Sintering Neck Research Articles

Effect of calcination temperature on the microstructure, composition and properties of nanometer agglomerated 8YSZ powders for plasma spray-physical vapor deposition (PS-PVD) and coatings thereof

Homemade nano-agglomerated powders 8YSZ powders for PS-PVD were prepared by the spray drying, then calcination processes at four different temperatures (500 °C, 700 °C, 900 °C and 1100 °C) were carried out on the spray-dried powders. Checked by laser particle sizer, scanning electron microscope (SEM) and X-ray diffraction (XRD), the physical properties, microstructure and phase constitutions of the calcined powders were investigated. The results show that the size of powders calcined at 500 °C is increased relative to the spray-dried powder, whereas the powders calcined at 700 °C, 900 °C and 1100 °C possess smaller size. The binding force of the primary particles tend to rise with the increase of calcination temperature. When the temperature was up to 900 °C and above, it was found that the sintering neck indicating with strong binding was formed between the primary particles. In parallel, the powders underwent an m-ZrO2 to t-ZrO2 transition as the calcination temperature rose. It is also found that the PS-PVD prepared coatings which were obtained by using the above powders undergo a transformation from a feather-like to a dense laminate structure as the calcination temperature rises. It is noteworthy that the coating obtained by the powders calcined at 700 °C have a special three-layer composite structure of near dense surface layer, columnar intermediate layer and dense sub-layer. The composite structural coating has excellent adhesion and thermal shock resistance, with a bonding strength of 81MPa and no major spalling when water quenched 100 cycles at 1100 °C.

Multi-particle molecular dynamics simulation: shell thickness effects on sintering process of Cu-Ag core-shell nanoparticles

As a substitute for pure silver nanoparticles, Cu-Ag core-shell nanoparticles (CS NPs) have received significant attention in the fields of electronic packaging and conductive inks. Shell thickness is one of the important factors affecting the sintering performance of CS NPs. In this work, we conduct a multi-particle molecular dynamics simulation to investigate the shell thickness effects on the sintering process of Cu-Ag CS NPs at different temperatures. The results show that the less the shell thickness, the lower the potential energy decrease during sintering. This study mainly involves two sintering mechanisms, plastic deformation mechanism, and diffusion mechanism. During the sintering process, the contribution of the plastic deformation mechanism decrease with the layer thickness decreases. As for the diffusion mechanism, the self-diffusion coefficient at 500 K is much lower than that at higher temperatures. However, the slow diffusion can increase the densification by filling the pores generated in the initial stage of sintering. The self-diffusion coefficient of surface atoms increases as the shell thickness decreases. The decrease of shell thickness can produce more lattice mismatches at the core-shell interface, which hinders crystallization during the sintering process. And we found more amorphous Ag atoms at the sintering neck. A higher sintering temperature can induce the diffusion of Cu atom near the core-shell interface, especially for the particle with thinner shell thickness. Some diffused Cu atoms were found in the neck area of sintered CS NP of Ag5Cu3.5 and Ag5Cu4 at 700 K. Moreover, the sintered neck partly composed of Cu atoms was observed in sintered Ag5Cu4 CS NPs at 900 K.

Effect of sintering atmosphere on the microstructure and properties of nano-WC modified copper-iron-based oil-containing bearing

Nano-WC modified copper-iron-based oil bearing was prepared by powder metallurgy. The effect of sintering atmosphere on the microstructure, crushing strength, hardness and oil content of the oil bearing were studied. The results show that under the sintering condition of 865 °C×30min, when the sintering atmosphere is 100%N2, the sintering process stays in the early stage of sintering neck growth, there are a large number of irregular pores in the microstructure. The sintering densification is insufficient, and the sample is in the state of undersintering. When the sintering atmosphere is 100%H2, the dezincification of matrix phase α(Cu) is serious, and there are many long strip microcracks between metal particles. The effective connection between α(Cu) and Fe phase is cut off, so the density of sample is very low. When the sintering atmosphere is 75%H2+25%N2, α(Cu) forms a united whole through sufficient sintering diffusion bonding, and encapsulates the Fe-rich phase. The sintering densification is very sufficient, the pores in the microstructure tend to be circular. The sintered samples have good comprehensive properties, the density and oil content are 6.60 g/cm3 and 18.21vol.%, the crushing strength and hardness are 235.81 MPa and 42.41 HB, respectively, showing the best sintering state.

