Showing papers on "Sintering published in 2013"
TL;DR: In conclusion, the recent literature, especially on in situ studies, shows that OR is the dominant process causing the growth of nanoparticle size, and this could help to develop sinter-resistant catalysts, with the ultimate goal of designing catalysts that are self-healing.
Abstract: Metal nanoparticles contain the active sites in heterogeneous catalysts, which are important for many industrial applications including the production of clean fuels, chemicals and pharmaceuticals, and the cleanup of exhaust from automobiles and stationary power plants. Sintering, or thermal deactivation, is an important mechanism for the loss of catalyst activity. This is especially true for high temperature catalytic processes, such as steam reforming, automotive exhaust treatment, or catalytic combustion. With dwindling supplies of precious metals and increasing demand, fundamental understanding of catalyst sintering is very important for achieving clean energy and a clean environment, and for efficient chemical conversion processes with atom selectivity. Scientists have proposed two mechanisms for sintering of nanoparticles: particle migration and coalescence (PMC) and Ostwald ripening (OR). PMC involves the mobility of particles in a Brownian-like motion on the support surface, with subsequent coalescence leading to nanoparticle growth. In contrast, OR involves the migration of adatoms or mobile molecular species, driven by differences in free energy and local adatom concentrations on the support surface. In this Account, we divide the process of sintering into three phases. Phase I involves rapid loss in catalyst activity (or surface area), phase II is where sintering slows down, and phase III is where the catalyst may reach a stable performance. Much of the previous work is based on inferences from catalysts that were observed before and after long term treatments. While the general phenomena can be captured correctly, the mechanisms cannot be determined. Advancements in the techniques of in situ TEM allow us to observe catalysts at elevated temperatures under working conditions. We review recent evidence obtained via in situ methods to determine the relative importance of PMC and OR in each of these phases of catalyst sintering. The evidence suggests that, in phase I, OR is responsible for the rapid loss of activity that occurs when particles are very small. Surprisingly, very little PMC is observed in this phase. Instead, the rapid loss of activity is caused by the disappearance of the smallest particles. These findings are in good agreement with representative atomistic simulations of sintering. In phase II, sintering slows down since the smallest particles have disappeared. We now see a combination of PMC and OR, but do not fully understand the relative contribution of each of these processes to the overall rates of sintering. In phase III, the particles have grown large and other parasitic phenomena, such as support restructuring, can become important, especially at high temperatures. Examining the evolution of particle size and surface area with time, we do not see a stable or equilibrium state, especially for catalysts operating at elevated temperatures. In conclusion, the recent literature, especially on in situ studies, shows that OR is the dominant process causing the growth of nanoparticle size. Consequently, this leads to the loss of surface area and activity. While particle migration could be controlled through suitable structuring of catalyst supports, it is more difficult to control the mobility of atomically dispersed species. These insights into the mechanisms of sintering could help to develop sinter-resistant catalysts, with the ultimate goal of designing catalysts that are self-healing.
929 citations
TL;DR: Transparent polycrystalline ceramics have found various applications, such as laser hosts, infrared windows/domes, lamp envelopes and transparent armors, due mainly to their processing flexibility in fabricating items with large sizes and complex shapes and more importantly costeffectiveness as mentioned in this paper.
453 citations
TL;DR: In this article, an additive manufacturing (AM) technique for high strength oxide ceramics was developed, which aimed at directly manufacturing fully dense ceramic freeform components with good mechanical properties.
Abstract: Purpose – The purpose this paper is to develop an additive manufacturing (AM) technique for high‐strength oxide ceramics. The process development aims at directly manufacturing fully dense ceramic freeform‐components with good mechanical properties.Design/methodology/approach – The selective laser melting of the ceramic materials zirconia and alumina has been investigated experimentally. The approach followed up is to completely melt ZrO2/Al2O3 powder mixtures by a focused laser beam. In order to reduce thermally induced stresses, the ceramic is preheated to a temperature of at least 1,600°C during the build up process.Findings – It is possible to manufacture ceramic objects with almost 100 percent density, without any sintering processes or any post‐processing. Crack‐free specimens have been manufactured that have a flexural strength of more than 500 MPa. Manufactured objects have a fine‐grained two‐phase microstructure consisting of tetragonal zirconia and alpha‐alumina.Research limitations/implications...
