Force For Sintering Research Articles

Recyclable Liquid Metal‐Based Circuit on Paper

AbstractPaper electronics is considered very environmentally friendly, but effective recyclability of metallic circuits on paper still needs more improvement. Therefore, liquid metal (Galinstan) circuits based on the reversible conversion from particles to wires are fabricated on paper using mechanical methods, i.e., mechanical sintering and sonication. Line‐width of liquid metal (LM) circuit is kept in the range of 10 µm to ≥0.5 mm by controlling the sintering force. The results demonstrate that LM circuits exhibit high electrical stability during deformation, as resistance changes only ≤4% after the passage of 10 000 folding cycles. Meanwhile, LM particles spread on paper's porous structures, have enhanced the thermal diffusivity of paper and make paper electronics work in a facile temperature when integrated with high density units. More importantly, the reborn circuits exhibit almost identical electrical stability under deformation and thermal characteristic with pristine ones, thus making LM circuits environmentally friendly during their whole life span.

Sintering forces acting among particles during sintering by grain‐boundary/surface diffusion

AbstractThe microstructural evolution during sintering involves the formation and pinch‐off of pore channels. The sintering of 3 particles in 3 dimensions is a simple model for studying the pinch‐off of a pore channel. We simulate the solid‐state sintering of 3 particles by using Brakke's Surface Evolver program, which incorporates coupled grain‐boundary diffusion and surface diffusion. The pinch‐off of pore channel divides the sintering process into 2 stages; the initial stage and the later stage. The contact area has a noncircular shape bounded by both surface triple junction and triple junction in the later stage. A general method is presented to determine the sintering force acting on the noncircular contact. The mechanical approach of densification, where the relative motion of particles is driven by both sintering force and applied force, is applicable not only for the initial stage, but also for the later stage.

Influence of the calcination procedure on the thermoelectric properties of calcium cobaltite Ca3Co4O9

Calcium cobaltite is one of the most promising oxide p-type thermoelectric materials. The solid-state reaction (or calcination, respectively), which is well known for large-scale powder synthesis of functional materials, can also be used for the synthesis of thermoelectric oxides. There are various calcination routines in literature for Ca3Co4O9 powder synthesis, but no systematic study has been done on the influence of calcination procedure on thermoelectric properties. Therefore, the influence of calcination conditions on the Seebeck coefficient and the electrical conductivity was studied by modifying calcination temperature, dwell time, particle size of raw materials and number of calcination cycles. This study shows that elevated temperatures, longer dwell times, or repeated calcinations during powder synthesis do not improve but deteriorate the thermoelectric properties of calcium cobaltite. Diffusion during calcination leads to idiomorphic grain growth, which lowers the driving force for sintering of the calcined powder. A lower driving force for sintering reduces the densification. The electrical conductivity increases linearly with densification. The calcination procedure barely influences the Seebeck coefficient. The calcination procedure has no influence on the phase formation of the sintered specimens.

A novel multiscale silver paste for die bonding on bare copper by low-temperature pressure-free sintering in air

Nanosilver sintering is expected to overcome the limitation of relatively high production cost and become widely available for the die bonding of power electronics. A potential application of nanosilver sintering is bare copper bonding, where replacing substrates with auxiliary silver or other plating that can damage bonding would be advantageous. Here, we introduce a novel multiscale silver paste containing both nanoparticles (20–100nm) and microparticles (1–5μm) for the bonding of high-power chips on a bare copper substrate by pressure-free sintering in air. The energy potential difference generated in the surface force field was critical in the formation of sintering necks between the nano and microparticles, which, together with other microparticles, formed the high-density sintered structure. Despite the development of a copper oxide film, the interfacial bonding was comparable to or higher than the sintering force due to the high surface energy of porous sintered structure and easy diffusion of nanoparticles occurred. A processing temperature of 265°C was considered optimal for bare copper joint (shear strength: 53MPa, transient thermal impedance: 0.132°C/W) considering the trade-off between achieving excellent mechanical and thermal properties while minimizing oxidation.

