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A small canister of metallic powder
Iron powder

A powder is a dry solid composed of many very fine particles that may flow freely when shaken or tilted. Powders are a special sub-class of granular materials, although the terms powder and granular are sometimes used to distinguish separate classes of material. In particular, powders refer to those granular materials that have the finer grain sizes, and that therefore have a greater tendency to form clumps when flowing. Granulars refer to the coarser granular materials that do not tend to form clumps except when wet.[citation needed]

Types

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Many manufactured goods come in powder form, such as flour, sugar, ground coffee, powdered milk, copy machine toner, gunpowder, cosmetic powders, and some pharmaceuticals. In nature, dust, fine sand and snow, volcanic ash, and the top layer of the lunar regolith are also examples.

Because of their importance to industry, medicine and earth science, powders have been studied in great detail by chemical engineers, mechanical engineers, chemists, physicists, geologists, and researchers in other disciplines.

Mechanical properties

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Microstructure of a steel powder particle (image 0.15 mm wide)

Typically, a powder can be compacted or loosened into a vastly larger range of bulk densities than can a coarser granular material. When deposited by sprinkling, a powder may be very light and fluffy. When vibrated or compressed it may become very dense and even lose its ability to flow. The bulk density of coarse sand, on the other hand, does not vary over an appreciable range.

The clumping behavior of a powder arises because of the molecular Van der Waals force that causes individual grains to cling to one another. This force is present not just in powders, but in sand and gravel, too. However, in such coarse granular materials the weight and the inertia of the individual grains are much larger than the very weak Van der Waals forces, and therefore the tiny clinging between grains does not have a dominant effect on the bulk behavior of the material. Only when the grains are very small and lightweight does the Van der Waals force become predominant, causing the material to clump like a powder. The cross-oversize between flow conditions and stick conditions can be determined by simple experimentation.[1]

Many other powder behaviors are common to all granular materials. These include segregation, stratification, jamming and unjamming, fragility, loss of kinetic energy, frictional shearing, compaction and Reynolds' dilatancy.

Transport

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Powders are transported in the atmosphere differently from a coarse granular material. For one thing, tiny particles have little inertia compared to the drag force of the gas that surrounds them, and so they tend to go with the flow instead of traveling in straight lines. For this reason, powders may be an inhalation hazard. Larger particles cannot weave through the body's defenses in the nose and sinus, but will strike and stick to the mucous membranes. The body then moves the mucus out of the body to expel the particles. The smaller particles on the other hand can travel all the way to the lungs from which they cannot be expelled. Serious and sometimes fatal diseases such as silicosis are a result from working with certain powders without adequate respiratory protection.

Also, if powder particles are sufficiently small, they may become suspended in the atmosphere for a very long time. Random motion of the air molecules and turbulence provide upward forces that may counteract the downward force of gravity. Coarse granulars, on the other hand, are so heavy that they fall immediately back to the ground. Once disturbed, dust may form huge dust storms that cross continents and oceans before settling back to the surface. This explains why there is relatively little hazardous dust in the natural environment. Once aloft, the dust is very likely to stay aloft until it meets water in the form of rain or a body of water. Then it sticks and is washed downstream to settle as mud deposits in a quiet lake or sea. When geological changes later re-expose these deposits to the atmosphere, they may have already cemented together to become mudstone, a type of rock. For comparison, the Moon has neither wind nor water, and so its regolith contains dust but no mudstone.

The cohesive forces between the particles tend to resist their becoming airborne, and the motion of wind across the surface is less likely to disturb a low-lying dust particle than a larger sand grain that protrudes higher into the wind. Mechanical agitation such as vehicle traffic, digging or passing herds of animals is more effective than a steady wind at stirring up a powder.

The aerodynamic properties of powders are often used to transport them in industrial applications. Pneumatic conveying is the transport of powders or grains through a pipe by blowing gas. A gas fluidized bed is a container filled with a powder or granular substance that is fluffed up by blowing gas upwardly through it. This is used for fluidized bed combustion, chemically reacting the gas with the powder.

Some powders may be dustier than others. The tendency of a powder to generate particles in the air under a given energy input is called "dustiness". It is an important powder property which is relevant to powder aerosolization. It also has implications for human exposure to aerosolized particles and the associated health risks (via skin contact or inhalation) in workplaces.

Explosion risk

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Main article: Dust explosion

Many common powders made in industry are combustible; particularly metals or organic materials such as flour. Since powders have a very high surface area, they can combust with explosive force once ignited. Facilities such as flour mills can be vulnerable to such explosions without proper dust mitigation efforts.

Some metals become especially dangerous in powdered form, notably titanium.

Comparison with other substances

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A paste or gel might become a powder after it has been thoroughly dried, but is not considered a powder when it is wet because it does not flow freely. Substances like dried clay, although dry bulk solids composed of very fine particles, are not powders unless they are crushed because they have too much cohesion between the grains, and therefore they do not flow freely like a powder. A liquid flows differently than a powder, because a liquid cannot resist any shear stress and therefore it cannot reside at a tilted angle without flowing (that is, it has zero angle of repose.) A powder on the other hand is a solid, not a liquid, because it may support shear stresses and therefore may display an angle of repose.[citation needed]

See also

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References

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Revisions and contributorsEdit on WikipediaRead on Wikipedia

