Conventional Pressureless Sintering Research Articles

Effect of sintering temperature on the microstructure and conductivity of Na0.54Bi0.46Ti0.99Mg0.01O3-δ

The present work examines the effect of sintering temperature on the phase formation and the conductivity of Na0.54Bi0.46Ti0.99Mg0.01O3-δ. The formation of the secondary phase and the poor grain boundary conductivity are a few major concerns in this recently developed material. Polycrystalline bulk specimens are fabricated through conventional pressureless sintering at different temperatures of 930 °C, 950 °C, 1000 °C, and 1050 °C for 2 h. A single rhombohedral perovskite phase is detected in the XRD profiles collected on all the sintered samples. However, the SEM micrographs reveal the existence of impurity phase in all the tested samples with the amount increasing considerably with the increase in temperature. The local elemental analysis of the impurity phase suggests the presence of non-stoichiometric Na2xTi2-xMgxO4 compound. However, the chemical composition of this compound varies across different specimens. The samples sintered at 950 °C exhibit the maximum bulk and grain boundary conductivities of 13.3 mS.cm−1 and 3.6 mS.cm−1, respectively, at 600 °C. A lower magnitude of both the conductivities is observed on sintering the samples at higher temperatures. The activation energy for bulk ionic conduction also decreases from 0.67 eV to 0.48 eV with the sintering temperature supporting the conjecture that higher Mg2+ amount is dissolved in the perovskite phase present in the samples sintered at 950 °C. The higher total conductivity in the samples sintered at 950 °C is attributed to the formation of comparatively less amount of Mg-rich impurity phase. Even though 950 °C appears to be an optimum sintering temperature, the grain boundaries of the samples continue to be resistive. At 600 °C, the grain boundary conductivity is less than one-third of the bulk conductivity. As a result, grain boundaries contribute a significant portion of the total resistivity offered by the Na0.54Bi0.46Ti0.99Mg0.01O3-δ samples.

A Novel Method for Densification of Titanium Using Hydrogenation-Induced Expansion Under Constrained Conditions

Abstract Full density of sintered Ti alloys is difficult to achieve through conventional pressureless sintering techniques. This paper introduces a novel approach for densifying powder metallurgy Ti alloys through constrained hydrogenation assisted densification (CHAD). This method utilizes hydrogenation-induced volumetric expansion and constrains a sintered Ti body in a rigid mold to achieve densification. The results demonstrate that the residual porosity in powder metallurgy CP-Ti and Ti-6Al-4V alloys can be effectively reduced or removed by CHAD, leading to improved tensile properties. The described method can be used as an efficient and cost-effective technique to make Ti and Ti alloys with high density and improved mechanical properties.

Transparent MgAl2O4 ceramics prepared by microwave sintering and hot isostatic pressing

Transparent MgAl2O4 ceramics have been successfully fabricated by microwave sintering and hot isostatic pressing (HIP) from high purity spinel nanopowders. The influence of microwave sintering temperature on densification and microstructure was studied, choosing 1400 °C as the sintering temperature. The comparative study of microwave sintering and pressureless sintering of MgAl2O4 ceramics was made to investigate the difference between the two sintering methods. Compared to the conventional pressureless sintering, the microwave sintered MgAl2O4 ceramics reach relative high density at lower temperature and much shorter time. The in-line transmittance of the transparent 1.5 mm thick MgAl2O4 ceramic obtained by microwave sintering (1400 °C, 80 min) and HIP (1650 °C/180 MPa/3 h) was 86.4% at 1064 nm and 80.2% at 400 nm, respectively. The samples have a Vickers hardness of 13.7 GPa.

Fabrication of Al-Si controlled expansion alloys by unique combination of pressureless sintering and hot forging

The present work reports the synthesis of Al-X wt.% Si (X = 20, 30, 40 and 50) alloys by a unique combination of pressureless sintering (PLS) and hot forging at 800 and 1000 MPa forging pressures with 37.5 and 50% deformation, respectively, in a specially designed die. The effect of hot forging parameters on densification, hardness, compressive strength and microstructures of the alloys was studied as well as suitably compared with the conventional PLS alloys. The ‘PLS + forged’ alloys, specifically 50% deformed at 1000 MPa, yielded excellent densification and subsequent very good mechanical properties. The combination of hot forging pressure and temperature enabled the high densification due to pore collapse, fracturing of Si particles leading to Al flow between the fractured Si particles and better interfacial diffusion between Si and Al. Moreover, the alloys showed excellent electrical conductivity (~67% of that of pure Al), low coefficient of thermal expansion (CTE) and high thermal conductivity (TC) at par with the same alloys prepared with other methods, like spark plasma sintering, hot pressing and Osprey techniques.

