Elimination Of Pores Research Articles

Densification and microstructure evolution of reactively sintered transparent spinel ceramics

Reactive sintering is an effective and simple method to prepare transparent spinel ceramics. In this research, transparent MgO·nAl2O3 (0.98 ≤ n ≤ 2) spinel ceramics were prepared via reactive sintering in air followed by hot isostatic press (HIP), using MgO and γ-Al2O3 powders as raw materials. The influence of composition on densification and microstructure evolution was systemically investigated. More importantly, the relationship between microstructure of presintered samples and final properties of transparent ceramics was singled out. Thermodynamically stable large pores were easily generated in magnesia-rich and stoichiometric samples after presintering in air, causing severe abnormal grain growth during the HIP treatment and poor optical quality of the resulting samples. The presintering temperature of alumina-rich samples widely varied with composition. No large pores were observed in the presintered sample, which was beneficial for the elimination of residual pores in the following HIP process. Highly transparent spinel ceramics with n = 1.1 and 1.3 were successfully fabricated with the transmittance above 84% even at the short wavelength of 400 nm, close to the theoretical value.

AC Electric Field Assisted Pressureless Sintering Zirconia: 3 mol% Yttria Solid Electrolyte

3 mol% yttria ionic conductors are sintered by applying AC electric fields with frequencies in the 0.5–1.0 kHz range at 1100 °C. The sintering experiments are conducted in pressed pellets positioned between platinum electrodes inside a dilatometer furnace. The dilatometer is modified in order to allow for the simultaneous monitoring of thickness shrinkage, electric voltage, and current across the pellet. The results show that the higher the frequency of the electric field, the higher the attained shrinkage and the apparent density of the pellets. Increasing the frequency of the applied electric field leads to an increase in the Joule heating promoted by the electric current pulse through the polycrystalline ceramic sample. A higher frequency therefore leads to higher amount of thermal energy delivered to the sample, favoring enhanced densification. The ionic resistivity decreases in pellets sintered with increasing frequency of the applied electric field. We suggest that Joule heating favors pore elimination and the removal of chemical species at the space charge region, inhibiting the blocking of oxide ions at the interfaces.

Feasibility of Using Diffusion Bonding for Producing Hybrid Printed Circuit Heat Exchangers for Nuclear Energy Applications

Hybrid printed circuit heat exchangers have been designed for Gen-IV nuclear power applications. While microchannel devices are commonly fabricated using diffusion bonding, diffusion bonding of these hybrid heat exchangers is challenging due to non-uniform stress distributions across its structure during bonding, which can lead to bonded regions with varied joint properties that can compromise the performance and safety of these devices. In this paper, we delineate a method for determining whether diffusion bonding conditions exist for bonding hybrid printed circuit heat exchangers with maximum joint strength, minimum creep and without yielding or buckling. Diffusion bonding feasibility is determined in two steps. First, an improved pore elimination model is used to identify a feasible set of diffusion bonding parameters that can give acceptable bonded area and compressive creep. Second, a structural analysis is performed on regions in the structure susceptible to yielding and buckling that can constraint the maximum permissible bonding pressure. Diffusion bonding conditions were experimentally validated for bond strength, porosity and creep based on shear test and metallographic examinations as per ASME standards. Two sets of bonding conditions were found to satisfy nuclear boiling and pressure vessel code requirements. Future efforts will involve the diffusion bonding of a hybrid device structure capable of meeting the hermeticity and dimensional integrity requirements of the device.

Laser clad NiCrBSi alloy wear-resistance coating with RE addition on heavy duty spur gear flank

In this research the wear-resistance composite coating successfully produced on heavy duty gear work surface by laser was reported. The coating containing 99 wt.% NiCrBSi alloy and 1 wt.% RE (rare earth element) oxidation powder. The RE addition coupled with laser operating parameters optimization caused elimination of both cracks and pores meanwhile further enhanced comprehensive properties of the laser layer. The coating microhardness, microstructure, phase construction and wear behaviors were tested by hardness tester, SEM equipped with EDS, XRD and tribometer, respectively. The results reflected the fact that the RE addition enhanced the coating ability of wear resistance and laser clad layer properly bonded with the gear flank. The wear volume loss rate of coating was half of that of the gear flank metal the COF curve of coating kept bellow that of the gear flank steel.

