Low Apparent Density Research Articles

Influence of polyethylene terephthalate (PET) flakes coating on the microstructure and mechanical properties of alkali activated composites

The objective of this research is the improvement of the mechanical strengths of the alkali activated slag composites with polyethylene terephthalate (PET) flakes content. The main drawback when large PET flakes (2–9 mm) are used in these composites is reduced adhesion between plastic surface and binding matrix. Therefore, the PET flakes were coated with two types of adhesives – based on polyvinyl acetate or sodium silicate solution, and then covered with fly ash, waste glass or slag powders. The microstructure (assessed by scanning electron microscopy) of interfacial transition zone between the PET flakes coated with waste glass and slag powders is denser than for uncoated PET flakes; this explains the improvement of the compressive strength of these mortars especially after longer curing times (28 days). The flexural strengths of the mortars with PET flakes are higher than for reference mortar (with sand aggregate) and the apparent density of the alkali activated composites with PET flakes are in general smaller than for the reference mortar. Thus, the coating of PET flakes improves the mechanical behaviour of mortars, preserving also a low apparent density. These results can lead to a wider recycling of PET waste, slag and waste glass powder, promoting circular economy and reducing the CO2 footprint without compromising the critical mechanical strength of mortars.

Novel B4C supports for ceramic membrane filtration

Novel complex-shaped porous B4C supports were fabricated for use in ceramic membrane filtration applications, the processing of which is described in detail. Firstly, an extrudable ceramic paste was formulated and homogenised, containing a high content of B4C of super-coarse and ultrafine grades, plus water as liquid medium and various organic additives as thickener, binder, plasticiser, and lubricant. Secondly, tubular honeycomb parts (i.e., cylinders of ∼26 mm outer diameter with 30 inner channels of 3 mm diameter each) were extruded at ∼25 bar and dried in air at 120 °C for 24 h, resulting in robust green B4C membrane supports without macro- or micro-defects. Thirdly, suitable debinding conditions were identified by thermogravimetry of the paste and its components, and the dry membrane supports were debinded at 700 °C for 1.5 h in high vacuum (<0.1 Pa) confirming that they retain their shape with minimal isotropic shrinkage (∼0.13–0.15 %). And fourthly, the debinded membrane supports were pressureless sintered at 2000 °C for 3 h in inert atmosphere undergoing only an additional 1 % isotropic shrinkage, resulting in B4C membrane supports with low apparent density (∼0.89(1) g/cm3), high porosity (∼38–39 %), open interconnected micrometre pores (in the range ∼0.5–16 μm, and d50∼1.5 μm), and low skeletal density (∼2.455(1) g/cm3). Importantly, their only partial densification makes them both sufficiently strong mechanically (∼68(2) MPa compressive strength) and highly permeable to water (∼13131(418) l/(m2·h·bar) permeability at ∼1.3 bar). These attributes, together with the intrinsic lightness and durability of B4C ceramics, make these novel supports in principle very appealing for ceramic membrane filtration and other applications.

Scientific analysis of folk contract documents from Tianshui region: insights of fiber use and preservation state

Folk contract documents (FCD) are valuable materials for studying social history, and the paper they use reflects the social realities of different eras and social classes. Research and scientific analysis of numerous FCD samples after the fourteenth century are rare. We conducted a study on 96 Tianshui folk contract documents (TFCD, 107 paper samples) from the Tianshui area of Gansu Province, Northwest China, taking into account both the textual content and the materiality of paper carriers, and interpreted the results from multiple lines of evidence and discussion. Physical performance analysis revealed that the paper used by the northern folk exhibits a lower apparent density, which is not conducive to the long-term preservation of paper. The preservation status investigation, curtain pattern analysis, and fiber analysis show that the paper used in the TFCD differs from traditional cultural paper regarding disease types, production precision, and fiber materials, providing a basis for its protection and restoration. The analysis of chemical components indicated that the aging and yellowing of paper can be correlated with the content of sulfur and carbonyl groups. The feasibility of using pyrolysis gas chromatography-mass spectrometry (Py-GC/MS) analysis to quickly identify papermaking fibers in a large number of paper samples was proposed. By utilizing various techniques to inspect the paper of documents, this study helps to enhance the academic understanding of FCD materials. In addition, it expands the knowledge base of Northwest handmade paper.

