High Sintering Density Research Articles

Design, synthesis and improved sintering behavior of core-shell Mo–Cu composite powders

Molybdenum-copper composites have attracted increasing attention due to their excellent properties in terms of electrical conductivity, thermal conductivity and mechanical property, which are closely related to the density and composition of the sintered material. However, high temperatures are required to obtain high sintering density, resulting in loss of copper in most cases. In this work, a novel core-shell Mo–Cu composite powder was prepared by a ball-milling method followed by hydrogen reduction. The phase compositions and microstructure of prepared powders were studied by XRD, TEM and EDS, and the formation process of core-shell structure of Mo–Cu powders was discussed in detail. It was shown that the significant difference in reduction temperatures between molybdenum and copper oxides contributes to the formation of core-shell structure.

Spray pyrolysis-based synthesis of pure molybdenum powder with nanoscale primary particles and its sintering characteristics

Molybdenum (Mo) is a refractory metal with exceptional high-temperature stability, resistance to creep, and low coefficient of thermal expansion, it thus has great potential for various high-temperature applications. Nevertheless, conventional methods of synthesizing Mo powder, such as mechanical milling and wet chemical processing, have encountered limitations in producing sintered bodies with high relative density and a fine grain structure while maintaining a low sintering temperature. Therefore, it is crucial to develop an efficient powder synthesis method that enables precise and uniform control over the powder's size and shape while minimizing contamination. Such an approach would contribute to achieving excellent formability and sinterability for the resulting Mo sintered body. In this study, we successfully demonstrated the ultrasonic spray pyrolysis (USP) technique in combination with hydrogen reduction for synthesizing pure Mo powder. The resulting Mo powder exhibited micro-sized secondary particles composed of nanoscale primary particles, leading to improved formability and sinterability, essential for fabricating high-quality sintered bodies. In order to comprehensively understand the properties of the as-prepared molybdenum oxide and hydrogen-reduced Mo powder, we employed various methods to analyze its physicochemical characteristics systematically. Subsequently, the fine Mo particles were subjected to a pressureless sintering process after a conventional uniaxial powder molding. Due to its excellent formability and sinterability, we achieved a Mo sintered body with a grain size of approximately 3 μm while maintaining a relatively high sintering density of 98.3% at a comparatively low sintering temperature of 1400 °C. The resulting microstructure contributed to a relatively high hardness of around 234 Hv. These results indicate the successful synthesis of pure Mo powder with desirable properties and highlight its potential in various applications, particularly those requiring high-quality sintered bodies with enhanced hardness.

Preparation and characterization of LLZO-LATP composite solid electrolyte for solid-state lithium-ion battery

All-solid-state lithium-ion batteries (ASSLB) demonstrate significant advancements in energy density and safety comparing the traditional lithium-ion batteries, which using liquid electrolyte. The performance of ASSLBs strongly depends on the characteristics of solid electrolyte materials. Currently, Li1.3Al0.3Ti1.7(PO4)3 (LATP) and Li7La3Zr2O12 (LLZO) have the best individual performances in the field of solid ceramic electrolytes. The purpose of this study is to investigate the characteristics of the LLZO-LATP composite electrolyte sintering at various temperatures. The dense LLZO-LATP solid composited electrolyte with acceptable electrochemical performance is successfully prepared using planetary ball-mill method. The phase and morphology of the samples were assisted by X-ray diffraction and Field-emission scanning electron microscopy. The LLZO-LATP composite sintered at 800 °C has less chemical reaction with small amount of second phase with high sintering density of 88.4%. The ionic conductivity of LLZO-LATP sintered at 800 °C is 1.3 × 10−6 Scm−1. This study addresses the lowering of sintering temperature of a composite solid electrolyte with two different structure electrolyte materials, is informative for its future research and to be promising in solid-state batteries.