Closed-loop optimized nanometal sintering method

Abstract Intense pulsed-light sintering is suitable for forming conductive patterns as it exhibits high efficiency and processing speed; however, it has problems such as destructive sinter necks and poor forming stability. To overcome these drawbacks, conductive pattern sintering based on noncontact height and temperature sensor feedback is proposed. This method involves using a dual-camera vision system for extracting the surface features of the scanned surface. In addition, a temperature sensor measures the surface temperature distribution of the sample to evaluate the sintering state. The two sets of data are fused using a variable weight method to comprehensively evaluate the sintered state of the conductive patterns and adjust the parameters of the next sintering operation. This system was utilized for printing and sintering microstrip antennas, and their return loss and patterns were determined experimentally and they were found consistent with those obtained using traditional manufacturing methods. Therefore, the sensor feedback of the intense pulsed-light sintering method improves the pass rate and reduces the duration of the production cycle. Moreover, this is an environmentally friendly method because unlike traditional etching processes, it does not generate large amounts of contaminated solutions.

Process Optimization Under Uncertainty for Improving the Bond Quality of Polymer Filaments in Fused Filament Fabrication

Abstract This paper develops a computational framework to optimize the process parameters such that the bond quality between extruded polymer filaments is maximized in fused filament fabrication (FFF). A transient heat transfer analysis providing an estimate of the temperature profile of the filaments is coupled with a sintering neck growth model to assess the bond quality that occurs at the interfaces between adjacent filaments. Predicting the variability in the FFF process is essential for achieving proactive quality control of the manufactured part; however, the models used to predict the variability are affected by assumptions and approximations. This paper systematically quantifies the uncertainty in the bond quality model prediction due to various sources of uncertainty, both aleatory and epistemic, and includes the uncertainty and the model discrepancy in the process parameter optimization. Variance-based sensitivity analysis based on Sobol’ indices is used to quantify the relative contributions of the different uncertainty sources to the uncertainty in the bond quality. A Gaussian process (GP) surrogate model is constructed to compute and include the model discrepancy within the optimization. Physical experiments are conducted for calibration and validation of the physics model and also for validation of the optimum solution. The results show that the proposed formulation for process parameter optimization under uncertainty results in high bond quality between adjoining filaments of the FFF product.

Recycling of construction and demolition waste to fabricate cost-effective anorthite ceramic membranes for enhanced separation of an oil-in-water emulsion

The recycling of construction and demolition waste to prepare cost-effective anorthite membranes was demonstrated by the in-situ synthesis of anorthite. The effects of sintering temperature and calcium loading on the physical properties (porosity, phase composition, microstructure, pore size, nitrogen permeation, and tortuous factor) of the as-synthesized porous membrane were investigated in detail. With the addition of coal cinder and natural bauxite, the anorthite phase was synthesized at 878 °C. Almost all samples had a high porosity of >35% at 900 °C–1000 °C. With the increasing temperature, the porosity of the samples (except for P2 and P4) decreased gradually to close to zero. The variation trend of pore size was consistent with that of the mass fraction of calcium-containing crystals. This occurred because the growth of these crystals as a sintering neck led to the expansion of the pore structure. The increasing temperature caused a decrease of the tortuous factor and an increase of permeance. Due to the presence of anorthite, the as-synthesized membrane became hydroxylated by the alkali-solution treatment and manifested improved hydrophilicity, resulting in an enhanced separation of the oil-in-water emulsion.

Molecular dynamics simulations of the alloying process of Cu/Au nanoparticles with different sizes

The coalescence of particles extensively exists in the industrial production and nature, which is of great research significance. This paper examined the alloying process of Cu/Au nanoparticles with different sizes by molecular dynamics (MDs) simulations. The coalescence process presents three stages which can be divided by the contact and fusion. The alloying processes of Cu/Au nanoparticles with different sizes had contacted with each other before the heating at 300 K. The Au atoms diffused through the outer area of the sintering neck before the nanoparticles were fused into one particle. The coalescence had become severe after the systems reached the melting temperature. The different systems showed different sintering rate.