304 citations
TL;DR: Calcium phosphate ceramics have become of prime importance for biological applications in the field of bone tissue engineering and this paper reviews the sintering behaviour of these bioceramics.
299 citations
TL;DR: My laboratory group's research toward the basic understanding of the effects of particle size and support material on catalytic properties is reviewed, with key insights into how and why specific structural properties of catalyst nanomaterials dictate their catalysttic properties.
Abstract: Transition metal nanoparticles on the surfaces of oxide andcarbon support materials form the basis for most solid catalysts and electrocatalysts, and have important industrial applications such as fuel production, fuels, and pollution prevention. In this Account, I review my laboratory group’s research toward the basic understanding of the effects of particle size and support material on catalytic properties. I focus on studies of well-defined model metal nanoparticle catalysts supported on single-crystalline oxide surfaces. My group structurally characterized such catalysts using a variety of ultrahigh vacuum surface science techniques. We then measured the energies of metal atoms in these supported nanoparticles, using adsorption calorimetry tools that we developed. These metal adsorption energies increase with increasing size of the nanoparticles, until their diameter exceeds about 6 nm. Below 6 nm, the nature of the oxide support surface reaches also greatly affects the metal adsorption energies. Usin...
294 citations
06 Feb 2013
TL;DR: Spark plasma sintering (SPS) or pulsed electric current (PECS) is a SPS technique that utilizes uniaxial force and a pulsed (on-off) direct electrical current (DC) under low at-mospheric pressure to perform high speed consolidation of the powder as discussed by the authors.
Abstract: Spark plasma sintering (SPS) or pulsed electric current sintering (PECS) is a sintering techni‐ que utilizing uniaxial force and a pulsed (on-off) direct electrical current (DC) under low at‐ mospheric pressure to perform high speed consolidation of the powder. This direct way of heating allows the application of very high heating and cooling rates, enhancing densifica‐ tion over grain growth promoting diffusion mechanisms (see Fig. 1), allowing maintaining the intrinsic properties of nanopowders in their fully dense products.
246 citations
TL;DR: The results of this study showed that the increase in sintering temperature increased the contrast ratio, but led to a negative impact on the flexural strength.
Abstract: This study investigated the effect of sintering temperatures on flexural strength, contrast ratio, and grain size of zirconia. Zirconia specimens (Ceramill ZI, Amann Girrbach) were prepared in partially sintered state. Subsequently, the specimens were randomly divided into nine groups and sintered with different final sintering temperatures: 1,300°C, 1,350°C, 1,400°C, 1,450°C, 1,500°C, 1,550°C, 1,600°C, 1,650°C, or 1,700°C with 120 min holding time. Three-point flexural strength (N = 198; n = 22 per group) was measured according to ISO 6872: 2008. The contrast ratio (N = 90; n = 10 per group) was measured according to ISO 2471: 2008. Grain sizes and microstructure of different groups were investigated (N = 9, n = 1 per group) with scanning electron microscope. Data were analyzed using one-way ANOVA with Scheffe test and Weibull statistics (p < 0.05). Pearson correlation coefficient was calculated between either flexural strength or contrast ratio and sintering temperatures. The highest flexural strength was observed in groups sintered between 1,400°C and 1,550°C. The highest Weibull moduli were obtained for zirconia sintered at 1,400°C and the lowest at 1,700°C. The contrast ratio and the grain size were higher with the higher sintering temperature. The microstructure of the specimens sintered above 1,650°C exhibited defects. Sintering temperatures showed a significant negative correlation with both the flexural strength (r = −0.313, p < 0.001) and the contrast ratio values (r = −0.96, p < 0.001). The results of this study showed that the increase in sintering temperature increased the contrast ratio, but led to a negative impact on the flexural strength. Considering the flexural strength values and Weibull moduli, the sintering temperature for the zirconia tested in this study should not exceed 1,550°C.
