Powder Grains Research Articles

Effects of rotational speed of the classifying wheel during jet milling on magnetic properties and thermal stability of sintered Nd-Fe-B magnets

Sintered Nd-Fe-B magnets with different grain sizes are prepared by adjusting rotational speed of the classifying wheel during the jet milling process. The microstructures of the magnets, the element contents of the magnets, the relationship between powder particle size and grain size and the magnetic properties of the magnets are studied by X-ray diffracton (XRD), scanning electron microscopy (SEM), optical microscopy (OM), electron backscattered diffraction (EBSD), inductively coupled plasma (ICP) and permanent magnet tester. The effect of grain size on thermal stability is characterized by the temperature coefficient of coercivity from room temperature to working temperature and the irreversible magnetic flux loss of the magnet at high temperature. The results show that the particle size of the alloy powder decreases with the increase of the rotational speed of the classifying wheel. When the alloy powder is formed and sintered, it is found that the grain size of the prepared magnet decreases with the decrease of the particle size of the alloy powder. The finer the grain size of the magnet, the clearer the grain boundary. As the grain size of the magnet decreases, the coercivity of the magnet increases, the remanence of the magnet decreases slowly and the thermal stability of the magnet becomes better. For the magnet with a rotational speed of the classifying wheel of 4900 r/min, the average grain size of the magnet is 4.70 μm, the coercivity (Hcj) is 15.47 kOe, and the remanence (Br) is14.09 kGs. At a temperature of 80 °C, the coercivity temperature coefficient of the magnet reaches −0.745 %/°C, and the irreversible loss of magnetic flux hirr is 16.49 %. In addition, the magnet prepared by the tail material produced a large number of voids and pores due to the high oxygen content. Tail material is not good enough in terms of magnetic property as well as thermal stability.

Nuclear magnetic resonance investigation of superconducting and normal state Nb3Sn

The superconductor Nb3Sn has important applications for construction of very high-field superconducting magnets. In this work we investigate its microscopic electronic structure with 93Nb nuclear magnetic resonance (NMR). The high-quality Nb3Sn powder sample was studied in both 3.2 T and 7 T magnetic fields in the temperature range from 4 K to 300 K. From measurement of the spectrum and its theoretical analysis, we find evidence for anisotropy despite its cubic crystal structure. Magnetic alignment of the powder grains in the superconducting state was also observed. The Knight shift and spin-lattice relaxation rate, T1−1 , were measured and the latter compared with BCS theory for the energy gap Δ(0)=2.7±0.3kBTc at 3.2 T and Δ(0)=2.33±0.07kBTc at 7 T, indicating suppression of the order parameter by magnetic field.

Enhanced mechanical properties of dispersed carbide-strengthened CrFeNi-based medium entropy alloys prepared via powder metallurgy

In this study, a set of CrFeNi -based medium-entropy alloys (MEAs) with varying carbon contents were prepared by spark plasm sintering (SPS) using atomized alloy powders as raw material. The microstructures of powders and as-sintered alloys were characterized using ECCI and EBSD. The mechanical properties of as-sintered alloys were tested and the strengthening mechanical were discussed. The results showed that the microstructures of the CrFeNi gas atomized powder and sintered alloy were both single FCC phase in an equiaxed state. However, the matrix grains of powder with carbon addition were mostly dendritic, and few eutectic carbides could be observed between matrix grains. Compared with the CrFeNi MEA, the addition of 8 at. % C led to an increase in the yield strength and tensile strength from 395 MPa to 630 MPa–590 MPa and 990 MPa, respectively. Orowan strengthening and grain refinement resulting micro/nano carbides are responsible for the improvement in mechanical properties.