Effect of ZnO on the microstructure and dielectric properties of BaTiO3 ceramic coatings prepared by plasma spraying

Three contents of zinc oxide (2 wt %, 4 wt % and 6 wt %) were added into raw BaTiO3 powders. The maximum relative permittivity value (εr) is in the range 230–406 and the dielectric loss (tanδ) varies between 0.18 and 0.09 for all ZnO modified BaTiO3 coatings for frequency1 kHz at ambient temperature. The reasons for enhancing the relative permittivity are attributed to the elimination of micro-cracks and pores. The detail discussion was conducted on the characteristics morphologies (cross section, fracture surface and spreading behavior of BaTiO3+xwt. % ZnO composite powders) and phase. With the analysis, the addition of ZnO leads to the improvement in the melting state and increment in amorphous phase. Zn existed in the form of secondary phase (ZnO) and substitution in the BaTiO3 lattice. The enhancement in structure and dielectric performance is highly discussed based on the phase equilibria and chemical defect.

The elimination of pores in laser welds of AISI 304 plate using different shielding gases

A laser power of 10kW was used to weld 304 stainless steel. Different shielding gases including Ar, N2 and no gas were used. The porosity in the laser welds was examined. The metallic plume, liquid melt pool and laser keyhole were observed by a high-speed video camera. Bubbles were produced in all the laser welds. Many pores were retained in the 304L stainless steel laser welds made in Ar. Almost no pores were found in the laser welds made in N2 or no shielding gas. The dissolution of N2 bubbles in the liquid melt pool led to the elimination of the pores in the 304L laser welds. The retention time of liquid melt pool was 1.1s at a laser power of 10kW and a welding speed of 0.8m/min. It took 30μs to absorb all the N2 bubbles in the liquid melt pool. The increase of the nitrogen content in weld metal was 3.4×10−5wt.%.

Fabrication of highly transparent AlON ceramics by hot isostatic pressing post-treatment

Highly transparent aluminum oxynitride (AlON) ceramics were fabricated by pressureless sintering with hot isostatic pressing (HIP) post-treatment. The experimental results showed that the optical transparency of AlON ceramics was improved markedly over the visible and near-infrared range by HIP at 1825°C for 3h in 200MPa argon gas, which derived from the elimination of residual pores in the prepared ceramics. For AlON ceramics pre-sintered at 1800°C, the transmittances of the sample increased from 63.6% to 84.8% at 600nm and from 75.4% to 86.1% at 2000nm, respectively. The average grain size of the HIPed sample was about 47.9μm.

Preparation of carbon nanotube film with high alignment and elevated density

Carbon nanotube (CNT) film with high alignment and elevated densities was prepared by taking CNT ribbons as component unit fabricated by floating catalyst chemical vapor deposition (FCCVD) method. The as prepared film showed good alignment but low density, however, the density of the CNT film increased by 109% after the rolling treatment due to elimination of pores and gaps in the films. The alignment measured by the ratio IG∥/IG⊥ obtained from polarized Raman scattering measurements increased from 1.44 to 1.90. The tensile strengths of the CNT film were recorded respectively as 765 ± 15 MPa and 184 ± 58 MPa in parallel and perpendicular directions to CNT ribbons alignment, and the corresponding electrical conductivities were (1.65 ± 0.15) × 105 S/m and (1.04 ± 0.10) × 105 S/m. These findings indicated that combination of FCCVD with the rolling treatment led to highly aligned CNT film with elevated densities, and thus improved tensile strengths and electrical conductivities.

Effects of calcination atmosphere on monodispersed spherical particles for highly optical transparent yttria ceramics

AbstractHighly sinterable powders are required for the fabrication of transparent ceramics. Here, we studied the effects of calcination atmosphere on the characteristics of monodispersed spherical Y2O3 powders, such as crystallite size and particle density, for high optical transparent ceramics. It was found that vacuum calcination around the crystallization temperature is the crucial step to eliminate intragranular pores in the spherical particle. The fast decomposition rate in a vacuum creates smaller crystallites, and the following higher calcination temperature results in the enhancement of pore elimination. The in‐line transmittance of the transparent Y2O3 ceramics, vacuum sintered at 1750°C, was improved by increasing the particle density of the as‐calcined powders. This result indicates that the high‐density starting particles effectively enhance the pore elimination during the fabrication of transparent Y2O3 ceramics.