Laser sintering technique to fabricate nano-Sn/graphite anode for lithium ion batteries: Microstructures and electrochemical properties

Nano-Sn/Graphite anodes for lithium-ion batteries are successfully fabricated by the method of laser sintering on the copper current collectors. Then microstructures and electrochemical properties of the electrodes are investigated in details. The results show that the microstructure of the electrode is featured with discontinuous pore networks attached to the grain boundaries and nano-Sn particles embedded in the graphite that connect through sintering neck. On the free surface of the electrode, the graphite develops from block-shape into sheets. Meanwhile, the electrode exhibits a firm interface bonding with the current collector benefitting from the formation of the mutual diffusion zone. Being attributed to the microstructure characteristics, the electrode exhibits high capacity, and long cycle life performance compared with those of the electrode before laser sintering, indicating that the laser sintering is an effective method to fabricate anode of lithium-ion batteries.

Fabrication and formation mechanism of vacuum cladding Ni/WC/GO composite fusion coatings

Nickel (Ni)/tungsten carbide (WC)/graphene oxide (GO) (Ni/WC/GO) composite cladding fusion coating was fabricated using vacuum cladding technique. The microstructural appearance characteristics and corresponding composition of the whole composite coating were observed under different temperatures conditions and analyzed through scanning electron microscopy, energy dispersive spectroscopy, and X-ray diffractometer. The forming process and mechanism of composite cladding fusion coating were also investigated systematically. Results show that four zones, namely, approximately 1.5 mm thick composite zone, about 300 μm thick metallurgical fusion transition zone, around 30 μm thick diffusion–fusion zone, and roughly 400 μm thick diffusion–affection zone, are found from the coating surface to the substrate. The main phases of the Ni/WC/GO composite cladding fusion coating are Cr7C3, FeNi3, WC, Cr23C6, Ni3Si, C, Fe7W6, and γ-Ni solid solution. In the composite zone, flocculent complex phase carbides that contain W, Cr, and Ni were formed. The diffusion–fusion zone was mainly composed of Ni-based solid solution and eutectic structure, whereas the diffusion–affection zone was mainly composed of pearlite. The phase size of the composite zone is smaller than that of the transition zone. During the formation of composite cladding fusion coating, the surface melting and fusion of Ni-based alloying particles in a certain thickness zone on the composite prefabricated coating surface were all prior to the transition zone. The formation of composite cladding fusion coating mainly depended on the formation, growth, wetting to WC particles, and fusion of the sintering neck, until to completely fusing of all the sintering necks each other in prefabricated layer to form the composite cladding fusion coating.

Oxide reduction and oxygen removal in water-atomized iron powder: a kinetic study

Reduction of oxides during sintering is a prerequisite for the manufacturing of powder metallurgy steels. Inadequate control of the sintering atmosphere may impede sinter neck formation and cause entrapment and growth of oxides in sinter necks, ultimately deteriorating the mechanical properties of sintered components. In this study, the oxide reduction and oxygen removal in water-atomized iron powder was investigated by means of thermogravimetric analysis in pure hydrogen. Two principal mass loss events were recorded, corresponding to the removal of the surface oxide layer at around 400 °C and reduction of internal and stable oxides in the range 600–1350 °C. The apparent activation energies of these mass loss processes were determined by means of kinetic analyses, giving values around 100 kJ mol−1 and 200–400 kJ mol−1, respectively. The validity of the results was asserted using hematite reference samples which displayed good correlation with the reduction of the surface oxide layer, thereby showing that the powder surfaces are covered by an Fe2O3 oxide. The high-temperature mass loss, with no analogy in the reference samples, is believed to originate from a combination of oxygen removal from internal oxides and stable oxide particulates on the surface. Analysis of the oxide reduction in iron powder compacts show a slightly lower activation energy for the oxide layer reduction, indicating an influence of the compaction step on the initial state of the powder and oxide layer. At the same time, the high-temperature mass loss event was shifted to higher temperatures, which is believed to be caused by the increasingly restricted mass transport of reduction products along the pores in the sintered compact.