Powder

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from Grokipedia
A powder is a dry, bulk solid composed of numerous fine, discrete particles, typically ranging from 100 nanometers to 1 millimeter in size, that can flow freely under agitation or gravity while exhibiting both solid-like rigidity and fluid-like behavior. These multiphasic materials consist of solid particles interspersed with interstitial gas and minimal liquid, often air and trace moisture, enabling unique mechanical properties such as packability and compressibility. Powders play a critical role across diverse industries due to their high surface area, tunable flow characteristics, and ability to form complex structures. In powder metallurgy, fine metal or alloy particles are compacted and sintered to produce precision components for automotive, aerospace, and electronics applications, minimizing material waste and enabling intricate geometries. The pharmaceutical sector relies on powders for formulating tablets, capsules, and inhalants, where particle size and distribution directly influence dissolution rates, bioavailability, and manufacturing efficiency. In the food industry, powders facilitate the creation of dehydrated products like milk powder, instant coffee, and seasoning blends, enhancing shelf life and ease of handling. Additional applications span cosmetics (e.g., talc and mica for makeup), chemicals (catalysts and pigments), and additive manufacturing (3D printing feedstock), underscoring powders' versatility in enabling innovation and efficiency. Key properties like bulk density, particle morphology, and cohesion are rigorously characterized to optimize performance and ensure reproducibility in these processes.

Definition and Fundamentals

Definition

A powder is a dry, bulk composed of many small, discrete particles, typically ranging from 100 nanometers to 1 millimeter in size, that can flow under the influence of or applied force. These particles are individual entities that maintain their discrete nature, forming a collection where interparticle spaces and potential internal voids contribute to the material's overall structure. Key attributes of powders include their and flowability, which arises from the relative movement of particles against one another under directional forces. Flowability can vary, with powders classified as free-flowing when particle interactions are minimal or cohesive when interparticle forces like van der Waals attractions lead to clumping. This behavior depends on factors such as , though powders remain fundamentally capable of bulk movement without forming a continuous phase. Powders are distinguished from dust, which consists of finer airborne particles generally below 75 micrometers that can remain suspended in air, and from granules, which are larger aggregates exceeding 1 millimeter in size, often formed by binding smaller particles for improved handling. These distinctions highlight powders' intermediate scale, enabling applications in where controlled flow and packing are essential without the dispersion risks of or the reduced mobility of granules.

Historical Development

The earliest recorded uses of powders date back to prehistoric times, when early humans employed —a natural ground into powder—for body adornment and cave paintings around 40,000 BCE. In , around 3000 BCE, powder was crushed and mixed with animal fats or oils to create green eye makeup known as kohl, serving both cosmetic and protective purposes against eye infections. A significant advancement occurred in during the 9th century CE, when Taoist alchemists accidentally invented while seeking an elixir of immortality; this black powder, composed of , , and saltpeter, marked the first known explosive mixture and revolutionized warfare. The 19th century saw the formal development of in Europe, with British chemist pioneering techniques in 1819 to produce malleable from platinum sponge powder by dissolving impure in , precipitating it, and fusing the resulting powder under and temperature. This method enabled the commercial production of pure for scientific instruments and jewelry, laying the groundwork for broader applications in metal processing. By the early , atomization techniques emerged as a key innovation for producing fine metal powders; gas and atomization, refined around the , involved disintegrating molten metal streams with high-pressure jets to create uniform particles for alloys like and aluminum, facilitating for electrical and automotive components. Scientific study of powders advanced in the late with Heinrich Hertz's 1882 theory on the between elastic spheres, which described the deformation and stress distribution at particle interfaces, providing a foundational model for understanding granular interactions. The field of granular physics emerged in the , driven by efforts to apply and continuum theories to dense assemblies of particles, addressing phenomena like jamming and flow in non-cohesive materials. Post-2000, computational modeling has transformed the discipline, with discrete element methods and simulations enabling predictions of large-scale granular behavior, such as force chains and segregation, validated against experiments in and industry.

Physical Characteristics

Particle Size and Morphology

Powder particles are classified by size into categories that reflect their behavior in various applications, such as coarse particles exceeding 100 μm, fine particles ranging from 10 to 100 μm, and ultrafine particles below 10 μm. These distinctions arise from standard practices in powder processing, where finer and ultrafine particles are suitable for applications requiring high structural integrity due to better densification and sintering, while also enhancing surface finish; however, they pose handling challenges due to increased cohesion, whereas coarser particles offer improved flowability but may compromise on density and strength. Particle size is measured using techniques like laser diffraction, which determines equivalent spherical diameters across a broad range from nanometers to millimeters by analyzing light scattering patterns, and sieve analysis, a mechanical method that separates particles through stacked sieves for sizes typically above 20 μm. Particle morphology encompasses and surface characteristics that significantly affect interparticle interactions. Common shapes include spherical, which promote efficient arrangement; irregular, often resulting from mechanical processes; and flaky or plate-like, characteristic of certain or metal powders. Spherical particles achieve higher packing densities compared to irregular or flaky ones, as their uniform geometry minimizes voids and enhances compactness during consolidation. Surface texture, particularly roughness, influences and flow; rougher surfaces increase between particles, while smoother ones reduce it, and this is quantified through scanning electron microscopy (SEM) imaging that visualizes topographic features at high resolution. Particle size distribution (PSD) describes the range and frequency of sizes within a powder sample, often following a log-normal distribution where smaller sizes occur more frequently on a logarithmic scale, reflecting natural grinding or atomization processes. Key metrics include d50, the median diameter where 50% of particles by volume are smaller; and span, calculated as (d90 - d10)/d50, which quantifies distribution width with narrower spans indicating greater uniformity. A uniform PSD ensures consistent processing outcomes, such as even sintering or coating thickness, by minimizing segregation during handling. Variations in PSD can lead to differences in bulk density, as explored in related sections.