Effect of Cathodic Polarization on the La0.6Sr0.4Co0.2Fe0.8O3-δ-Cathode/Gd-Doped Ceria-Interlayer/YSZ Electrolyte Interfaces of Solid Oxide Fuel Cells

Evaluation of the interfaces is critical for improved long-term stability and performance of solid oxide fuel cells (SOFC) under required operating conditions. In this work, cells consisting of LSCF-cathode|GDC-interlayer|8YSZ-electrolyte|Pt-anode were tested under open-circuit voltage (OCV) and cathodic polarization at a constant current of −0.3 A at 800°C for 300 h in air. Dense GDC films by pulsed laser deposition (PLD) were used as reaction barrier layers. The results showed that after cell tests, SrZrO3 formed at the LSCF/GDC interface in consequence of the fast Zr migration in the PLD-grown GDC interlayers. However, SrZrO3 formation and GDC microstructural feature differ between OCV and cathodic polarization. Under OCV, SrZrO3 laterally formed at the O2/LSCF/GDC triple phase boundary with GDC interlayer showing increased porosity near the YSZ electrolyte. Both of these led to a drastic increase in polarization resistance with time. Under cathodic polarization, localized transverse growth of SrZrO3 was observed, and an apparent pore elimination occurred in GDC. Cells also showed no performance degradation under cathodic polarization. Based on the above observation, a plausible cation diffusion behavior and oxygen transport is discussed.

Zirconia nano-powder – a useful precursor to dense polycrystals

The main problem of utilization of ceramic nanopowders consists in their tendency to form hard agglomerates. The inter-agglomerate space forms pores surrounded by numerous elementary particles (crystallites). Such pores cannot be easily removed from the system during pressureless sintering. So, the obvious strategy for a technologist is to eliminate such pores at the level of shaping operations. Dry pressing is a frequently applied shaping technique of ceramic materials. That is why mechanical strength of agglomerates should be as low as possible. It enables the elimination of inter-agglomerate pores to occur under moderate pressures, applied during the shaping process. This problem will be illustrated using the really nanometric (below 10nm) zirconia-yttria solid solution powder. The methods of effecting agglomerate strength will be shown. One of the elaborated powder preparation rout results in extremely soft agglomerates.

Elimination of pores in Ag–Sn TLP bonds by the introduction of dissimilar intermetallic phases

Low-temperature transient liquid phase (TLP) bonding is a very promising technology for achieving die attachment in high-temperature power devices. However, the Ag–Sn TLP-bonded joint has high sensitivity to shrinkage pores, which will lead to the deterioration of mechanical, thermal, and electrical properties. In this study, we have proposed two novel methods to solve the problem of pores in Ag–Sn TLP bonding through the introduction of second phases. The first method is replacing Ag substrate on one side with Cu substrate to create dual layers of dissimilar intermetallic compounds (IMCs). Consequently, the dual layers of Cu6Sn5 (or Cu3Sn) and Ag3Sn IMCs emerge on the cross-sections of bonded joint, resulting in the effective elimination of pores, which can be attributed to the change of the microstructure and the interface migration between two dissimilar IMC layers (e.g., Cu6Sn5/Ag3Sn). The second method is coating a thin Cu film on the Ag substrate to introduce Cu–Sn IMC particles. As a result, a great number of Cu6Sn5 particles disperse in the middle of the Ag3Sn layer, filling the micropores efficiently, such that a significant decrease in shrinkage pores is achieved. Both methods have been experimentally verified to improve the mechanical properties, and they have high potential to be implemented in other TLP systems.

INFLUENCE OF SINTERING TEMPERATURE ON TRANSLUCENCY OF YTTRIA-STABILIZED ZIRCONIA FOR DENTAL CROWN APPLICATIONS

This study aims to investigate the effect of sintering temperature on the translucency of yttria-stabilized zirconia (YSZ) for dental crown applications. YSZ suspension was treated by colloidal processing and 24 h of sedimentation to eliminate agglomerates and aggregates. The green bodies of YSZ were then shaped into pellets through slip casting. These bodies were sintered into a final shape at 1450 °C–1650 °C. The densities of the specimens were measured using Archimedes method. Light transmission of the YSZ specimen was also evaluated using a spectrophotometer with an integrating sphere. Morphological analysis was conducted with field-emission scanning electron microscopy. Results showed that sintering temperature significantly influenced the density, light transmission, and microstructure of YSZ. High sintering temperatures produced YSZ with a compact and homogeneous microstructure and a high density. Furthermore, the low light scattering effect on the porosity-free microstructure yielded light transmission as high as 37% in YSZ sintered at 1650 °C. The optimal sintering temperature was found to be 1600 °C, at which 34% light transmission was generated. In conclusion, high sintering temperatures improved the translucency of YSZ. This effect was attributed to effective densification of grains and elimination of pores at high temperatures, thereby alleviating the light scattering effect of the pores. At the optimal temperature, YSZ with high density and translucency and a compact microstructure was formed

