Liquid metal switches

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Pure metals that have melting points near room temperature are mercury (Hg), caesium (Cs), rubidium (Rb), francium (Fr) and gallium (Ga).  Cs, Rb and Fr are all in column 1 in the modern periodic table.  Mercury is in column 12, and  gallium is in column 13. When you are considering any form of product design, you are told to look for the simplest idea that works.  We'd like to add to that: look for the simplest, sustainable non-toxic idea that works.

The original liquid-metal switch used mercury, it was used in thermostats and "silent" light switches.  Mercury has become a major polutant,everytime you eat fish you get a dose these days.  Howard Stern, well-known low-brow radio personality contracted mercury poisoning from salmon lunches and had to change his diet.  Here's something to think about when you buy your kids shoes with LED lights in them.  What's in the batteries, and where is it going when you throw them out? Does Gramps know he should be recycling those hearing aid batteries? Speak up, I can't hear you, whippersnapper!

Mercury used to be a play toy for kids, and it was used in hat manufacturing.  Recall the "Mad Hatter" in Alice in Wonderland, he became "mad" from mercury exposure. More recently, a Dartmouth researcher died from exposure to the organic compound dimethly mercury.  There is no shortage of junk science videos where fools play with mercury. We'd prefer that these people play with Tide Pods instead, to reduce the possibility of environmental damage.


The family that hazmats together...

Another cause of mercury pollution is gold assaying.  "Forty-niners" brought vats of mercury to California, and much of it spilled up in the mountains. It will work its way up the food chain for the next 10,000 years. "Up the food chain" means "into your lunch box".

Mercury is still used in commercial switches. But mercury is not really used in RF applications, so why are we even talking about it?

Galinstan switches

RF reseachers have created designs using galinstan, an alloy made from a mixture of gallium, indium and tin.  Galinstan typically transitions from solid to liquid at 11C (52F). It is said to be "eutectic" which means that the transition is at the same temperature in both directions. Galinstan is also said to be nontoxic, but we recommend that you don;t add it to a cocktail.

MEMS have been used to move galinstan to form RF switches

Galinstan has been used to change the operation of resonators

The advantage of a galinstan switch is low loss.  It can potentially handle high power (hundreds of watts)

One disadvantage is that temperature has to be carefully controlled.  Switching speed is slow. And what happen under vibration, shock and G-loading?