Explore the Susceptor

This is a part of the first law of thermodynamics, aka the law of conservation of energy. It states that the total energy of an isolated system cannot be changed. Energy cannot be created nor destroyed only converted from one form to another.  A susceptor merely changes one form of energy into another. A susceptor is a material that can absorb electromagnetic energy (i.e. radio waves) and convert it to heat energy. The most recognizable example of the susceptor in your home is the one presently wrapped around a burrito in your microwave.

It's intuitive when you think about the physics. The heating element in the toast oven operates on a similar principle. When electricity is applied to the element, the electrical resistance of the metal converts some of the current into heat energy. But converting RF is another more nuanced animal. In that same context a susceptor is a lossy material with a resistivity of around 200 Ω/sq. The one which ensconces your Hot Pocket consists of a paper substrate coated in aluminum or nickel and coverd in a layer of polyethylene terephthalate or PET, a polyester film. Similar warmers units might use molybdenum, niobium, or even stainless steel.

Believe it or not, but susceptors do have a use beyond crisping the underside of cheap pizza at your local 7-11. They can be used to apply heat through induction to non-conductive materials. There are situations in manufacturing where a metal surface would interact with the material being produced and a non-conductive container, crucible etc. is preferable. For example, in metalorganic vapour phase epitaxy (MOCVD) a chemical vapour deposition method is used to deposit a polycrystalline film in the manufacture of semi-conductors. In this case a graphite susceptor is often used but carborundum is sometimes also used.

Sometimes radio waves are good for unexpected things...

LED

Courtesy of the fine geeks at Make.com here is a nice little video on making LEDs from scratch; impractical but fun. They refer to a few engineers I've written about in the past including Oleg Losev, Nick Holonyak and Henry Joseph Round. It's worth spending a few minutes to watch, and to remember that electronics are just for geeks. You too can enjoy wholesome educational fire hazards in the home.



MAKE presents: The LED from MAKE magazine on Vimeo.

A Note on Carborundum

Certain naturally-occurring minerals can be used to detect radio signals, including galena, zincite, silicon, bornite and others. Carborundum was unique among the early crystals because it was synthetic. It was durable, and at 9 mohs much harder than most available crystals. More here. Also interestingly it requires it requires a negative potential of 1 volt to be used as a diode. Carborundum was not created with this purpose in mind. It was created in the early search for artificial diamonds. How it got into radio is truly arcane.

If you look at the Arlington Cemetery Website, they refer to General Henry Harrison Chase Dunwoody as "The inventor of Heart Of Radio."  He was once the Chief Signal officer of the United States Navy.  The website doesn't explain what the "heart" is it at all.  So I turned to his patents. In 1906 he patented a detector using silicon carbide aka Carborundum.  I can see calling that the heart of radio. It was none other than Lee DeForest who first marketed the invention commercially. DeForest installed it at 120 of his telegraph stations. At the time there were only 145 telegraph stations in the whole of the United States, and 130 of them switched to Carborundum by the end of 1907.  This is because of two things, it really was more durable, and it was a drop in component that could be used with existing hardware. The voltage it required could be applied by a battery. All this thanks to Dunwoody. But Dunwoody didn't invent Carborundum, he just figured out how to use it as a detector. So how did that end up in his hands?

The invention of the light-emitting diode (LED) is usually credited to Nick Holonyak in 1962. This is wrong. The real credit should go to Captain Henry Joseph Round at Marconi Labs.  Henry was a personal assistant to Marconi, but he racked up 117 patents on his own. In 1907 he applied voltage to a crystal of silicon carbide (Carborundum) and a cat's-whisker detector that it glowed. We call it electroluminescence. That diode he was using was General Henry Harrison Chase Dunwoody's detector (or something else nearly identical). Henry was so excited he wrote a letter to the editors of Electrical world. the published it February 9th 1907. I quote most of it below:

"During an investigation of the unsymmetrical passage of current through a contact of carborundum and other substances a curious phenomenon was noted. On applying a potential of 10 volts between two points on a crystal of carborundum, the crystal gave out a yellowish light. Only one of two specimens could be found which gave a bright glow on such a low voltage, but with 110 volts a large number could be found to glow. In some crystals only edges gave the light and others gave instead of a yellow light green, orange or blue. In all cases tested the glow appears to come from the negative pole, a bright blue-green spark appearing at the positive pole. In a single crystal, if contact is made near the center with the negative pole, and the positive pole is put in contact at any other place, only one section of the crystal will glow and that same section wherever the positive pole is placed.  There seems to be some connection between the above effect and the e.m.f. produced by a junction of carborundum and another conductor when heated by a direct or alternating current..."

This was huge. More interesting to me is that since it's a diode it naturally goes that it could be used as a crystal detector. Dunwoody later made that leap.  Back in 1907 an issue of Mineral Industry reveals that the Carborundum Company of Niagara Falls NY as the sole producer of carborundum in America. It then goes on to list the quantity of carborundum produced each year in the United States all the way back to the year 1891, all by the Carborundum Company. Let me list that off.

