Subsequent Sintering Research Articles

Microhardness Optimization of Al–TiC Nanocomposite Produced by Mechanical Milling and Heat Treatment

In this study, the Al–TiC nanocomposite was produced by the mechanical milling and sintering process. Also, the optimization of the milling parameters was performed by the Taguchi method. The X-ray diffraction analysis, scanning electron microscopy, and microhardness test were used to analyze the phase characterization, microstructure, and mechanical properties of the Al–4% TiC nanocomposite. At first, the milling speed, milling time, and ball to powder weight ratio were considered as the input data, and the microhardness was considered as the output value of the Minitab software. According to the design of the experiment, 27 experiments must be performed, which were reduced to 9 by the Taguchi method. After the milling, the powders were subjected to the cold pressing and subsequent sintering at 450 °C. The microhardness results showed that the Al–4% TiC nanocomposite was formed with a maximum microhardness of 271 HV. Furthermore, a proper model was proposed and the results indicated that there was a good agreement between the experimental and predicted microhardness.

Structure, the Grain Structure, and the Dielectric Properties of Ceramics of Solid Solutions YCuxMn1 – xO3

Solid solution YCuxMn1 – xO3 ceramics of with x = 0.05, 0.10, and 0.15 have been produced by the solid-phase reaction method with subsequent sintering by common ceramic technology. The structure, the microstructure, and the dielectric properties of the ceramics have been studied, including the measurements of the temperature–frequency dependences of the relative dielectric permittivity and the dielectric loss tangent, and also the electrical conductivity. It is found that the mainly hexagonal phase forms in the ceramics; in this case, their grain structures are inhomogeneous and their synthesis occurs with the formation of liquid phases of eutectic origin. It is shown that the anomalous behavior of the dielectric characteristics at T = 30–200°C in all these ceramics is related to the effects of the interlayer polarization, and at T = 222°C (x = 0.05), 234°C (x = 0.10), and 247°C (x = 0.15), with the “isostructural” transition that is intermediate one between the ferroelectric and paraelectric phases.

Use of Microwave Radiation for Denitration of Uranyl Nitrate Solution and Subsequent Sintering of Uranium Dioxide Fuel Pellets

Use of Microwave Radiation for Denitration of Uranyl Nitrate Solution and Subsequent Sintering of Uranium Dioxide Fuel Pellets

Origin of low thermal conductivity in Nb1-xTixFe1.02Sb half-Heusler thermoelectric materials

Nb1-xTixFe1.02Sb half-Heusler thermoelectric materials were synthesized trough arc melting and subsequent spark plasma sintering (SPS). Doping effect of Ti at Nb-site results in high power factor due to the optimization of hole carrier concentration. Further, the lattice thermal conductivity reduced as the Ti content increased, which mainly resulted from point defects and mass fluctuation by Ti doping as well as coherent nano-scale second phase, Ti-rich and Fe-deficient Ti1.18Fe0.57Sb. For the sample of Nb0.6Ti0.4Fe1.02Sb, a minimum lattice thermal conductivity of ∼2.08 W/mK was obtained. This is attributed to the enhanced phonon scattering at the alternating white and dark lamellar interphase boundaries of (Nb0.6,Ti0.4)FeSb half-Heusler and Nb-doped Ti1.18Fe0.57Sb. A maximum thermoelectric figure of merit, ZT, of ∼0.81 was obtained at 973 K, which is comparable with values previously reported for this system.

Red mud-molten salt composites for medium-high temperature thermal energy storage and waste heat recovery applications

Red mud (RM) is an industrial waste of the aluminum industry with presently estimated worldwide legacy-site stockpiles of 4 billion tones. RM is typically disposed in the sea, dams or dykes, posing a significant environmental hazard due to its high alkalinity and traces of heavy metals. Despite recent valorization efforts, only 15% of RM deposits are currently utilized. In this work, a novel use of RM to formulate composite phase change materials (CPCMs) is proposed. The CPCM is formulated by milling nitrate salts with RM, compressing and subsequent sintering of the two. Overall good performance over the temperature range of 25–400 ℃ is observed. Maximum latent heat of the CPCMs is 58 J/g, while average thermal conductivity and Cp are in the range of 0.77–0.83 W/mK and 1.03–1.31 J/g ℃, respectively. No variations in the melting point or latent heat are observed after 48 cycles. Energy storage density is calculated to be up to 1396 MJ/m3. The working temperature of this novel CPCM make it ideal for waste heat recovery of medium-high temperature waste heat streams providing a valorization pathway and valorization for RM as a by-product for energy-related applications.

