering at temperatures at which it cannot be reproduced conventionally.

Synthesis and electrical field-assisted sintering behaviour of yttria-stabilized tetragonal ZrO2 nanopowders by polyacrylamide gel method

The tetragonal ZrO2 nanopowders stabilized with 3 mol% Y2O3 (3YSZ) were synthesized using a polyacrylamide gel method. The mean particle size of the 3YSZ nanopowders was found to decrease with increasing molecular ratio of monomer to the precursor salt. The 3YSZ nanopowders with mean particle size of 12 nm can be densified in 1 h at 800∘C, by the application of a d.c. electrical field. Under a constant d.c. electrical field, the current density through the specimen of 3YSZ rose rapidly when the temperature increased to a certain value. In the sintering process, the current density was restricted when the sharp increase occurred. By limiting current density to different values for one hour, it was found that current density was the most important factor in electrical field-assisted sintering process. The grain size of 3YSZ bulk increased with the enhanced current density. The stable stage of electrical field-assisted sintering process can be explained by Joule heating. Corresponding real temperature of specimens is estimated by applying black body radiation theory.

Synthesis and electric field-assisted sintering behavior of Al2O3–ZrO2 composite nanopowders by polyacrylamide gel method

Al2O3–ZrO2 composite ceramic nanopowders were synthesized using a polyacrylamide gel method. The mean particle size of the Al2O3–ZrO2 nanopowders was found to decrease with increasing mole ratio of monomer to the precursor salt. The Al2O3–ZrO2 nanopowders with mean particle size of 10 nm can be densified in 1 h at 965 °C, by application of a dc electrical field. Under a constant dc electrical field, the current density through the sample of Al2O3–ZrO2 rose rapidly when the temperature increased to a certain value. In the sintering process, the current density was restricted when a sharp increase occurred. It was found that current density had a great effect on the sintering process in electric field-assisted sintering. Joule heating was proposed to be the main mechanism of the electric field-assisted sintering process. The Al2O3–ZrO2 composite ceramic nanopowders were synthesized using a polyacrylamide gel method. The Al2O3–ZrO2 nanopowders can be densified in 1 h at low temperature, by the application of a dc electrical field. Under a constant dc electrical field, the current density through the sample of Al2O3–ZrO2 rose rapidly when the temperature increased to a certain value.

Formation of secondary phase at grain boundary of flash-sintered BaTiO3.

Recently, Raj et.al. have developed a very unique sintering technique, called flash-sintering [1]. According to their report, fully densified ZrO2-3mol%Y2O3 ceramic bulks were successfully obtained only at 800°C for 5sec. Considering the conventional sintering condition around 1500°C for a few hours necessary to obtain ZrO2-3mol%Y2O3 ceramic bulks, their sintering technique is very attracting from a viewpoint of sintering temperature, soaking time and further the physical phenomena. The flash-sintering is a technique that green compacts were heating under application of high electric field. When furnace temperature reaches at a critical temperature, the electric current abruptly increases and the compact sinters near full density with a very high shrinkage rate. So far, a few studies about flash-sintering were reported for Y2O3 [2], SrTiO3, MgO-Al2O3. To understand the detail mechanism of flash-sintering, more case studies must be necessary. In this study, we focused BaTiO3 widely used for electro-ceramics, which has not been investigated from a viewpoint of flash-sintering.Green compacts were prepared from BaTiO3 raw powders (0.1-m, 99.9%, SAKAI chemical industry Co. Ltd., Lot. No.1308607) after uniaxially pressed at 100MPa into a rectangular shape with 2x10x30mm(3). The green compacts were suspended into a box type furnace by Pt-wires with Pt-based paste. Then, the furnace temperature was raised at 300°C/h under application of electric field ranged from 25V/cm to 350V/cm with monitoring the specimen current. After sintering, the shrinkages, microstructure of the sintered compacts were investigated.Sintering rates at all electric fields were found to be accelerated by applying electric field in BaTiO3. The appearance of abrupt current increment was confirmed over the application of 75V/cm. For example, a density of green compact reached about 90% relative density of BaTiO3 only at 1020°C for 1min at 100V/cm. However, the final shrinkages were revealed to decrease with an increase in electric fields, which is very different from the case of ZrO2-3mol%Y2O3 and Y2O3 ceramics. This fact means that application of high electric fields does not effectively operate for enhancement of shrinkage rates in the case of BaTiO3. In contrast, only gradual current increment was observed at 25V/cm, which is categorized in field-assisted sintering (FAST) process. The density of the green compact at 25V/cm was more than 95%.To investigate the mechanism of the decrease in a total shrinkage with electric fields, the microstructure of flash-sintered compact was observed. As a result, it was found that discharge occurs during flash-sintering process, indicating that the input power due to high electric fields does not work effectively. A typical example of the microstructure near the discharge area is shown in Fig. 1. Fig. 1 is a TEM bright field image taken from BaTiO3 flash-sintered at 100V/cm. As seen in the image, the formation of a secondary phase along the grain boundary can be clearly seen. Diffractometric and EDS analysis have revealed that the secondary phase is BaTi4O9, one of compounds between BaO and TiO2 system. By discharging, grain boundaries partially melt and a part of Ba vaporizes to form BaTi4O9 with cooling. To investigate flash-sintering behaviors, it was concluded that FAST process play an important role to enhance the shrinklage rate in the case of BaTiO3.jmicro;63/suppl_1/i19/DFU048F1F1DFU048F1Fig. 1.TEM bright field image of a secondary phase and the electron diffraction pattern taken from the secondary phase.

