In my story, I've been drafting a character with abilities based on fire and heat. I had this idea that he could have gloves that can increase in heat on the outside while the inside temperature stays mostly the same. My question is: would these be possible to make (out of any material)? If it helps, this story takes place in a futuristic setting.
It would be trivial to make a portable flamethrower with the nozzle attached to a glove's palm. In fact, some people with enough spare parts and time have already done it.
While there are many Darwin Awards (and honorable mentions) that relate to death by fire, a quick search indicates that not one has been awarded so far having to do with homemade flamethrowers. If you are quick enough to release your novel, it may be cited when someone tries to lit a cigarette with such a glove.
Seriously now, having heated hands is a bad idea™. If your hands are just as hot as a good cup of tea, the most you can do with those is becoming a very unique reiki practicioner, which you can do without the gloves anyway.
But if you hands are hot enough to do anything truly superhuman - from softening PVC to melting solder - you are just a distraction away from the ER. If your eye itches, if you pick your nose, if you use your fingers to whistle, if you scratch your butt... It will not be pleasant.
And if you need to grab someone to prevent them from falling from a ledger... You won't be making many friends, nor getting many teamwork awards.
You will always be better served by having a good soldering iron and a heat gun.
How hot are we talking, and how long does it need to work for?
There are two ways you can deal with this:
Materials with a vacuum in between two layers of material are basically as good as you can get with insulation. With this you will have a heat conductivity of about 0.006-0.008 W/(m·K). Insulation will only slow down how fast the inside heats up.
You can calculate this with the following equation W = kA/L (W is the Joules of heat every second (work), k is the heat conductivity, A is the surface area, and L is the thickness).
So if the glove was 1cm thick, and the surface area of our hand is about .05m^2 according to google, and we plug in a theoretical ideal 0.004 for our conductivity we get.
W = 0.004*0.05/0.01 = 0.02 J/s
This is the energy being transfered and getting through every second per degree Kelvin (Celsius) the gloves are heated up.
For a male each hand make up about 0.65% body weight (0.5% for females). That means for a 200lb man, 1 hand would have a mass 0.58kg. I was going to treat humans as water, but a google for "heat capacity of human flesh" actually gave me a result (3500 J/(kg*K)), which is actually pretty close to water.
So to heat up your hand 1 degree will take about 2030 J of energy.
So if the gloves where only 1 degree hotter on the outside it would take 28 hours to get your hand to the same temperature (though I imagine your body can dump the heat in sweat. I am not sure what the limits are for that.)
So how hot can these gloves be before you get burned. Temps of between 40-80 degree Celsius will cause burns over varying amounts of time (40 over long time, 80 over a second). Let's say the user wants to be able to use the gloves for a minute before it reaches 50 degrees and they start burning their hands. That is 30 degrees over room temperature.
So that means 2030*30 = 60,900 J of energy need to get through the glove to burn our user. Over the span of a minute, that means a full 1000 watts of energy can get through.
Looking at our easier equation, that means the outside of the gloves can be a full 50,000 degrees Kelvin. This is really hot. Like crazy hot. You can melt steel and stone. It is hot enough that I thought I made a mistake. Then I looked up the kevlar gloves that can withstand lava. The big problem is not the heat getting through, its melting the gloves themselves.
Looking up high temp polymers Looks like we can get up to 600 degrees Celsius currently without melting with some newer ones. We are constantly making better ones and you could probably say "something something carbon" to say the gloves have a melting point of around 1500 degrees Celcius. At this point you can melt steel just fine.
At those temps you could use the gloves for about 30 min before the internal heat got bad.
The other method you can keep them going is with active cooling. Water tubing could carry heat up your arms to big heat sinks on your back. To balance the heat getting through the gloves with the insulation above, the gloves would need to pump about a hundredth of a liter of water a minute.
More traditional insulations might require about a liter of water a minute pumped through to maintain thermal balance.
The bigger problem is where you are getting the energy to heat the gloves.
No need for science fiction, we have substances that can do that now. There are a variety of thermal insulators that would be capable of being made into gloves. Aerogel would be one example. There are a variety of ceramic heat shields that would work as well. Just google thermal insulators.