Modern material processing

Kragen Javier Sitaker, 02020-09-24 (updated 02020-09-26) (8 minutes)

A lot of laboratory apparatus for processing exotic materials hasn’t changed much since the time of Bunsen — though electric heating mantles are new, and now stir bars are covered in teflon instead of glass, many other things are still the same. What might it look like if we designed it from scratch today?

Perhaps we would focus more on continuous-flow processing rather than batch processing, handling very small quantities of liquid or other material at a time. Maybe we’d use teflon gaskets more. Our lego blocks might be smaller than traditional flasks; we might have micro-channel blocks that swing open at a parting line to expose the interiors of the channels for cleaning, pressed together by a clip. Captive gaskets between them would separate the cavities along the parting line.

Some blocks could incorporate their own electrical heating elements, catalyst foams, valves, and so on. For thermal insulation, they could incorporate vacuum “panels” containing multilayer insulation, for example of gold leaf or aluminum foil.

Microfluidic devices are capable of executing many experiments at once, and are widely used for this in biology today.

At ordinary temperatures gaskets would be made of teflon, but higher-temperature blocks would use gaskets that are brittle at room temperature, softening as their Tg is exceeded. Alternatively, modern fabrication technology is capable of shaping mating faces to match with submicron precision, which by itself may reduce leakage sufficiently without gaskets. If not, greasing the mating faces with liquid (whether water, table salt, sulfur, H₂SO₄, lithium with its delightfully low vapor pressure, other liquid metals such as lead-tin solder, or something else) may be an option, although they probably couldn't be separated cold.

There is in most cases no need for the surface of vessels to have a significant affinity for liquids; superhydrophobic or omniphobic surface treatments would significantly reduce corrosion, cross-contamination, and cleaning effort. Fluorinated surfaces are another, cheaper alternative. Moreover, in many cases, such a surface would prevent the entry of water or other liquids into small channels without pressure being applied.

The use of such blocks could make high-pressure processing routine, since they can easily be made to contain high-pressure materials.

Glass is fragile; in many cases it could be replaced by fluorinated plastics or fluorinated ceramics. Modern fabrication techniques are capable of mechanically carving channels through many materials, and often of molding them as well.

One possible candidate replacement for glass is fluorite, which cost US$300 per tonne in 02020, can be optically clear even into infrared and ultraviolet, and has good resistance to some corrosive agents. It doesn’t melt until 1418°. Although it is noticeably more fragile than the traditional borosilicate, its major flaw, which is fatal for many uses, is that it cannot deal with strong acids.

Crystalline alumina is better than borosilicate in every way except that it is very difficult to shape. There is no known alumina glass, and polycrystalline sintered alumina ceramics are in wide use, but they are never transparent. General Electric’s 01961 “Lucalox” sodium-vapor lightbulbs (“transLUCent ALuminum OXide”) were translucent sintered alumina; the 1959 Nature report on them said they were “presumably sintered in vacuo”, and that objects viewed from a distance were “blurred as though through frosted glass”. A 01996 article by J.E. Burke, one of its inventors explained that it was doped with a fraction of a percent of MgO before sintering at 1800°, preventing pore retention by preventing discontinuous grain growth that traps pores within grains, and that pore-free alumina is translucent because alumina is birefringent. He also notes that to bring out grain boundaries by etching, he resorted to etching the alumina with molten exotic K₂S₂O₇, and that sintered pore-free alumina is “the only successful material found to date [01996] for containing the plasma of the high-pressure sodium-vapor lamp”.

Yttria-stabilized zirconia is already seeing some use in labware, but usually in a non-transparent form — though transparent YSZ is widely used for jewelry. It is outstandingly resistant to corrosion, fairly hard (though less so than alumina), and above all, tough. When hot (900°–1300°, maybe lower) it conducts electricity in the form of oxygen ions, and thus can be used to electrolytically add or remove oxygen from a hot reagent.

Other transparent ceramics include yttria doped with 10% thoria to prevent discontinuous grain growth, analogously to Lucalox (the former Yttralox, which contains no alumina, despite its name) and lanthana-doped yttria, melting at 2430°. Unfortunately yttria is vulnerable to concentrated hydrochloric acid with ammonium chloride, thoria is radioactive, and lanthana is soluble in dilute acids. Also, Rosenflanz’s alumina-containing rare-earth glass-ceramics from 2004; yttria–alumina garnet (YAG); aluminum oxynitride spinel; magnesium aluminate spinel, aka just “spinel”; topaz; silicon carbide; chrysoberyl; beryl, such as emerald; berlinite; chalcogenide and phosphate glasses; transparent glass-ceramics like Corning VISION; zircon (zirconium silicate); rutile; boron nitride; and so on.

Fluorinated polymers may be a reasonable inert alternative to ceramic materials for many low-temperature purposes, as well. Tubes of materials like PVDF (Kynar) can carry most materials with impunity, though only up to 150° in that particular case, and also not very aggressive materials. Teflon can withstand both higher temperatures and more corrosive materials, and many plastics and even metals can have their surfaces fluorinated in order to make them more inert to their contents, as well as in most cases decreasing wettability, as mentioned earlier.

Blocks can be instrumented with not only stir bars and heaters but pumps, thermometers, pH meters, dielectric spectrometers, regular spectrometers, imaging spectrometers, sonar (to measure specific acoustic impedance, speed of sound (from which density can be inferred), and distance to a liquid surface), immersion densitometers, ion exchange beds, flow meters, chromatography columns, distillation columns, NMR equipment, reflectometers, X-ray fluorescence and diffractometry equipment, and strain gauges for both pressure and weight.

Pumps need not use a shaft through a sliding seal; like stir bars, their impeller can be driven by a magnetic field from without, a trick even easier to do with a flow meter. Also, piezoelectric devices can set up oscillations in the fluid that drive a "fluidic diode" type of pump; in some cases pure fluidic pumping driven by a stream of a friendlier fluid can be used; and in some cases peristaltic pumping is applicable. In the special case of conductive liquids, magnetohydrodynamic pumping can be used.

By moving a supporting rigid support along an unchanging beam and measuring the changing torque at the support, the mass distribution of the beam can be easily calculated, most precisely toward the center of the beam. In general there is a tradeoff between precise measurement of weight and sealed couplings that permit easy fluid passage through the apparatus, but (at least without a pressure difference between inside and outside) it can be eased somewhat with flexible couplings, in particular plane-like couplings, like flexible circuit boards with fluid channels bored through them.

Topics