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While playing old games, I encountered a friend I knew from SciFi series and games alike, the Merculite missile. While thinking about what Merculite might actually be (and linking it to a mineral from the planet Mercury in my head), I had a basic idea for a short story about a freighter transporting a mineral that is not dangerous while hot (but still non-liquid), but get dangerously explosive if its temperature drops below a certain point.

This might not work out at all and does not necessarily require hard science, but I was wondering if there are any known components or minerals that exhibit such behaviour.

My basic school and university chemistry tells me that it is counter-intuitive as any exothermic reaction starting once the temperature drops below a certain point would heat up the mass again, but maybe there are some weird chemical or physical things that may allow for this, like

  1. It really simply explodes once the temperature drops too low or
  2. it is only stable above a certain temperature and thus seperates itself into two or more components that form an explosive liquid that explodes once it reaches a critical mass or
  3. I have absolutely no idea.

Are there any weird substances with such or similar attributes, are such substances even possible with our current understanding of physics and chemistry or would I need to use a pseudoscience mineral?

UPDATE Wow! I'm really impressed and thankful. Please give me some time to pick the answer - I'm not sure if I should pick the one that I like best, but that goes a little bit in a different direction than my original question, or pick the one that answers my original question best.

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    $\begingroup$ Half serious: a soda bottle left in the freezer. $\endgroup$ – Brian McCutchon May 2 '17 at 22:36
  • $\begingroup$ @BrianMcCutchon as in the Answer posted 2 hours ago, with picture? $\endgroup$ – JDługosz May 2 '17 at 22:47
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    $\begingroup$ Certainly, you could believably handwave a mineral that explodes if cooled too fast. My mum did this with a glass dish in which she had made a trifle, so it held boiling water (in the jelly). She put it in the fridge to set it quickly - later there was a loud band and the fridge shelf was full of broken glass and not-quite-set-jelly. (Which she wouldn't let us eat!) $\endgroup$ – Grimm The Opiner May 3 '17 at 9:14
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    $\begingroup$ At least you could rock out with the loud band! $\endgroup$ – Willk May 3 '17 at 13:07
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    $\begingroup$ Something else to consider is that maybe heating it normally makes it explode, but heating it in some special way (in an oxygenless environment, like wood -> charcoal, or slowly) above a threshold makes it inert. Then, you can use normal hot storage techniques to keep the whole thing above a certain temperature, but if it slips below, it would explode in the same scenarios. (Note: I have no idea if there's anything like this, but it seems, intuitively, more possible than something that explodes as it cools) $\endgroup$ – Nic Hartley May 3 '17 at 17:40

18 Answers 18

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Use Iron's allotropic forms

Iron has a wide variety of allotropes. Two interesting ones are austenite, with atoms aligned face-centered cubic (FCC) and hexaferrum, where the atoms align in a hexagonal close packed (HCP) form. Here is the phase chart for iron:enter image description here

Both Austenite and Hexaferrum are denser than the Ferrite that they will phase transition into at lower temperatures. Thus, as the material cools (at constant pressure) and the phase change occurs, the material will expand (much like water does when it freezes). In the case of hexaferrum, this transition will happen quickly.

An additional interesting property of these allotropes is that they have differing solubility for various materials. Austentite is can dissolve much more carbon than ferrite, so it is used to make high carbon or stainless steels where you want some other material to dissolve in an iron solution.

The last piece of the puzzle is that hexaferrum is not well known. It requires a pressure of ~10 GPa to maintain. One interesting property of it is that it is martensitic, giving it excellent magnetic properties.

Ok here is a technology idea that I've been saving up for a while; I'm going to share it just with you. What if you wanted a material that could serve as the 'core' of a fusion generator that had the following properties:

  • Acts as a neutron shield
  • Does not readily undergo nuclear fusion
  • Can be magnetically compressed
  • Hydrogen readily dissolves in it

What I just described is hexaferrum. You could (using a little pseudo-science perhaps) use it as a 'core' of a fusion reaction, with the hydrogen fuel dissolved in the hexaferrum core. The core shields the outside from most of the radiation and compression from powerful magnetic fields keeps the core at high enough pressure that fusion can occur and it stays solid.

Your ship has a hexaferrum core that is cooling down and about to explode.