Artificial lightweight aggregate made from alternative and waste raw materials, hardened using the hybrid method

Lightweight aggregates are a material used in many industries. A huge amount of this material is used in construction and architecture. For the most part, lightweight construction aggregates are obtained from natural resources such as clay raw materials that have the ability to swell at high temperatures. Resources of these clays are limited and not available everywhere. Therefore, opportunities are being sought to produce lightweight artificial aggregates that have interesting performance characteristics due to their properties. For example, special preparation techniques can reduce or increase the water absorption of such an aggregate depending on the needs and application. The production of artificial lightweight aggregate using various types of waste materials is environmentally friendly as it reduces the depletion of natural resources. Therefore, this article proposes a method of obtaining artificial lightweight aggregate consolidated using two methods: drum and dynamic granulation. Hardening was achieved using combined methods: sintering and hydration, trying to maintain the highest possible porosity. Waste materials were used, such as dust from construction rubble and residues from the processing of PET bottles, as well as clay from the Bełchatów mine as a raw material accompanying the lignite overburden. High open porosity of the aggregates was achieved, above 30%, low apparent density of 1.23 g/cm3, low leachability of approximately 250 µS. The produced lightweight aggregates could ultimately be used in green roofs.

Influence of Forest-Pasture Conversion on the Erodibility of a Dystrophic Red Yellow Latosol with Different Deforestation Temporalities in the Southwestern Amazon

The objective was to evaluate the erodibility of a Dystrophic Red Yellow Latosol from the Mutum Paraná River sub-basin to understand the influence of forest conversion and different pasture temporalities on its physical structure and the potential for sediment release after surface samples undisturbed were subjected to the Inderbitzen experiment. Tests of Mechanical Resistance to penetration and physical parameters under forest and pasture conditions were carried out. A simulation of surface runoff by water spraying (Inderbitzen) was carried out to analyze the disaggregation and transportability of particles. The erodibility equation (K factor) was applied, and the Limits of Consistency were defined to measure the Plasticity Index of the soil that qualifies its erodibility. The pasture samples submitted to the experiment indicated soils of low susceptibility to erosion, even under conditions of maximum slope of the area (17°) and under intense simulated flow (3 L and 6 L min-1) of sprinkler, not showing large losses of sediments or intense surface degradation. There was a relation between soil physical properties and the response of the sample in the experiment. Samples with higher total porosity (r = 0,89) and lower apparent densities (r = -0,88) showed higher disaggregation. There was a significant correlation between the simulated samples that showed lower sediment losses, with a higher percentage of clay (r = -0,65). Samples with a higher percentage of total sand indicated higher percentages of disaggregation (r = 0,68). Erodibility data obtained by the K-factor equation and Plasticity Index endorsed the low soil erodibility

Preparation of Filter Paper from Bamboo and Investigating the Effect of Additives.

As air pollution escalates, the need for air filters increases. It is better that the filters used be based on natural fibers, such as non-wood fibers, which cause low damage to the environment. However, the short fiber lengths, low apparent densities, and high volumes of non-wood materials can make it challenging to prepare filter paper with the required mechanical and physical properties. In that context, this study focused on utilizing bamboo fibers to fabricate filter paper by employing the anthraquinone soda pulping method. The pulp underwent bleaching and oxidation processes, with the incorporation of cationic starch (CS) and polyvinyl alcohol (PVA) to enhance resistance properties, resulting in the creation of handmade filter papers. The findings revealed that the tear, burst, and tensile strength of filter paper increased with the oxidation and addition of CS and PVA. Air permeability increased with addition of PVA and combination of CS and PVA. FTIR demonstrated the conversion of hydroxyl groups in cellulose chains to carboxyl groups due to oxidation. SEM images illustrated alterations in the fiber structure post-oxidation treatment, with CS reducing pores while PVA and the CS-PVA combination enlarged pore size and enhanced porosity. The BET surface area surface area expanded with oxidation and the addition of the CS-PVA blend, indicating heightened filter paper porosity. Notably, the combined inclusion of CS and PVA not only augmented mechanical strength but also increased porosity while maintaining pore size.