IMPACTS of SUPERSONIC and SUBSONIC AMMUNITION on GUNSHOT RESIDUE DISTRIBUTION

In incidents where firearms are involved, the determination of the firing distance is one of the basic steps of the analysis aimed at determining the origin of the incident with Forensic Shot Analysis studies. Shot residues consist of partially burned and unburnt powder grains, gunpowder gas, capsule residues and inorganic and organic residues. As a result of determining the presence and density of shot residues on the target, the distance from which the shot was fired can be determined. The pattern and density of shot residues exhibit a direct correlation with the specifications of firearms and ammunition. The intended use and traits of the weapon affect the properties of the ammunition. An instance of specialised ammunition is subsonic cartridges which have lower powder burn rates and amounts compared to supersonic cartridges. To ensure accurate gunshot residue analysis, gunshot residue density and pattern from reference test-fires are necessary. The key factors affecting the GSR analysis are the type of weapon and ammunition used during test-fires, which should closely match those used in the incident. Any variation in these variables can reduce the precision of the residue analysis. In the study, an independent T-test was applied to determine whether there is a significant difference between the gunshot residue density on the target during test firing of 9×19mm diameter and type Supersonic and Subsonic cartridges with semi-automatic pistols at distances of 0cm, 15cm, 30cm, 60cm and 100cm. It was determined that a significant difference existed between supersonic and subsonic cartridges in the density and patterns of gunshot residue on the target resulting from shots fired at the mentioned distances.

Effect of Cold Isostatic Pressing and VC Grain Growth Inhibitor Addition on WC Grain Size and Mechanical Properties of WC-8Co Cemented Carbide

In this study, sub-micron WC-Co cemented carbides were fabricated by cold isostatic pressing using recycled WC powder, Co powder, and VC grain growth inhibitor as the initial composite powders. The raw WC-8Co and WC-8Co-0.3VC compositions were prepared according to conventional technology. After that, part of the green compacts was CIP-ed under pressure of 200 MPa. Sintering was carried out in an industrial vacuum sintering furnace at a temperature of 1420 ºC simultaneously for all samples. The microstructures and properties, including density, coercivity, transverse rupture strength, A-type Rockwell hardness and fracture toughness were investigated, characterized and compared systematically. The experimental results showed that the action of cold isostatic pressing in combination with grain growth inhibitor addition promoted ultrafine grains with the finest WC grain size of 0.68 μm and an increase in the fracture toughness up to 15.4 MPa·m1/2 ± 0.2 compared to rest cemented carbides, showing the techniquehas potential to fabricate cermets with both small average WC grain size and high fracture toughness, which is key in providing excellent performance.

Evaluation of sweet spots for a tight sandstone reservoir: A quantitative study of diagenesis in the fourth member of the Oligocene Huagang Formation, Xihu Depression, East China Sea Shelf Basin

Reservoir sweet spot evaluation is very important for tight sandstone gas exploration and development. Due to the characteristics of large burial depth, strong diagenesis heterogeneity, and prominent importance of diagenetic facies, sweet spot evaluation is the focus and difficulty of unconventional oil and gas exploration and development. By combining with core observation, conventional core analysis, clay x-ray diffraction analysis, bulk rock x-ray diffraction analysis, powder grain size analysis, thin section quantitative statistics, fluid inclusion homogenization temperature, scanning electron microscopy, scanning electron microscopy mineral quantitative evaluation, well logging data, and pre-stack seismic data, integrated simulation approach of seismic diagenetic facies and diagenetic numerical simulation was used to simulate porosity evolution and spatial distribution of tight sandstone of second submember (Smbr 42) of the fourth member (Mbr 4) of the Oligocene Huagang Formation in the Xihu Depression in the East China Sea Shelf Basin, and to evaluate reservoir sweet spot distribution. Based on mineral texture relationship and diagenetic sequence of Smbr 42 sandstone compaction, clay mineral transformation, calcite cementation, quartz cementation, and dissolution, five diagenetic facies were identified. Based on various seismic elastic parameters and core-derived diagenetic facies, the distribution of seismic diagenetic facies was interpreted by seismic diagenetic facies prediction method. Seismic diagenetic facies distribution can provide a “hard-constraint” three-dimensional diagenesis heterogeneity model for diagenetic history reconstruction, and seismic diagenetic facies distribution provides constraints on grain size and quartz grain content distribution in diagenetic numerical model. Based on the detailed consideration of input parameters of burial history, thermal history and diagenetic history, integrated diagenetic numerical simulation was used to reproduce porosity evolution and spatial distribution of Smbr 42 sandstone, and was applied to reservoir sweet spot evaluation of tight sandstone. Smbr 42-1 sublayer developed type III reservoir sweet spot and non-sweet spot. Smbr 42–2 to Smbr 42-6 sublayers mainly developed type II1, type II2, type III, and minor type I reservoir sweet spot. Simulation results were consistent with the locations of multiple sets of drilled gas layers and proven gas reservoirs.