Bulk Properties

Bulk density refers to the mass of a powder divided by the volume it occupies in a loose, untapped state, encompassing both the volume of the particles and the interparticulate voids. Poured density, a form of , is determined by gently pouring the powder into a without compaction, reflecting the initial packing efficiency influenced by particle interactions. In contrast, tapped density measures the same ratio after subjecting the powder to mechanical tapping, which settles the particles and reduces void volume, typically resulting in a higher value than poured density due to compaction. These are standardized in pharmaceutical and materials testing to assess powder packing behavior. The Hausner ratio quantifies powder compressibility by comparing tapped and poured densities, calculated as H=ρtρbH = \frac{\rho_t}{\rho_b}, where ρt\rho_t is the tapped density and ρb\rho_b is the bulk (poured) density. Values closer to 1 indicate low and good flowability, while higher ratios (e.g., above 1.25) suggest poor flow due to greater volume reduction upon tapping. This metric, originally developed by Hausner in the , provides a simple index for predicting handling characteristics in industries like pharmaceuticals and . Flowability of powders is often evaluated using the angle of repose, which measures the maximum angle at which a pile of powder remains stable without slumping, calculated as θ=tan⁡−1(hr)\theta = \tan^{-1}\left(\frac{h}{r}\right), where hh is the height of the pile and rr is the radius of its base. Angles below 30° typically indicate excellent flow, while those exceeding 56° suggest cohesive, poor-flowing powders. Another key metric is Carr's index, derived from densities as CI=100×ρt−ρbρtCI = 100 \times \frac{\rho_t - \rho_b}{\rho_t}, where values under 15% denote free-flowing powders and over 25% indicate cohesive ones with handling challenges. These indices, standardized in pharmacopeial methods, help characterize bulk flow without advanced equipment. Cohesion in powders arises from interparticle forces such as , which dominate in , dry particles by inducing attractive potentials over short ranges, leading to clumping and reduced flow. exacerbates cohesion through bridging, forming forces that bind particles, while electrostatic charges contribute to by generating repulsive or attractive fields between charged surfaces. These forces collectively increase interparticle attraction, making powders (e.g., sub-micron sizes) prone to agglomeration. to container walls follows similar mechanisms, often combining and electrostatic effects, complicating discharge in processing equipment. Shear cell testing assesses cohesion by applying normal and shear stresses to a powder sample, generating yield loci that plot the minimum required for flow against applied normal stress, as pioneered by Jenike in the . The yield locus curve, often linear for cohesive powders, reveals the unconfined yield strength, where higher values indicate greater cohesion and poorer flowability. This method, using devices like the annular shear cell, enables derivation of flow functions to classify powders from easy-flowing to very cohesive, informing design and process optimization.

Production Methods

Mechanical Processes

Mechanical processes for powder production involve the physical of bulk materials into finer particles through the application of mechanical forces, without inducing chemical alterations to the material's composition. These methods are essential for creating powders from raw ores, minerals, polymers, and other solids, where the goal is to achieve a desired (PSD) while minimizing energy input and material degradation. Common techniques rely on principles of impact, attrition, shear, and compression to and reduce particle size, often requiring subsequent separation steps to refine the output. Grinding and milling represent the core of mechanical powder production, employing specialized equipment to break down larger feedstocks. Ball milling, for instance, uses rotating cylinders filled with grinding media such as balls to induce attrition and impact, effectively reducing particle sizes to sub-micron levels in high- variants. Jet milling, on the other hand, accelerates particles in a high-velocity gas stream, where collisions between particles generate the grinding action, making it suitable for heat-sensitive materials due to the absence of mechanical contact. The required for these processes follows Rittinger's , which posits that the work input is proportional to the increase in surface area created during size reduction; mathematically, this is expressed as E=K(1d2−1d1)E = K \left( \frac{1}{d_2} - \frac{1}{d_1} \right) where EE is the energy per unit mass, d1d_1 and d2d_2 are the initial and final particle diameters, respectively, and KK is a material-specific constant. This law holds particularly well for fine grinding operations, where surface creation dominates the energy demand. Following grinding, sieving and classification ensure the powder achieves a uniform PSD by separating particles based on size. Vibratory sieves apply mechanical agitation to a stacked series of mesh screens, allowing finer particles to pass through while retaining coarser ones, typically effective for sizes above 75 μm. Air classifiers, conversely, use aerodynamic forces in a gas stream to segregate particles by terminal velocity, enabling precise control for ultra-fine fractions below 10 μm and integrating well with milling circuits for continuous operation. These steps are critical for tailoring the PSD to specific applications, such as enhancing flowability or reactivity in downstream processing. In mineral processing, ore crushing exemplifies mechanical powder production, where primary jaw crushers reduce run-of-mine ore to manageable sizes, followed by secondary cone or impact crushers to yield fine mineral powders for beneficiation. For polymers, cryogenic grinding mitigates heat buildup by cooling the material with liquid nitrogen to below its glass transition temperature, enabling brittle fracture and preventing thermal degradation during milling, which is particularly advantageous for elastomers and thermoplastics. The resulting particle morphologies, such as irregular shapes from impact milling, influence powder behavior and are further characterized in the particle size and morphology section.