Facile Production Method of PbS Nanoparticles via Mechanical Milling of Galena Ore

In this research, some physical properties such as the density, specific heat capacity, and micro-hardness of galena ore lumps purchased from the public market were determined. The microscopic study, using the scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS), confirmed that the as-received galena ore was mostly lead sulfide (PbS). The XRD pattern of the galena powder also elucidated that all the peaks were assigned to PbS. In addition, the as-received galena was roughly crushed, and fine-milled using a high-vibration milling machine with tungsten carbide rings. Nanoscale particles of about 90 nm were produced in a very short milling time of around 15 min. The obtained nanoscale powder was well investigated in the SEM at low and high magnifications to assess the exact range of particle size. Meanwhile, the SEM was employed to investigate the microstructure of sintered samples, where a part of the milled galena powder was compacted and sintered at 700 °C for 2 h. Again, the result of this investigation proved the formation of PbS with even smaller grain size compared with the grain size of the starting galena ore. A high relative sinter density of approximately 97% for galena powder was achieved by sintering under vacuum.

The effect of compaction pressure, sintering time, and temperature on the characterization of an aluminum/alumina composite with rising alumina proportions

The purpose of this article is to investigate the effect of various process parameters such as compaction pressure, sintering temperature, and time on the physio-mechanical properties of a powder metallurgy-fabricated composite made of pure aluminium/alumina. Temperatures (580°C, 600°C, and 630°C), periods (1.5, 2, and 2.5 hr), compacting loads (30KN-65KN), and alumina percentages (2, 4, 6, and 8weight percent) are all considered. X-ray diffraction (XRD) and X-ray fluorescence spectroscopy (XRF) studies are carried out to determine the phases present and their proportions. Crystallite size study is performed using XRD data, and the Al+4 weight % alumina composite has the smallest size of any composite tested. For optimization, sintering density, porosity, and microhardness are calculated. Scanning electron microscopy (SEM) is used to analyse the different microstructures. At 600°C, 2 hr of operating time, and 4weight% alumina additions, the highest sintering density and microhardness are found.

Pure Ti fabricated by cold isostatic pressing and sintering TiH2 powder

ABSTRACT The use of TiH2 as initial powder can produce nearly full-density titanium with good mechanical properties. In this paper, compaction and sintering behaviors of TiH2 and HDH Ti powders were investigated at various combinations of particle size distributions, pressures, and compact methods. Main attention was given to a manufacturing route that included cold isostatic pressing (CIP) is potentially capable of producing complicated shape components with homogeneous density distributions. The results showed the priority of using TiH2 powder as a starting material in the CIP-and-sinter manufacturing route. Compared to materials processed from HDH Ti powder, the TiH2-based materials showed lower porosity, more homogeneous distribution of fine spherical pores, and lower oxygen content, reaching a property balance that met requirements of CP Ti specifications. Advantage of CIP over uniaxial die pressing (UDP) was also shown and explained by specific features of the two compaction methods. The higher sintered density (99.1%) and acceptable oxygen content (0.18%) of the materials processed from TiH2 powder of 0–45 µm provide mechanical characteristics (UTS 516.1 MPa, Elongation 26.8%) suitable for practical application.

Effects of the Sn4+ Substitution and the Sintering Additives on the Sintering Behavior and Electrical Properties of PLZT

(Pb, La)(Zr, Ti)O3 (PLZT) with antiferroelectric properties can be applied as a capacitor whose capacitance increases in a high electric field. From this, we obtained a high sintering density at 950 °C by adding low-temperature sintering additives, 8.0 wt% of PbO and 2.5 wt% of ZnO, simultaneously to a (Pb0.88, La0.12)(Zr0.86, Ti0.14)O3 composition. The change in electrical characteristics was confirmed in terms of Sn4+ substitution, resulting in no change in the sintering density by Sn4+ substitution. However, as the amount of Sn4+ substitution increases, the dielectric constant gradually decreases from 1300 to 700, and the grain size decreases from about 4 to 1 µm in terms of microstructure. In the crystal structure analysis, the general formation of a single perovskite structure was confirmed. The results of the hysteresis curve measurement revealed that the breakdown electric field increases from 4 to 9 kV·mm−1 as the amount of Sn4+ substitution gradually increases. However, polarization decreases in the same way as the permittivity trend. The composition exhibits excellent electrical properties when the ratio of Sn4+ is 0.4: a high energy storage density of 3.5 J·cm−3, energy efficiency of 80%, and breakdown electric field of about 8.5 kV·mm−1.