Low traces of acetone detection with WO3-based chemical sensors

This work presents the ability of WO3-based sensors to detect low traces of acetone, specifically within the range of 0.25–5 ppm, specific to the in-field atmosphere. The WO3 powder was synthesised through the hydrothermal method. Morpho-structural investigations showed a monoclinic structure and a good crystallization of the WO3 powder, containing well-grown and faceted grains along low-index crystallographic planes. The paste obtained by mixing the powder with propanediol was screen-printed as a thick layer onto commercial alumina substrates, obtaining the chemical sensors. A dynamic computer-controlled Gas Mixing System was utilized to ensure controlled airflow with variable relative humidity and acetone concentrations. The sensor response was explained based on physico-chemical equations, taking into consideration pre-adsorbed species of oxygen and water, both of which are relevant constituents of atmospheric conditions. The results highlight the applicative potential of WO3, having a good signal-to-noise ratio in relative humidity conditions up to 90% and a pronounced sensitive selectivity to acetone.

Study on the pathway of performance improvement and mechanism of Gd2O2S:Tb3+ green phosphor

To improve the luminescence performance of Gd2O2S:Tb3+, Gd2O2S:Tb3+, Tm3+, Gd2O2S:Tb3+, Dy3+, Tm3+ and Gd2O2S:Tb3+, Dy3+, Tm3+@SiO2 phosphors have been prepared by molten salt method. The samples are characterized using XRD, SEM, TEM, FT-IR, UV-Vis absorption spectroscopy, photoluminescence, cathodoluminescence and thermal stabilization tests. XRD tests and refinement results show that the crystal structure of the phosphor is still the hexagonal Gd2O2S crystalline phase and the lattice parameter decrease due to Dy3+/Tm3+ occupying the Gd3+ sites. SEM images and mapping of each element show that the particle grain and all expected elements distribute uniformly. TEM results illustrate that SiO2 coating is in an amorphous form on the surface of the powder grains. FT-IR tests indicate that SiO2 is present on the powder surface through Si-O-Gd bonding. The results of photoluminescence and cathodoluminescence show that the luminescence intensity increase 50% while adding Tm3+ into Gd2O2S:Tb3+, Gd2O2S:Tb3+, Dy3+ phosphors caused mainly by the energy transfer from Dy3+/Tm3+ to Tb3+. The core-shell structure of Gd2O2S:Tb3+, Dy3+, Tm3+@SiO2 has better photoluminescence and cathodoluminescence and the fluorescence lifetime remains unchanged while SiO2 coats phosphor. And it has good thermal stability. Its luminescence intensity at 425 K reaches 71.4% of that at room temperature, which is better than the corresponding value (46.4%) for the uncoated sample.

Structural evolutions of MX80 bentonite pellet/powder mixtures under wetting and suction-controlled oedometer loading

The compression behaviour of MX80 bentonite pellet/powder mixture was investigated by performing suction-controlled oedometer tests. The suction from 113 to 4.2 MPa was imposed by vapour equilibrium technique (VET), and zero suction by water circulation (WC). After instantaneously unloaded from various target vertical stresses, the structural observation was conducted by mercury intrusion porosimetry and micro-computed tomography. Results showed that during wetting, the intra-grain micro- and macro-pores increased due to the swelling of pellets and powder grains. During loading, the variation of intra-grain micro-pores was characterised by the loading effect, while that of intra-grain micro-pores was controlled by the suction and loading effects. The inter-grain pores, for the VET samples, were reduced by the swelling of pellets and powder grains during wetting on one hand, and by the movement of pellets and powder grains during loading on the other hand. For the WC samples, most inter-grain pores were closed during wetting, with a few pores in the top closed by further loading. The global compression of bentonite mixture samples during loading was mainly governed by the filling of inter-grain pores for the VET samples, but controlled by the compression of intra-grain pores for the WC samples.

