Likely-feasible non-flux-deposition powder-bed 3-D printing processes

Kragen Javier Sitaker, 2015-09-11 (updated 2019-10-08) (20 minutes)

I just wrote this long thing in Flux deposition for 3-D printing in glass and metals about a powder-bed 3-D printing technique that deposits a binder that’s completely inert at room temperature, but upon firing the print in a kiln, becomes active. (See also 3-D printing by flux deposition.)

I think there are a variety of other possibilities in powder-bed 3-D printing that have not yet been fully explored.

Powder-bed 3-D printing, in general, consists of depositing one layer after another of powder, alternating with selectively applying some kind of treatment to the top layer of powder which results in causing it to solidify. The classic inkjet-binder-deposition 3-D printing is one example, but selective laser sintering and selective laser melting are other processes in this category.

Magnesium oxychloride (Sorel cement) or zinc oxychloride

Sorel cement is a combination of highly water-soluble magnesium chloride (nigari) with highly water-insoluble magnesium oxide (milk of magnesia); it’s a cement similar to Portland cement, but more refractory, less water-resistant (and won’t harden underwater), and nearly twice as strong.

So, although I’d have to investigate more, I think you could use an aqueous solution of magnesium chloride to moisten a powder bed of sand and dry magnesium oxide to form a very strong mortar.

Zinc oxychloride might work in the same way: zinc oxide is insoluble, like magnesium oxide, while zinc chloride is so soluble it’s deliquescent; and zinc oxychloride or zinc hydroxychloride formed in precisely this way was formerly used as a dental cement, like the zinc phosphate mentioned below. Zinc chloride, however, is acidic, corrosive, and a skin irritant, while magnesium chloride is free of these problems. In fact, Sorel investigated zinc oxychloride before settling on magnesium oxychloride!

Selective hammering

Instead of squirting binders onto a powder bed like an inkjet printer, you could bang the shit out of it with hammers like a dot-matrix printer, ideally under vacuum so that you don’t generate explosive gas expulsions. The impact will stick together the particles in the vicinity, affecting a total mass of powder material similar to the total mass of the hammer. (This suggests that low-mass hammers are in some sense optimal.)

Selective electrical sintering

For beds of metal particles, instead of squirting binders, you could touch the surface of the powder with an electrode and drive a large current into it, sintering the nearby particles together through joule heating of their contact points, like an old-fashioned coherer.

The electrode would probably have to be a carbon rod, since any other plausible material is likely to stop working due to surface oxidation.

This probably won’t produce a strongly bonded part, but might be enough to produce a solid part that can then be solidified further by other means.

Cement precipitation by cross-linking with calcium or other polyvalent cations

A number of anions, such as phosphate, carbonate, and alginate, form water-soluble compounds with monovalent cations like those of the alkali metals (sodium, potassium) and ammonium, while forming water-insoluble compounds with divalent cations like those of the alkaline earth metals (calcium, magnesium). Calcium and magnesium also have highly water-soluble salts, such as their nontoxic chlorides. Phosphate is also water-soluble in the form of phosphoric acid.

This means that by mixing two liquids you can precipitate a solid through a double ion replacement reaction. This is used in molecular gastronomy spherification of foods, forming a flexible calcium alginate membrane around a liquid center with sodium alginate dissolved in it.

(I’m pretty sure this is because these anions are polyvalent and are strongly enough bonded to their cations that they are solvated together with them, rather than separately, so that once the cations are also polyvalent, the individual anions floating around with their individual cation harems are replaced by endless chains in which each cation links together different anions. But I’m no chemist.)

Candidate cements and fillers

Other polyvalent cations, like Cu₂₊, Zn₂₊, Fe₃₊, and Fe₂₊, should also work for this. Most of these also have relatively innocuous water-soluble salts; ZnCl₂, Fe(NO₃)₃, Cu(NO₃)₂, FeCl₃, as well as blue, white, and green vitriol, of course, which last are innocuous enough to use as nutritional supplements, but are subject to onerous reporting paperwork in places nowadays; acetates of calcium, magnesium, copper, zinc, and ferrous iron (II) are also all soluble, though acetate of zinc only a bit, and acetate of ferric iron (III) not at all. Ferrous citrate is also soluble.

So the plan is that you precipitate a solid cement in the interstices of an aggregate or filler, such as quartz, carbon black, fumed silica, mullite needles, aluminum oxide crystals, rutile needles, zircon crystals, chopped carbon fiber, chopped basalt fiber, powdered graphite, powdered copper, powdered silver, hollow glass spheres, hollow steel spheres, chopped cellulose fiber (such as sawdust), clay (especially finely dispersed bentonite), diatomaceous earth, etc.; or a mixture. Different possible resulting cements include the following; I’m including Mohs hardnesses as an imprecise but readily available and roughly accurate guide to strength:

So you should be able to get relatively high strength, almost as high as portland cement (whose strength comes mainly from belite, which is known as larnite in nature, Mohs 6), by precipitating calcium phosphate crystals from a water-soluble calcium salt such as calcium chloride and a water-soluble phosphate salt such as monoammonium phosphate; you may be able to get a highly refractory bond by calcining the phosphate or carbonate of magnesium into magnesia; you can get an instant nontoxic aqueous elastomeric gel with calcium alginate; you can get biocompatibility (and guaranteed-working recipes) from zinc and magnesium oxides with buffered aqueous phosphoric acid; and there are thirteen other combinations that will probably work as well.