Chemical and Thermal Processes

Chemical and processes for powder production involve transforming precursors through reactions or application to create fine particles, distinct from mechanical fragmentation. These methods enable precise control over particle composition, size, and morphology, often yielding high-purity powders for like metals and ceramics. Key techniques include atomization, , sol-gel synthesis, , and , each leveraging chemical or thermal drivers to form powders from liquids or solids. Atomization produces metal powders by disintegrating molten metal into droplets that solidify rapidly. In gas atomization, a stream of molten metal, such as , is ejected through a into a chamber where high-velocity inert gas jets (e.g., or ) impinge upon it, breaking the stream into fine droplets typically smaller than 150 microns. The process begins with melting the raw material via induction or arc methods, followed by atomization in a controlled environment to minimize oxidation. Droplet solidification occurs during flight, involving stages of undercooling, , recalescence (rapid temperature rise due to release), and segregation solidification, with smaller droplets exhibiting lower nucleation temperatures and higher solid fractions at recalescence due to faster cooling rates. This results in spherical particles suitable for additive manufacturing. Water atomization, a cost-effective variant, uses high-pressure jets (20–200 bars) to shatter molten metal streams into irregular droplets that solidify into powders ranging from 50 μm to 1 mm, commonly applied to iron and for sintered components. The rapid in promotes oxidation but allows high throughput, up to 30 tons per hour in industrial setups. Chemical precipitation methods generate ceramic powders by inducing simultaneous precipitation of multiple metal ions from solutions, ensuring compositional homogeneity. of s, for instance, involves adding a precipitant like ammonium hydrogen to aqueous solutions of metal salts (e.g., and aluminum chlorides), forming amorphous precursors that are then to yield pure phases such as yttrium aluminum garnet (YAG). Optimal conditions, such as 0.5 M precipitant concentration, produce soft agglomerates with no secondary phases after at 1000°C, enabling high sinterability and translucent ceramics upon densification. This technique is favored for ceramics requiring uniform , as it avoids segregation issues in solid-state mixing. The sol-gel process extends to nanoparticles via controlled and of metal s or salts. converts groups (M-OR) into hydroxyls (M-OH) in the presence of and catalysts, forming a sol of colloidal particles less than 100 nm. Subsequent links these via M-O-M bonds, releasing or alcohol and creating a three-dimensional gel network that, upon drying and , yields nanoparticles with tailored sizes and high purity. Acidic conditions accelerate linear for fibrous structures, while basic media promote branching for particulate gels, making sol-gel ideal for applications like catalysts and . Thermal methods further refine precursors into powders through or . converts solutions or slurries into dry powders by atomizing the feed into a hot gas stream (typically 150–250°C), where rapid of the forms hollow or dense spherical particles in seconds. The process includes feed preparation for homogeneity, atomization via nozzles (e.g., or two-fluid types) to create fine droplets, and drying in a co-current chamber, followed by separation with cyclones to collect the powder. This yields free-flowing powders with controlled morphology, widely used for pharmaceuticals and food additives. heats solid precursors, such as carbonates, to decompose them into oxides while releasing gases like CO₂, typically at 800–1000°C in controlled atmospheres. For example, (CaCO₃) decomposes endothermically above 900°C to form CaO, with the interface stabilizing near the decomposition point despite bulk heating. This thermal treatment enhances phase purity and reactivity, essential for producing oxide powders in ceramics and battery materials.

Material Properties

Mechanical Properties

Powders exhibit distinct mechanical behaviors under applied forces, influenced by interparticle interactions and material composition. These properties determine how powders withstand shear, compact under pressure, and resist frictional wear during handling. Understanding these responses is essential for applications in manufacturing and processing, where powders must endure stress without excessive deformation or failure. Shear strength in powders is commonly modeled using the Mohr-Coulomb failure criterion, which describes the onset of yielding under combined normal and shear stresses. The criterion is expressed as τ=c+σtan⁡ϕ\tau = c + \sigma \tan \phi, where τ\tau is the shear stress at failure, cc is the cohesion (often negligible for dry, non-cohesive powders), σ\sigma is the normal stress, and ϕ\phi is the internal friction angle, typically ranging from 20° to 45° for granular materials like sands or metal powders. This linear relationship captures the frictional nature of powder failure, where higher normal stress increases resistance to shear, as derived from soil mechanics and extended to granular flows in quasi-static conditions. In confined storage systems such as silos, the Janssen effect illustrates how wall friction limits vertical stress transmission in powders. Initially proposed through experiments on corn silos, this phenomenon occurs when frictional forces between the powder and silo walls support a portion of the material's weight, causing vertical pressure to saturate at a characteristic depth rather than increasing linearly with height. The effect is particularly pronounced in tall, narrow containers, where the saturated pressure PP balances the material density ρ\rho, gravity gg, silo cross-section AA, wall friction coefficient μ\mu, and perimeter UU, following a differential equation solution akin to exponential decay. For cohesive powders, this leads to arching and reduced base loads, impacting design in bulk storage. Compaction of powders under uniaxial proceeds through distinct stages of densification, reflecting progressive particle interactions. In the initial rearrangement stage, low (typically below 50 MPa) allow particles to slide and rotate, filling voids and increasing without significant deformation, leading to a rapid initial rise. As escalates (around 100-300 MPa), the deformation stage dominates, where elastic recovery and plastic yielding at contact points close remaining pores, with brittle materials showing localized flow. At higher (above 500 MPa), fragmentation occurs, fracturing particles to further densify the compact, though this increases flow resistance due to . These stages are influenced by and , with finer powders favoring rearrangement over fragmentation. The plasticity of powders during compaction is quantified by the Heckel equation, which models the of under : 1−ρbρt=Ae−kP1 - \frac{\rho_b}{\rho_t} = A e^{-kP}, where ρb\rho_b is , ρt\rho_t is true , PP is applied , AA is an initial factor, and kk is a material-specific constant related to yield strength. The reciprocal of kk yields the mean yield , providing a measure of deformability; for example, ductile metal powders like iron exhibit kk values around 0.01-0.02 MPa^{-1}, indicating easier compaction than brittle ceramics. This equation, derived from first-order kinetics analogy for pore closure, applies primarily to the deformation stage and correlates with performance in pharmaceuticals. increases notably during these stages, linking to overall packing efficiency. Interparticle governs load transmission and stability in powders, with coefficients typically ranging from 0.1 for smooth spheres to 0.5-1.0 for angular granules, affecting shear resistance and elastic moduli. Higher enhances tangential forces at contacts, strengthening the chain network and increasing bulk , as seen in discrete element simulations where reduced leads to more isotropic stress distribution and lower . In angular powders like or ores, promotes dilatancy during shear, expanding volume under stress. Abrasion during powder handling arises from particle impacts and sliding against equipment surfaces, causing material loss through micro-cutting or ploughing mechanisms. In low-stress scenarios, such as pneumatic conveying of fine powders like fly ash, wear rates follow Archard's law, scaling with hardness mismatch; alloys (500-1000 HV) reduce volume loss by up to 80% compared to mild . High-stress abrasion in mixers or mills involves impact, where ductile materials form protective tribolayers, while brittle powders like silica accelerate via repeated . Selection of wear-resistant linings, such as martensitic , mitigates these effects in industrial flows of minerals.