Fusion stopped or was stopped in the core, due to sabotage, accident, or routine shutdown. However, the hexaferrum is cooling down too fast. When the hexaferrum core reaches the critical temperature, it will suddenly increase in volume by 20%, causing it to fracture and spit out all the irradiated elements in it. Also, all the hydrogen dissolved in it will come out of solution, potentially destroying the ship.

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    $\begingroup$ The stuff coming out of solution would itself be enough to cause the explosion. I think that is actually why beer and soda explode so violently: ice does not hold the CO2. $\endgroup$ – Willk May 3 '17 at 16:38
  • $\begingroup$ Austenite is FCC, Ferrite is BCC. Not this is really relevant, but since you specifically included diagrams and text talking about this... Well, actually ferrite being BCC is relevant, but the error is when talking about austenite in the beginning. $\endgroup$ – Ville Niemi May 7 '17 at 7:47
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Water enter image description here

ok, beer. Watery beer.

An explosion occurs when matter changes so as to occupy more volume. We are used to the explosions where a solid or liquid suddenly changes into a gas, which takes up loads more volume.

When water freezes, the solid form takes up more volume than the liquid. If confined in a space which cannot deform, the expanding ice can explode the container. Expanding ice can break stone.

No-one is going to make grenades out of freezing water. But this might work in your story. A space going water tanker is super plausible. What if environmental control fails and it starts to freeze?

I am envisioning the ruptured freighter in a ball of ice like these beers, trails of sublimating vapor drifting off into space.

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    $\begingroup$ I'd combine this with something else, transport something dangerous stored in water or that uses water and if it freezes it causes an explosion. $\endgroup$ – adaliabooks May 2 '17 at 21:36
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    $\begingroup$ This happened to me with a soda bottle. The bottle went off like a torpedo, shot the fridge door open and proceeded to distribute its contents all over the kitchen. Now you just need a reason to transport soda bottles on a space ship. $\endgroup$ – Sumyrda May 3 '17 at 6:29
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    $\begingroup$ Of course, water is optimally transported in dehydrated form. $\endgroup$ – Jeremy May 3 '17 at 12:05
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    $\begingroup$ @MadPhysicist: But when water expands during freezing, it doesn't "scale up" geometrically. $\endgroup$ – ruakh May 3 '17 at 15:20
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    $\begingroup$ Now I'm thinking about really making this a beer transporter, as putting not only the crew at the risk of dying but also putting tons of beer at risk of being lost to space forever adds so much more tension to the story. $\endgroup$ – Blutkoete May 3 '17 at 18:10
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Polymorphic explosives.

Materials with exact the same chemical composition can exist in several phases, allotropes or polymorphic forms which are only different for their crystal structure.

One well-known example is diamond and graphite: Image by Rob Lavinsky, Wikicommons, CC-SA 3.0 license

Diamond ist one of the hardest substances, highly transparent, a semiconductor and a good heat conductor.
Graphite in contrast is soft, black and completely opaque. Both are essentially carbon.

This polymorphism is also existent in explosives. TNT is well known that it has two polymorphs: a yellow, stable variant and an orthorombic orange variant, the orange variant changes into yellow if heated. The paper counts other substances, picryl bromide has e.g. five known polymorphs. It is also known that some polymorphs are even disappearing and appearing: Turanose was once liquid at room temperature, the current form is solid.Paroxetine is a substance which caused a patent litigation because a new, unpatented form (hemihydrate) occured and even more vexing for the firm, seed crystals forced the patented form (anhydrate) into the unpatented form.

While I am not aware of an example, it is possible that heating a substance will cause a reaction, the reaction creates a new substance which is in this phase harmless but will change into a highly explosive phase if cooled.

While not an explosive, phosphorus is a good example. The white variant is highly toxic and extremely flammable and self-igniting, heating it (!) transforms it into the much more stable and non-toxic red form.