Selective Ammonia Detection at Ppb-Level with Si-Doped MoO3 Sensors for Food and Environmental Applications

Introduction Ammonia (NH3) is an important marker for food spoilage[1] as well as a toxic environmental pollutant emitted from concrete walls, fertilizers or refrigeration systems[2]. Therefore, low-cost compact sensors for the detection of hazardous NH3 are urgently needed. Metal-oxide (chemo-resistive) gas sensors are attractive for NH3 detection as they offer simple operation, low power consumption and can be integrated into portable devices.[3] In specific, the α-phase of MoO3 is promising for detection of NH3 down to 50 parts per billion (ppb) at dry conditions. However, due to the high operating temperatures of MoO3 gas sensors (250 to 500 °C), the material must be stabilized by doping or the addition of foreign oxides. Here, we show how tailoring material morphology, phase composition and particle size enables high NH3 sensing performance[4]. Method Pure and Si-doped MoO3 (0 – 20 wt%) nanoparticles were made by flame spray pyrolysis (FSP) and directly deposited onto Al2O3 sensor substrates with interdigitated Pt electrodes[4]. Subsequently, the sensor films were annealed at 450 °C for 5 h in an oven for thermal stabilization. The nanoparticles were analyzed using X-ray diffraction (XRD) patterns, nitrogen adsorption (Brunauer-Emmett-Teller, BET) as well as transmission electron microscopy (TEM) and electron diffraction (ED). The sensing performance was characterized as a function of SiO2 content with synthetic gas mixtures at a sensor temperature of 400 °C on a dynamic mixing setup. Furthermore, the selectivity towards possible interfering molecules of different chemical classes (e.g. acetone, CO and NO) was evaluated. Results and Conclusions The as-prepared, pure MoO3 powder shows fine crystallinity confirmed by transmission electron microscopy (TEM) (Figure 1a) and ED patterns (Figure 1b). By annealing the sensor substrates at 450 °C for 5 h, pure MoO x particles grow to ribbon-like structures (Figure 1d) with large crystallites, as indicated by XRD (triangles) and BET (open circles) and confirmed by bright spots in ED patterns (Figure 1c). Doping with Si reduces particle and crystal growth significantly, decreasing the crystal size from 147 to 65 nm and the particle size from 272 to 83 nm resulting in superior thermal stability. Excess Si (above 3 wt%) is not incorporated into the MoO3 lattice, but forms amorphous SiO2 domains, inhibiting further crystal growth, as subjected by the constant dXRD (Figure 1) and visible by TEM. Due to those structural changes, the sensor response and selectivity depend on the SiO2 doping content.Figure 2 shows the response of pure and Si-doped MoO3 sensors to 1000 ppb NH3 (triangles), acetone (circles), NO (squares) and CO (diamonds) at 400 °C. Increasing the SiO2 content from 0 to 3 wt% almost triples the NH3 response from 0.22 to 0.53 with only minor changes for the other analytes. This results from the reduced sinter neck size of MoO3 due to segregated SiO2 domains which locally narrow the conduction channel leading to an increased electron depletion and change the morphology resulting in a higher surface-to-volume ratio[4]. Therefore, charge carrier mobility and thus film resistance are dominated by surface phenomena increasing the sensor’s sensitivity. Moreover, Si-doping (3 wt%) improved the NH3 selectivity toward other interfering gases achieving a response ratio to acetone (S NH3 / SA ) of 3.6 and to NO of 7.9 and no sensitivity for CO (Figure 2). Above 3 wt% SiO2 content, the film resistance continuously increases (inset in Figure 2) due to the formation of large and inert SiO2 domains. In addition to the enhanced response and selectivity towards NH3, the sensor could clearly detect NH3 down to 400 ppb even in the presence realistic relative humidity (up to 90%)[4]. As a result, this sensor is promising for integration into a portable device for food spoilage detection and environmental monitoring.