Ultralight and strain-sensitive bacterial cellulose derived carbon fiber-reinforced graphene aerogel for broadband sound absorption

The growing severity of urban noise pollution poses a threat to the physical and mental health of residents, prompting the need for the development of effective sound-absorbing materials. Herein, we present a versatile method to fabricate ultralight and flexible graphene-based aerogels by introducing one-dimensional carbon fibers as reinforcements. These carbon nanofiber-reinforced graphene aerogels (CNF/GAs) were obtained by the carbonization of freeze-casted bacterial cellulose/graphene oxide (BC/GO). Our CNF/GAs possess a low apparent density (∼3.05 mg cm−3) and exhibit broad-band sound absorption performance with a superior noise reduction coefficient (NRC) as high as 0.65 at a thickness of 30 mm. Meanwhile, their excellent elasticity endows the CNF/GAs with strain-sensing capability, enhancing their capacity for detecting deformations when utilized as sound absorbers in engineering applications. Additionally, the highly porous structure makes the CNF/GAs excellent thermal insulators, with a thermal conductivity as low as 0.011 W m−1 K−1, further expanding their environmental reliability. Therefore, our approach develops CNF/GAs with excellent broad-band sound absorption performance and mechanical-environmental reliability, showing significant practical potential.

Ultra-light weight proppant of diatomite-reinforced poly(methyl methacrylate-co-styrene) enabling high compressive strength, heat and acid resistance toward hydraulic fracturing

The growing emphasis on environmental protection in petroleum exploitation results in a high demand for ultra-lightweight (ULW) proppants. This work involves fabricating ULW proppant utilizing suspension polymerization, in which alkyl chain decorated natural clay diatomite (Dia) is incorporated into poly(methyl methacrylate-co-styrene) (poly(MMA-co-St)). The obtained poly(MMA-co-St)/Dia composite beads exhibit a low apparent density ranging from 1.025 to 1.213 g cm−3, and show an impressively low crushing rate of 0.99 % under 52 MPa. The rigid and thermally stable Dia crosslinks with poly(MMA-co-St) matrix, which increases the network density and restricts the movement of the molecular chain, thereby enhancing both the mechanical and thermal stability of poly(MMA-co-St)/Dia. Additionally, these composite beads display minimal acid solubility and low turbidity. The exceptional mechanical and thermal stability (attributed to the reinforcement effect and thermal barrier effect of inorganic Dia, as well as the synergistic effect of the copolymerization of MMA and styrene), along with the low acid solubility and turbidity, enabling the poly(MMA-co-St)/Dia composite to replace traditional proppants and be applied in hydraulic fracturing for oil and gas exploitation.

Valorization of Agricultural Rice Straw as a Sustainable Feedstock for Rigid Polyurethane/Polyisocyanurate Foam Production.

Agricultural rice straw (RS), often discarded as waste in farmlands, represents a vast and underutilized resource. This study explores the valorization of RS as a potential feedstock for rigid polyurethane/polyisocyanurate foam (RPUF) production. The process begins with the liquefaction of RS to create an RS-based polyol, which is then used in a modified foam formulation to prepare RPUFs. The resulting RPUF samples were comprehensively characterized according to their physical, mechanical, and thermal properties. The results demonstrated that up to 50% by weight of petroleum-based polyol can be substituted with RS-based polyol to produce a highly functional RPUF. The obtained foams exhibited a notably low apparent density of 18-24 kg/m3, exceptional thermal conductivity ranging from 0.031-0.041 W/m-K, and a high compressive strength exceeding 250 kPa. This study underlines the potential of the undervalued agricultural RS as a green alternative to petroleum-based feedstocks to produce a high-value RPUF. Additionally, the findings contribute to the sustainable utilization of abundant agricultural waste while offering an eco-friendly option for various applications, including construction materials and insulation.

Durability of Recycled Concrete after Reinforcing the Aggregates with Permeable Crystalline Materials

The utilization of recycled aggregate can significantly mitigate the extraction of natural sand and gravel. However, the practical application of recycled aggregate in engineering is impeded by its inherent characteristics, encompassing high water absorption, high crushing, and low apparent density. This study employed a soaking and air-drying method to enhance the strength of three types of aggregates with varying initial strengths by utilizing permeated crystalline materials. The durability of recycled aggregate concrete (RAC) was studied with three different aggregate replacement rates (0%, 50%, and 100%). The test results demonstrate that the slump, compressive strength, freeze resistance, and carbonation resistance of RAC exhibit a decreasing trend as the aggregate replacement rate increases. The freeze resistance and carbonation resistance of RAC are notably enhanced after incorporating permeated crystalline material. This study contributes to a sustainable and efficient solution for the treatment of construction waste, thereby enhancing the utilization rate of recycled concrete.