Further alternative polycations might include nickel, mercury, and vanadium ions, but these have some disadvantages (carcinogenicity, higher toxicity) and not much in the way of available information. Further alternative polyanions might include sulfate (which does have some insoluble salts, notably calcium sulfate (gypsum) and barium sulfate), oxalate, silicate (see below), sulfide (soluble with lithium, sodium, and ammonium, but should precipitate transition metals) and perhaps some carrageenans.

Iron sulfide in particular — fool’s gold — is 6–6.5 on the Mohs scale, harder than apatite. It has the disadvantage of gradually oxidizing in air, though, with corrosive results, and of course the soluble sulfides are toxic.

Liquid tank systems

It might be advantageous to work with a mixture that is liquid until the cement is precipitated, rather than consisting mostly of a packed granular filler. This doesn’t exclude the use of fillers; especially bentonite clay can remain in suspension in water up to fairly high concentrations of clay without solidifying the water. It might be worthwhile to mix a little sodium or potassium alginate in with the phosphate so that the initial introduction of the calcium donor will gel things in place in milliseconds and prevent the liquid from flowing further, even if the calcium phosphate or other cement takes some time to fully crystallize. (This might be useful to limit diffusion even in a powder-bed system.)

(The advantage of Newtonian or at least non-thixotropic liquids is that their surfaces are reliably quite flat and horizontal; they have no angle of repose.)

Other plant gelling agents such as pectins and carrageenans can also be precipitated into a gel by pH control and in some cases by polyvalent cations (though there are many different types of pectin and many different types of carrageenan, and they can sometimes react in opposite ways to pH changes), and aluminum sulfate precipitates insoluble, gelatinous aluminum hydroxide when the water is insufficiently acidic.

Nucleation control

It may be desirable to prevent homogeneous nucleation in order for the cement particles to be big enough to bridge the gaps between grains of filler. For of these most cements, if the temperature is kept high enough, cement particles will only nucleate on the surfaces of grains of filler; this may help to produce a solid mass. (More speculatively, pressure control is another possible lever to control nucleation, but this would probably require a liquid-filled chamber.) It may also be possible to solve this problem by making the precipitation mass-transport-limited.

Filler particles with more extreme aspect ratios — clays such as bentonite being the champion here, though a less expansive clay may be more practical for this use — should lower the critical percolation threshold needed to form a solid mass, thus placing less stringent demands on the nucleation process.

Densification

Once you have the “green” article made out of filler grains cemented together, you can use water to wash off the unhardened mixture of filler (“powder”) and unprecipitated solute, as well as washing out leftover reaction products other than the cement. Densification may be needed after the initial precipitation, since when the cement precipitates from solution, the water and other solute remain. (For example, if reacting aqueous dipotassium phosphate (which dissolves 150 g per 100 mℓ of water) with calcium chloride to produce hydroxyapatite, you have potassium chloride and water taking up space in the result.) Densification can be carried out by passing a supersaturated solution of the same cement, or a compatible cement, over the printed object once it is removed from the powder bed; or it can be carried out by infusing the pores with a different material, perhaps a melt, again after powder removal.

Electrolytic injection of cations

As an alternative source of polyvalent cations, you could use small anodes of suitable metals (zinc, copper, manganese, or iron, although maybe it might be possible with a suitable alloy of calcium or magnesium) with a controllable current; this might allow you to switch on and off the cementing action with much higher precision and frequency than pumping solute liquids in and out of a pipette or inkjet, and would avoid the need for the extra water content to maintain those cementing ions in solution.

This approach should be especially suitable to introducing controlled amounts of impurities into particular places in the printed object — for example, copper or iron ions would probably produce a bright blue color, or manganese ions a rose-red color. You could probably get a wide variety of other colors by using other metals not otherwise mentioned here; cations introduced for the purpose of adding color need not be polyvalent or form physically strong compounds.

More generally, the precise control of mixing provided by the electrolytic mechanism can be used to produce precisely controlled gradients of material properties in the cementing material, for example to produce controllable optical or acoustic refraction.

In theory you could also use a sacrificial cathode that released anions such as phosphate or carbonate when electrolytically reduced, but that seems much more difficult; I know of no such material.