Sintering Behavior and Hardness of Tungsten Prepared by Hard Metal Sludge Recycling Process without Ammonium Paratungstate

In this paper, we suggest a novel recycling process for hard metal sludge that does not use ammonium paratungstate. Ammonia, which in the conventional recycling process is essential for removing sodium and crystallized tungstate, was not used in the novel process. Instead of ammonia, acid was used to remove the sodium and crystallized tungstate resulting in the formation of tungstic acid (H2WO4). Tungsten powders were successfully synthesized by hydrogen reduction of the tungstic acid through H2O decomposition, WO3 to WO2 reduction, and tungsten metal formation. The tungsten powders prepared from tungstic acid were spherical in shape and had a higher sintering density than the facet-shaped tungsten powders prepared from tungsten oxide. The spherical shape of the tungsten powders enhanced their sinterability and resulted in an increase in the size of grains. This is a result of the high diffusion rate of the atoms along the particle surfaces. Despite having a higher density, the hardness of the sintered tungsten was lower than that of tungsten from tungsten oxide. High energy milling effectively reduced grain size and improved hardness. The hardness of the tungsten prepared from milled tungstic acid was enhanced to a value (max. 471 HV) higher than the best previously reported value (389 HV). In sum, tungsten can be hardened, thereby improving its sinterability and reducing grain size, with tungstic acid prepared using the proposed recycling process.

Development of a High-Performance-Ferrite Magnet Fabrication Process without Sintering Additives

Sintered M-type hexaferrites with the chemical formula of Sr0.3Ca0.4La0.3Fe9.8Co0.2-xMnxSi0.135O19-d (<i>x</i> = 0, 0.05, 0.1, 0.2) and Sr0.3Ca0.4La0.3Fe9.8-yCo0.2MnySi0.135O19-d (<i>y</i> = 0.05, 0.1, 0.2) were prepared by conventional solid station reaction routes. A high sintering density of more than 95% of the theoretical density was achieved in all hexaferrite samples when calcination was carried out at 1100 oC for 4 h, followed by sintering at 1230-1250 oC for 2 h without the use of sintering additives. High saturation magnetization and coercivity were achieved simultaneously at the <i>x</i> = 0.05 composition, where Mn replaces part of the Co. The secondary phase Fe2O3 generated by the initial addition of SiO2 was gradually reduced when the Fe contented was decreased in the Sr0.3Ca0.4La0.3Fe9.8-zCo0.15Mn0.05Si0.135O19-d samples, and a single M-type hexaferrite phase was confirmed in the Sr0.3Ca0.4La0.3Fe8.3Co0.15Mn0.05Si0.135O19-d (<i>z</i> = 1.5) sample, which also exhibited optimized hard magnetic properties, with a saturation magnetization of 4581 G and coercivity of 4771 Oe. Anisotropic sintered magnets were fabricated using the optimized composition, and showed excellent hard magnetic properties, with a remanent magnetic flux density of 4400 G and intrinsic coercivity of 4118 Oe, and a maximum energy product of 4.72 M·G·Oe. This result is very promising because high magnet performance can be achieved with a single batch process without the need for sintering additives during the process.

Investigation of the effect of the SiC particle size on the properties of the AlN–SiC composite ceramic

In this paper, high density AlN–SiC composite ceramic with Y2O3 as the sintering additive was prepared by pressureless sintering at 2000 °C in an inert gas atmosphere. The effects of original SiC particle size on the properties of the AlN–SiC composite ceramic, such as density, thermal conductivity, electrical resistivity and dielectric properties at X band were characterized and investigated. With the increase of SiC particle size, the density of AlN–SiC composite ceramic decreased, the electrical resistivity increased, and the dielectric constant decreased while the dielectric loss (tgδ) increased. XRD analysis showed no solid solution formed between AlN and SiC. SEM showed a uniform microstructure in the AlN–SiC composite ceramic with micron SiC powder. In this study, AlN–SiC composite ceramic with 2 μm SiC powder showed the best performance, with the highest thermal conductivity of 45.53 W/(m·K) at room temperature, relatively higher tgδ value of 0.37–0.47 and lower dielectric constant of 14–17, reasonably high sintering density and electrical resistivity.