3D multifunctional porous pine carbon aerogels coupled with highly dispersed CoFe nanoparticles for robust electromagnetic wave response

Functional carbonaceous materials with controllable morphology, low apparent density, large surface area, and high porosity starting from natural precursors using environmentally friendly processes are an appealing topic in the electromagnetic wave (EMW) field. In this work, renewable pine woods with ordered pore channels are selected to load highly dispersed CoFe alloy nanoparticles formed by in-situ pyrolysis reaction between Fe3O4 nanospheres and ZIF-67 nanoparticles. The constructed three-dimensional (3D) porous CWA-CoFe-NC aerogel inherits the characteristics of highly dispersed small CoFe alloy nanoparticles, porous carbon aerogel with rectangular honeycomb-like structure, and abundant N heteroatoms. Therefore, CWA-CoFe-NC aerogel achieves an excellent EMW absorption performance with reflection loss (RL) values of −61.6 and −58.2 dB at matching thicknesses of 3.7 and 1.2 mm, respectively. Benefiting from the reasonable design of the composite structure and composition, 3D porous aerogel also enables great potential for multifunctional applications. Particularly, good lightweight and mechanical properties are realized in the CWA-CoFe-NC aerogels due to their ordered pore channels and abundant rectangular pores. Furthermore, good flame retardant performance can ensure the serviceability of the target device in high/low-temperature environments. In addition, CWA-CoFe-NC aerogels show good thermal stability and thermal management characteristics. This work provides a novel and effective method for the preparation of lightweight, high-performance, and multifunctional EMW absorbers.

Impact of air jet impingement technology on the strength of tissue paper

Impinging air jets can be used to dewater, heat, and dry the web of tissue paper. High velocities of the air jets degrade the paper, and appropriate adjustments to the jet velocity and the distance of the nozzle from the surface of the wet web are crucial to obtain the highest quality product. This work investigated the correlation between the velocity of the air jet and the strength of paper subjected to the impingement method. Papers with an initial moisture content of 20% and various pulp mixes were tested, and the physical properties of papers were explored. After impinging an air jet, different tensile strength limits were obtained in the machine and cross directions. The paper had lower apparent density and higher roughness compared to classical pressing. The dependence of tensile strength and roughness on the fibers composition also was determined. Increasing the amount of eucalyptus fibers in impingement dewatered paper resulted in a decrease in its tensile strength and roughness. The value of elongation before breaking was the highest for softwood papers after the impingement method. The maximum velocity of an air jet that can be used to dewater or dry paper without the risk of damage to the papers was determined.

Delicate control of graphite flakes alignment in the copper matrices via powder selections and filling processes

Due to the anisotropy properties of graphite flakes (Gf), the control of their orientation inside copper (Cu) matrice is strictly correlated with the final properties of such metal matrix composite (MMC). In this study, MMCs are fabricated by powder metallurgy process using three types of Cu powder particles (flake, dendritic, and spherical) as well as different powder filling methods. By using Cu flake powder with a relatively lower apparent density as compared to the two other Cu powder types (dendritic and spherical), high aligned Gf could be easily obtained, after uniaxial hot pressing, in a one-step powder filling approach. For the other two types of powders, a comparable orientation degree of Gf could be achieved only via several steps of delicate powder pressing. In-plane thermal conductivities of Cu/Gf composites were enhanced by improving the orientation degree of Gf, agreeing with effective medium approximation predictions (Maximum TC up to 540 W mK−1). A modeling, based on the apparent density of Cu powders, was discussed to show the effect of the alignment process on the final thermal property of such MMCs. This model supplies a basic guideline to obtain highly orientated Gf.