Solid-state Synthesis and High Magnetostriction Performances of Heavy Rare Earth–Free Sm0.88Nd0.12Fex Particulate Composites

The magnetostrictive materials are widely applied as modern smart materials converting electrical to mechanical energy. Here, the pseudobinary Sm0.88Nd0.12Fex compounds, free of both (expensive) heavy rare earths and B/Co stabilizer, have been prepared with mechanical alloying and subsequent solid-state sintering process. An amorphous phase can be formed by high-energy ball milling, followed by the formation of single-phase Sm0.88Nd0.12Fe1.80 alloy at an annealing temperature of 690 °C. A high anisotropic magnetostriction λa as large as 1280 ppm for the as-sintered alloy is obtained at an external field of 12 kOe. The oriented pseudo 1-3 type particulate composite is fabricated by embedding and aligning the as-sintered alloy particles in an epoxy matrix under an oriented magnetic field. The composite with a volume faction of 30% shows large magnetostriction (λa ~895 ppm at 12 kOe). In particular, λa and the longitudinal magnetostriction λ|| at a low field (3 kOe) reaches a value as high as 630 and 450 ppm, respectively, 78% and 84% larger than that of its as-sintered polycrystalline alloy although Vp is only 30%. Ultimately, it is anticipated that this work will open a new design paradigm to develop Sm-based materials with negative-magnetostriction performance.

Interfacial Engineering in a Cathode Composite Based on Garnet‐Type Solid‐State Li‐Ion Battery with High Voltage Cycling

AbstractGarnet‐type solid electrolyte is a promising candidate for the fabrication of high energy all‐solid‐state Li‐ion batteries (ASSLIBs), but its use is hampered by a large interfacial resistance. Herein, we propose a surface modification and subsequent sintering to enhance the interfacial connection between the cathode and the solid electrolyte. The ASSLIB prepared by this method delivered an initial discharge capacity of ∼66 mAh g−1 (80 °C) at a rate of 0.1 C. However, the poor contact between the cathode and electrolyte triggered the increase of the interfacial resistance, which caused severe capacity decay upon cycling. The encapsulation of LiCoO2 particles with LiBO2 using a single‐step sintering process dramatically increased the interfacial contact, resulting in a higher discharge capacity of 116 mAh g−1 with good cycling behavior. Therefore, surface modification of the cathode offers a reduction of resistance and promotes efficient Li‐ion transfer pathways across the cathode/solid‐electrolyte interface.

Microstructure and mechanical properties of WC-(Ti,W)C-Co hardmetals fabricated by in-situ carbothermal reduction of oxides and subsequent liquid phase sintering

WC-(Ti,W)C-Co hardmetal was fabricated using carbon black, WC, Co, WO3, TiO2 as raw materials, through an energy-efficient and cost-efficient method that the process of carbothermal reduction of oxides and the liquid phase sintering of hardmetal were combined in one heating cycle. Phase transformation and microstructure characteristics of the hardmetals were examined by XRD, SEM, EDX, and TEM. Moreover, mechanical properties were tested with related mechanical property testing instruments. Results demonstrated that the in-situ carbothermal reactions and outgassing processes had finished below 1200 ℃. Microstructures of hardmetal consisted of WC grains, (Ti,W)C solid solution, and Co binder phase. Moreover, the mean size of WC and (Ti,W)C grains were both smaller than those of hardmetal prepared with TiC powders. A coherent relationship between Co and (Ti,W)C grain was noticed. Moreover, a coherent relationship between two (Ti,W)C grains was observed. Compared with hardmetal prepared with TiC powders, TRS and KIC were improved. The increase of TRS was chiefly due to the smaller grain size and the coherent interface structure. Moreover, the increase of KIC was ascribed to the (Ti,W)C solid solution and the coherent interface structure. Hardmetals prepared through this method showed good mechanical properties with a hardness of 90.7 ± 0.2 HRA, a TRS of 3007 ± 87 MPa, a KIC of 13.1 ± 0.2 MPa·m1/2.