Low temperature sintering ZnO - Bi2O3 based varistor ceramics with low electrical breakdown voltage and high nonlinear coefficient

In this study, a novel pre-synthesized Bi–Sb–O phase was produced and doped into the ZnO– Bi2O3– Co2O3– MnO2 (ZBCM)- based varistor to make an improvement in nonlinear coefficient (α) and a decrease in breakdown voltage (E1mA). Results showed that the ZBCM- Bi–Sb–O ceramics can be well sintered with high sintering density above 0.98 of the ZnO theoretical density at a low sintering temperature of 900 °C. A large average grain size (d) of 6.89 μm was obtained in the ZBCM – 3 wt% Bi–Sb–O varistor, which leads to a low E1mA of 235.0 V/mm. At the same time, the α increases to 53.0 due to the improved barrier height (ϕb) of 1.85 eV. Furthermore, the lowest dielectric loss (tanδ) at low frequency as well as the increased grain resistance (Rg) made the smallest leakage current density (IL) of 0.005 μA mm−2 in the same Bi–Sb–O doping content. Besides, a considerable low E1mA of 168.91 V/mm with a mildly α of 37.4 was obtained in the ZBCM– 1 wt% Bi–Sb–O varistor. Hopefully, this novel research provided a new strategy to prepare low-voltage ZnO– Bi2O3- based varistors at low sintering temperature.

W-Y2O3 composite nanopowders prepared by freeze-drying method and its sintering characteristics

In order to prepare high performance oxide-dispersion-strengthened tungsten based alloys, the W-Y2O3 composite powder precursors were synthesized by novel freeze-drying method. The average W grain size of prepared W-Y2O3 composite powder is 14.2 nm, and corresponding grain size distribution is extremely narrow. The low-temperature sintered W-Y2O3 alloys possess an ultrafine W grain size of about 410 nm while maintaining a comparatively high sintering density of 98.1%. The Y2O3 particles (20–50 nm) with a W-Y-O diffusion layer and Y2WO6 particles (<20 nm) distribute uniformly within W grains and especially at W grain boundaries. These W-Y-O phases adsorb free oxygen impurity, resulting in the purification and strengthening of W matrix. What's more, coherent or semi-coherent interfaces are observed between Y2WO6 particles and W matrix, reinforcing the bonding strength of phase boundary. The combined action of the factors mentioned above leads to the hardness of sintered W-Y2O3 alloys in our work as high as 690.2 ± 32.0 HV0.2. These results indicate that the freeze-drying method and subsequent low temperature sintering is a promising method for preparing high performance W-Y2O3 alloys with ultrafine grains.

Fabrication of fine grained molybdenum by fast resistance sintering under ultra-high pressure

A novel fast resistance sintering method has been developed to fabricate pure Mo bulk material with fine grain size and high sinter density. A pure Mo compact with more than 98.5% theoretical density and grain size less than 2 μm has been successfully fabricated without any sintering additive or grain growth inhibitor under ultra-high pressure of 9000 MPa, sintering temperature of 1300 °C for only 60 s. The relationships between microstructure, basic mechanical property and processing parameters, including pressure, input power and sintering time, were investigated.

Elaborating the Cu-network structured of the W–Cu composites by sintering intermittently electroplated core-shell powders

The tungsten–copper composites are widely used in heat sink materials for high power circuits, electrical contact materials, and electrode materials because of their excellent thermal and mechanical properties. We elaborate the Cu-network structured of the W–Cu composites by sintering the intermittently electroplated core-shell powder. After hot-pressing sintering, the W particles dispersed uniformly within the continuous Cu phase. Thus, the bulk of the dense W–Cu composites demonstrated the desired Cu-network structure with high thermal conductivity. No distinct voids were found in the connection between W and Cu. Thermal conductivity increased with the increase of Cu content, and the maximum value was 274.23 W/(m·K). The high sintering density, the Cu-network structure, and the good interface of W and Cu are the main factors ensuring the excellent thermal performance of composites. The maximum values of bending strength (995 MPa) and hardness (240 HV) were achieved with the 20 wt % Cu sample. The deposition process of Cu coating during intermittent electroplating was also discussed.