Experimental and numerical simulation study of porosity on high-pressure aluminum die casting process

The resulting porosity is responsible to 70% of failures on the high-pressure aluminum die casting process. The determination of the origin and setting the pore elimination is a complex mechanism. There are several factors that induce their occurrence. Thus, it is common to employ process and engineering alternatives to try to solve the issue. Faced with such complexity we tried to understand how to apply the finite element methods to minimize the occurrence of pores in high-pressure die casting products. The objective of this study was to develop a methodology to generate an equation that represents the porosity behavior aiming to determine the best engineering and process settings to reduce the pore volume in aluminum injected products. The aim of this study was to develop a methodology to generate an equation that represents the porosity behavior. To do that, the results obtained with the variation of some boundary conditions which were applied to computer simulations in commercial dedicated software were analyzed. It was observed that a flow and solidification analysis of the product in the mold can determine the probability of occurrence of pores in the product already during injection process.

Electric field assisted sintering of electroceramics and in situ analysis by impedance spectroscopy

Since the first report several years ago on pressureless sintering of yttria-stabilized zirconia with several seconds at relatively low temperatures by application of an electric field, an increasing number of scientific reports have been published on sintering ion conducting, semiconducting and insulating polycrystalline electroceramics. The electric field-assisted sintering consists in applying an electric field either during heating up or under isothermal conditions at temperatures well below the ones applied during conventional sintering. Besides the lower temperatures, shorter times are required to achieve full densification without considerable grain growth, evidencing the potential of this technique for obtaining functional electroceramics with improved mechanical and electrical properties at lower costs. Even though some mechanisms have been suggested, the description of the phenomenon at the microscopic level leading to full densification with seconds remains a challenge. We report in situ electrochemical impedance spectroscopy of an ionic conductor (ZrO2: 8 mol% Y2O3), a proton conductor (BaCe0.8Zr0.1Y0.1O3-δ) and a semiconductor (SnO2: 0.5 mol% MnO2) performed during conventional and electric field-assisted sintering experiments. Attention is also directed on the description of the experimental setups and procedures and to the evaluation of microstructural details by scanning electron microscopy. The analysis of the in situ impedance spectroscopy diagrams under heating (before either conventional or electric field-assisted sintering) and under cooling (after) provides evidence of densification with pore elimination and welding of grains. Prospects for future experimental and simulation research work are outlined.

Effect of the long-term heat treatment on the cyclic oxidation behavior of Fe-based amorphous/nanocrystalline coatings prepared by high-velocity arc spray process

Abstract In this study, Fe-based nanocrystalline coatings were obtained by crystallization of amorphous coatings through heat treatment with different annealing time of 0.5 h, 5 h, 10 h and 100 h, respectively. Cyclic oxidation behaviours of the Fe-based amorphous coating as well as these annealed ones at 750 °C in still air were investigated and compared. The results showed that the grain size of the annealed Fe-based coatings was significantly increased as the pre-annealing time prolonged. However, the cyclic oxidation kinetic curves indicated that the cyclic oxidation resistance of the annealed Fe-based coatings was unexpectedly improved as the increase in grain size. The coating, which was annealed at 700 °C for 100 h, exhibited the lowest accumulated weight gain of 2.8 mg/cm2 and parabolic rate constant kp values of 1.7 × 10− 11 g2 cm− 4 s− 1, indicating the highest oxidation resistance. The enhanced oxidation resistance of the annealed coatings was mainly attributed to the elimination of cracks, pores and splat boundaries caused by the intersplat sintering during heat treatment. Cracks and splat boundaries were the primary diffusion paths for the oxygen, resulted in the different diffusion models of oxygen or metal atoms in Fe-based nanocrystalline coatings.

Pressureless Sintering of ZrB2Prepared by Colloidal Processing: Particle Packing, Sintering Conditions, and Additives

Nonaqueous colloidal processing in cyclohexane and slip casting allowed the preparation of green ZrB2compacts with 67.3%TD(theoretical density), which upon pressureless sintering at 2100°C densified to a final density of 92.4%TD. The effect of controllable sintering parameters (temperature, time, and heating rate) on the sintered properties of ZrB2compacts prepared using the nonaqueous colloidal processing method was also studied, to maximize the sintered density and tailor the desired microstructure (i.e., dense final object with small grain size). The competition between pore elimination and grain growth mainly controls the final density achievable. ZrB2compacts with maximum sintered densities of 93%TDcould be achieved by controlling the sintering conditions. Finally, the use of carbon and B4C additives to improve the sintered density and microstructure of ZrB2compacts was also investigated. The additives, compatible with colloidal processing, allowed compacts with greater sintered densities (96–98%TD) and finer grain sizes to be achieved by pressureless sintering, even at a lower temperature of 1900°C.