Microstructural and micro-mechanical properties of Mo–Si–B alloyed with Y and La

Nanoindentation and atomic force microscope were used to study the mechanical properties of the individual phases of Mo–9Si–8B–0.2Y and Mo–9Si–8B–0.2La (at%) alloys produced by field assisted sintering. SEM, EBSD, and TEM analysis revealed that the alloys were comprised with the three major phases, molybdenum solid solution (Moss), Mo5SiB2 (T2) and Mo3Si (A15), along with the uniform dispersion of Y or La rich fine-sized precipitates. Both the Y and La-doped Mo–Si–B alloys consisted of fine grain (0.25–0.6μm) microstructures, and they possessed a superior oxidation resistance compared to the unalloyed Mo–Si–B at higher temperatures. The A15 phase showed the highest hardness and elastic modulus values of the order of 34.9GPa and 390.4GPa, respectively for Y-alloyed material. The average modulus values for Moss and T2 phases obtained from these materials were found to be higher than those reported for arc melted Mo–Si–B. The load–displacement curves for T2 and A15 phases showed pop-ins, which could be attributed to the presence of dislocation structures formed as a result of compressive loading during the field assisted sintering process.

Study on ZrB<sub>2</sub>-SiC Prepared by Field Assisted Sintering

Zirconium diboride and silicon carbide are thought to have a low intrinsic sinterability due to their strong covalent bonds, low bulk and grain boundary diffusivities. ZrB2-SiC ceramic composites were prepared by a field assisted pressureless sintering process in the present work. The densification behavior and the effect of sintering temperature on microstructure and properties of sintered samples were studied. Pellets were in-situ formed by dry uniaxial pressing in the graphite die at a pressure of 50MPa for 3min and then sintered at a sintering temperature ranged from 1650 °C to 1950 °C with fixed heating rate and holding time. The current, voltage, temperature and displacement data were all collected by the real-time acquisition system. The bulk densities were determined by Archimedes method and the microstructure of samples was characterized by SEM. The onset of some measurable shrinkage of the green body was recorded at around 1400 °C regardless of the sintering temperature and significant shrinkage took place at higher temperature of around 1600 °C. For the sample sintered at 1950 °C, no shrinkage occurred after ~2min holding time. The relative density increased significantly with increasing temperatures and samples could be densified to a relative density of more than 99% at 1950 °C by the field assisted sintering process without obvious grain growth.

A sintered nanoparticle p-n junction observed by a Seebeck microscan

A nanoparticular p-n junction was realized by a field-assisted sintering process, using p-type and n-type doped silicon nanoparticles. A spatially resolved Seebeck microscan showed a broad transition from the positively doped to the negatively doped range. Overshoots on both sides are characteristic for the transition. Despite the tip size being much larger than the mean particle size, information about the dopant distribution between the particles is deduced from modeling the measured data under different assumptions, including the limited spatial resolution of the tip. The best match between measured and modeled data is achieved by the idea of doping compensation, due to the sintering process. Due to a short time at high temperature during the field-assisted sintering process, solid state diffusion is too slow to be solely responsible for the observed compensation of donors and acceptors over a wide range. Therefore, these measurements support a densification mechanism based on (partial) melting and recrystallization.

Development of master sintering curve for field-assisted sintering of HfB2–20SiC

Field assisted sintering (FAST) or spark plasma sintering (SPS) has emerged as a promising technology for densification of ultra high temperature ceramics like HfB2–20SiC at relatively low temperatures and shorter times. In the present study the concepts of master sintering curve (MSC) was applied to model the densification behavior of HfB2–20SiC during FAST process. An activation energy of 300kJ/mol was estimated for field assisted sintering of HfB2–20SiC. The densification curves at various heating rates merged for activation energy of 300kJ/mol confirming the applicability of MSC concepts to FAST process. The developed master sintering curves can be used to design sintering cycles and predict the densification of HfB2–20SiC during FAST process.

Inductive hot-pressing of titanium and titanium alloy powders

Inductive hot-pressing is a field-assisted sintering process (FAST) in which an electrical current is used to enhance the densification of the powder. Inductive hot-pressing could be employed to enable titanium powder to reach a higher density in less time than the pressing and sintering process. In this study, titanium and titanium alloy powders with different features were processed by means of inductive hot-pressing. The influence of processing temperature on density, microstructure, quantity of interstitial elements and hardness was investigated. Generally, practically fully dense materials were obtained without any carbon pick-up, even if the materials were in contact with the graphite matrix during processing. Nevertheless, there was an increment of the nitrogen content and some oxygen pick-up, especially for the powders with smaller particle size. Hardness is not significantly affected by the pressing temperature, but it strongly depends on the amount of interstitials.

Cementite-iron nanocomposite

Nanocomposites Fe-85 vol.% Fe3C were prepared by mechanosynthesis (MS) of elemental Fe and graphite powders. The crystallite size in the MS powder was about 7 nm for Fe3C and about 8 nm for Fe. These powders were densified by a field assisted sintering process. Near full density was achieved by consolidation at 450 °C for 3 minutes in air under a pressure of 63 MPa. A final grain size of 45 nm was retained. This small grain size is responsible for significantly enhanced mechanical properties. Hardness values reach 10.8 GPa. Conversely, physical properties (i.e., magnetic) are not altered by the fine grain size achieved in this cementite-based nanocomposite.

Bandgap enlargement in anodic oxide films on titanium

Thin anodic oxide films on titanium in sulphuric acid show bandgap enlargement of at least 0.2 eV provided that the anodization rate is slow and the formation potential limited. The origins of this effect are the occurrence of quantum-size effects and Burstein shifts. Upon aging of films at their formation potentials higher than 1.5 V, a field-assisted sintering process sets in that induces a relaxation of the bandgap to its nominal value.