Relevance of inter-particle interaction in directed energy deposition powder stream

Blown powder dynamics is one of the aspects of Directed Energy Deposition (DED) with a major influence on deposition quality and potential for improvement in simulation. Most currently employed computational models discard powder grain collisions as negligible, although little explicit evidence for that claim exists. A Discrete Element Method approach is thus employed in the present work to simulate the actual number of grain collisions in a powder stream of a commercial discrete coaxial nozzle and how that number varies with the key processing parameters. While the number of powder grain collisions is found to in fact be negligible at one side of the usable parameter spectrum, the opposite side results in as many as 84% of all the powder grains being involved in inter-granular collisions with an average rebound angle of 14°, challenging the established hypothesis of the negligibility of this phenomena.

EFFECT OF FILLER CONTENT AND SIZE ON THE FLEXURAL STRENGTH AND HARDNESS OF PURE IRON POWDER FILLED LAMINATED FIBERGLASS EPOXY COMPOSITES

The aim of this work is to evaluate the effect of the amount of pure iron powder (99.9987%), introduced as a filler in a glass-epoxy composite laminate, as well as the variation of its grain dimensions, on its flexure strength and hardness. Contact molding was used to produce laminate plates of 15, 20, and 25% of weight content with three different dimensions 92, 64, and 32 μm, and the fiberglass content was set at 30%. For the bending test, the specimens were prepared according to ISO 14125 standard specification, the test was carried out with three different speeds, and the required number of specimens were manufactured according to Taguchi's orthogonal array L9 (3 × 3) by varying three parameters (iron powder content, grain size, and test speed). The results were studied with the Taguchi method; however, the hardness was measured on plates with the same iron powder content while varying the dimensions of its grains (92, 64, and 32 μm). The results show that the pure iron powder content and dimension variations have a direct and clear influence on the composite material's mechanical characteristics, in such a way that: a specific percentage of filler and grains size reduction could widely increase the hardness as well as the material flexure strength. The results of the elaborated composite material's bending test were compared to a 45% glass-epoxy laminate. Whereas the hardness of the elaborated laminate was compared to a resin plate and the 45% glass-epoxy laminate.

Pycnometric-Additive Determining of the Degree of Coating of High-Strength Synthetic Diamond Grinding Powders using the Actual 3D Morphology of their Grains

The methodological components of direct and indirect analytical determining of the degree of coating of synthetic diamond grinding powders are analyzed. It has been established that the weight method most used in practice for determining this technological property of grinding powder is not universal for different methods of applying the coating. More universal in this regard, as the review of publications showed, is the well-known indirect-analytical method based on the pycnometric-additive approach. An improved variant of this method is proposed, aimed at application to high-strength synthetic diamond grinding powders. The method takes into account the peculiarities of the 3D morphology of the grains of such powders. Using the example of grinding powder AC300 500/400, the grains of which were coated with a solution of a mixture of boron oxide, sodium silicate, and titanium carbide, the advantages of using the proposed method are illustrated. The results of a comparison of determining the degree of coating by a known method and its improved variant are presented.

The Effect of Powder Temperature on Semi-Solid Powder Rolling AA2024 Based on Experiments and Numerical Simulation

Semi-solid powder rolling (SSPR) is widely used to produce alloy strips with fine grains and excellent performances in the automotive, aerospace and shipbuilding industries. During SSPR, powder temperature, as a very important parameter, greatly affects strips’ microstructures and mechanical properties, which have been investigated by many researchers, but its effect on the forming process and mechanism has rarely been studied. Therefore, based on online experimental detection and transient simulation, the microstructures, strip temperatures, relative densities and rolling forces at different conditions were, respectively, measured, calculated, compared and analyzed in order to study the deformation process and mechanism during SSPR. The result shows that with the increase in powder temperature, the strip temperature and relative density increase, while the rolling force decreases. The grains of the strips are refined after SSPR, and fine and dense microstructures are obtained at 600 °C, which is the optimum powder temperature. In the main deformation sections (II and III), when the contact normal force exists and reaches a maximum, the relative density and rolling force increase rapidly. At these sections, the strips rolled at 600 °C are mainly in a porous solid state, and powder crushing dominates the strip deformation. Therefore, SSPR at 600 °C and below can be considered porous or powder hot rolling, integrating powder crushing, solidification, deformation, densification and grain coarsening. Moreover, as the simulated values are basically consistent with experimental values, the thermomechanical coupling model based on the Fourier equation and its parameters are confirmed to be reasonable.