237 citations
TL;DR: In this article, the feasibility of flash sintering a covalent ceramic, SiC, has been investigated for the first time, and a near-theoretical density was achieved using Al 2 O 3 ǫ+ǫ Y 2 Oǫ 3 Sintering aids at a furnace temperature of only 1170°C and in a time of 150
Abstract: The feasibility of flash sintering a covalent ceramic, SiC, has been investigated for the first time. Flash sintering involves the application of an electrical potential difference across a powder compact during heating, which leads to sintering at low furnace temperatures in a few seconds and has only been demonstrated with ionic ceramics previously. Near-theoretical density was achieved using Al 2 O 3 + Y 2 O 3 sintering aids at a furnace temperature of only 1170 °C and in a time of 150 s. Specimen temperatures were significantly higher than the furnace temperature owing to Joule heating and consequently heat loss limited densification in the near surface region. It was not possible to reach high densities using “ABC” sintering aids (aluminium–boron–carbon) or pure SiC. The mechanisms involved and potential commercial advantages are briefly discussed.
206 citations
TL;DR: In this paper, for the first time, the authors have observed an incubation time for the onset of flash sintering in experiments carried out at isothermal furnace temperatures, where the incubation times vary highly nonlinearly with the strength of the DC field, akin to a nucleation-like phenomenon.
Abstract: We report, for the first time, the observation of an incubation time for the onset of flash sintering in experiments carried out at isothermal furnace temperatures. The incubation time varies highly nonlinearly with the strength of the DC field, akin to a nucleation-like phenomenon. The setting of the maximum current at the power supply has a significant influence on the extent of densification. The hold time at these current settings has an influence on the grain size increasing it with time and the magnitude of the current setting. The experiments were carried out on 3 mol% yttria-stabilized zirconia. In all instances the specimen temperatures during flash sintering, estimated from a previously validated blackbody radiation model remain well below the temperatures that would be required for conventional sintering of yttria-stabilized zirconia. Taken together these observations imply a nucleation of defect avalanche as a possible mechanism for flash sintering.
198 citations
TL;DR: In this paper, carbon-coated Li4Ti5O12 (LTO/C) particles were synthesized via a simple solid-state reaction using a hydrothermally prepared TiO2/C precursor and the effects of the sintering temperature and carbon content on the electrochemical properties of the as-prepared materials were systematically investigated.
Abstract: Carbon-coated Li4Ti5O12 (LTO/C) particles were synthesized via a simple solid-state reaction using a hydrothermally prepared TiO2/C precursor The effects of the sintering temperature and carbon content on the electrochemical properties of the as-prepared materials were systematically investigated Among the temperature examined, the sample treated at 800 °C showed the best performance due to the combination of relatively high crystallinity, small particle size, and high electrical conductivity In addition, the ionic transport mechanism in the carbon coating layer was studied by in situ Raman analysis It is proposed that the defects and vacancies in the carbon are responsible for the efficient Li ion transportation The results indicate that the enhanced electrode properties can be achieved by optimizing the content of the coated carbon due to the balance between the electric conduction and the ionic transport
190 citations
TL;DR: In this paper, a simple and upscalable method is described to produce graphene/alumina composites by spark plasma sintering (SPS) with a significant improvement on both mechanical and electrical properties of monolithic Al2O3.
Abstract: A simple, fast and upscalable method is described to produce graphene/alumina (G/Al2O3) composites by spark plasma sintering (SPS) with a significant improvement on both mechanical and electrical properties of monolithic Al2O3. Graphene oxide (GO) was mixed with Al2O3 using a colloidal method obtaining an excellent dispersion of GO in the alumina matrix. The material was consolidated by SPS that allowed, in one-step, the in situ reduction of the GO during the sintering process. A detailed Raman analysis was found to be very useful to study the orientation of the graphene in the composite and to evaluate and optimise its thermal reduction. Graphene platelets acted as elastic bridges avoiding crack propagation and providing this material with a crack bridging reinforcement mechanism. A very low graphene loading (0.22 wt%) led to a 50% improvement on the mechanical properties of the alumina and to an increase of the electrical conductivity up to eight orders of magnitude.
TL;DR: In this article, the authors confirmed that the catalyst preparation methodologies have a significant effect on the activity and stability of Ni/SiO2 catalyst for methanation of syngas.