Microstructure and mechanical properties of Fe matrix composites reinforced by nickel–chromium double-layer coated ZTAP ceramics

The preparation, microstructure and mechanical properties of ZTA particles reinforced Fe matrix composites (ZTAP/Fe composites) were systematically investigated. The X-ray diffraction (XRD), scanning electron microscopy (SEM) and electron probe microanalysis (EPMA) were used to analyze the microstructure and phase constituents, the hardness (H), Young's modulus (E) and plasticity factor (δA) were determined as well. The nickel-chromium double-layer coated ZTAP of core-shell structure was prepared by electroless plating and multi-arc ion plating technologies. The modified ZTAP is consisted of the α-Al2O3, t-ZrO2, α-Cr and γ-Ni phases. In addition, the compressive strength of ceramic preforms is high due to a compact sintering neck formed among ZTAP during high temperature sintering. The preferred sintering process for ZTA ceramic preforms is determined as 1350 °C + 1 h, in this case the preforms show the remarkable porosity and highest compressive strength which are 27.8% and 35.0 MPa, respectively. The interfacial metallurgical chemical reactions occurred during infiltration casting process, which ensure the excellent metallurgical interfacial bonding of the ZTAP/Fe composites. The interface contains double transition layers, which refer to silicate and CrFe phases, respectively. The H, E and δA of interfacial double transition layers were measured by the nano-indentation method, the values of which are situated between the ZTAP and iron matrix.

The interaction of nanoparticulate Fe2O3 in the sintering process: A molecular dynamics simulation

In this paper, molecular dynamics (MD) simulations have been used to investigate the sintering process of nanoparticulate Fe2O3 at 300 K and 1300 K. The main growth mechanism of the sintering neck is the surface atom diffusion with the diffusion coefficient of O (DO = 0.00417 nm2/ps) and Fe (DFe = 0.00333 nm2/ps), respectively. It is proposed to evaluate the sintering degree by the effective sintering area (Ae) and the effective sintering volume (Ve). The migration mechanism of the atom at different stages is further explored by the atomic displacement vector diagram. The evolution of crystal structure has also been observed by the common neighbor analysis. The crystal structure gradually changed from HCP and BCC structures to amorphous structures with a proportion of 66%.

Molecular dynamics simulation of sintering of Cu and Au nanoparticles

Particle coalescence has wide applications in nature and industry. In this study, molecular dynamics (MDs) simulations were employed to examine the sintering of Cu and Au nanoparticles, as well as two other systems, namely, Cu nanoparticles and Au nanoparticles. The results suggested that, the Au atoms diffused through the outer area of the sintering neck before the nanoparticles were fused into one sphere. The possible reason was that the Au atoms resembled fluid, which could be ascribed to the local thermal energy at the contact area. Typically, the change in energy per atom from 300 K to the contact temperature denoted that less energy was required for the atoms in the pure Cu system to contact with each other than those in the other two systems.

Liquid phase enhanced sintering of porous aluminum for cylindrical Al-acetone heat pipe

In this study, porous aluminum is successfully sintered on the inner surface of a slim aluminum pipe utilizing liquid phase enhanced sintering method, and cylindrical heat pipe is successfully fabricated using acetone as the working fluid. The process mainly includes sintering, acetone filling, vacuuming, secondary degassing and argon welding. The sintering experiments indicate that pure aluminum powders are poorly sintered; however apparent sintering necks between aluminum powders are formed with copper particles added. Moreover, aluminum powders agglomerate and average pore size increases obviously when copper mass fraction reaches 4.5 wt%. Thermal performance test results of Al-acetone heat pipe demonstrate that the temperature nonuniformity is small and the maximum heat transfer capability is 7 W in horizontal orientation. The total thermal resistance of the heat pipe is 0.6 °C/W within the heat load range of 3–7 W.

Sintering neck growth mechanism of Fe nanoparticles: A molecular dynamics simulation

In this paper, the sintering neck growth mechanism of Fe nanoparticles is studied by molecular dynamics method. The accuracy of the EAM potential function is verified. The relationship between the sintering temperature of the nanoparticles and the particle size has been investigated. The sintering temperature of the particles is linearly positively correlated with the size. The whole sintering process can be divided into three stages: the initial sintering, the stable growth and the sudden increase. The atoms are divided into surface atoms and internal atoms depending on the recognition method. By calculating the mean square displacement of the particles and observing the vector displacement of the atoms, the growth mechanisms of the sintering neck at different stages have been analyzed. Similarly, the evolution of crystal structure during sintering is captured by structure analysis. These simulation results firstly reveal the rule of atomic migration and the growth mechanism of the sintering neck.