Temperature stable and high-Q microwave dielectric ceramics in the Li2Mg3−xCaxTiO6 system (x=0.00–0.18)

Microwave dielectric properties of Li2Mg3−xCaxTiO6 (x=0–0.18) ceramics were studied using a conventional solid-state route to find temperature stable and high Q microwave ceramics. As the calcination temperature was 500°C, the Li2TiO3 phase with monoclinic rock salt structure in C2/c space group started to form. When the samples were calcined from 600°C to 900°C, the XRD patterns exhibited a remarkable chemical reaction between the MgO and Li2TiO3 phases, which eventually formed the Li2Mg3TiO6 phase. The results indicated the Li2Mg3TiO6and CaTiO3 co-existed with each other and formed a stable composite system when the calcium content was added. The SEM photographs indicated that the pores caused by the Li evaporation could be effectively reduced due to the appearance of CaTiO3. As x was increased from 0 to 0.18, the relative density was significantly improved due to the elimination of pores. As the Ca content increased, the dielectric constant (εr) increased from 14.8 to 20.6; the quality factor (Q×f) decreased from 148,713GHz to 79,845GHz, and the temperature coefficient of resonant frequency (τf) significantly increased from −42.4 to +10.8ppm/°C due to the increased amount of CaTiO3. Therefore, at x=0.12, the LMCxT ceramics sintered at 1280°C for 6h displayed excellent comprehensive properties of εr=17.8,Q×f=102,246GHz and τf=−0.7ppm/°C.

Active release of pneumolysin prepores and pores by mammalian cells undergoing a Streptococcus pneumoniae attack

BackgroundStreptococcus pneumoniae is a potent human pathogen. Its pore-forming exotoxin pneumolysin is instrumental for breaching the host's epithelial barrier and for the incapacitation of the immune system. Methods and resultsUsing a combination of life imaging and cryo-electron microscopy we show that pneumolysin, released by cultured bacteria, is capable of permeabilizing the plasmalemma of host cells. However, such permeabilization does not lead to cell lysis since pneumolysin is actively removed by the host cells. The process of pore elimination starts with the formation of pore-bearing plasmalemmal nanotubes and proceeds by the shedding of pores that are embedded in the membrane of released microvesicles. Pneumolysin prepores are likewise removed. The protein composition of the toxin-induced microvesicles, assessed by mass spectrometry, is suggestive of a Ca2+-triggered mechanism encompassing the proteins of the annexin family and members of the endosomal sorting complex required for transport (ESCRT) complex. ConclusionsS. pneumoniae releases sufficient amounts of pneumolysin to perforate the plasmalemma of host cells, however, the immediate cell lysis, which is frequently reported as a result of treatment with purified and artificially concentrated toxin, appears to be an unlikely event in vivo since the toxin pores are efficiently eliminated by microvesicle shedding. Therefore the dysregulation of cellular homeostasis occurring as a result of transient pore formation/elimination should be held responsible for the damaging toxin action. General significanceWe have achieved a comprehensive view of a general plasma membrane repair mechanism after injury by a major bacterial toxin.

Solid-state synthesis of high performance Na-β″-Al2O3 solid electrolyte doped with MgO

The use of a small amount of MgO dopant can help to improve the performance of Na-β″-Al2O3 electrolyte by enhancing the relative density, promoting the formation of homogeneous and compact microstructure, and increasing the content of β″-Al2O3 phase. In this work, the effects of the amount of MgO dopant and the sintering temperature on the relative density, β″ phase content, and performance of the Na-β″-Al2O3 electrolyte were studied in detail, respectively. The samples were characterized in respects of relative density, closed-cell porosity, XRD, SEM, bending strength, and AC Impedance. The results demonstrated that the addition of MgO dopant significantly improved the densification rate of Na-β″-Al2O3 ceramics. The crucial temperature phase for pore elimination was 1500–1550°C. Samples doped with 0.2–2.0wt% MgO formed homogeneous microstructures associated with grain size refinement of the final compact at 1550°C. Macro defects appeared when sintering temperature reached to 1580°C. The total conductivity showed a linear dependence on the temperature (200–700°C), which fitted well with the Arrhenius equation. Na-β″-Al2O3 ceramics doped with 0.4wt% MgO showed the best bending strength (283MPa) and preferable ionic conductivity.