Influence of porosity and impurities on the thermal conductivity of pressure-less sintered Cu powder green bodies

ABSTRACT Copper as a material with a high electrical and thermal conductivity awakes large interest for many applications in industry, e.g. thermal management of electronic components. Powder-based manufacturing techniques (e.g. Selective Laser Melting, Binder Jetting, Fused Filament Fabrication and Metal Injection Molding) enable the complex shaping of metals. Especially the methods without melting processes like Binder Jetting, Fused Filament Fabrication and Metal Injection Molding have a great potential for complex Cu structures. These techniques built up a powder-based green body and require a subsequent sintering step to reach a high density with maximum properties. This work reports the development of the heat conductivity during pressure-less sintering of Cu powder green bodies. The experimental results are compared to analytical models and a numerical simulation and show the limits of the reachable heat conductivity depending on the remaining porosity and the impurity concentration.

Improving the uniformity of strength distribution in cylindrical specimens formed from iron based powders

The distribution of microhardness in cylindrical samples formed from industrial iron-based powders and not subjected to sintering is experimentally studied. This distribution is described by a simple mathematical model. It is shown that the measured hardness has a significant correlation with the strength. Microhardness is correlated with the local strength. Because, during the subsequent sintering and heat treatment of the powder sample, phase transformations occur within the individual grains, the microhardness distribution that occurs before the sample is sintered will remain after sintering. The technology of microhardness equalization in the sample at the stage of its forming, before sintering, is described.

ВЛИЯНИЕ ДИСПЕРСНОСТИ ГЛИНОЗЕМА НА ТЕРМИЧЕСКИЕ ПРЕВРАЩЕНИЯ И СПЕКАНИЕ МНОГОКОМПОНЕНТНОЙ КЕРАМИЧЕСКОЙ МАССЫ

The effect of alumina wet mechanical grinding time on its fineness as well as subsequent solid-phase transformations and sintering of multicomponent ceramic mass based on it upon heating to 1000 °C was studied using x-ray diffraction, differential thermal analysis, nitrogen adsorption, granulometry and dilatometry.

ИЗГОТОВЛЕНИЕ И ИССЛЕДОВАНИЕ ТАБЛЕТОК СМЕШАННЫХ НИТРИДОВ УРАНА, ПЛУТОНИЯ, АМЕРИЦИЯ И НЕПТУНИЯ

The method of carbothermic synthesis produced 260 g tablets of mixed nitrides of uranium, plutonium, americium and neptunium with a mass fraction of americium 0.61%. For the production of tablets were used uranium oxide manufactured by the water method, as well as plutonium dioxide containing impurities of uranium and americium oxides with a mass of 0.9 g americium, obtained by volumetric crystallization method in a NaCl — 2CsCl melt. Neptunium dioxide and americium oxide were added to the mixture of uranium and plutonium oxides before carbothermic synthesis. The resulting tablets had a density of 11.6-11.9 g / cm3. By methods of gamma-spectrometry, scanning electron microscopy and x-ray microanalysis is showed that the heat treatment of the mixture of the source chemicals - oxides of uranium, plutonium, americium, neptunium with carbon black at a temperature 1600-1500 °C for 72 h (24 h in nitrogen and 48 h in nitrogen-hydrogen atmospheres), and also the subsequent sintering synthesized by the press powders for 48 h at a temperature of 1800°C in nitrogen-hydrogen atmosphere doesn't leads in average to the significant losses of americium.

Influence of binder content on physical and mechanical properties of composite materials such as metal powder-waxy substance