Effects of Si and Oxide Additives on Phase Evolution, Microstructure and Mechanical Properties of Si3N4–Mo Cermet Fabricated by Conventional Powder Metallurgy

Silicon nitride (Si3N4) based cermets are very popular due to their versatile applications like high temperature engine parts, cutting tools, refractory and structural applications. Here, in this work, Si3N4–Mo (Si3N4 as matrix and molybdenum as reinforcement) and Si3N4–Mo–Si (Si3N4 as matrix, Mo and Si as reinforcement) cermets are fabricated through powder metallurgy route and phase evolution, micro-structure and mechanical properties are analyzed. Three initial powder compositions of 90Si3N4–10Mo (S90M10), 75Si3N4–25Mo (S75M25) and 75Si3N4–15Mo–10Si (SMS) (all in wt%) are prepared through planetary milling and total additive amount of 5, 10 and 15 wt% of Y2O3 and Al2O3 are mixed (equal wt ratio of each) with milled base powder, cold compacted and followed by conventional pressure-less sintering at 1500 °C for 1 h. The effects of additives and reinforcements on phase evolution, microstructure and mechanical properties are discussed. From XRD and SEM analysis of sintered pellets, the presence of molybdenum silicide (i.e. Mo3Si, Mo5Si3 and MoSi2) in all the cermets is confirmed. It has been observed that the relative density and hardness values are increased with the increase in additive wt%. Comparing all the cermets, 75Si3N4–15Mo–10Si with 10 wt% additives has shown the maximum compression strength and fracture toughness of 134.2 MPa and 8.89 ± 0.34 MPa.m1/2 respectively. The maximum relative density of 75.24% is observed in S90M10 and hardness of 8.81 ± 0.13 GPa is found in S75M25 both cermets containing 15 wt% of additives. The transformation from α- to β- Si3N4 is observed in all the cermets after sintering.

Microstructure and mechanical properties of spark plasma sintered SiC ceramics aided by B4C

Silicon carbide ceramics were densified by spark plasma sintering (SPS) using Silicon carbide (SiC) powders with the sintering aid Boron carbide (B4C). Dense solid-phase sintered SiC ceramics were obtained by SPS with a holding time of 5 min at a temperature of ~1800 °C, which is ~350 °C lower than that required by conventional pressureless sintering (PS). The microstructure and mechanical properties of the ceramics was systematically investigated for different sintering temperatures. The sample relative density, flexural strength, hardness, elastic modulus and fracture toughness were determined to be 98.6%, 592 MPa, 28.3 GPa, 461 GPa and 3.6 MPa m1/2, respectively. The spark plasma sintered specimens showed superior mechanical properties and finer grains compared to the pressurelessly sintered specimen. The densification kinetics for the spark plasma sintered SiC ceramics and the role of B4C in the sintering process were discussed.

Preparation and properties of spodumene/silicon carbide composite ceramic materials