Characteristics of water-retaining porous ceramics with sandblasting waste

The study focuses on the characteristics of water-retention porous ceramics with sandblasting waste. Recycling of sandblasting waste collected from the surface treatment process in the solar industry. Sand blasting waste is mainly composed of Al2O3, and also contains very small portion of silicon and SiC. Water-retaining porous ceramic (WRPC) samples, comprising waste diatomite and sandblasting waste, with excellent humidity control performance were prepared. The appearance and structural properties of the samples were investigated through analysis of water absorption and compressive strength, X-ray diffraction, and scanning electronic microscopy. Moreover, the water retention performance levels of the samples at various heating temperatures were investigated. The WRPC samples prepared at the heating temperature of 1,270°C meet the Standards for Japan Interlocking Block Pavement Engineering Association (compressive strength>3MPa, water absorption>70%). At high heating temperatures, the quartz of the WRPC samples completely changed to cristobalite, thus affording the WRPC samples favorable chemical properties and heat stability. When the WRPC samples were sintered at high temperatures (1,200–1,270°C), because of the driving force of sintering, the Scanning Electron Microscope (SEM) micrographs exhibited neck growth. The samples featured high compressive strength, low water absorption, and high water-retention performance. The results revealed that the WRPC samples could maintain a good water-retention performance for 10–15h and better than in the foamed glass material (4h). WRPC samples containing 5–20% sandblasting waste exhibited excellent water-retention performance at various heating temperatures.

Interface topology for distinguishing stages of sintering

Sintering is a common process during which nanoparticles and microparticles are bonded, leading to the shrinkage of interstitial pore space. Understanding morphological evolution during sintering is a challenge, because pore structures are elusive and very complex. A topological model of sintering is presented here, providing insight for understanding 3-D microstructures observed by X-ray microtomography. We find that the topological evolution is described by Euler characteristics as a function of relative density. The result is general, and applicable not only to viscous sintering of glasses but also to sintering of crystalline particles. It provides criteria to distinguish the stages of sintering, and the foundations to identify the range of applicability of the methods for determining the thermodynamic driving force of sintering.

Densification and Crystallization in Fe–Based Bulk Amorphous Alloy Spark Plasma Sintered in the Supercooled Liquid Region

Spark plasma sintering of Fe48Cr15Mo14Y2C15B6 bulk amorphous alloy at a range of temperatures in the supercooled liquid region (SLR) and above yielded near fully dense compacts. Upon sintering in the temperature range of 570–630 °C in SLR, large increments in density are observed due to enhanced sintering resulting from drastic reduction in the viscosity of the alloy. Above 630 °C, the absence of sufficient driving force for sintering and stiffening of the amorphous matrix from partial crystallization leads to sluggish densification. Analysis of the temperature profile in the sample and the die reveals that the temperature at the center of the sample is higher than that at the inner wall of the die as recorded by the thermocouple and the difference between the two is estimated to be between 31 and 53 °C.

Effect of magnetic CoFe2O4 component on sintering densification process of Bi3.15Nd0.85Ti3O12 ceramics

Sintering densification processes of the composite ceramics (Bi3.15Nd0.85Ti3O12 (BNdT)/CoFe2O4 (CFO)) have been investigated using dilatometric experiments combining with the TG-DTA, density measurements and microstructure studies. Microstructures analyses and quantitative calculations show that the composite ceramics achieve densification at low temperatures (<1150°C). The formation of coherent-lattice interfaces between (200)/(020)BNdT and (310)CFO are considered to play an important role on such densification. The intrinsic preferred orientation of BNdT grains is suppressed by CFO phase because of this coherent relationship. Although the sintering activation energies of 0.8BNdT-0.2CFO are about 2.7 times larger than those of pure BNdT due to the pinning effect, the composite ceramic could still be densified, indicating the formation energy of coherent-lattices provided the extra sintering force. The even coercive electric fields of the resulting pure BNdT and 0.8BNdT-0.2CFO ceramic are approximately 89 and 97kV/cm, respectively, at 250kV/cm. The polarization of 0.8BNdT-0.2CFO reaches saturation around 430kV/cm.

