Beru Glow Plug CGP-002

This would make a great hot end for printing metals plastics and ceramics. If it only had a small center bored hole in the middle…Possibly the theoretical best possible hot end. 
In fact,I decided to actually make it into a hotend here:
 

I have included the available manufacturer technical information regarding these glow plugs, their material design and internal structure...
I have not modeled the interior because I have not had occasion to section and examine them closely as such. I do however provide temperature-current-voltage and TCR curves I have managed to collect up ~1200 degrees C, here.

These are, to my knowledge, the highest performance ceramic glow plugs on the open market to date. 
As far as I know, they are capable of a maximum operating temperature of ~1800 degrees C for as long as a few hours in dry air at atmospheric pressure. 
At ~1200-1300 degrees C, their lifetime in dry air at atmospheric pressure can be > 100,000 hours.

if you raise their temperature above about 1500 degrees C, depending on the humidity in the environment, they will begin to bubble and fiz with a visible and audible foam at the ceramic tip this foam will drip off the tip. This is presumably silicon-dioxide converting to silicon oxide, and silicon hydroxide and thus gasifying and melting in the presence of water and oxygen. This is probably bad for the heater, but the lifetime, even under these conditions appears to be at least a few tens of hours. 
The ceramic is made of a mixed matrix of MoSi2-Si3N4, (molybdenum disillicide in silicon nitride) this mixture seems to be varied in specific molar ratio to allow for varying conductivity of the matrix which gives the plugs their nearly metallic electronic character and forms the heater itself. 

These devices are extremely durable. I was really impressed. They do not handle impact well.
They are capable of directly melting small quantities of platinum with just a benchtop DC power supply capable of 10 AMP output at 12 volts.

Interestingly the tip will resist wetting by molten metals such as aluminum, silver, gold, platinum, zinc, copper, brass, bronze, and mild steel. perhaps would serve well as a low cost small scale immersion heater for molten metals or an at home high temperature furnace build?

Their positive temperature coefficient is self stabilizing, and is sufficiently sensitive that it can serve as a thermistor input, if a sense resistor is used on the supply side of the heater. 
this can be good for closed loop temperature control. 

The body of the plug appears to be made of a high temperature resistant stainless steel, or more likely some inconel in the 600 family I would presume. I do not know this for sure.

These glow plugs are the subject of several interesting patents.
WO2010060616A3
https://patentimages.storage.googleapis.com/47/03/35/2e8a9987957099/WO2010060616A3.pdf

Patent No.: US 7,160,584 B2
https://patentimages.storage.googleapis.com/02/f1/b9/0fa1cb87058b66/US7160584.pdf

US 8,513,570 B2
https://patentimages.storage.googleapis.com/d7/11/e9/d6e08156c5840d/US8513570.pdf

 these have been largely superseded in the US by :
GM Genuine Parts 68G