Chemical and Thermal Properties

Powders exhibit enhanced chemical reactivity primarily due to their high (A/V), which accelerates oxidation and other surface-mediated reactions compared to bulk materials. The reaction rate is proportional to this ratio, as smaller s expose more reactive sites per unit mass, facilitating faster interaction with oxidants like oxygen or . For instance, in powders, increased surface area from milling has been shown to elevate nitridation rates while altering the alpha to beta phase ratio during oxidation processes. This heightened reactivity is particularly pronounced in metal powders, where fine particles can become pyrophoric, igniting spontaneously in air. Pyrophoric aluminum powders, especially nanoscale variants, can ignite at ambient temperatures or as low as 30–100°C depending on and layer thickness, generating sufficient from rapid oxidation to sustain . The thermal conductivity of loose powders is notably low, typically ranging from 0.01 to 0.1 W/m·, owing to limited contact between particles and the insulating effect of interstitial gas (e.g., air or inert atmospheres). Heat transfer occurs mainly through point contacts between irregular particle surfaces and conduction via the surrounding , resulting in poor overall efficiency compared to solid forms of the same material. For metal hydride powders, effective thermal conductivities fall between 0.1 and 1.5 W/m·, influenced by packing and , with gas-phase conduction dominating in low-density beds. Effective (EMT) provides approximations for these properties by modeling the powder bed as a heterogeneous composite, averaging the conductivities of solid particles and the fluid phase while accounting for and ; percolation-based EMT variants further refine predictions for porous media by incorporating connectivity thresholds. Thermal stability in powders is governed by processes like and phase transformations, which occur at elevated temperatures and can alter material integrity. Sintering temperatures for metal powders generally range from 60–80% of the material's (T_m); for example, iron powders sinter effectively at 1000–1200°C (T_m ≈ 1538°C), while powders require 700–900°C (T_m ≈ 1085°C), promoting particle necking and densification without full . powders, such as alumina or zirconia, sinter at higher homologous temperatures, often 1200–1600°C, due to stronger ionic bonds, with additives like yttria lowering the onset for enhanced densification. In pharmaceutical powders, heating can induce polymorphic transitions, where metastable forms convert to more stable polymorphs, potentially affecting ; for instance, indomethacin undergoes form II to form I transition around 100–150°C, while faster heating rates can suppress endothermic solid-solid changes, leading to direct of lower-temperature polymorphs. These transitions are kinetically influenced, with reversible enantiotropic behavior observed under controlled .