1891 - 50 lbs

1892 - 2,145 lbs (1 ton)

1893 - 15,200 lbs (7 tons)

1894 - 52,190 lbs (24 tons)

1895 - 225,930 lbs (102 tons)

1896 - 1,190,600 lbs (540 tons)

1897 - 1,242,929 lbs (564 tons)

1898 - 1,594,152 lbs (724 tons)

1899 - 1,741,245 lbs (791 tons)

1900 - 2,401,000 lbs (1,089 tons)

1901 - 3,838,175 lbs (1,742 tons)

1902 - 3,741,599 lbs (1,698 tons)

1903 - 4,760,000 lbs (2,160 tons)

1904 - 7,060,380 lbs (3,203 tons)

1905 - 5,596,280 lbs (2,539 tons)

1906 - 6,225,280 lbs (2,824 tons)

1907 - 7,532,670 lbs (3,418 tons)

What's interesting is that the invention of the carborundum detector by Dunwoody in 1906 barely produces a blip in the volume of carborundum being produced. What were we doing with all this carborundum? They were used to make grinding wheels and sharpening stones. Sharpening things was way more common than radio.

Carborundum was created accidentally by Edward Acheson during what is usually described as "attempts to create artificial diamonds."  It's not exactly accurate. Mr. Acheson was born march 9th 1856 in Washington, PA.  His dad was a grocer and later became the manager of a furnace in Monticello, PA. In 1873, his father died and the furnace shut down.  At the age of 17 he had to work to support the family.  He worked for the oil companies as it was booming in western PA at the time. In 1880 he managed to get a job working for Thomas Edison at Menlo park. He later worked for Westinghouse, and the Standard Underground Cable Company.

A stone cutter working for the Tiffany Company named George Kuntz by chance told Acheson about his need for a better abrasive to cut, shape and work stones. Acheson  already knew that some carbon minerals were very hard, specifically diamond. In his electric work Acheson had already noticed that on some carbons were naturally-occurring tiny dark crystals. Acheson began to save these and sell them as an abrasive powder. But he didn't know how to produce it deliberately. He did know that an electrical arc between two carbon elements produced a lot of heat. So he mixed coke, sand, sawdust and salt and applied tremendous voltage. When the mixture vaporized it cooled and formed carborundum.  It was 1891.

He also noticed that in the uneven heating the hottest parts of the mixture had become graphite.  He then devised that applying additional current to heat the carbide produced graphite. He went on to also make a graphite lubricant with a similar method.  He patented the furnace, those methods and the products. US patent numbers 492,767 and 615,648 etc. In 1894 he incorporated the Carborundum Company, and relocated it to Niagara Falls in 1895 because he needed more power to make more Carborundum. Acheson died in New York City,1931.

Perikon detector

Strange as it may seem, the crystal in crystal radios wore out. The rectifier was a contact metal–semiconductor point-contact junction, aka a Schottky diode. I mentioned this once before so I wont get too much into the diode. But suffice it to say that not every part of the crystal would rectify the circuit. Rectification in this case means "act like a diode." More here.
Because the whisker was made of wire or a needle, and the crystal was softer, the surface was worn by use. OK hypothetically not every "crystal" was softer, but they were almost inherently of different harnesses. The very soft Graphite was used as a whisker sometimes. and at the other end, carborundum was considered durable. So either the whisker or the crystal was experiencing wear and usually it was the crystal. So hobbyist chose a crystal and whisker based both on it's sensitivity as a detector but also it's longevity. The hunt was on from the start for a "permanent detector."

Lets look at the hardness of some known whiskers and crystals:
WHISKERS
Graphite: 1-2 Mohs
Bornite: 3 Mohs
Iron: 4 Mohs
Steel: 7-8 Mohs
Bronze: 3 Mohs

CRYSTALS
Galena: 2.5 Mohs
Zincyte: 4 - 4.5 Mohs
Molybdenite: 1 - 1.5 Mohs
Iron Pyrite: 6-7 Mohs
Carborundum: 9 Mohs
Silicium: 7-8 Mohs
Chalcopyrite: 2.5 Mohs

As narrow as that variation is, remember that the Mohs scale is only a 10 point same topping out with diamond. not every whisker and crystal worked well together and even when they did, their mismatched harnesses could be problematic. Some combinations were proprietary and many of those were discovered by Mr. Greenleaf Whittier Pickard. He found both the Zincite /Bornite rectifier and the Zincite / rectifier. He called it the Perikon Detector.

He also tested versions with iron pyrite and silicon crystals. Silicon was novel enough he patented it's use in 1906. His Zinc Oxide detector was patented in 1909. He developed one with Molybdenite the same year. More here.