TL;DR: In this article, the structure and Li + transport process of Li 7−2 x La 3 Zr 2− x Te x O 12 ( x ǫ = 0.125 and 0.25) lithium garnets are reported.
TL;DR: Infrared spectroscopic measurements and scanning tunneling microscopy studies of trimethylaluminum ALD on copper show that the remarkable stability imparted to the nanoparticles arises from selective armoring of under-coordinated copper atoms on the nanoparticle surface.
Abstract: Atomic layer deposition (ALD) of an alumina overcoat can stabilize a base metal catalyst (e.g., copper) for liquid-phase catalytic reactions (e.g., hydrogenation of biomass-derived furfural in alcoholic solvents or water), thereby eliminating the deactivation of conventional catalysts by sintering and leaching. This method of catalyst stabilization alleviates the need to employ precious metals (e.g., platinum) in liquid-phase catalytic processing. The alumina overcoat initially covers the catalyst surface completely. By using solid state NMR spectroscopy, X-ray diffraction, and electron microscopy, it was shown that high temperature treatment opens porosity in the overcoat by forming crystallites of γ-Al2O3. Infrared spectroscopic measurements and scanning tunneling microscopy studies of trimethylaluminum ALD on copper show that the remarkable stability imparted to the nanoparticles arises from selective armoring of under-coordinated copper atoms on the nanoparticle surface.
TL;DR: In this paper, the effect of graphene nanoplates reinforcement on densification behavior, microstructure, and mechanical properties (microhardness, biaxial flexural strength, and indentation fracture toughness) of the composites are presented.
TL;DR: Different sintering conditions resulted in differences in grain size and light transmittance, which should be considered to obtain more translucent dental zirconia restorations.
Abstract: PURPOSE. This study aimed to identify the effects of the sintering conditions of dental zirconia on the grain size and translucency. MATERIALS AND METHODS. Ten specimens of each of two commercial brands of zirconia (Lava and KaVo) were made and sintered under five different conditions. Microwave sintering (MS) and conventional sintering (CS) methods were used to fabricate zirconia specimens. The dwelling time was 20 minutes for MS and 20 minutes, 2, 10, and 40 hours for CS. The density and the grain size of the sintered zirconia blocks were measured. Total transmission measurements were taken using a spectrophotometer. Twoway analysis of variance model was used for the analysis and performed at a type-one error rate of 0.05. RESULTS. There was no significant difference in density between brands and sintering conditions. The mean grain size increased according to sintering conditions as follows: MS-20 min, CS-20 min, CS-2 hr, CS-10 hr, and CS-40 hr for both brands. The mean grain size ranged from 347-1,512 nm for Lava and 373-1,481 nm for KaVo. The mean light transmittance values of Lava and KaVo were 28.39-34.48% and 28.09-30.50%, respectively. CONCLUSION. Different sintering conditions resulted in differences in grain size and light transmittance. To obtain more translucent dental zirconia restorations, shorter sintering times should be considered. [J Adv
TL;DR: In this article, a cubic yttria-stabilized zirconia is shown to sinter at 390°C, more than 1000°C below nominal sintering temperatures, by using a DC electric field of 2250 V/cm.
Abstract: Using the flash sintering technique, cubic yttria-stabilized zirconia is shown to sinter at 390°C, more than 1000°C below nominal sintering temperatures, by using a DC electric field of 2250 V/cm. Furthermore, flash sintering experiments performed with electric fields between 60 and 2250 V/cm were used to show that the relationship of the temperature at the onset of flash sintering (TOnset) and the applied field (E) follows the power relationship TOnset (K) = 2440 E−1/5.85(V/cm). Using this relationship, and considering the sintering of the sample in the absence of an electric field, the critical electric field to enter the flash sintering regime is shown to be 24.5 V/cm. For electric fields between this critical electric field and 2250 V/cm, the onset of flash sintering occurs in the same range of critical volumetric power dissipation, between 1 and 10 mW/mm3, suggesting this is a material property. Despite the volumetric power dissipation being the critical value for the onset of flash sintering behavior, the current density achieved during sintering appears to be more critical for densification rather than maximizing power dissipation.