<sec>Silicon carbide (SiC) is widely used due to the lower coefficient of thermal expansion (CTE), high thermal conductivity and excellent mechanical properties. However, the self-diffusion coefficient of SiC relative to that of oxide ceramics is very low, it is difficult to sinter at lower temperature. The <i>β</i>-spodumene has ultra low or even negative thermal expansion coefficient combined with good thermal and chemical durability, which melts at 1423 ℃. Accordingly, the present study focuses on the use of <i>β</i>-spodumene as a flux at lower sintering temperature and the preparation of lower CET composite ceramics. The effects of spodumene on the sintering behavior, phase relations, thermal expansion and mechanical properties of spodumene/silicon carbide composites are discussed.</sec><sec>A high pure <i>β</i>-spodumene LiAlSi<sub>2</sub>O<sub>6</sub> compound with nearly zero thermal expansion coefficient is synthesized via solid phase sintering. Spodumene/silicon carbide composites are fabricated by the adding 25, 30, 35 and 40 mass% synthesized <i>β</i>-spodumene powder to 75, 70, 65 and 60 mass% <i>α</i>-SiC matrix, respectively. Both <i>β</i>-spodumene and SiC are fabricated by conventional pressureless liquid sintering technique, and the batches are uniaxially pressed into discs and rectangular bars, then sintered at 1550 ℃ for 2 h in an Ar atmosphere.</sec><sec>The results show that the SiC and <i>β</i>-spodumene do not react during sintering, and the <i>β</i>-spodumene changes from tetragonal phase into hexagonal phase, the cell volume has a considerable shrinkage. A certain amount of liquid phase can help to enhance the density, improve Young’s modulus and promote the sintering behavior of SiC. When the feedstock contains 35% <i>β</i>-spodumene, the Young’s modulus reaches to (204.2 ± 0.5) GPa. Excess porosity is formed when liquid phase is too much during sintering, The Young's modulus of the sample 40SP is (119.6 ± 0.5) GPa. It is determined that the Young’s modulus of these materials are affected by porosity and internal microcracks. This study indicates that the content of <i>β</i>-spodumene and porosity are the dominant factors to control the CET of composites, but the porosity has a stronger effect. Besides, the microcracks, which are formed by the interaction of various internal stresses, are also an impotant factor. The materials with nearly zero thermal expansion are developed in a lower temperature range from –150 ℃ to 25 ℃, the spodumene content in the most stable composite reaches 40 mass%, and the CET of composite is close to that of Si (α<sub>25 ℃</sub> = 2.59 × 10<sup>–6</sup> ℃<sup>–1</sup>) in a temperature range of 25–480 ℃.</sec>

Effects of TiO2 Content on Microwave-Attenuating Properties of TiO2-Al2O3 Composite Ceramics

Microwave attenuation ceramic is a kind of dielectric loss material, which is used to absorb some unnecessary microwave electromagnetic field energy. For this attenuation material, it is required to have a large attenuation ratio, a sufficient mechanical strength and a good thermal stability. In this work, TiO2-Al2O3 composite ceramics were prepared via the conventional pressureless sintering method. The addition of CuO as liquid phase sintering aid was used to lower the sintering temperature of TiO2-Al2O3 composite ceramics. The effects of TiO2 content on sintering properties, phase composition, microstructure, mechanical properties and microwave-attenuating properties were investigated. With the increase of TiO2content, the sintering temperature decreased from 1300°C to 1100°C, meanwhile the dielectric constant rose from 15.42 to 23.80 and the dielectric loss increased from 0.042 to 0.314, which improved the attenuation of the TiO2-Al2O3 attenuation ceramics. When the content of TiO2 was 10 wt%, TiO2-Al2O3 composite attenuation ceramics with good properties could be obtained.

A novel alternating current-assisted sintering method for rapid densification of Al2O3 ceramics with ultrahigh flexural strength

A novel alternating current-assisted sintering (ACS) technology was applied to prepare Al2O3 ceramics. The results show that, compared to conventional pressureless sintering (PS), the ACS method has remarkable advantages in reducing the sintering temperature and shortening sintering time. Specifically, submicron dense Al2O3 ceramics could be obtained by ACS with a soaking time of 5 min and a sintering temperature of 1400 °C. A higher sintering temperature (1500 °C) and longer soaking time (1 h) were necessary for PS to reach a similar density. The maximum flexural strength and hardness of alternating current-assisted sintered (ACSed) samples were 864 MPa and 19.24 GPa, respectively. For the ACSed and pressureless sintered (PSed) samples whose densities and grain sizes were similar, the flexural strength of the ACSed ones was more than 50% higher than that of the PSed ones. The results suggest that the key to achieving rapid densification is the fast heating rate and mechanical load. In conclusion, the ACS technology, whose cost is lower than spark plasma sintering (SPS), is a viable alternative for preparing high performance Al2O3 ceramics.