Alternative solvents

Water is a terribly convenient solvent for facilitating such double-metathesis reactions, since it’s capable of dissolving a very wide variety of ions, it’s fairly nontoxic, and it is liquid at room temperature. But it has the major disadvantage that it contains oxygen, so to metals like calcium, water is utter death. Other polar solvents might be feasible alternatives; for example, anhydrous ammonia at low temperature and/or high pressure, or molten-salt mixtures like FLiNaK and FLiBe at somewhat higher temperatures, or the truly outlandish polar organic solvent systems used in current lithium-ion batteries.

Bicarbonate as a hydroxyl donor

Cyanoacrylates polymerize in the presence of hydroxyl ions; dripping cyanoacrylate onto NaHCO₃, stealing hydroxyl ions and converting it to sodium carbonate, is a well-known manual additive manufacturing technique which can probably be improved by adding filler to the NaHCO₃.

Bicarbonate as a CO₂ donor

Waterglass (sodium or potassium silicate) forms a silica gel rapidly upon exposure to CO₂; maybe you can use NaHCO₃ as a CO₂ donor for this purpose. Certainly you can harden it with acids instead, or with ethanol.

There are other materials that harden or recrystallize upon exposure to CO₂, most notably Ca(OH)₂, slaked lime, which produces calcium carbonate. Normally they harden fairly slowly once wet by absorbing CO₂ from the air, but maybe you could get them to harden faster by supplying them with NaHCO₃.

Metastable redox systems such as thermites

Rather than using chemicals that react immediately on contact, as in the above, or initiating some kind of interaction by slowly heating the entire powder bed after careful deposition, as in Flux deposition for 3-D printing in glass and metals and 3-D printing by flux deposition, it might be worthwhile to use chemicals that can react quite energetically, but which remain almost completely inert during the printing process; and, once the printing is complete, ignite them and allow the self-sustaining reaction to run to completion. The trick is to identify reactions that would produce enough heat to produce interesting materials, but without producing enough gas to blow the nascent object to bits.

Thermites, such as the classic aluminum-powder/magnetite system formerly widely used for welding, are one example; you could selectively deposit aluminum powder into a bed of magnetite, and then ignite the thermite once the printing is done (traditionally, using magnesium ribbon). This produces molten iron and molten (!!) aluminum oxide, which I expect would then quickly quench in the much larger body of magnetite, producing a solid object consisting of a magnetite shell around a core consisting of phases of iron and amorphous or cryptocrystalline corundum; plausibly both phases might initially be continuous, as in an open-cell foam, but the corundum would almost certainly fracture severely during cooling. With some luck, the purified iron thus produced will be sufficiently ductile to remain intact.

(The temperature is 2500° when the oxidizer is hematite rather than magnetite, but I think this is limited by aluminum boiling at 2519° rather than by the energy available.)

Magnetite has some disadvantages; it will melt onto the outside of the printed object, its own properties are not all that desirable, and it adds iron (thus, weight) to the piece. Other oxygen donors might solve or at least ameliorate these problems. However, the traditional alternatives are hematite (red iron oxide), silica, diboron trioxide (boria), a mixture of manganese dioxide with manganese monoxide, lead tetroxide, cupric oxide (CuO, the toxic tenorite), and viridian. Of these, I think silica is the one with the highest melting point (1600°), and it has the benefit of being transparent; but the metallic silicon thus formed is even more brittle than corundum. Viridian and cupric oxide offer the fascinating prospect of 3-D printing in purified chromium and copper, but cupric-oxide thermite can be explosive. Additionally, chromite (FeCr₂O₄) might work — I think aluminothermic reduction of chromite is used for commercial chromium smelting.

Sometimes people use teflon instead of an oxygen donor, thus producing a metal fluoride (and carbon) rather than a metal oxide.

Typically when burning aluminum with quartz as the oxidizer, sulfur is included in an aluminum–sulfur–sand composition; WP claims this functions as an extra oxidizer to add energy, as well as to ease ignition. Sulfur is sometimes used with magnetite, aluminum, and barium nitrate to make “thermate,” a higher-temperature thermite with mostly military uses.

Aluminum is not the only possible fuel metal, only one of the cheapest and safest; other possibilities include zirconium, calcium (!), zinc, titanium, silicon, boron, and magnesium.

Common fillers for thermite welding include high-carbon steel, cast iron, or pig iron, which melt and mix with the purified iron to produce a steel with the desired level of carbon.

Alternatively, at somewhat higher cost, you could attempt to make the oxidizer rather than the metal the limiting reagent — for example, depositing a small amount of magnetite powder in a bed of aluminum powder, rather than the reverse; then, the newly formed material will quench in the aluminum, acquiring an aluminum coating rather than a magnetite coating. This is very risky, though, because the aluminum powder burns fiercely in air. You’d need to do it under an inert or reducing gas, or in vacuum.

The reaction between zinc and sulfur, every chemistry teacher’s favorite, is another candidate. The sphalerite or wurtzite thus produced is a reasonably strong mineral (Mohs 3.5–4). Other metals, such as aluminum and I think iron, have similar reactions, but the sulfides thus formed are less stable and tend to hydrolyze.

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