[Enthalpy]

Against oxidation, a soldering flux like rosin is very effective. Upon heating, it de-oxidises copper alloys, permitting the solder metal to wet them. Maybe the producer uses a similar compound (but without leftovers) for metal clay? This would permit long storage, needing a reductive atmosphere only in the kiln.

Sintering: it can achieve the full strength of an alloy, but only if using extreme pressure, hence my interrogation. Some alloys are produced that way, especially refractory ones which would be impractical to melt: molybdenum, tantalum, tungsten...

Produce a powder: wearing now my mechanical engineer hat, I'd say "grind it". Though, many metals catch fire then: titanium, iron. Neither did I try with gold, which may well clog the grinding wheel and produce zero powder.

I wouldn't be surprised if only a mix of powders with different grain sizes sinters well without pressure, because such a mix leaves less void between the grains - just like concrete uses sand, gravel and crushed stones.

Thanks for the links and explanation! I didn't know such materials. Maybe I find uses in industrial applications.

[Enthalpy]

One standard way to pulverize metals is to melt them and let an injector make the spray in a cooler gas that solidifies the droplets. Mind the nozzle material to resists liquid metal.

The gas shouldn't oxidise the metal. Nitrogen is doubtful, dry argon and hydrogen are candidates. Some hydrogen adsorbs on metals; to degas, put the powder in a gentle oven, like 10mn at 300°C.

[Enthalpy]

The mix of powders with different grain sizes would also shrink less when sintered. 30% presently is a drawback for jewellery parts.

[Enthalpy]

Mixing powders of different grain sizes would help sintering and reduce shrinkage when sintering under high pressure too, not only for metal clay - and for ceramics and polymers in addition to metals. I have an intuition that it's already done.

[Enthalpy]

Said metal clay could be interesting for 3D printing too. Or was that the original intention?

Used much like a polymer, but as a clay instead of molten: the fine printing head deposits it where metal is wanted. The printed part must be sintered afterwards, which puts constraints on the materials and the objects' shape. But a seducing alternative to present methods, which involve local laser sintering of the metal powder as each complete layer is deposited.

Marc Schaefer, aka Enthalpy

[Enthalpy]

Wondering how a metal powder ("metal clay") can constitute one solid part upon heating but keep its shape... The temperature range must be narrow, more so without applying the strong pressure typical of true sintering.

This should be much easier if the powder comprises initially separated components that combine in a eutectic. Heating between the melting points of the eutectic and the components produces a brazed joint of eutectic between the components. This process usual in microelectronics should work well with powders.

The "metal clay" could comprise powders of different components, or grains of one components covered with the other by displacement in an electrolyte, vapour deposition or any method.

This applies to jewelry and to more technical products and includes 3D printing. Mixing different grain sizes applies too.

Or is it already usual?

Marc Schaefer, aka Enthalpy

[Enthalpy]

Some parts of cast titanium alloy get pressed in hot silicone to compact and harden them. The process is known as HIP for Hot Isostatic Pressure.

I've already suggested to apply the process to polymer parts made by 3D printing
  chemicalforums

Hot Isostatic Pressure could also densify and harden parts obtained from metal powder, be it from metal clay or by 3D printing, or other processes that make porous parts. Silver and its alloys, first discussed here, melt well below titanium, so it's a candidate, as well as copper and its alloys, gold and its alloys, pure or alloyed Zn, In, Sn, Sb, Te, Pb, Bi and more.

Marc Schaefer, aka Enthalpy

[Enthalpy]

Electrically conductive glue uses to contain a fine powder of silver or other metal. It too would improve if mixing different grain sizes to make more contact points. A matrix that shrinks much upon curing should stabilize the contacts too, but I suppose this is banal.

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Thermal paste often contains a fine powder of thermally conductive ceramic in a liquid matrix, and here too, mixing different grain sizes should improve the conductivity while keeping an acceptable viscosity.

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In all uses, a continuous distribution of grain sizes doesn't suffice. It takes well separated sizes to fill the matrix better, as is done for concrete.

Marc Schaefer, aka Enthalpy

[Enthalpy]

Most conductive paints include powder of graphite or metal in a binder. They serve to electrodeposit metal on or in an insulating mold, and other purposes.

Some polymer parts also include powders, to shield against electrostatic discharges or electromagnetic interferences, improve tribological properties, adjust the expansion ratio, reduce the cost, and more.

A powder mix of very different grain sizes lets increase the volume fraction of powder, to give a higher and more reliable conductivity, reduce further the thermal expansion or the cost, and so on.

Fullerenes can provide the finest grain.

Marc Schaefer, aka Enthalpy

[Enthalpy]

More filler and less matrix, as enabled by a mix of very different grain sizes, can have more uses.

A cermet, which embeds a ceramic powder in a metal matrix, would have properties closer to the ceramic: thermal expansion, stiffness, hardness and more.

A lubricating grease could embed more graphite, MoS₂... or be more fluid.

Marc Schaefer, aka Enthalpy

[Enthalpy]

Molds can be made of wax, paraffin and other easily molten materials. Gypsum and other fillers improve the stiffness, strength, thermal expansion.

With mixes of very different grain sizes, more powder can fill the matrix for better behaviour.

Marc Schaefer, aka Enthalpy

[Enthalpy]

Solid propellants for launchers comprise aluminium powder, ammonium perchlorate powder, and a prepolymer that makes a castable paste, cures, and adds combustion gases. Casting without cracks imposes far less perchlorate than the combustion optimum.

Maybe the perchlorate could consist of several very different grain sizes, so it fills more volume and the paste can still be cast. Or maybe not, because the combustion speed depends on grain size, I suppose at aluminium more.

Vega and Ariane 6 achieve a higher perchlorate proportion than Ariane 5 does. Maybe they apply this idea already? Aluminium grain size very different from perchlorate, or perchlorate of several grain sizes?

Marc Schaefer, aka Enthalpy

[Enthalpy]

I mentioned here on 21 Feb 2023 that fillers can reduce the thermal expansion of polymer parts. Epoxy is filled with fine silica powder to encapsulate semiconductor chips. This reduces the shrinkage at polymerization as well as the thermal expansion to protect the chip and its fragile connection against big epoxy movements.

Several very different grain sizes would further improve the dimension stability, or make the encapsulating paste more fluid, possibly improve the shock resistance, or all.

Marc Schaefer, aka Enthalpy

[Enthalpy]

Silica brings an extremely low expansion coefficient, but if different grain sizes increase enough the powder's volume fraction, chip encapsulation can use other powders with different benefits, alone or mixed with silica.

For instance alumina conducts heat better than silica.

I already suggested a high optical index:
  scienceforums

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Was it done before? Around 2001, I suggested to mix a high-index nanopowder in a polymer to produce lenses.

Different grain sizes would make lenses thinner (increased powder fraction) or less brittle (fewer contacts between the grains) or easier to inject. Vision glasses and others improve.

The biggest grain size must stay <<λ/4.

Marc Schaefer, aka Enthalpy