Er3+ and K0.5Na0.5NbO3 modified Ba0.85Ca0.15Ti0.9Zr0.1O3: Novel translucent ceramics with reversible photochromism

Er3+-doped xK0.5Na0.5NbO3-(1-x)Ba0.85Ca0.15Ti0.9Zr0.1O3 (x = 0.04, 0.05, 0.06 and 0.07) translucent ceramics have been first prepared by conventional pressureless sintering. Owing to the effective suppression of grain growth induced by both Er3+ and K0.5Na0.5NbO3, the ceramics possess dense and fine-grained microstructure. The XRD patterns demonstrate cubic-like phase structure with minimal optical anisotropy. The ceramics perform good photochormic (PC) properties under 407-nm light irradiation and superior reproducibility after heat treatment (200 °C, 5 min). And PC reaction is responsible for coloration and decoloration processes. The up-conversion photoluminescence before and after irradiation have been studied. With increasing the K0.5Na0.5NbO3 content or elevating the sintering temperature of the ceramics, the luminescent quenching degree (ΔRt) gradually increases. The color centers induced by irradiation can be reversibly created or eliminated, resulting in invertible luminescent modulations. This work can guide other multifunctional PC transparent ferroelectric materials for optoelectronic applications.

Effect of Y2O3 and Al2O3 sintering additives during fabrication of Si3N4–Mo–Si cermet by conventional pressure-less sintering

Abstract The present paper reports the individual and combined effect of Y2O3 and Al2O3 as sintering additives during fabrication of Si3N4–Mo–Si cermet by powder metallurgy route. Si3N4 + Mo + Si cermet composition is prepared and different proportions of yttria (Y2O3) and alumina (Al2O3) are added to that composition as sintering additives. Powder mixture is cold compacted and finally sintered by conventional pressure-less sintering at 1500 °C for 1 h in Argon gas atmosphere. The effect of the additives on the mechanical properties as well as microstructure of cermet has been investigated. Hardness and fracture toughness are found to be improved with combined addition of both the additives than that of individual addition. The maximum hardness and fracture toughness values are found in case of 5 wt. % Y2O3 and 10 wt. % Al2O3 added cermet with 11.41 ± 0.43 GPa and 7.005 ± 0.58 MPa m1/2 respectively. However, the optimal results are found in case of 7.5 wt % Y2O3 and 7.5 wt % Al2O3 added cermet with relative density of 62.25%, micro-hardness of 8.07 ± 0.55 GPa and fracture toughness of 4.57 ± 0.44 MPa m1/2. From microscopic study it is observed that initial α- Si3N4 transforms to β-Si3N4 after sintering.

Rapid manufacturing of copper components using 3D printing and ultrasonic assisted pressureless sintering: Experimental investigations and process optimization

The present work deals with the comprehensive study of ultrasonic and sintering parameters effect on the rapid manufacturing of pure copper components by ultrasonic assisted pressureless sintering and 3D printing. The customized sintering setup with the stepped horn was designed and fabricated for the experiments. First, the ultrasonic assisted sintering process was compared to conventional pressureless sintering process in regards of physical and mechanical properties of fabricated samples. The study revealed that the ultrasonic vibrations increased the contact area between the particles and raised the local heating for large sintering necks. Thus, the ultrasonic assistance enhanced the properties of the fabricated parts. Further, response surface methodology was used to design the set of experiments with sintering temperature, heating rate, soaking time and ultrasonic power percentage intensity to study the parameters effect. The analysis of variance was performed to analyze the significant contribution of parameters on relative density, compressive yield strength, and volumetric shrinkage. The sintering temperature and ultrasonic power percentage intensity dominated the other parameters with respect to responses. The less effect of low ultrasonic power intensity and short soaking time at high sintering temperatures was revealed by the significant interactions. Furthermore, the genetic algorithm based multi-objective optimization was used for the parameters optimization on maximizing relative density, compressive yield strength and minimizing the volumetric shrinkage. The efficacy of the developed method was tested by the fabrication of a customized heat sink on the optimized parameters.