Influence of Surface Preparation on the Microstructure and Mechanical Properties of Cold-Sprayed Nickel Coatings on Al 7075 Alloy

This work presents the effect of surface roughness (Al 7075) on the microstructure and mechanical properties of cold-sprayed nickel coatings. Coating analysis included substrate surfaces and coating geometry, microstructure characterization, microhardness, nanohardness, elastic modulus, and adhesion. The results show that the surface preparation had a significant effect on coating adhesion and microstructure. The coating deposited at the highest gas temperature revealed a dense microstructure, showing very good adhesion of the impacting powder particles to the substrate and good bonding between deposited layers. The Ni grains with different shapes (elongated, equiaxed) and sizes of a few dozen to several hundred nanometres were present in the splats. An increase in temperature caused significant growth in coating thickness as a result of the powder grains’ higher velocity. Moreover, higher gas temperature resulted in the enhancement of micro- and nanohardness, elastic modulus, and adhesion. The adhesive bond strength of Ni coatings in the tested temperature ranges from 500 °C to 800 °C increased with the increase in the surface roughness of the substrate. For the Al 7075 coarse grit-blasted (CG) substrate with the highest roughness, the adhesion reached the highest value of 44.6 MPa when the working gas was at a temperature of 800 °C. There were no distinct dependencies of surface roughness and thickness on the mechanical properties of the cold-sprayed nickel coating.

Synthesis, Characterization of Chromium Oxide Powders and Coatings

The chromium oxide powders are transformed into plasma sprayable particles by using synthetic polymers for agglomeration. In order to carry out the agglomeration process, spray drying technique was employed. This research work highlights the significance of the process variables that control the synthesis of plasma spray powder and consequently, the properties that were suited for plasma spray coating. Energy Dispersive Spectroscopy (EDS) was used to characterize the elemental composition, while scanning electron microscopy (SEM) was used to analyse the morphology and powder grain sizes and X-ray diffraction (XRD) was used to identify the phase structure. And for the development of coatings on the substrates, Atmospheric plasma spray (APS) technique was used. The plasma sprayable powders were created with the intention of investigating for use as corrosion-resistant coatings.

Elucidating the role of pulse shaping on defects formation in aluminum alloys fabricated by powder bed fusion

The Powder Bed Fusion (PBF) additive manufacturing technique, although widely studied and implemented in the industry, still presents issues of porosity and lack of fusion even when optimal processing parameters are used. The benefits and drawbacks of PBF utilizing pulsed lasers are both consequences of the transient temperature field induced by laser-matter interaction. During laser processing with nanosecond pulse durations, intense heating and cooling rates are induced, setting the melt pool dimensions, which are directly related to the occurrence of lack of fusion, and ablation of the processed material, leading to recoil pressure that induces the keyhole effect, increasing the occurrence of porosity. Consequently, controlling the transient temperature field is a major challenge in incorporating pulsed wave laser technology into PBF to minimize porosity and lack of fusion defects. In this study, to reduce porosity and lack of fusion, a smooth heating rate able to sinter powder grains is designed to facilitate heat flow through the heat affected zone, followed by a sharper heating rate that generates a fully molten region while reducing ablation at the same time. This work represents the first attempt to control the transient temperature field, by shaping a nanosecond laser pulse in the time domain, thereby affecting the formation of defects. The effects of the pulse shape on build quality quantified by lack of fusion and high-energy spherical pores were investigated using X-ray microtomography, molecular dynamics (MD), X-ray diffraction (XRD), Electron Backscatter Diffraction (EBSD), and finite element simulations. The variation of porosity and lack of fusion within manufactured samples were revealed by the high-resolution 3D X-ray micro computed tomography. The differences in the transient temperature field, resulting residual stresses, and microstructure are attributed to the differences in the energy delivery rates within the time domain shaped laser pulses. Laser pulse shaping is an additional parameter that plays an important role in optimizing the PBF as-processed part's production opening a new research niche.