The scientific basis for the development of working materials from waxy highly filled composites for FDM 3D printing using is not yet sufficiently developed. The need to develop such composites is due to the fact that highly filled waxy composites, on the one hand, have low 3D printing temperatures, and on the other hand, the fact that the waxy substance has a low ash content and is completely removed during annealing and subsequent sintering of the residual framework from the filler. Therefore, in this work, the influence of the binder content on the physical and mechanical properties of composite materials such as metal powder - waxy substance in the ratio of metal to waxy substance, as 50/50, 60/40, 70/30, 80/20 % (by volume), as well as the shape change of composite samples when they are heated at a temperature from 61 to 230 °C. Carbonyl nickel and carbonyl iron were used as metal powder, and beeswax and paraffin were used as a binder. It was found that the developed surface of particles significantly affects the density dependence, the microhardness and compressive strength of composites from the binder content, the actual density of the composite samples after pressing is less than the calculated by the additivity formula with the binder content up to 40 % (by volume) and only with an increase in the binder content to 50 % (by volume) does the actual density approach the calculated additive; the developed surface of nickel particles several times increases the strength of the composite in comparison with iron at the same binder content, at the same time, the dependences of the microhardness differ significantly: on Ni-wax samples, it tends to decrease with an increase in the wax content, and on samples Fe-paraffin - to an increase, which is due to the influence of such phenomena as adhesion and cohesion, mechanical adhesion. The level of strength of composite samples with a binder content of 40-50 % (by volume) is sufficient so that they do not collapse not only in 3D print head The resulting experimental data can be extended to other similar systems when creating working materials for FDM 3D printing based on carbonyl nickel and iron powders.

Constitutive Modeling of a Fine‐Grained Ti2AlNb‐Based Alloy Fabricated by Mechanical Alloying and Subsequent Spark Plasma Sintering

The flow stress evolution of a fine‐grained Ti2AlNb‐based alloy fabricated by mechanical alloying and subsequent spark plasma sintering is experimentally acquired for different deformation conditions. Isothermal uniaxial compression test is conducted in the deformation temperature range of 950–1070 °C and the strain rate range of 0.001–1 s−1. Based on the experimental flow stress with friction correction, the modified Johnson–Cook model and strain‐compensated Arrhenius‐type model are established in the α2 + β/B2 + O triple‐phase and α2 + B2 two‐phase fields. The average absolute relative error (AARE) and determination coefficient (R 2) are 9.78% and 0.9858 for the modified Johnson–Cook model, and 4.19% and 0.9927 for the strain‐compensated Arrhenius‐type model. Compared with the modified Johnson–Cook model, the strain‐compensated Arrhenius‐type model is more suitable for predicting the high‐temperature flow stress of a fine‐grained Ti2AlNb‐based alloy fabricated by mechanical alloying and subsequent spark plasma sintering.

Superhard B4C-ReB2 composite by SPS of microwave synthesized nanopowders

The challenge to develop new robust materials inspired the current research on a microwave synthesis and subsequent spark plasma sintering of a lightweight superhard B4C-27wt.%ReB2 ceramic composite. A strategy for fabricating high density parts was developed by sintering the microwave synthesized B4C-ReB2 composite without additives. The bulk material produced at 1900 °C temperature, 50 MPa pressure and dwell time of 10 min exhibited microhardness of 50 ± 3 GPa, estimated in the asymptotic-hardness region at 49 N load, exceeding the hardness of both constituents. Fractured surfaces of fully dense B4C-27wt.%ReB2 composite evidenced neither obvious pores nor undersintered areas. The high value of hardness ranks the composite among the superhard materials supposedly due to the strong interphase formation via increased number of boron atoms and increased number of sigma bonds in ReB2 unit cell.

Microstructure refinement and mechanical properties improvement in the W-Y2O3 alloys via optimized freeze-drying

Oxide-dispersion-strengthened (ODS) tungsten based alloys with high performance were synthesized by optimized freeze-drying with PVP and PEG polymer dispersant additives and subsequent low temperature sintering. The pre-freezing principle developed firstly in our work shows that fast cooling rate and polymer dispersant additives contribute to the refinement and uniformity of precursor powders. The optimized freeze-dried W-Y2O3 composite powders possess an average grain size of 9 nm, which can be attributed to the steric hindrance provided by polymer dispersants during freeze-drying and voids left by the decomposition of polymer dispersants during calcination. After sintering at low temperature, the W-Y2O3 alloys have ultrafine W grain (210 nm) and nano second phase particles (<50 nm) dispersed in W matrix while maintaining a high relative density of 97.8%. These factors mentioned above lead to the microhardness and compressive strength as high as 721 ± 31 HV0.2, 814 MPa respectively, accompanied by a high total deformation plasticity of above 50%. These results indicate that the optimized freeze-drying and subsequent low temperature sintering is a promising way to fabricate ODS tungsten based alloys with high performance.