Computational homogenization of liquid‐phase sintering based on a mixed variational format

AbstractIn this paper a mixed velocity‐pressure variational formulation is adopted for the subscale modeling of sintering stemming from micropores with surface tension. The macroscopic response is obtained from variationally consistent homogenization. As to the driving force for sintering, the model framework allows for a non‐spherical sintering stress, derived via the homogenization procedure, which contrasts traditional macroscopic modeling. The proposed method can seamlessly handle the transition from macroscopically compressible to incompressible response.For the Representative Volume Elements (RVEs) the weakly periodic, Neumann, and Dirichlet type boundary conditions are established, and it is shown that the Hill‐Mandel condition is satisfied. The numerical examples show the transient behavior of a 2D unit‐cell subjected to free sintering. Effective macroscale properties pertinent to different boundary conditions are compared for a 3D microstructure. (© 2016 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Synthesis mechanism and microwave dielectric properties of Co0.5Ti0.5NbO4 ceramics

Co0.5Ti0.5NbO4 (CTN) microwave dielectric ceramics with low loss were prepared by the conventional solid-state method. The process of phase formation was investigated by X-ray diffraction, it was found that the structure transformed from orthogonal phase to tetragonal phase along with the increasing of temperature from 800 to 1080 °C. Only when temperature was over 1080 °C, the pure CTN phase could be synthesized. However, high calcining temperature would lead to inadequate driving force of sintering. The influences of calcining temperature on the microstructure, densities and microwave dielectric properties were discussed systematically. Calcining at 800 °C for 3 h could achieve optimal microwave dielectric properties in CTN ceramics, permittivity for 64.7, quality factor (\({\text{Q}} \times f\)) for 12,141 GHz and the temperature coefficient of resonant frequency (τf) for 202 ppm/°C. The lower sintering temperature and excellent microwave dielectric properties would make CTN ceramics become a potential material for microwave applications via tailoring high τf.

Sintering and Nanostability: The Thermodynamic Perspective

Sintering and nanostability (defined as the stability against sintering) are critical phenomena present in the processing and application of nanoparticles. With important implications in obtaining high‐quality dense ceramics with fine grains or in enabling high surface areas in nanoparticles for catalytic applications, the control of these interrelated phenomena has been the focuses of several studies. From a thermodynamic perspective, it is recognized that surface energy is a fundamental parameter in both cases, since it is the main driving force for sintering and also the reason that nanoparticles are thermodynamically unstable and have the tendency to coarsen at elevated temperatures. The role of grain‐boundary energies is less recognized as relevant, but is also connected to densification, grain growth, and nanoparticle stability. In this paper, we review the critical aspects of the role of interfacial energies in the microstructure evolution, in particular addressing them as parameters to allow better control in addition to more conventional kinetic parameters. The concept is based on the nonsingularity of interfacial energies in a given system, which varies with temperature, atmosphere, and most importantly, chemical composition—this last offering a method to induce particular microstructural evolutions. While the model assumes isotropic grain boundaries but consequences to anisotropy are also discussed. The paper presents examples of the role of dopants on interfacial energies, how this is quantitatively related to their segregation at the interfaces, and the impact in sintering and nanostability. Given the importance of interface energetics to these phenomena, we also present a short review on the current methods used to obtain reliable interface thermodynamic data.

Sintering force behind the viscous sintering of two particles

The mechanical principles of viscous sintering were analyzed by finite element simulation of a simple model: the coalescence of two identical spheres. The sintering force in the non-equilibrium process of viscous sintering was defined as the difference between the average pressure on the contact area and the surface tension along its circumference. The average strain rate on the contact plane was proportional to the sintering force, so that the sintering force was the thermodynamic driving force for both neck growth and shrinkage. Conversely, a theoretical method was proposed to elicit the sintering force from neck growth curves. The simulation also shows that the tensor–virial equation, which is an alternative method to describe the overall anisotropic deformation of aggregates of particles, is valid for viscous sintering.