Discrete element model for powder grain interactions under high compressive stress

Discrete element model for powder grain interactions under high compressive stress

Coprecipitation of Co and La2O3 coprecipitation on WC for tailoring the grain distribution and boundary of high-performance coarse-grained WC-10Co cemented carbide

The development of excellent performance coarse-grained WC-Co cemented carbides has emerged as a hot topic for cutting tools, drills, molds, wear-resistant parts, etc. In this study, the WC powder was coated by Co and La2O3 simultaneously using a new coprecipitation strategy, and its beneficial effects on the densification, microstructure and mechanical properties of WC-10Co was systematically investigated. Firstly, due to the successful introduction of La2(C2O4)3 precursor, the surprising transformation of rod-like loosely CoC2O4 to regular-spherical-like dense one was observed, further inducing the dense and uniform coating of La2O3 with Co on the surface of WC after the H2 reduction treatment. The addition of La2O3 by this way was found to improve the grain uniformity of WC in the sintered cemented carbide, inhibiting the appearance of abnormally grown grains, and preventing the conversion of CoFCC to CoHCP effectively, which should be ascribed to the segregation of La2O3 on the grain boundary of WC, produced pinning effect and inhibited the dissolution and growth of WC grains. The mechanical properties of coarse-grained WC-Co cemented carbides, including hardness, strength, fracture toughness and elastic modulus were systematically investigated, validating the beneficial effects of Co and La co-precipitation apparently. In general, this study presents a new strategy for enhancing the performance of coarse-grained WC-Co cemented carbides by both powder mixing design and grain boundary tailoring.

Harmonizing nanomaterial exposure methodologies in ecotoxicology: the effects of two innovative nanoclays in the freshwater microalgae Raphidocelis subcapitata

Layered double hydroxides (LDHs) are innovative nanomaterials (NMs) with a typical nanoclay structure (height <40 nm) consisting of layers of metallic cations and hydroxides stabilized by anions and water molecules. Upon specific triggers, anions can exchange by others in the surrounding environment. Due to this stimuli-responsive behavior, LDHs are used as carriers of active ingredients in the industrial or pharmaceutical sectors. Available technical guidelines to evaluate the ecotoxicity of conventional substances do not account for the specificities of NMs, leading to inaccuracies and uncertainty. The present study aimed to assess two different exposure methodologies (serial dilutions of the stock dispersion vs. direct addition of NM powder to each concentration) on the ecotoxicological profile of different powder grain sizes of Zn–Al LDH-NO3 and Cu–Al LDH-NO3 (bulk, <25, 25–63, 63–125, 125–250, and >250 µm) in the growth of the freshwater microalgae Raphidocelis subcapitata. Results revealed that the serial dilutions methodology was preferable for Zn–Al LDH-NO3, whereas for Cu–Al LDH-NO3 both methodologies were suitable. Thus, the serial dilutions methodology was selected to assess the ecotoxicity of different grain sizes for both LDHs. All Zn–Al LDH-NO3 grain sizes yielded similar toxicity, while Cu–Al LDH-NO3 powders with smaller grain sizes caused a higher effect on microalgae growth; thus, grain size separation might be advantageous for future applications of Cu–Al LDH-NO3s. Considering the differences between exposure methodologies for the Zn–Al LDH-NO3, further research involving other NMs and species must be carried out to achieve harmonization and validation for inter-laboratory comparison.

Evaluation of Plant Powder/ Oil Against Pulse Beetle (&lt;i&gt;Callosobruchus chinensis&lt;/i&gt; L.) on Green Gram

Six plant powders viz. neem leaf powder, neem seed powder, neem cake powder, curry leaf powder, chili fruit powder and bael leaf powder each at 5 g/ kg and mustard oil at 5 ml/ kg of powder grains were evaluated for their effectiveness as grain protectants against Callosobruchus chinensis on green gram in storage. The results revealed that mustard oil and neem seed powder were more effective. The order of efficacy of remaining treatments was neem cake &gt; neem leaf powder &gt; curry leaf powder &gt; chili fruit powder &gt; bael leaf powder which were significantly superior over control. It was also found that treatments do not affect the germination of seeds.