You may know that miniaturization of electronic circuits is what makes modern, hand held devices possible. In your lifetime you have seen computers and phones get smaller and more powerful by several fold. Eventually, the limit for circuit dimension set by technology will converge to the one set by physical law. One annoying obstacle that continues to present a problem for circuit designers is the capacitor. Capacitors are an old idea, as old as Faraday and Leyden, and Coulomb. If you make a three layer sandwich of conductor/insulator/conductor then send current into one of the conductors, it will “fill” with charge until it can “hold” no more. On the other side of the insulator the opposite charge (+ or -) will be induced in the conductor.

They can’t neutralize one another across the insulator but if you provide a path for them to meet through a wire via  a switch for example, current will flow in the circuit. The device provides the circuit with the capacity to produce current by storing charge. The bucket is filled, then emptied. When the circuit is shorted as described, the charge can dump out at very high speed. This is how flash lamps, strobe lights, and Tasers work. It is also a favorite prank among electronics students to toss a charged capacitor to a hapless noob on whom it will discharge when caught. One of my former students told me about tossing a fully charged 1000 Farad (Capacitance is measured in Farads after Faraday. Most reasonable caps have values of micro and pico Farads!!) off of a roof, the roof of his place of employment. I don’t know what his job was but it sounds like he was overqualified and/or underutilized. This thing must have been the size and shape of a can of driveway sealer. Anyway, a lightning bolt, temporary blindness and deafness, and a hole in the parking lot were the result. Keep in mind, it takes 33,000 volts to arc through air. Oh for the days when I could do demos like that! 

That was the direct current example. What if the current is alternating like in your home electrical system? (At way less voltage!) instead of filling the bucket completely you just fill it partially at a slow rate  then empty it partially at a slow rate. this makes the induced charge on the other side go up and down too, but it won’t go up and down in sync with the one driving it. It will lag behind a little because it takes time to respond. If you constrict the flow of current out of or into the capacitor with a resistor you will slow the rate at which it can fill and empty. This enables the capacitor to act like a damper. You can set it up so that only slow moving signals can make it through undamped or that only fast moving signals can make it through. Thus the two components make a device that can filter out different frequencies or select only a narrow range of them, the RC filter. Pretty useful for communication or audio reproduction . They’re also great for constant current sources which you need for reliability in low voltage devices like phones.

RC circuit 

When an engineer designs a circuit requiring a capacitor it is assumed that a standard size capacitor will be used, that is, one with a fixed capacitance. If you want your filter to work in  a specific range you need capacitors of specific values. Maybe you need a big one and you can’t design around it. To get more capacity out of a capacitor you have two options, make the area of the conductors and insulator bigger, or make the insulator thinner. No insulator is perfect so all will break down and pass some charge at a certain limiting thickness depending on how hard you push on it (voltage). So at some point you can only get bigger capacitance with a bigger capacitor. There goes your miniaturization scheme. capacitors are almost always the biggest things on a “mixed component board” that is, one with microscale chips and macroscale resistors, capacitors, transistors etc. Every circuit board made is still “mixed” like this. There’s no way around it. If humans are going to use it it can’t be so small that said human can’t poke at it with its stubby fingers and expect to hit what it is poking at.

 upright black cylinders are capacitors

Enter the search for new materials. If only there were a better conductor/insulator pair that could give high capacitance in a small package…

Chemistry in the 1800’s was still an art. It involved practical skills and patience unknown in the contemporary field. If you were trying to identify a new element from a rock, or pile of sand, or lump of clay you had a limited set of tools to do it.  Not to mention that if you wanted a piece of glassware you made it your damn self and if you needed  a vacuum you pumped the air out of a chamber by turning the crank on a pump by hand, all day. If you still had two eyes and ten fingers by the time you became  a full professor you either had tremendous luck or really dedicated assistants. 

Most elements are metals. metals combine with things in a predictable way. If you make your mixture react with: oxygen, fluorine, chlorine, etc., you can try to distinguish the subtle differences between what you recognize and what you don’t. Almost everything will form an oxide and not all oxides are created equal. Some will be more susceptible to acids, some will dissolve in organics. All will have  a unique density. So, make the oxides (they’re probably already oxides) separate them by solubility or flotation. Try to take the oxygen off, not always easy or possible, and see what you get. Two different metals? An uncooperative alloy that you can’t get apart? Something already known for the last hundred years? Any and all are possible. Thus it was with the mineral columbium. Charles Hatchett “discovered ” this “element” in 1801. A year later, Anders Ekeberg discovers an element he calls tantalum. It can’t be “slaked” (hydrated)  by acidic solution so like Tantalus, it’s thirst cannot be slaked. (Scientists knew the classics back then.) Eight years later William Hyde Wollaston compares the oxides of the two elements and decides that one contains some of the other. Thirty seven years after that, (progress was hard won in those days.) Heinrich Rose claims that there are actually two additional elements in the tantalum ore, He names them niobium and pelopium after the children of Tantalus, Niobe and Pelops. Another twenty years, another pantheon of Europeans with hyphenated and compound names, like Henri Etienne Sainte-Claire Deville and Jean Charles Galissard de Marignac to name a couple, determine the empirical formulas of some compounds of tantalum and niobium confirming that there are only two elements. Columbium, it turns out, was pure niobium, and pelopium is a mixture of tantalum and niobium. Finally, in 1864, de Marignac reduces tantalum to the metallic form in a hydrogen furnace. I’m sure that was totally safe to be around.

Wikipedia article on tantalum

So here’s what makes this special. Metals that will form oxides easily in thin layers are good for making capacitors. The oxide becomes the insulating dielectric layer. Aluminum is good at this but the oxide that forms naturally in atmospheric oxygen is too thick for real miniaturization. Thin dielectric means more capacitance. Tantalum on the other hand will make a really thin layer just by exposure to a salt solution. The solution itself becomes the other conducting layer of the sandwich. Voila! The wet tantalum capacitor, microfarad capacitance in a package millimeters on edge. (This is also called an electrolytic capacitor or a bipolar capacitor). Dry capacitors are also manufactured from tantalum. 

 aluminum electrolytic capacitor

 tantalum dry capacitor

 various tantalum capacitors

This statement from Cabot Corporation sums up the utility of this metal:

More than 60 percent of the world's tantalum, a high performance metal, is used in electronics products. The largest application is electronic capacitors, where tantalum's ability to form stable oxide films creates highly efficient, highly reliable and environmentally versatile components.

In semiconductors, tantalum has emerged as an ideal barrier solution, since copper is replacing aluminum as the material of choice for interconnects.

Strength, ductility, toughness, corrosion resistance, thermal conductivity and high melting point make tantalum important in chemical and pharmaceutical processing, aerospace, energy and ballistic applications.

Tantalum is also used as alloy in turbine blades, in power turbines and jet engines, where it brings structural integrity at higher temperatures, increasing fuel efficiency.

Some four million pounds of tantalum are consumed annually in the form of metal powder and wire, ingot, fabricated forms, compounds and alloys. Cabot Supermetals is one of just a few fully integrated producers of tantalum products in the world, with two ISO certified manufacturing facilities in the United States and Japan. Cabot statement

Cabot, among other manufacturing companies buys its tantalum from sources in various parts of the world. It turns out that some of these source countries in Africa use the profits from the sale of ore to fund genocidal wars. It is not always possible or a priority for companies to be selective on the basis of political or moral issues. More on that angle in the next part of the story...

This post turned out to be much too big to put in one installment. I never read anything that long myself, too short an attention span.