Outstanding optical temperature sensitivity and dual‐mode temperature‐dependent photoluminescence in Ho3+‐doped (K,Na)NbO3–SrTiO3 transparent ceramics

AbstractHigh optical temperature sensing properties based on rare‐earth‐doped (K,Na)NbO3‐based ferro‐/piezoelectrics have attracted much attention due to their potential application in novel optoelectronic devices. Here, we fabricated Ho3+‐doped (K0.5Na0.5)NbO3–SrTiO3 transparent ceramics by conventional pressureless sintering. Their microstructures, transmittances, up‐conversion photoluminescence, and optical temperature sensing properties have been characterized in details. Because of the cubic‐like phase, dense, and fine‐grained structure as well as relaxor‐like feature, the ceramics exhibit high transmittance (~70%) in the near‐infrared region. Owing to Ho3+, green and red up‐conversion emissions have been observed, which can be easily modulated by temperature. The ceramics have stable emission colors (<200°C) and superior temperature‐modulating emission color‐tunable performance (>200°C). Furthermore, the temperature sensing behavior based on the thermally coupled levels (5F4, 5S2) of Ho3+ has been analyzed by a fluorescence intensity ratio technique. The transparent ceramics possess outstanding optical temperature sensitivity (~0.0096/K at 550 K), higher than most rare‐earth‐doped materials (e.g., ceramics, glasses, and phosphors).

Optimization of Process Parameters for Preparing Metallic Matrix Diamond Tool Bits by Microwave Pressureless Sintering Using Response Surface Methodology.

The process of preparing metallic matrix diamond tool bits by microwave pressureless sintering (MPS) was exclusively studied in this paper. The effects of the sintering temperature, the cold pressure, and the holding time on the mechanical properties of the bit were determined by using the response surface methodology (RSM) with Box-Behnken Design (BBD). In addition, with RSM, the second-order polynomial equation of mechanical properties was obtained. The solutions were well matched with the experimental values. This indicates that major variations in mechanical properties of the sintered sample could be predicted by the models, which shows that the applied model is accurate. Conventional pressureless sintering (CPS) experiments were also conducted to make a comparison. The experimental results showed that the MPS can enhance the mechanical properties of sintered samples. A possible MPS mechanism is proposed in this work after analyzing all the experimental results.

Fabrication of Cu Based Metallic Binder for Diamond Tools by Microwave Pressureless Sintering

Microwave pressureless sintering (MPS) method is successfully applied in the fabrication of Cu based metallic matrix for diamond tools. The main purpose of this work is to obtain better mechanical properties when the metal binder of the diamond tools was prepared by the MPS method. The orthogonal experimental method is adopted to design the sintering process parameters. The optimized experimental conditions are suggested as 880 °C of sintering temperature, 375 MPa of cold pressure, and 35 min of withholding time. The contrastive investigation of the MPS and conventional pressureless sintering (CPS) are performed under optimized conditions. The microstructures information are obtained by scanning electron microscopy (SEM), X-ray diffraction (XRD), electron probe microanalysis (EPMA), and the necessary mechanical properties, such as relative density, hardness, and flexural strength are tested. Experimental results show that the MPS method, compared with CPS, can significantly improve the mechanical properties of the metallic matrix. The factors of relative density, hardness, and flexural strength increase 1.25%, 3.86%, and 6.28%, respectively. The possible sintering mechanism of the MPS method is also discussed. This work may provide a reference for the fabrication of metal-based diamond tools by microwave heating method.

An ultrafast synthesis method of LiNi1/3Co1/3Mn1/3O2 cathodes by flash/field‐assisted sintering

AbstractLiNi1/3Co1/3Mn1/3O2 as a promising cathode material in lithium‐ion batteries was synthesized by flash/field‐assisted sintering technique for the first time. This study showed that the current‐limited synthesis of LiNi1/3Co1/3Mn1/3O2 could be carried out at temperatures less than 400°C for only 8 minutes, compared with the conventional pressureless sintering at 850°C for 12 hours. X‐ray diffraction results showed the phase evolution from precursor mixtures to the final LiNi1/3Co1/3Mn1/3O2 products during flash/field‐assisted sintering process and a well‐layered structure without undesirable cation mixing in the as‐formed LiNi1/3Co1/3Mn1/3O2. Combined with the lowered sintering temperatures and reduced sintering time, the excellent electrochemical performance of flash/field‐assisted sintered LiNi1/3Co1/3Mn1/3O2 materials suggested that this technique could be an energy‐efficient approach for the synthesis of lithium‐ion battery cathode materials and other materials requiring high‐temperature heat treatment.