Harmonic Structure Design: A Strategy for Outstanding Mechanical Properties in Structural Materials

Structured heterogeneous materials are ubiquitous in a biological system and are now adopted in structural engineering to achieve tailor-made properties in metallic materials. The present paper is an overview of the unique network type heterogeneous structure called Harmonic Structure (HS) consisting of a continuous three-dimensional network of strong ultrafine-grained (shell) skeleton filled with islands of soft coarse-grained (core) zones. The HS microstructure is realized by the strategic processing method involving severe plastic deformation (SPD) of micron-sized metallic powder particles and their subsequent sintering. The microstructure and properties of HS-designed materials can be controlled by altering a fraction of core and shell zones by controlling mechanical milling and sintering conditions depending on the inherent characteristics of a material. The HS-designed metallic materials exhibit an exceptional combination of high strength and ductility, resulting from optimized hierarchical features in the microstructure matrix. The experimental and numerical results demonstrate that the continuous network of gradient structure in addition to the large degree of microstructural heterogeneity leads to obvious mechanical incompatibility and strain partitioning, during plastic deformation. Therefore, in contrast to the conventional homogeneous (homo) structured materials, synergy effects, such as synergy strengthening, can be obtained in HS-designed materials. This review highlights recent developments in HS-structured materials as well as identifies further challenges and opportunities.

Товщина покриття відновленого контакту електричного апарату та методика її визначення

The regularities of the formation of the main variants of coatings obtained by gas-plasma spraying with subsequent liquid-phase sintering, depending on the conditions of wetting the powder particles with the melt and the porosity of the sprayed layer, have been established. The analysis of methods for controlling the thickness of thermal gas coatings, which indicates the advantages and disadvantages of three methods of measuring the thickness: magnetic, electromagnetic (eddy currents) and metallographic. The research of the dependence of the thermal power of the plasma jet (directly affects the thickness of the sprayed coating) on the thermal equivalent of the electric power of the arc was carried out; dependence is characterized by an effective efficiency. As a result of the research, the thickness of the sprayed coatings of the contacts and the tolerances for their processing were calculated depending on the height of the contact piece. Coating thickness is the shortest distance between the coating surface at a given point and the base metal surface. As a technological characteristic, the thickness affects such an important performance property as the strength of the bond with the base metal. Increasing the thickness of the coating above the optimum leads not only to a deterioration in quality (decrease in bond strength), but also to economic costs. The coating was applied to a given area due to the linear movement of the spray device in combination with either the transverse displacement of the spray device or contact piece. With a single linear movement of the spray device, the sprayed material forms a uniform coating layer, the cross-section of which is described by the Gaussian curve. The requirements for the thickness of the sprayed layer are indicated in the technological documentation, based on the uniformity of the distribution of the coating thickness on the contact surface. The required thickness of the sprayed coating was determined based on the thickness of the working layer, the tolerance for subsequent processing and the tolerance for the dimensions of the contact piece and the thickness of the sprayed layer. The thickness of the sprayed coating on the contacts, in which the lost dimensions are restored, provided for a tolerance for their following processing: turning and grinding. As a result, the thicknesses of the sprayed coatings of the contacts and the tolerances for their machining were calculated depending on the height of the contact piece (the number of switching cycles of the sprayed contact pieces). In thermal spraying of coatings, the build-up of parts occurs at a relatively high rate, that is, approximately 10 - 20 times faster than electrolytic deposition of steel and approximately 100 - 200 times faster than electrolytic chromium plating.

Effect of sintering aids on microstructure and properties of textured SiC ceramics prepared in 6 T

ABSTRACT C-axis textured α-SiC ceramics were prepared by gel-casting in a strong magnetic field of 6 T and subsequent sintering at 1950 °C for 2 h. The effects of sintering aids on the microstructure and properties of textured SiC ceramics were discussed. Results showed that highly textured α-SiC green bodies were obtained by gel-casting in a strong magnetic field of 6 T. The sintering process slightly suppressed the texture development of α-SiC ceramic. The degree of texture in the α-SiC green bodies and sintered bodies both showed a decreasing trend with the increase of sintering aids content. And the increase of the sintering aids content promoted the densification and improved the bending strength of α-SiC ceramics. Besides, the bending strength of textured SiC ceramics showed an anisotropy based on the grain orientation.