Handling and Transport

Transport Techniques

Pneumatic conveying is a widely used method for transporting powders through pipelines using gas streams, particularly air, to suspend and move the material. This technique is suitable for fine to granular powders and offers flexibility in routing around obstacles. There are two primary modes: dilute-phase and dense-phase conveying. In dilute-phase conveying, powders are suspended in a high-velocity gas stream (typically 20-40 m/s) at low concentrations (solids loading ratio of 1:1 to 15:1), resulting in a suspension-like flow that minimizes particle but can cause higher on due to particle impacts. In contrast, dense-phase conveying operates at lower velocities (3-10 m/s) with higher solids concentrations (loading ratios up to 100:1 or more), where powders move as plugs or slugs in a fluidized or semi-fluidized state, reducing energy consumption and abrasion while being ideal for fragile or abrasive materials like or plastics. Pressure drop in pneumatic conveying systems is a critical design parameter, calculated to ensure efficient operation and prevent blockages. The total pressure drop (ΔP) depends on factors such as pipeline length (L), diameter (D), gas density (ρ), and velocity (v), often expressed through hydrodynamic models that account for frictional losses, particle acceleration, and elevation changes. For dilute-phase systems, a common approach uses the Darcy-Weisbach equation adapted for two-phase flow, where ΔP ≈ f(L/D) * (ρ v² / 2) for gas friction plus solids-related terms, with empirical correlations for saltation velocity to avoid deposition. Dense-phase calculations incorporate voidage and plug flow dynamics, typically yielding lower ΔP per unit length due to reduced velocity. Mechanical systems provide robust alternatives for horizontal or inclined powder transport, relying on physical contact rather than fluid suspension. Belt conveyors consist of a continuous flexible belt loop supported by rollers, transporting powders in open or enclosed configurations at speeds up to 3 m/s, suitable for high-volume, non-abrasive materials like or minerals to minimize generation. Screw feeders, or augers, use a rotating helical within a trough to push powders forward, offering precise metering and compaction for cohesive materials at rates of 0.1-100 tons/hour, though limited to shorter distances due to shear-induced degradation. Vibration-induced flow in hoppers complements these by applying oscillatory forces (typically 10-50 Hz) to overcome arching or rat-holing in storage vessels, promoting mass flow through the outlet without full , as seen in pharmaceutical and where gentle agitation prevents segregation. Fluidization techniques enable powder transport by passing gas upward through a bed of particles, transforming the static powder into a fluid-like state for easy movement. Gas fluidized beds are employed in transport lines or reactors, where air or at controlled velocities suspends powders, allowing horizontal or vertical conveyance with minimal mechanical parts. This method is particularly effective for fine, aeratable powders (Geldart A/B classes) in processes like or , with bed expansion providing uniform mixing. The minimum fluidization velocity (u_mf), the threshold for initial suspension, is determined by balancing drag and gravitational forces, often using the Ergun equation-derived correlation: umf=μρgdpRemfu_{mf} = \frac{\mu}{\rho_g d_p} Re_{mf} where μ is gas viscosity, ρ_g is gas density, d_p is particle , and Re_mf is the at minimum (typically 0.4-20 for small particles, via Wen-Yu approximation Re_mf ≈ √(33.7² + 0.0408 Ar) - 33.7, with Ar = ρ_g (ρ_p - ρ_g) g d_p³ / μ²). Simplified correlations adjust for particle properties, ensuring u_mf remains below to avoid entrainment.

Flow and Processing Challenges

Powder flow and processing present significant challenges due to the particulate nature of powders, which can lead to inconsistent handling and production inefficiencies in industrial applications. One primary obstacle is segregation, where particles of different sizes or densities separate within a during flow or agitation, compromising uniformity. This arises from various mechanisms influenced by particle interactions and external forces, often resulting in demixing that affects product quality in processes like blending and . Segregation mechanisms include sifting, a size-based process where smaller particles percolate through voids between larger ones under gravity or , allowing fines to sift downward while coarser particles rise. , closely related to sifting, involves small particles moving into gaps created by shear or shaking in the bed, particularly prominent when fines constitute a minor fraction of the mixture. In binary mixtures subjected to vertical , the effect occurs, whereby larger, denser particles (intruders) rise to the top as surrounding smaller particles percolate around them, driven by a displacement mechanism; this effect is reversed under certain conditions like high amplitude, where large particles may sink. These mechanisms are well-documented in granular systems and highlight the role of disparity in promoting spatial inhomogeneity during processing. Another critical challenge is jamming and arching, especially in hoppers, where cohesive forces between particles cause the formation of stable bridges that obstruct flow. Cohesion, arising from factors like van der Waals forces, electrostatics, or moisture, enables particles to support the weight of overlying material, leading to blockages near discharge outlets. The span of such cohesive arches can be predicted using the relation B=k(σcρg)0.5B = k \left( \frac{\sigma_c}{\rho g} \right)^{0.5}, where BB is the arch size, kk is a material-dependent constant, σc\sigma_c is the cohesive strength, ρ\rho is the bulk density, and gg is gravitational acceleration; this formula underscores how higher cohesion or lower density exacerbates arch formation, necessitating larger outlet diameters to ensure reliable discharge. Mechanical arching, involving particle interlocking without strong cohesion, is less common but can occur in non-cohesive systems with outlets smaller than 8–12 particle diameters. To mitigate these flow challenges, several strategies are employed to enhance powder mobility and prevent segregation or blockages. Additives such as glidants, exemplified by , are incorporated to reduce interparticle cohesion by adsorbing onto particle surfaces, thereby improving flowability without altering bulk composition significantly. For arching in , mechanical aids like apply localized oscillations to disrupt bridges, though arches may reform post-vibration if underlying cohesion persists. Air cannons deliver impulsive air blasts to dislodge adhered material from hopper walls, promoting consistent discharge in cohesive powders. These interventions, often combined, address the root causes of poor flow while maintaining efficiency.

