Sharif Digital Repository

In this research, effects of two-step sintering parameters, such as first and second step sintering temperatures (T1 and T2, respectively) and soaking time in the second step, on microstructure and electrical properties of ZnO varistors have been studied. In addition, slow rate of grain growth in the second step of sintering has also been investigated. Near fully dense (higher than 99%) and sub-micron grain size (1-0.75 μm) ZnO varistors have been fabricated by different two-step sintering cycles. Decreasing T1 and T2 result in longer necessary time in the second step to reach near full density with smaller final grain size. The superior electrical properties of varistors (VB: 2050-1022 V/mm... 

In order to improve the performance of thin film devices, it is necessary to characterize their mechanical, as well as electrical, properties. In this work, a model is developed for analysis of the mechanical and electrical properties and the prediction of residual stresses in thin films of silver nanoparticles deposited on silicon substrates. The model is based on inter-particle diffusion modeling and finite element analysis. Through simulation of the sintering process, it is shown how the geometry, density, and electrical resistance of the thin film layers are changed by sintering conditions. The model is also used to approximate the values of Young's modulus and the generated residual... 

Alumina-MWCNT composite was densified by microwave sintering. CNTs were coated with boehmite nanoparticles to enhance their distribution in composite samples. Calcination temperature of composite powder was determined by TGA analysis (5 °C/min). Samples containing 0 and 1vol%CNT were produced by cold isostatic pressing at 180 MPa. Microwave sintering (1520 °C for 45 min) was conducted under the flow of argon. Phase analysis of the calcined composite powder showed complete transformation of boehmite into gamma-alumina. The relative densities were 99.3% and 98.1% for monolithic alumina and composite, respectively. CNT addition improved the fracture toughness of alumina by ~37%. SEM images... 

Sintering process at temperature intervals close to the melting point of polymers is greatly important due to its role in synthesizing porous materials. During sintering, particles of polymeric materials coalesce throughout a process called interdiffusion. On the contrary, crystallization strongly affects the interdiffusion process at temperature intervals below and close to the melting point. Apparently, the outcome of the contention between these two factors would determine the interfacial width. Therefore, the current study presents a model, which takes both crystallization and interdiffusion into account, to predict sintering kinetic. Consequently, interfacial strength was assessed with... 

In this research, fine-grained SnO2-based varistors with a simple microstructure and the mean grain size of 0.8 µm were obtained by the microwave sintering method. The breakdown electric field measured for these varistors was 12 kV/cm that is 3 times higher than the conventional-sintered varistors. The high nonlinear coefficient (60) and low leakage current density (9 µA/cm2) were achieved in fine-grained SnO2 varistors. The fine-grained SnO2 varistors appropriately clamped pulse current surges and the calculated value of the clamping voltage ratio of these varistors was 2. The excellent electrical properties and protective effect of fine-grained SnO2 varistors showed that these varistors... 

In this study, PZT-PCN ceramic was made via sol–gel seeding method and effects of conventional sintering (CS) as well as two-step sintering (TSS) were investigated on microstructure, phase formation, density, dielectric and piezoelectric properties. First, high quality powder was achieved by seeding method in which the mixture of Co3O4 and Nb2O5 powder was added to the prepared PZT sol to form PZT-PCN gel. After drying and calcination, pyrochlore free PZT-PCN powder was synthesized. Second, CS and TSS were applied to achieve dense ceramic. The optimum temperature used for 2 h of conventional sintering was obtained at 1150 °C; finally, undesired ZrO2 phase formed in CS procedure was removed... 

To fabricate ceramic/metal complex-shaped part by co-powder injection moulding process (2KPIM), it is essential to tailor the sintering parameters in order to gain a low mismatch shrinkage, i.e. high sintering compatibility. In the present work, nanocrystalline 3Y-TZP and commercial 430L MIM grade stainless steel powders were co-sintered at various sintering cycles. Isothermal and non-isothermal sintering behavior of the individual and composite layers in argon and vacuum atmospheres were examined. High resolution scanning electron microscopy (HRSEM) coupled with energy dispersive X-ray (EDX) analysis was used to study the bonding interface between the ceramic and metal joints. It is shown... 

Manufacturing of complex-shaped bimetals utilizing two-color powder injection molding (2C-PIM) and three-dimensional printing (3DP) processes, which basically involve sintering of a powder/binder mixture, has been attracted a great interest. This article addresses sintering of biocompatible Co-Cr-Mo alloy for manufacturing stepwise porosity-graded composite structures. Such composite structures provide strength at the core and a porous layer for the tissue growth. To evaluate the process, two grades of gas atomized Co-Cr-Mo powder with an average particle size of 19 and 63 μm were used. Isothermal and nonisothermal sintering behavior of the loose powders under hydrogen and argon atmospheres... 

In this study the effects of HfB2 addition were investigated on the sintering behavior, densification and microstructure of ZrB2-SiC in the presence of B4C and MoSi2 as a sintering catalyst. 5, 10, 15 and 20 wt% of HfB2 was substituted with ZrB2 in ultra-high temperature ceramic composites of ZrB2-SiC. Additionally, the effect of using nano-scale against common micron size SiC was compared. All compositions were sintered for 1 h in 2100 °C and 2150 °C. Relative and bulk density and open porosity percentage of samples were calculated by Archimedes method. Dimensional changes of pellets and their weight reduction after pyrolysis and sintering proved that increasing of HfB2 leads to increasing... 

ABSTRACTs: The need for fully dense material with well-engineered microstructures has led to the promising emergence of innovative sintering technologies among which the Spark Plasma Sintering (SPS) is one of the most favorite. Unlike the conventional sintering processes, SPS takes advantage of a current flow passing through the sintering die and metallic powders by which fast densification with minimal grain growth and enhanced physicomechanical properties can be obtained. Albeit there is a growing interest in the exploitation of SPS in producing sufficiently consolidated metallic parts, no analytical review has been released over the effects of SPS parameters on the densification behavior,... 

