Modeling solid-state sintering with externally applied pressure: a geometric force approach

The problems of the Thomson-Freundlich equation in the solid state sintering are pointed out, which has been used as the relationship concerned with the chemical potential difference, surface curvature and vacancy concentration gradient. Based on the view point of the variation in total system energy as the driving force of the solid state sintering, the new concept of the reduction in local surface energy is introduced, and with the new introduced concept of the reduction in local surface energy, the change of surface configuration by surface diffusion is two dimensionally discussed.

Effect of rigid body motion in phase-field models of solid-state sintering

The densification of ceramic compacts of YBa2Cu3O7−x (123) was studied with a vertical dilatometer. The runs effected under isothermal conditions (ISO) covered the 920–970∘C range and were performed under static air atmosphere. Also, controlled heating rate (CHR) runs, from about 800 to 1050∘C, were conducted at 5∘C/min under either flowing oxygen or static air. The ISO data could be satisfactorily fitted by the solution-precipitation (SP) model giving an activation enthalpy of 221 kJ/mol. Furthermore, the CHR data for 920–970∘C was also fitted with the same model giving 207 kJ/mol as the activation energy. From analysis of CHR data, the initial stage sintering is driven by solid state sintering between 827–894∘C (823–908∘C in O2). Then, in the interval 902–920∘C (914–934∘C in O2) the intermediate stage driven by grain growth (GG), competes with the rearrangement process associated to the presence of a liquid phase. This last process applied because the next sintering stage in the range 922–970∘C (938–990∘C in O2 flow) could be fitted by the SP model with an activation enthalpy of 207 kJ/mol (229 kJ/mol in O2). In the range 972–995∘C (990–1014∘C in O2), the solid state (GG) intermediate stage mechanism and/or viscous flow competes with the SP process.

Mechanism of the liquid-phase sintering for Nd:YAG ceramics

Ultrafine and nanoscaled hardmetals show a significant grain growth already before reaching liquid phase sintering temperature. To limit grain growth during this stage of sintering the known grain growth inhibitors, mostly metal carbides like Cr3C2 or VC can be used. To investigate grain growth inhibition and dissolution of these carbides both the solid state sintering without Co (binderless tungsten carbide) and with 10 wt-% Co were investigated in the temperature range between 600°C and 2000°C and 1500°C, respectively. It could be shown that e.g. the Cr3C2 starts to decompose already below 800°C into a Cr-rich and C-rich phase. In both types of material with and without Co this leads to earlier reduction of W-oxides and the formation of Cr-based oxides which alter the chemical processes during sintering and change the grain growth behaviour. By using thermoanalytical methods as well as interrupted sintering experiments and their characterisation by FE-SEM and XRD, the direct influence of Co and grain growth inhibitors regarding the change in the chemical processes happening during sintering was studied.

16 - Sintering

The presented special edition is devoted to materials and technologies used in modern manufacturing and will be helpful to a wide range of engineers and researchers from materials science, processing technologies, machinery and opto- and microelectronics.

Slip cast Y2O3 stabilized Zirconia (8YSZ) ceramics was subjected to Conventional Ramp and Hold (CRH), Rate Controlled (RCS) and Two Stage Sintering (2SS) methodologies. Sintered samples were characterized for their densities and grain size analysis by Scanning Electron Microscopy. The slip cast samples sintered by CRH and 2SS have achieved 98 - 99 % of theoretical densities while RCS samples have exhibited a low density of 97 %. The samples exhibited an average grain size of 2.64 ?m by 2SS sintering in comparison to 8.83 ?m in case of CRH and 3.45 ?m in case of RCS. Controlled pore growth associated with RCS, when compared to CRH methodology is mainly responsible for the relatively smaller grain size observed with RCS. A four fold decrease in grain size i.e. 2.64 ?m observed with the two step sintering can be attributed to the fact that the first heating step to high temperature of 1550?C for a shorter duration closes the porosity without significant grain growth. The second step at 1375?C for a longer period of time imparts densification with limited grain growth.

ABSTRACTSolid electrolyte interface (SEI) layer plays a key role in lithium-ion batteries’ degradation research. However, SEI layer microstructure prediction still needs further investigation, especially the lithium-ion diffusion in SEI layer considering its morphology evolution during the growth of SEI. Due to the unique advantage of avoiding explicitly tracking the interfaces with sharp composition gradients, a phase field model is developed to simulate the SEI formation and its morphology evolution that is regarded as a solidification process. Fick’s law and mass balance are applied to investigate lithium-ion concentration distribution and diffusion coefficients of different SEI layers (i.e., compact and porous SEI layers) predicted by the developed phase field model. The simulation results show lithium-ion diffusion coefficients between 298K and 318K are 1.34-1.87(10-16) m2/s and 1.73-2.18(10-12) m2/s for compact SEI and porous SEI layer, respectively. The developed model has great potential to be extended to three dimensional spaces for SEI layer spatial growth investigation and other interfaces with complex morphology evolution.

Microlens arrays are key elements for light management in optoelectronic devices. The recent advancement in the wearable intelligent electronics has driven the development of flexible microlenses. In this work, we show a controllable and scalable surface-droplet-based strategy to create unconventional flexible polymer microlens arrays. The technique is underpinned by the morphological transition of femtoliter liquid on the surface of a microlens surrounded by a planar area. We found that the droplet liquid wetted the rim of the microlens first and gradually moved upward to the microlens surface with an increase in the liquid volume. The morphology evolution of the droplet is in good agreement with the predication from our simulations based on the interfacial energy minimization under the condition of the pinned boundary. The shape of the droplet on the microlens is well controlled by the droplet volume, aspect ratio of the microlens, and the interfacial energy of the droplets on the microlens. As a result, the obtained structures of one microlens partially covered by a droplet can be produced in arrays over a large scale, serving as templates for fabricating transparent polymer double microlens arrays for improved light emission from the optoelectronic device.

Laplace-Fourier transform solution to the electrochemical kinetics of a symmetric lithium cell affected by interface conformity

With high energy, high brilliance and eminent collimation, synchrotron X-ray imaging enables in situ observation of dendrite growth during the solidification of Sn-Bi alloy under an applied electric field such as direct current (DC) and electric current pulse (ECP). In present study, the application of DC leads to the suppression of dendrite growth and the degeneration of the secondary and higher order dendrites. Due to the induced Joule heat, the sharp dendrite tips are modified to be round or flat. By applying ECP to the sample during the whole solidification process, a significant grain refinement is observed.

The spreading of cobalt, nickel and iron on tungsten carbide and the first stage of hard metal sintering

Two-stage sintering inhibits abnormal grain growth during β to α transformation in SiC

A simple diffuse interface approach for compressible flows around moving solids of arbitrary shape based on a reduced Baer–Nunziato model