Safety and Risks

Explosion and Combustion Risks

Combustible powders pose significant explosion risks when their fine particles form airborne suspensions in the presence of an ignition source. The dust explosion pentagon outlines the five essential elements required for such an event: a combustible dust as fuel, an oxidizer typically from ambient air, an ignition source such as sparks or hot surfaces, dispersion of the dust particles to create a flammable cloud, and confinement within an enclosure that allows pressure buildup. Removing any one of these elements can prevent an explosion, emphasizing the importance of controlling dust accumulation and ignition sources in industrial settings. Key parameters characterizing the severity of dust explosions include the maximum (Pmax) and the index (Kst). Pmax represents the highest achieved during a in a closed vessel, typically ranging from 5 to 12 bar for most combustible s, and is independent of vessel volume if the enclosure withstands the . Kst, measured in bar·m/s, quantifies the maximum rate of rise normalized to a 1 m³ vessel and is used to classify explosivity: St 0 (non-explosive, Kst = 0), St 1 (weak, 1–200 bar·m/s), St 2 (strong, 201–300 bar·m/s), and St 3 (very strong, >300 bar·m/s). Ignition sensitivity is assessed by the minimum ignition energy (MIE) and minimum ignition temperature (MIT). MIE is the lowest electrical or electrostatic energy, in millijoules (mJ), capable of igniting a dust-air mixture, with lower values indicating higher sensitivity; for example, grain dust typically has an MIE of around 30 mJ. MIT is the lowest temperature of a hot surface or cloud that can initiate ignition, such as approximately 430°C for grain dust. Prevention strategies focus on eliminating explosion elements through standards like NFPA 660 (effective December 6, 2024), which provides comprehensive guidelines for managing fire and risks in facilities handling combustible particulate solids, including hazard analysis, , and equipment design to minimize accumulation and ignition. For instance, a combustible at Horizon Biofuels in , on July 29, 2025, resulted in fatalities and is under investigation by the U.S. Board, underscoring ongoing risks. venting directs release through panels or ducts, with the required vent area (Av) calculated as Av = C (V2/3 Kst1/2 / Pred1/2), where V is the enclosure in m³, Pred is the maximum reduced in bar, and C is a constant depending on dust type and vent geometry per NFPA 68. Suppression systems, using chemical agents to interrupt within milliseconds, offer an alternative for protecting equipment where venting is impractical.

Health and Environmental Hazards

Exposure to fine powder particles poses significant health risks primarily through , as these particles can penetrate deep into the . The most penetrating particle size (MPPS) for lung deposition is approximately 0.5 μm, where particles are least likely to be filtered by upper airways and can reach the alveoli via and . Inhalation of respirable crystalline silica from , common in and powders, leads to , a progressive disease characterized by and scarring that impairs breathing. The (OSHA) sets a (PEL) of 0.05 mg/m³ for respirable to mitigate this risk. Similarly, prolonged of fibers, often released as fine powder during handling, causes , an resulting in thickened tissue and reduced oxygen exchange. Powder exposure can also irritate the skin and eyes, particularly with alkaline substances that disrupt tissue integrity. Contact with lime () powder, for instance, generates heat upon reaction with moisture, leading to chemical burns on the skin and severe corneal damage in the eyes due to of cell membranes. In pharmaceutical settings, certain powders enable dermal absorption, allowing systemic uptake of potent compounds through the skin barrier and potentially causing or in workers. Environmentally, powder handling generates dust emissions that contribute to ambient air pollution, elevating particulate matter (PM) levels and affecting ecosystems and beyond industrial sites. Fine particles from processes like grinding or pneumatic conveying can disperse widely, exacerbating formation and deposition in water bodies. Remediation techniques such as wet scrubbing, which use liquid sprays to capture in gas streams, effectively reduce these emissions by up to 99% for particles larger than 1 μm.

Applications

Industrial and Manufacturing Uses

Powder metallurgy is a key industrial process for fabricating complex metal components from fine metal powders, offering advantages in material efficiency and near-net-shape production. In this method, metal powders are first compacted under high pressure to form a green body, typically at with pressures ranging from 100 to 700 MPa, depending on the powder's characteristics and desired . The compacted part is then at elevated temperatures to bond the particles, achieving densities up to 95% of theoretical values. For tools, sintering occurs around 1400–1500°C to form hard, wear-resistant cermets used in cutting tools and dies. (HIP) further enhances densification by applying uniform pressure of 100–200 MPa and temperatures from 900–1250°C in a gas atmosphere, eliminating residual in high-performance parts like components. In ceramics and composites , powders enable precise forming techniques for intricate shapes. Slip involves suspending clay powders in water to create a slip, which is poured into porous molds where draws out liquid, leaving a solid clay layer that forms the desired shape after drying and firing. This method is widely used for producing sanitaryware, , and electrical insulators from materials like kaolin and ball clay. Additive , or 3D printing, with metal powders emerged in the as a layer-by-layer fusion process using lasers or beams to build components from digital designs, revolutionizing prototyping and production of and alloys for automotive and biomedical applications. Food processing utilizes powders for efficient preservation and transformation of raw materials. Milling grains such as involves cleaning, conditioning, and grinding the kernels through roller mills to separate and germ from the , yielding fine particles suitable for and other uses. converts liquid into powder by atomizing concentrated milk into a hot air chamber, rapidly evaporating to produce stable, non-perishable skim milk powder with extended up to two years under ambient conditions. These processes must address flow challenges, such as powder bridging in lines, to ensure consistent throughput.