Study on buckling behavior of multilayer pyramid lattice structures

Three-dimensional lattice structures with high specific strength, specific stiffness and low apparent density, are widely employed across multiple engineering fields. Under uniaxial compressive loading, the lattice structure may experience local buckling or global buckling. This paper conducted a systematic parametric study of the various factors affecting the buckling behavior of multilayer pyramid lattice structures to investigate the mechanism of the two main instability modes, local and global buckling. It was found that the critical buckling load increases with the increase in the size of the unit cell and decreases with the increase in the total height of the structure for the same relative density. As the relative density increases, the buckling resistance of the lattice structure increases. It is possible to examine various buckling modes by varying the geometrical properties of the pyramid unit cell (slenderness ratio, rod inclination angle, cross-sectional size of strut connection parts). Finally, numerical simulations were performed to calculate the yield strength and buckling strength of the lattice structure under uniaxial compression load, in order to estimate the threshold relative density for structural buckling and yield failure. The results demonstrated that buckling failure should be considered for aluminum alloy pyramid lattice when the relative density is below 4.07%. This study provides design criteria for lattice structures dominated by buckling and offers ideas for improving the buckling capacity of truss-type lattice structures.

Enhanced Low-Density Silicone Foams Blown by Water-Hydroxyl Blends.

Water, alcohols, diols, and glycerol are low-cost blowing agents that can be used to create the desired silicone foam structures. Although their combined use can be beneficial, it remains unclear how it affects the physical properties of the resulting materials. We conducted a comparative study of these hydroxyl-bearing blowing agents in fumed silica- and mica-filled polymer composite systems for simultaneous blowing and crosslinking to obtain a low-density, uniform porosity and superior mechanical properties. The foams were optimized for a uniform open-pore structure with densities ranging from 75 to 150 kg‧m-3. Varying the diol chain length (Cn) from one to seven carbons can alter the foam density and structure, thereby enhancing the foam tensile strength while maintaining a low density. Replacing 10 mol% of water with 1,4-butanediol decreased the density by 26%, while increasing the specific strength by 5%. By combining glycerol and water blowing, the resulting foams exhibited a 30% lower apparent density than their water-blown analogs. The results further showed that Cn > 4 alkane chain diols had an odd-even effect on the apparent density and cell wall thickness. All foamable compositions had viscosities of approximately 7000 cSt and curing times below 2 min, allowing for quick dispensing and sufficient time for the foam to cure in semi-industrial volumes.

Stability and Detectability of Exomoons Orbiting HIP 41378 f, a Temperate Jovian Planet with an Anomalously Low Apparent Density

Moons orbiting exoplanets (“exomoons”) may hold clues about planet formation, migration, and habitability. In this work, we investigate the plausibility of exomoons orbiting the temperate (T eq = 294 K) giant (R = 9.2 R ⊕) planet HIP 41378 f, which has been shown to have a low apparent bulk density of 0.09 g cm−3 and a flat near-infrared transmission spectrum, hinting that it may possess circumplanetary rings. Given this planet’s long orbital period (P ≈ 1.5 yr), it has been suggested that it may also host a large exomoon. Here, we analyze the orbital stability of a hypothetical exomoon with a satellite-to-planet mass ratio of 0.0123 orbiting HIP 41378 f. Combining a new software package, astroQTpy, with REBOUND and EqTide, we conduct a series of N-body and tidal migration simulations, demonstrating that satellites up to this size are largely stable against dynamical escape and collisions. We simulate the expected transit signal from this hypothetical exomoon and show that current transit observations likely cannot constrain the presence of exomoons orbiting HIP 41378 f, though future observations may be capable of detecting exomoons in other systems. Finally, we model the combined transmission spectrum of HIP 41378 f and a hypothetical moon with a low-metallicity atmosphere and show that the total effective spectrum would be contaminated at the ∼10 ppm level. Our work not only demonstrates the feasibility of exomoons orbiting HIP 41378 f but also shows that large exomoons may be a source of uncertainty in future high-precision measurements of exoplanet systems.

Preparation of Elastic Macroporous Graphene Aerogel Based on Pickering Emulsion Method and Combination with ETPU for High Performance Piezoresistive Sensors.