Sintering of Lead-Free Piezoelectric Sodium Potassium Niobate Ceramics.

The potassium sodium niobate, K0.5Na0.5NbO3, solid solution (KNN) is considered as one of the most promising, environment-friendly, lead-free candidates to replace highly efficient, lead-based piezoelectrics. Since the first reports of KNN, it has been recognized that obtaining phase-pure materials with a high density and a uniform, fine-grained microstructure is a major challenge. For this reason the present paper reviews the different methods for consolidating KNN ceramics. The difficulties involved in the solid-state synthesis of KNN powder, i.e., obtaining phase purity, the stoichiometry of the perovskite phase, and the chemical homogeneity, are discussed. The solid-state sintering of stoichiometric KNN is characterized by poor densification and an extremely narrow sintering-temperature range, which is close to the solidus temperature. A study of the initial sintering stage revealed that coarsening of the microstructure without densification contributes to a reduction of the driving force for sintering. The influences of the (K + Na)/Nb molar ratio, the presence of a liquid phase, chemical modifications (doping, complex solid solutions) and different atmospheres (i.e., defect chemistry) on the sintering are discussed. Special sintering techniques, such as pressure-assisted sintering and spark-plasma sintering, can be effective methods for enhancing the density of KNN ceramics. The sintering behavior of KNN is compared to that of a representative piezoelectric lead zirconate titanate (PZT).

Capillary equilibrium and sintering kinetics in dispersed media and catalysts

The evolution of an aggregate of particles embedded in a fluid phase, no matter whether a liquid, a vapor, or a mixture of both, is determined by the dependence of the equilibrium interface area on porosity volume fraction. In system with open porosity, this equilibrium can be analyzed using a model representing the particles as a collection of cones of revolution, the number of which is the average particle coordination number. The accuracy of the model has been assessed using in situ X-ray microtomography. The model makes possible the computation of the driving force for sintering, commonly called sintering stress. It allows the mapping of the domains of relative density, coordination number, and dihedral angle that bring about aggregate densification or expansion. The contribution of liquid/vapor interfaces is enlightened, as well as the dependence of the equilibrium fluid phase distribution on particle size. Applied to foams and emulsions, the model provides insight into the relationship between osmotic pressure and coordination. Interface-governed transport mechanisms are considered dominant in the macroscopic viscosity. Both sintering stress and viscosity parameters strongly depend on particle size. The capacity of modeling the simultaneous particle growth is thus essential. The analysis highlights the microstructural parameters and material properties needed for kinetics simulation.

Highly Stable Bimetallic AuIr/TiO₂ Catalyst: Physical Origins of the Intrinsic High Stability against Sintering.

It has been a long-lived research topic in the field of heterogeneous catalysts to find a way of stabilizing supported gold catalyst against sintering. Herein, we report highly stable AuIr bimetallic nanoparticles on TiO2 synthesized by sequential deposition-precipitation. To reveal the physical origin of the high stability of AuIr/TiO2, we used aberration-corrected scanning transmission electron microscopy (STEM), STEM-tomography, and density functional theory (DFT) calculations. Three-dimensional structures of AuIr/TiO2 obtained by STEM-tomography indicate that AuIr nanoparticles on TiO2 have intrinsically lower free energy and less driving force for sintering than Au nanoparticles. DFT calculations on segregation behavior of AuIr slabs on TiO2 showed that the presence of Ir near the TiO2 surface increases the adhesion energy of the bimetallic slabs to the TiO2 and the attractive interactions between Ir and TiO2 lead to higher stability of AuIr nanoparticles as compared to Au nanoparticles.