TL;DR: In this article, a unified mechanism for field-assisted phenomena such as enhanced rapid flash sintering, reduction in flow stress and grain growth retardation is proposed, where defect segregation causes enhanced ionic and electronic transport along dislocations and grain boundaries.
10 Oct 2013-Materials Science and Engineering A-structural Materials Properties Microstructure and Processing
TL;DR: In this article, a graphene nanoplatelet reinforced tantalum carbide composites are synthesized by spark plasma sintering (SPS) at processing conditions of 1850°C and 80-100 MPa.
Abstract: Graphene NanoPlatelets (GNP) reinforced tantalum carbide composites are synthesized by spark plasma sintering (SPS) at processing conditions of 1850 °C and 80–100 MPa. The GNP addition enhances the densification of TaC–GNP composites to 99% theoretical density, while reducing the grain size by over 60% through grain wrapping mechanism. Survival and structure retention of GNP is confirmed through scanning electron microscopy and micro-Raman spectroscopy. Nanoindentation and high load (20–30 N) microindentation are utilized to evaluate elastic modulus and hardness. GNP improves fracture toughness of TaC by up to 99% through toughening mechanisms such as GNP bending, sheet sliding, cracking bridging, and crack deflection.
TL;DR: In this paper, the effect of Fe addition on catalytic activity and stability of LaNixFe1−xO3 perovskite catalyst was investigated for hydrogen production via steam reforming of tar using toluene as a model compound.
TL;DR: In this paper, the effect of energy density on the formation of single tracks from CoCrMo powder, using a noncommercial machine to select process parameters with open source technology, is analyzed.
Abstract: Selective laser melting (SLM) is an advanced manufacturing technology based on layer by layer building to produce solid parts from metallic powder (Kruth et al., CIRP Ann Manuf Technol 56:730–760, 2007). Commercial SLM machines are configured to use specific parameters to process different metallic powder grades. This paper proposes a methodology that makes it possible to analyze the effect of energy density on the formation of single tracks from CoCrMo powder, using a noncommercial machine to select process parameters with open source technology. Full factorial experimental work was carried out to produce single tracks under different combinations of process parameters. A major application of this research work is to develop more flexible machine configurations applicable to different materials.
TL;DR: In this paper, the influence of sintering parameters on pellet composition, microstructure and conductivity is discussed in addition to the analysis of the mechanical behavior of the grains interfaces.
Abstract: NaSICON-type lithium conductor Li1.3Al0.3Ti1.7(PO4)3 (LATP) is synthesized with controlled grain size and composition using solution chemistry. After thermal treatment at 850 °C, sub-micronic crystallized powders with high purity are obtained. They are converted into ceramic through Spark Plasma Sintering at 850–1000 °C. By varying the processing parameters, pellet with conductivities up to 1.6 × 10−4 S/cm with density of 97% of the theoretical density have been obtained. XRD, FEG-SEM, ac-impedance and Vickers indentation were used to characterize the products. The influence of sintering parameters on pellet composition, microstructure and conductivity is discussed in addition to the analysis of the mechanical behavior of the grains interfaces.
TL;DR: In this paper, a method to achieve rapid pressureless low-temperature sintering of Ag nanoparticles for bonding was described, in which organic shells adsorbing on the surface of nanoparticles to stabilize them were thinned to create a sparse protecting layer.
01 Jan 2013
TL;DR: The Spark Plasma Sintering (SPS) method is of great interest to the powder and powder metallurgy industry and material researchers of academia for both product manufacturing and advanced material research and development.