High-performance pressure sensors provide the necessary conditions for smart shoe applications. In this paper, the elastic Macroporous Graphene Aerogel (MGA) was synthesized via the modified Hummers' method, and it was further combined with Expanded-Thermoplastic polyurethane (ETPU) particles to assemble MGA-ETPU flexible sensors. The MGA-ETPU has a low apparent density (3.02 mg/cm3), high conductivity (0.024 S/cm) and fast response time (50 ms). The MGA-ETPU has a large linear sensing range (0-10 kPa) and consists of two linear regions: the low-pressure region (0 to 8 kPa) and the high-pressure region (8 to 10 kPa), with sensitivities of 0.08 kPa-1, and 0.246 kPa-1, respectively. Mechanical test results show that the MGA-ETPU sensor showed 19% reduction in maximum stress after 400 loading-unloading compression cycles at 40% strain. Electrical performance tests showed that the resistance of MGA-ETPU sensor decreased by 12.5% when subjected to sudden compression at 82% strain and returned to its original state within 0.05 s. Compared to existing flexible sensors, the MGA-ETPU sensors offer excellent performance and several distinct advantages, including ease of fabrication, high sensitivity, fast response time, and good flexibility. These remarkable features make them ideally suited as flexible pressure sensors for smart shoes.

Investigation of various curing methods on the properties of red mud-calcium carbide slag-based artificial lightweight aggregate ceramsite fabricated through alkali-activated cold-bonded pelletization technology

Industrial solid wastes red mud and calcium carbide slag are used to prepare lightweight aggregate ceramsite by cold-bonded pelletizing technology in this study. The effects of mixing water and curing method on the physical properties of ceramsite were investigated, including natural curing (in air at 20 °C), sealed curing, water curing, hot air oven curing, steam curing, and CO2 curing. The results showed that the lightweight aggregate had the lowest cylinder crush strength, the lowest apparent density, and the highest water absorption after natural curing. The highest cylinder crush strength of 8.7 MPa was obtained by steam curing at 60 °C for 12 h, which was 180.65% higher than that of natural curing because the high temperature promoted the hydration reaction thus the formation of C-(A)-S-H gel and hydrotalcite. CO2 curing at 60 °C significantly reduced the water absorption of the lightweight aggregate by 50% and by 38.03% as compared to that obtained by natural curing and steam curing at 60 °C for 12 h, respectively. During CO2 curing, the reaction between CO2 and Ca(OH)2 from the calcium carbide slag can form a dense layer of CaCO3, increasing the structural compactness and reducing the water absorption.

Sugarcane inspired polydopamine @ attapulgite-based aerogels with vertical pore structure for solar-driven steam generation

Desalination using solar-driven photothermal evaporators (SPEs) is of great significance for solving mankind's current shortage of fresh water. Herein, inspired by the vascular bundles for the directed and rapid transport of water and organic pollution such as inorganic salts in the vascular system of sugarcane, a polydopamine (PDA) @ attapulgite (ATP) based biomimetic aerogel (named as PAS aerogels) was prepared for efficient desalination. In PAS aerogels, the ATP gives the aerogel excellent mechanical properties, whereas the PDA gives the aerogel excellent light absorption properties. In the wet state, PAS aerogels have low thermal conductivity (0.24 W ∙ m−1∙ K−1) and low apparent density (0.13 g ∙ cm−3). Indicating that the photothermal evaporation rate of PAS-1.5 aerogel is 1.71 kg ∙ m−2∙ h−1 in pure water under 1 sun irradiation. Meanwhile, it can maintain excellent evaporation performance even in high salt solution with 20% concentration (about 1.50 kg ∙ m−2∙ h−1). Owing to the vertical pore structure of the aerogel, which is similar to that of sugarcane transport tissues, the aerogel has both an adequate supply of water and excellent salt tolerance. Furthermore, outdoor experiment of Yellow Sea water evaporation demonstrates its feasibility in the actual production of desalination.

Effect of ball mill time and wet pre-milling on the fabrication of Ti powders by recycling Ti machining chips by planetary milling

In this study, Cp-Ti particles were fabricated from titanium turning chips by wet-pre milling assisted mechanical milling with different ball to powder ratio and milling time. The minimum particle size fabricated with a ball to powder weight ratio of 15:1 is approximately 15 times smaller than powders produced with 5:1. TiO2 (Anatase and Rutile) peaks were found in the structure of Cp-Ti particles fabricated after 4 h of milling at a ball-to-powder ratio of 15:1. However after the milling with low ball to powder ratios (5:1 and 10:1) the oxidation was not observed in results of XRD analysis. The lowest apparent density and hardness was measured as 2.45 g/cm3 and 126 HV for 0.5 h milled powders, respectively. The maximum apparent density and hardness was measured as 3.45 g/cm3 and 325 HV for 7 h milled powders, respectively.