Structural variation in improving thermal stability of transition alumina with La dopant

Structural variation in alumina directly determines the thermal stability of transition alumina with La dopant at high temperature. In this work, the thermal stability and alumina structure from well-crystallized ultrafine boehmite doped with various La-bearing compounds were studied. La2O3, LaAlO3, and LaAl11O18 could not efficiently inhibit transition alumina changing into α-Al2O3, α-Al2O3 seed benefited α-Al2O3 formation in the presence of La dopant. La(NO3)3 doping into boehmite effectively improved the thermal stability of θ-Al2O3 above 1200 °C. The nano-diameter active spots with low melting point led to the formation of α-Al2O3 with a smooth edge and sintering neck from the blank boehmite, whereas La dopant contributed to rhombus θ-Al2O3 with considerable pores, jagged edge, and wedge at 1200 °C for 2 h. Moreover, the Al and La contents fluctuated asynchronously during calcination, and the atomic ratio of Al/O decreased with the addition of La dopant. These results are attributed to La migration, assembling in active spots and increasing their melting point. Furthermore, relative to nearly 100% AlO6 coordination in α-Al2O3, La dopant significantly increased the AlO4 content above 900 °C. Therefore, La dopant increased the melting point of the active spots, hindered AlO6 formation, and inhibited α-Al2O3 nucleation on the alumina surface, thereby improved the thermal stability of the transition alumina.

Novel sintering indexes to evaluate and correlate the crystal characteristics and compressive strength in magnetite pellets

This paper aims to develop sintering indexes to evaluate and correlate the sintering degree and compressive strength in pellets. Based on image processing techniques by using the watershed algorithm, sintering indexes, such as pellets consolidation index (CI), grain growth index (GI), and grain uniformity index (UI) are proposed for the first time to evaluate the sintering degree during the induration of pellets. The results showed that with the increase of roasting temperature from 0–1300 °C, CI and GI of ore #1 increase from 0 to 0.47 and 0 to 1.72 respectively. In contrast to ore #2, CI and GI increased from 0 to 0.41 and 0 to 1.75, respectively. CI and GI of ore #1 were higher than #2 ore under the same roasting temperature, which indicated that the Fe2O3 particles in #1 ore are easier to form sintering neck than #2. CI, GI and compressive strength of pellets have an obvious linear correlation, and the linear correlation between GI and the compressive strength of pellets is stronger with the R-Square 0.98 for #1 ore and 0.99 for ore #2.

Study on the microwave sintering characteristics of spherical tin-silver alloy powder

In this study, the sintering behavior of a tin–silver alloy powder (Sn–3% Ag) is investigated during microwave melting, and a two-sphere model is used to study the main diffusion mechanism of sintering. The effect of microwave on the sintering process, microstructure and composition distribution of alloy was studied. The sintering neck growth of the tin-silver alloy powder suggests that the diffusion process of alloy components has been realized through the viscous flow mechanism during the sintering stage before melting under microwave irradiation. The rapidly heating and melting of tin-silver alloy powder can be realized by microwave heating, and the heating rate can reaches to 153.4 °C·min−1 at microwave power of 1.2 kW. In addition, microwave also showed a good heating effect on the metal melt. Microwave can promote the homogenization of alloy microstructural, and the precipitated phase of the alloy is refined and the element distribution is more uniform under microwave irradiation compared to the traditional process. These technology provides a new way for the preparation and performance improvement of alloy materials.

Influence of the Compaction Pressure and Sintering Temperature on the Mechanical Properties of Porous Titanium for Biomedical Applications

In the present work, the use of porous titanium is proposed as a solution to the difference in stiffness between the implant and bone tissue, avoiding the bone resorption. Conventional powder metallurgical technique is an industrially established route for fabrication of this type of material. The results are discussed in terms of the influence of compaction pressure and sintering temperature on the porosity (volumetric fraction, size, and morphology) and the quality of the sintering necks. A very good agreement between the predicted values obtained using a simple 2D finite element model, the experimental uniaxial compression behavior, and the analytical model proposed by Nielsen, has been found for both the Young’s modulus and the yield strength. The porous samples obtained by the loose sintering technique and using temperatures between 1000 °C −1100 °C (about 40% of total porosity) are recommended for achieving a suitable biomechanical behavior for cortical bone partial replacement.