Abstract: Since two decades ago, Spark Plasma Sintering (SPS) method is of great interest to the powder and powder metallurgy industry and material researchers of academia for both product manufacturing and advanced material research and development. A rapid sintering is one of remarkable advantage of SPS. The SPS also features to provide a microstructure controlled sintering. Structurally tailoring effect in SPS processing was verified in the consolidation of nano-alumina. Therefore, it is generally well known that the SPS is an advanced processing technology to produce a homogenous highly dense nano-structural sintered compact, Functionally Graded Materials (FGM), fine ceramics, composite materials, new wear-resistant materials, thermo-electric semiconductors and Bio materials. It is considered that the ON-OFF DC pulse energizing method generates; (1)spark plasma, (2)spark impact pressure, (3)Joule heating, and (4)an electrical field diffusion effect. In the SPS process, the powder particle surfaces are more easily purified and activated than in conventional electrical sintering processes and material transfers at both the micro and macro levels are promoted, so a high-quality sintered compact is obtained at a lower temperature and in a shorter time than with conventional processes. This technique was originally invented in Japan as “Spark Sintering (SS)” in 1962. Today, a number of SPSed products for different industries are now being realized in Japan. The application has been getting into the practical industry use product stage through the scientific academia and/or R&D proto-type materials level such as in the field of mold and dies industry, cutting tools industry, electronics industry and automotive industry. It is a novel sintering process featuring energy saving and high speed consolidation and has a low power consumption of between 1/5 and 1/3 that of conventional sintering techniques such as pressureless sintering (PLS), hot press (HP) sintering and hot isostatic pressing (HIP).
TL;DR: In this paper, a facile and green synthesis approach of Ag/SnO2 composite nanotubes is developed by the photocatalytic in situ reduction of Ag+ using SnO2 nanotsubes as a template.
TL;DR: In this paper, the synthesis and functional characterization of dense bulk nanometric oxides are reviewed, with particular emphasis on the modifications that a grain size in the low nanometric range introduces in their physical properties.
Abstract: The synthesis and functional characterization of dense bulk nanometric oxides are reviewed, with particular emphasis on the modifications that a grain size in the low nanometric range (10–50 nm) introduces in their physical properties. The preparation of ceramics with low porosity and extremely small grain size is particularly challenging and mostly relies on the sintering of extremely fine nanopowder. The most popular methods for the preparation of the starting nanopowders are introduced and briefly discussed as well as the most widely employed densification techniques. The role of nanostructure in controlling phase stability, electrical and thermal transport, optical and magnetic properties of nano-oxides is discussed in details. Several examples are given where bulk materials prepared with grain size equal or below 50 nm show characteristics that are either enhanced or, in some cases, completely different from those possessed by the same materials, but with larger grain sizes.
TL;DR: In this article, a graphite metal matrix composites (MMCs) were prepared by conventional powder metallurgy route using conventional and spark plasma sintering (SPS) techniques.
TL;DR: In this paper, the effects of molar ratio of OH − / NO 3 − (RO/N), the atomic ratio of Fe/Sr (RF/S) and sintering on the phase composition and magnetic properties of as-synthesized specimens are studied.
TL;DR: In this article, the authors proposed that applied high pressure, as an external force, assisted in the breakdown of surface oxide layers that enveloped the powder particles in the early stage of sintering.
Abstract: Metallic glass powder of the composition Al86Ni6Y4.5Co2La1.5 was consolidated into 10mm diameter samples by spark plasma sintering (SPS) at different temperatures under an applied pressure of 200MPa or 600MPa. Theheating rate and isothermal holding time were fixed at 40°C/min and 2 min, respectively. Fully dense bulk metallic glasses (BMGs) free of particle-particle interface oxides and nano-crystallization were fabricated under 600MPa. In contrast, residual oxides were detected at particle-particle interfaces (enriched in both Al and O) when fabricated under a pressure of 200MPa, indicating the incomplete removal of the oxide surface layers during SPS at a low pressure. Transmission electron microscopy (TEM) revealed noticeable nano-crystallization of face-centered cubic (fcc) Al close to such interfaces. Applying a high pressure played a key role in facilitating the removal of the oxide surface layers and therefore full densification of the Al86Ni6Y4.5Co2La1.5 metallic glass powder without nano-crystallization. It is proposed that applied high pressure, as an external force, assisted in the breakdown of surface oxide layers that enveloped the powder particles in the early stage of sintering. This, together with the electrical discharge during SPS, may have benefitted the viscous flow of metallic glasses during sintering.
TL;DR: In this article, two different hydroxyapatite based composites reinforced by oxide ceramic nano crystals were synthesized by high-energy ball milling and sintered by pressure less technique.