Apparent Activation Energy in High Frequency Microwave Sintering of Alumina Ceramic

The observed enhancements in the high frequency microwave processing alumina compared to those in conventional method at the same sintering temperatures have been studied. Some possible mechanisms responsible for the enhancements were investigated and are discussed in this paper. The enhancements could be associated with the increase either in driving force or in apparent activation energy of diffusion. An experiment to estimate the driving force of sintering is not easy to perform because the force depends not only on the density and grain size but also on the electric field in the materials. Therefore, a series of experiments to estimate the apparent activation energy were performed in high frequencies microwave and in conventional sintering method, as well. It was observed that the apparent activation energy in conventional sintering was much lower than that of microwave sintering. These activation energy values suggested that diffusion rate in microwaves sintering was faster, which led to the higher densification.

Flash Microwave Sintering of Transparent Yb:(LaY)2O3 Ceramics

Yb:(LaY)2O3 ceramic samples have been sintered to almost full density in a fast microwave heating process with zero hold time. Rapid densification is observed at heating rates ranging from 50°C to 2400°C/min. The estimated value of the power absorbed in the materials per unit volume is from 10 to 400 W/cm3, which is similar to the processes of flash sintering under an applied dc/ac voltage. The microstructure of the sintered samples exhibits traces of a liquid‐like intergranular phase. The observed flash microwave sintering effect is associated with both preferential microwave absorption and enhanced mass transport within the softened grain boundaries arising due to an elevated concentration of defects and impurities therein. The volumetric nature of microwave heating gives rise to thermal stresses which can act as an additional driving force for sintering. The advantage of the microwave flash sintering process is that no electrodes are needed to supply the power to the articles undergoing sintering.

Sintering force behind shape evolution by viscous flow

A non-spherical amorphous/glass particle relaxes to its equilibrium shape by viscous flow driven by capillarity at elevated temperatures. The shape evolution of an axisymmetric ellipsoidal particle, which is a model of viscous sintering of two spheres in the later stage, was simulated by the finite-element method. A force inside the particle is defined on a plane which cuts the center and perpendicular to the semi-major axis. It is given by the surface tension along the circumference and the average pressure on the plane. The velocity field near the center of the particle is a pure straining motion. The simulation showed that the radial growth rate was proportional to a stress which is given by the force divided by the cross-sectional area. This result suggests that the force can be regarded as the sintering force. Conversely, the force can be determined from the experimental observation of the length of semi-minor axis.

Role of Precursor Reactivity in Crystallization of Solution-Processed Semiconductors: The Case of Cu2ZnSnS4

We study the formation of Cu2ZnSnS4 (CZTS) films from various liquid-phase precursors. Our experimental data point to the significant role that reactivities of precursor components play in the quality of the final material. Although reactive molecular precursors favor formation of CZTS under milder conditions, the formation of large crystalline domains requires using less reactive nanostructured precursors. We explain this effect using kinetics of nucleation and growth. We have also demonstrated a strategy to effectively enhance grain growth of CZTS using solid-state phase transition as the driving force for nanocrystal sintering. We hope this contribution will provide a useful guide toward the rational design of liquid-phase precursors for inorganic semiconductors for electronic and optoelectronic applications.

Driving force for low temperature sintering of interstitial and intermetallic powders and induced applications

Intermetallic as well as (carbides and nitrides) interstitial compounds present functional and structural properties which make these materials necessary for advanced technologies. Meanwhile fabrication routes based on melting and casting or plastic deformation, are usually far to compete with powder metallurgy processes likely to provide near net shaped parts and components. The present study is devoted to a model system exemplified by nitrided iron and steel powders for which the thermal stability of nitrides is severely decreasing when temperature exceeds critical values during densification treatments. Thanks to the analysis of the thermal treatment induced transformations mainly characterised by Mössbauer spectroscopy, the consequences and impact on driving forces for densification under critical thermal conditions are discussed in order to achieve an optimised sintering process and an actual development.