An oscillatory pressure sintering of zirconia powder: Densification trajectories and mechanical properties

AbstractThe densification trajectories and mechanical properties of zirconia ceramics obtained by oscillatory pressure sintering (OPS) process were investigated, during the sintering process an oscillatory pressure was applied at three stages. Current results indicated that at intermediate stage the oscillatory pressure revealed a favorable improvement of mechanical properties compared with conventional hot pressing (HP) and pressureless sintering (PS) procedures, while the enhancement was not obvious at initial stage. When the oscillatory pressure was applied at final stage, the OPS specimens exhibited the highest bending strength and hardness of 1455 ± 99MPa and 16.6 ± 0.31GPa compared with the PS and HP specimens. Considering the high elastic modulus and Moiré patterns observed in the OPS specimen, the oscillatory pressure applied at intermediate and final stages was detected to facilitate the sliding of grain boundary, plastic deformation of monolithic grains, the removal of pores and the strengthening of atomic bonds.

Lead‐free (K,Na)NbO3‐based ceramics with high optical transparency and large energy storage ability

AbstractHighly transparent lead‐free (1‐x)K0.5Na0.5NbO3–xSr(Zn1/3Nb2/3)O3 (KNN–xSZN) ferroelectric ceramics have been synthesized via a conventional pressureless sintering method. All samples are optically clear, showing high transmittance in the visible and near‐infrared regions (~70% and ~80% at 0.5 mm of thickness, respectively). This exceptionally good transmittance is due to the pseudo‐cubic phase structure as well as the dense and fine‐grained microstructure. In addition, a high energy storage density of 3.0 J/cm3 has been achieved for the 0.94K0.5Na0.5NbO3–0.06Sr(Zn1/3Nb2/3)O3 ceramics with submicron‐sized grains (~136 nm). The main reason is likely to be the typical relaxor‐like behavior characterized by diffuse phase transition, in addition to the dense and fine‐grained microstructure. This study demonstrates that the 0.94K0.5Na0.5NbO3–0.06Sr(Zn1/3Nb2/3)O3 ceramic is a promising candidate of lead‐free transparent ferroelectric ceramics for new areas beyond transparent electronic device applications.

Enhanced properties of pure alumina ceramics by oscillatory pressure sintering

A novel oscillatory pressure sintering (OPS) process to consolidate high-quality pure alumina ceramics is reported. The microstructure of the ceramics prepared by OPS develops into a higher final density, a smaller and a narrower distribution of grain sizes compared with those prepared by conventional pressureless sintering (PS) and hot-pressing (HP) processes. Enhanced mechanical properties of alumina ceramics were investigated by OPS process. The bending strength, hardness and elastic modulus of the OPS specimen reached about 546MPa, 19.1GPa and 374GPa, respectively, i.e values significantly higher than that of the specimens by PS and HP. XRD analysis indicates the strengthening of atomic bonds aided by oscillatory pressure. The results suggest OPS to be an effective technique for preparing high-quality pure alumina ceramics.

Tetragonal Er3+-doped (K0.48Na0.48Li0.04)(Nb0.96Bi0.04)O3: Lead-free ferroelectric transparent ceramics with electrical and optical multifunctional performances

The lead-free Er3+-doped (K0.48Na0.48Li0.04)(Nb0.96Bi0.04)O3 (KNLNB-Er-x) ceramics were fabricated by conventional pressureless sintering. They possess a tetragonal perovskite phase with dense microstructure. High transmittances of the ceramics are obtained both in the visible and infrared regions. The optical band gap energies of them are 3.07–3.11eV, close to other KNN-based materials. The relaxor-like characteristics, good dielectric, ferroelectric and piezoelectric properties of the ceramics have been obtained. The up-conversion photoluminescence spectra have been studied and obvious color-tunable emissions have been observed by modulating temperature. The fluorescence intensity ratio (FIR) value based on green emissions at 533 and 555nm in the temperature ranging from 300 to 750K have been investigated, giving the maximum sensitivity of ~ 0.0038K−1. Furthermore, the FIR technique opens up a method to detect the Curie temperature of the ceramics. Owing to both optical and electrical properties, the KNLNB-Er-x transparent ceramics could be a promising candidate in the application of color-tunable solid-state lightings, optical temperature sensors, and electrical-optical coupling devices.