Surface analysis of iron and steel nanopowder

High sinter density is desired in powder metallurgy components as the requirement for performance is increasing day‐by‐day. One of the promising ways to achieve improved densification during sintering is through the addition of nanopowder to the conventional micrometer sized metal powder. It is well known that the surface chemistry of the powder has a decisive effect on sintering and consequently the properties of the components produced. Extensive research has hence been conducted to elucidate the surface chemistry and its influence on sintering for powder used in conventional powder metallurgy. Nanopowder, owing to high surface to volume ratio, can contribute to the activation of sintering at lower temperatures and enhance the sinter density. In this context, the surface chemistry of the nanopowder is also expected to exhibit substantial influence on sintering. The present investigation is aimed at establishing a methodology to study the surface chemistry and oxide thickness of nanopowder. For this purpose, iron nanopowder of 3 different size fractions: 35 to 45, 40 to 60, and 60 to 80 nm with core‐shell structure were studied. Different approaches were adopted to evaluate the shell thickness of the iron nanoparticles. The methodology was developed and tried on low alloy steel nanopowder to measure oxide thickness. X‐ray photoelectron spectroscopy, thermogravimetry, and high‐resolution scanning electron microscopy techniques were used to study the nanopowder. Results from different core‐shell models for iron nanopowder were found to be consistent except in the case where depth profiling was taken into account. The results were in agreement with the values obtained from thermogavimetry‐surface area correlation.

Effects of hydrogen content on powder metallurgy characteristic of titanium hydrides

The titanium dihydride (TiH2) powder metallurgy has been attracted a lot of attention, but TiH2 powder is difficult to press moulding. In this paper, the titanium hydride powder metallurgy including TiH2 and unsaturated titanium hydrides (TiH1.5) was investigated simultaneously compared with pure titanium metal powder metallurgy. The results indicates that the titanium hydride powder metallurgy is accompanied by the deoxidation self-purification effect during dehydrogenation process for both of TiH2 and TiH1.5, which have higher sintering density than pure titanium. There are the three stages relative to densification rate, namely the slow, rapid and full densification stages for all of three materials. The compressive yield strengths increase rapidly in the rapid densification stage and are unchangeable almost in the full densification stage after holding 2 h at 1300 °C. The titanium hydride powder metallurgy is helpful to obtain much better mechanical properties than the pure titanium metal powder metallurgy. Here the compressive yield strength of the as-sintered TiH2 compact with the maximum hydrogen content is the best but has very small difference compared with that of the as-sintered TiH1.5 compact after full sintering densification.

Synthesis of nanosized composite powders via a wet chemical process for sintering high performance W-Y2O3 alloy

With the aim of preparing high performance oxide dispersion strengthened tungsten-based alloys by powder metallurgy, the W-Y2O3 composite nanopowders were prepared by an improved bottom-up wet chemical method. Ultrasonic treatment and anionic surfactant sodium dodecyl sulfate (SDS) addition were innovatively introduced into this wet chemical method in order to fabricate homogeneous, ultrafine W-Y2O3 composite nanopowders. As a result, the average tungsten grain size of 40–50nm was obtained for this composite nanopowders. For comparison, W-Y2O3 composite powders were also prepared by traditional mechanical milling. After that, spark plasma sintering (SPS) was employed to consolidate the powders prepared by either mechanical milling or wet chemical method to yield high density as well as suppress grain growth. It is found that the W-Y2O3 alloy prepared by wet chemical method and subsequent SPS possesses smaller grain size (0.76±0.17μm) and higher relative density (99.0%) than that prepared by mechanical milling and subsequent SPS. Moreover, the oxide nanoparticles (about 2–10nm) are dispersed within tungsten grains and at grain boundaries more uniformly in W-Y2O3 alloy prepared by wet chemical method and subsequent SPS. Due to the ultrafine grains, high sintering density and homogeneously distributed oxide nanoparticles, the Vickers microhardness of yttria dispersion strengthened tungsten-based alloy prepared in our work reaches up to 598.7±7.3HV0.2, higher than that reported in the previous studies. These results indicate that the improved bottom-up wet chemical method combined with ultrasonic treatment and anionic surfactant addition developed in our work is a promising way to fabricate high performance oxide dispersion strengthened tungsten-based alloys with ultrafine grain and high density.