This paper presents a novel method for fabrication of bone-like Ti6Al4V foamy core@compact shell composite for utilization as substitutive implant for cortical bone having porous core. Seven prototypes with core-diameters of 0, 6, 8, 10, 12, 14 and 16 mm surrounded by dense shells of respective thicknesses 8, 5, 4, 3, 2, 1 and 0 were produced by an innovative two-stage packing/compaction sintering method. Density, porosity, Young's modulus and compression strength of the prototypes depended on the core diameter. Mechanical strength and Young's modulus of 10@16 (10 mm core, 16 mm diameter prototypes) resembled that of the human ulna bone. Creation of foam at the center was achieved by... 

In the present study, SiC samples with 3, 5, 7.5 and 10 wt% Al2O3–Y2O3 as additives were made by powder metallurgy method, then sintering was performed by microwave-assisted process. β–SiC samples sintering was performed for 100 min. The highest sintered relative density 91.06% was achieved at 10 wt% additives content. The maximum values of hardness and toughness were up to 23.3 GPa and 6.14 MPa.m− 1/2. α–SiC and α–SiC/β–SiC samples sintering was performed for 120 min. The maximum value of density and hardness were up to 96.38% T.D and 24.88 GPa in α–SiC with 7.5 wt% additives, whereas the highest toughness was achieved at 10 wt% additives content in β–SiC samples. The α–SiC samples... 

We have studied the effect of sintering temperature on the weak link behavior of polycrystalline Bi2223 superconductors with different intergranular coupling using the XRD, electrical resistivity and AC susceptibility techniques. The XRD results show that by increasing sintering temperature up to 865°C the Bi2212 phase fraction decrease. It was found that the Bi2212 phase on the grain boundaries is likely to play the role of weak links and consequently reduces the intergranular critical current densities. © 2006 WILEY-VCH Verlag GmbH & Co. KGaA  

Production of NiTi alloy from elemental powders was conducted by mechanical alloying (MA) and sintering of the raw materials. Effects of milling time and milling speed (RPM) on crystallite size, lattice strain, and XRD peak intensities were investigated by X-ray analysis of the alloy. Powder compaction and sintering time and temperature effects on pore percentage of the as-mixed and the mechanically alloyed samples were empirically evaluated. The crystallite size of the mechanically alloyed Ni50Ti50 samples decreased with MA duration and with the milling speed. Depending on the crystal structure of the raw materials, the lattice strain increases with the milling duration. Metallographic... 

In the present work, co-sintering of various powder injection molding (PIM) feedstocks including iron, low-alloy steels, high-alloy steels, and hardmetals was investigated. Dilatometric analysis was used to evaluate the sintering behaviour of co-injection moulded feedstocks after debinding. An approach based on the data of dimensional change versus time and temperature was proposed to determine the compatibility of two materials system for the two component powder injection molding (2C-PIM) process. The similarity of sintering behaviour of the twobody parts produced by 2C-PIM was quantified trough definition of a parameter termed as Apparent Co-Sintering Index (ACSI). The results determined... 

In this research, (X)PZT-(1-X)PMN ceramics (where X is 0.25, 0.5 and 0.75) were achieved from a conventional mixed-oxide method. Lead Zirconate titanate (PZT) powders with composition near the morphotropic phase boundary have been synthesized via a microemulsion processing route. Modified mixed oxides method was used for obtaining single phase Lead Magnesium niobate (PMN) powder with high relative density (98% of T.D.) and good electrical properties (dielectric constant of ≈10 4 at room temperature). Two-step sintering (TSS) has been applied to suppress the accelerated grain growth of PZT-PMN composite compacts in the last sintering stage. The produced samples with high PZT content showed... 

Nanocrystalline hydroxyapatite (nHA) of 50 nm average diameter and length to diameter ratio of >3 was synthesized by biomimetic method. Non-isothermal sintering improved densification behavior and mechanical properties of apatite to 0.88 maximum fractional density, 70MPa bending strength, 148MPa compressive strength and 2.53GPa microhardness at sintering temperature of 1250°C. Higher sintering temperatures resulted in the decomposition of the apatite and in-situ biphasic calcium phosphate HAP/TCP formation. This process lowered apatite densification and weakened mechanical properties of the sintered specimen. Transmission electron microscopy (TEM), x-ray diffraction (XRD) and field emission... 

This article reports experimental results on laser sintering of A356 aluminum alloy and A356/SiC composite powders. Effects of scan rate, sintering atmosphere, hatch spacing, and SiC volume fraction (up to 20%), and particle size (7 and 17 μm) on the densification were studied. The phase formation and microstructural development were analyzed by X-ray diffraction (XRD) and scanning electron microscopy (SEM) coupled with energy-dispersive X-ray spectroscopy (EDS). Laser sintering under argon atmosphere exhibited higher densification compared to nitrogen. A faster sintering kinetics was observed as the scan rate decreased. Except at a low SiC content (5 vol%), the composite powders exhibited... 

The combined effects of SiC and carbon additives on the densification, structure and characteristics of pressureless sintered ZrB2-SiC ceramic composites were studied. The ZrB2-SiC (1-10wt%) composite powders were mixed by 1-2wt% carbon. The prepared powder mixtures were then cold-consolidated and sintered in argon environment in the temperature range of 1800-2100°C for 2h. The constituted phases and microstructural evolutions were studied using x-ray diffraction and scanning electron microscope equipped with an energy dispersive x-ray detector. The obtained results concluded that the densification was increased by extending the sintering time and also by prior holding...