Pharmaceutical and Consumer Uses

In pharmaceutical formulations, powders play a vital role in systems, particularly through dry powder inhalers (DPIs) designed for respiratory conditions such as . These inhalers disperse fine drug particles directly into the lungs for rapid onset of action, bypassing gastrointestinal absorption. For instance, sulphate, a short-acting beta-agonist used to relieve , is micronized to particle sizes typically below 5 μm to ensure aerodynamic deposition in the lower airways, with optimal sizes around 1-3 μm for deep lung penetration. This formulation approach enhances while minimizing systemic side effects, as the powder is often blended with carrier particles like to improve dispersibility and flow properties. Powders are also essential in solid dosage forms like tablets, where they are compressed using excipients to achieve uniform distribution and mechanical stability. monohydrate serves as a primary filler and in these processes, providing bulk volume, aiding powder flow during , and acting as a binder to enhance tablet hardness without interfering with release. It is incorporated in 60-70% of oral solid , often comprising 100-200 mg per tablet to facilitate direct compression or wet techniques. These excipient-powder interactions ensure consistent disintegration and dissolution, critical for controlled therapeutic efficacy. In consumer products, powders contribute to and personal care items by providing texture, coverage, and functional benefits. , a finely milled magnesium , is widely used in makeup formulations like face powders and foundations for its oil-absorbing properties and ability to create a smooth, matte finish on the . , another mineral powder, imparts a shimmering, pearlescent effect in products such as eyeshadows and highlighters due to its platelet-like that reflects light. For sun protection, forms of (TiO2) and zinc oxide (ZnO), with particle sizes under 100 nm, are suspended in powders or lotions to offer broad-spectrum UVA and UVB blocking without the opaque white cast of larger particles; these nanoparticles primarily scatter UV radiation while minimizing penetration. Household applications leverage powders for their reactivity and cleaning efficiency. , a staple in food preparation, consists of (NaHCO3) combined with dry acids such as or sodium acid pyrophosphate, along with a stabilizer; upon and heating, the acid-base reaction releases gas, promoting leavening in batters and doughs. In cleaning products, powdered detergents incorporate like linear alkylbenzene sulfonates to reduce , emulsify oils and soils, and enable effective removal of grease from surfaces, often augmented by builders such as sodium tripolyphosphate for and enhanced performance. While handling pharmaceutical powders requires precautions against risks, as noted in safety guidelines, consumer formulations prioritize stability and ease of use.

Comparisons

With Other States of Matter

Powders differ fundamentally from liquids in their flow behavior, as they lack the continuous characteristic of fluids but instead display a yield stress that must be exceeded for flow to occur. This yield stress arises from interparticle forces such as and cohesion, allowing powders to behave as solids under low stress conditions, in contrast to liquids which flow readily under minimal shear. Unlike liquids, dry powders do not exhibit flow driven by in narrow spaces, though partial can induce bridges between particles, enhancing cohesion. When liquids are added to powders, they transition into pastes or slurries in the pendular state, where discrete liquid bridges form between particles, increasing strength and altering flow properties without achieving the uniform fluidity of pure liquids. The stability of piled powders is governed by the angle of repose, a measure of frictional resistance that determines the slope at which material remains static, whereas liquids form menisci shaped by and angles. In comparison to gases, powders comprise discrete solid particles that interact through collisions and contacts, forming a discontinuous medium rather than the molecular continuum of gases. This discreteness leads to distinct settling dynamics; for fine particles in a gaseous or carrier, the sedimentation vv is described by : v=(ρp−ρf)gd218μv = \frac{(\rho_p - \rho_f) g d^2}{18 \mu} where ρp\rho_p is the particle density, ρf\rho_f the fluid density, gg gravitational acceleration, dd the particle diameter, and μ\mu the fluid viscosity, highlighting how particle size and density drive settling absent in gaseous diffusion. Phase transitions involving powders often occur in mixtures with other states, such as powder-liquid slurries where solid particles are suspended in a carrier fluid, enabling pumpable flows for transport or processing while maintaining higher bulk densities than either phase alone. In freeze-drying, frozen aqueous solutions or slurries undergo sublimation, directly converting ice to vapor under vacuum to yield dry, porous powders without passing through the liquid phase, preserving structure in applications like pharmaceuticals.

With Granular Materials

Powders are distinguished from larger granular materials primarily by , with powders typically consisting of particles smaller than 1 mm, often below 100 μm, where interparticle cohesion forces such as van der Waals interactions dominate over gravitational or inertial effects. In contrast, granular materials like or feature particles exceeding 1 mm, where inertial forces prevail, leading to flow behaviors governed by particle collisions and rather than bonds. This scale threshold influences the in granular flows; for powders, the grain is often low (typically <1), emphasizing viscous drag and cohesion, while for larger granules, higher (>100) promote inertial-dominated, quasi-frictionless dynamics. Behaviorally, powders exhibit greater susceptibility to aerosolization, where fine particles readily become airborne suspensions upon disturbance, forming dust clouds that can persist due to their low settling velocities. Electrostatic effects are also more pronounced in powders, as triboelectric charging during particle contacts leads to aggregation or adhesion to surfaces, complicating handling in processes like pharmaceutical mixing. Granular materials, however, form more stable piles with narrower ranges of angle of repose, typically 30–45° for non-cohesive examples like dry sand, reflecting reliance on geometric stability and . Powders display a broader and often higher range of repose angles (20–65°), influenced by cohesion, which can result in steeper but less predictable piles; for instance, has an angle of repose of approximately 45° due to van der Waals bonding, while less cohesive powders like coarse salt settle at 30–45°.https://www.engineeringtoolbox.com/dumping-angles-d_1531.html Despite these differences, powders and granular materials share phenomena like jamming, a transition to a rigid, solid-like state under confinement or shear cessation, driven by packing and in both systems. However, powders tend to jam more readily due to cohesive bridges forming arches, whereas larger granules require higher stresses to initiate jamming, often behaving more fluid-like under moderate agitation. , the suspension of particles in an upward gas flow, highlights further distinctions: cohesive powders resist initial fluidization, demanding higher gas velocities to overcome interparticle forces, unlike larger granules which transition to a fluidized state more easily at lower velocities. Examples illustrate these overlaps and contrasts; , as a granular material with millimeter-scale crystals, forms stable drifts with jamming under compression but minimal cohesion, while , a powder, shows enhanced jamming in hoppers and easier during sieving.

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