Production of Crystal Oriented Neodymium Silicate Oxyapatite Ceramics and Their Conductivity

Production of oxide ion conductor ceramics having high conductivity is important for applications of solid oxide fuel cells (SOFC) and chemical sensors. Particularly, the ceramics with high ionic conductivity around 873 K have attracted attention for development of SOFC and sensors operated at middle-temperature.Neodymium silicate oxyapatite, denoted by NSO hereafter, was known to show very high oxide ion conductivity along to the c-axis below 873 K [1]. To utilize this property, production of the crystal-oriented ceramics is necessary. In this research, we produced crystal-oriented NSO green bodies by a slip casting under a magnetic field less than 1 T. Dense and crystal-oriented NSO ceramics were obtained by subsequent conventional sintering under an ambient atmosphere. In addition, NSO ceramics having different degree of crystalline orientation were successfully produced. Using these ceramics, the degree of the crystal orientation and its dependence on conductivity were discussed on the basis of experimental data.Composition of NSO was Nd9.43(SiO4)6O2, which was confirmed to be close to the oxyapatite single phase (NSO has a wide composition range [2], [3]). The raw materials were Nd2O3 and SiO2. NSO was synthesized by solid state reaction method at 1473 K or 1773 K.For production of crystal-oriented NSO green body, controlling the distribution of particle size was important to prepare a slurry used for slip casting. Therefore, the synthesized NSO powder was grinded by planetary ball mill. The slurry was prepared by mixing of the grinded NSO powder and polyethyleneimine in distilled water. The c-axis oriented NSO green body was prepared by slip casting under a magnetic field of 1 T and 0.5 T. These green bodies were denoted by NSO-1T and NSO-0.5T, hereafter. The green body produced without magnetic field was denoted as NSO-0T. The green bodies were sintered at 1873 K for 6 h in air in order to obtain the dense ceramics.X-ray diffraction (XRD) patterns on surfaces of (a) NSO-0T, (b) NSO-0.5T and (c) NSO-1T are shown in the Figure. XRD profile of (a) NSO-0T is good agreement with that of the oxyapatite single phase (standard data (d), ICSD No. 01-075-7767). Regarding to the patterns of (b) NSO-0.5T and (c) NSO-1T, intensities of (00l) peaks were very high, compared to those of (a) NSO-0T and standard data. The degree of crystal orientation was evaluated by Lotgering factor [4] calculated from these XRD patterns. Lotgering factors of the densified NSO-1T and NSO-0.5T were 0.93 and 0.95, respectively. In the case of perfect crystal-orientation, Lotgering factor is unity. Contrary, the factor is zero for random crystalline orientation. The factor values of 0.93 and 0.95 indicate that the degrees of the c-axis orientation are very high.Conductivity was measured by electrochemical impedance method between 773 K and 1273 K under air. The conductivity was found to be depended on the Lotgering factor values. Conductivities at 773 K of the samples sintered from NSO-1T, NSO-0.5T, and NSO-0T were 3.97×10-3, 3.04×10-3, and 3.88×10-4 S cm-1, respectively. The conductivities of the samples with highly c-axis orientation was about 10 times higher compared to that of randomly oriented samples. The conductivities of sintered NSO-0.5T and NSO-1T along to the c-axis at 773 K are confirmed to be close to that of a single crystal (6.4×10-3 S cm-1) [5].Considering into high magnetic susceptibility of neodymium, the crystal-oriented NSO ceramics can be fabricated under magnetic fields less than 1 T and high conductivity was achieved comparable to that of single crystal.Reference[1] S. Nakayama, et al, Journal of the European Ceramic Society, 19, 507-510, (1999).[2] R. O. Miller, et al, J. Am. Ceram. Soc., 47, 653-654, (1964).[3] G. J. McCathy, et al, J. Inorg. Nucl. Chem., 29, 253-254, (1967).[4] F. K. Lotgering, J Inorg. Nucl. Chem., 9, 113-123 (1959).[5] S. Nakayama, et al, Journal of materials science letters, 20, 913-915, (2001). Figure 1