Structure, B-site short-range ordering and dielectric properties of Ba(Zn1/3Ta2/3)O3 microwave ceramics with sub-micron sized grains by spark plasma sintering

Ba(Zn1/3Ta2/3)O3 microwave ceramics with sub-micron sized grains (<300 nm) and high sintering density (>98%) are synthesized by spark plasma sintering (SPS) with significantly reduced sintering temperature (1300–1450 °C) and a very short sintering time (15 min) under 30 MPa applied pressure. Raman spectra suggest that B-site short-range 1:2 ordering is present in SPS-ed Ba(Zn1/3Ta2/3)O3 ceramics, but it occurs on length scales that are not detectable by XRD. The density, 1:2 ordering, and Q × f values of Ba(Zn1/3Ta2/3)O3 ceramics are highly sensitive to sintering temperature rather than applied pressure. The Q × f values of SPS-ed Ba(Zn1/3Ta2/3)O3 ceramics increase with annealing time. The Q × f value of SPS-ed Ba(Zn1/3Ta2/3)O3 ceramics at 1400 °C for 15 min with 50 Mpa applied pressure is further improved to ~39 865 GHz by annealing at 1200 °C for 20 h in O2.

Microstructures and properties of CuZrAl and CuZrAlTi medium entropy alloys prepared by mechanical alloying and spark plasma sintering

Equiatomic CuZrAl and CuZrAlTi medium entropy alloys were designed and synthesized by mechanical alloying and spark plasma sintering technique. The alloying behavior, phase evolutions, microstructures and properties of samples were investigated by X-ray diffraction, differential scanning calorimetry, field emission scanning electron microscopy, microscopy/Vickers hardness testing and electrochemical polarization measurement. The results indicate that the final products of as-milled alloys consist of amorphous phases. Ti addition improves the glass forming ability of as-milled alloys. The as-sintered CuZrAl alloy contains face-centered cubic (fee) solid solution, Al1.05 Cu0.95 Zr and AlZr2 phases at different sintering temperatures. With Ti addition, the as-sintered sample is only composed of intermetallics at 690 °C, while feel, fcc2 and CuTi3 phases are formed at 1100 °C. CuZrAlTi-1100 °C alloy exhibits the highest hardness value of 1173 HV0.2 owing to the high sintering density, solid solution strengthening and homogeneous precipitation of nano-size crystalline phase. CuZrAlTi-690 °C alloy presents a similar corrosion resistance with 304L stainless steel in seawater solution and further possesses the lower corrosion rate.

Temperature effects on cycling stability of Li plating/stripping on Ta-doped Li7La3Zr2O12

Abstract This paper reports on the cycling stability of Li plating/stripping on Li6.6La3Zr1.6Ta0.4O12 [LLZ(Ta0.4)] with high sintering density at 25, 60, and 100 °C. Plated/stripped Li thicknesses are sequentially increased to 200 nm, 1.0 μm, and 2.0 μm after every ten cycles. The overpotential gradually increases with cycling Li plating/stripping processes, and the cells eventually short-circuit at 25 and 60 °C. However, the cycling stability of Li plating/stripping significantly improves at 100 °C, and the short-circuiting is prevented perfectly. Moreover, the cycling stability of Li plating/stripping at 25 °C is dramatically improved by pre-cycling the cell at 100 °C. Only heating a cell with Li metal at 100 °C for the same duration does not improve the cycling stability of the cell. It is hence considered that forced migrations of Li atoms and ions across entire Li/LLZ(Ta0.4) interfaces at 100 °C change the properties of the interfacial regions even for the following plating/stripping cycles at 25 °C.