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    $\begingroup$ Sounds like this is enough to make a totally plausible hand-wavium that does exactly what OP wants :) $\endgroup$ – Mad Physicist May 2 '17 at 22:07
  • $\begingroup$ Note the use of cryogenic treatment of steel knives. Various Steel alloys is one such material! It undergoes a phase change when cooled to a specific point. $\endgroup$ – JDługosz May 2 '17 at 22:30
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    $\begingroup$ Phosphorus is nasty nasty stuff. Most explosives like gun-powder are pretty stable until you add phosphorus. Bad, bad news. $\endgroup$ – Green May 3 '17 at 2:48
  • $\begingroup$ Could you attribute that image? $\endgroup$ – Tim May 6 '17 at 14:26
  • $\begingroup$ @Tim Done, I could not find proper attribution for the original image, even after Google Search. $\endgroup$ – Thorsten S. May 6 '17 at 17:41
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You could leverage some macroscopic properties, such as stresses which form as an object cools. Prince Rupert's Drops are a fascinating example. During cooling, they form a head which is virtually indestructable and a tail which, if tapped slightly, causes the entire drop to explode as it releases the mechanical forces pent up inside it.

A Prince Rupert's Drop suspended in a vial of nitroglycerin could be an excellent real life analog for the fictitious mineral you want. If the tail is tapped, the whole drop explodes with great force, and that would likely provide enough energy to set off the nitroglycerin. Obviously you wouldn't want exactly that structure, but you might be able to construct something which fractures as it cools embedded in a matrix which, when initiated by the fracture, undergoes a true explosion. Something involving a temperature sensitive protein might work, if biology is fair game for your explosive.

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    $\begingroup$ I spent hours on YouTube because of your answer, thank you so much for bringing these drops to my attention. $\endgroup$ – Blutkoete May 3 '17 at 18:11
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    $\begingroup$ "and a tail which, if tapped slightly, causes the entire drop to explode as it releases the mechanical forces pent up inside it." According to some YouTube videos, it takes a bit of force to break the tail. $\endgroup$ – k-l May 4 '17 at 16:57
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This sounds a bit like methylcellulose, a substance that can be used to create a gel with an unusual property: it sets up when heated and melts when cooled. It can be used to make such things as marshmallows that remain solid when hot, but melt when they cool down. Kind of the opposite of how normal marshmallows work.

freshly baked starting to melt (source)

A methylcellulose gel won't explode on its own, but it could be part of a trigger mechanism of a larger explosive device. Perhaps there's some high explosive dissolved in the gel that could be detonated when the gel melts. Or perhaps the gel is the explosive- it's suspended in a box made of an alkali metal, such as cesium. When the gel melts, it becomes able to contact the cesium, which then reacts exothermically with the water in the no-longer-gel, producing large amounts of heat and hydrogen gas in the process. Boom.

You'd probably also want to have some more conventional explosives in there (such as TNT), as the water-cesium reaction is limited by how quickly the two materials can mix. And if you're looking for a material that is inherently explosive when cooled, some of the other answers here are closer to that. But methylcellulose could certainly be used to set off an automated self-destruct mechanism of some sort- when the power goes out, the heater built into the mechanism stops working, and the block of methylcellulose cools and melts, dripping onto a piece of cesium and setting off a stack of TNT.

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    $\begingroup$ You didn't mention that it's the main ingredient in K-Y Jelly. Wonder what kind of ship that was… $\endgroup$ – JDługosz May 3 '17 at 4:43
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Hydrazine Perchlorate mixed with Hydrazine

There are several real world chemicals that explode when cooled enough. For example, Triazadienyl Fluoride explodes as soon as it reaches -196 C. (1) However, it also explodes when it is warmed, when it is bumped, or when it is looked at funny.

I think the most plausible example for your rocket to be carrying is Hydrazine Perchlorate mixed with Hydrazine. This solution was once investigated as a high performance rocket fuel, so it fits your setting. As the Hydrazine freezes out, the concentration of Hydrazine Perchlorate in the liquid part should increase, and if it increases too much, the whole lot will explode. (2)

  1. http://blogs.sciencemag.org/pipeline/archives/2008/10/21/things_i_wont_work_with_triazadienyl_fluoride

  2. Ignition!: An informal history of liquid rocket propellants. Page 78

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    $\begingroup$ About the sentence following your (1), I think it also explodes when it is not looked at for a sufficiently long period of time. Ever tried heating milk on the stove, where the milk has a tendency to barely get warm no matter how long you warm it, but will boil over the moment you turn around or otherwise direct your attention elsewhere? Same basic principle. (Said in Willy Wonka voice.) $\endgroup$ – a CVn May 4 '17 at 11:04
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    $\begingroup$ The book Ignition! An Informal History of. Liquid Rocket Propellants is a terrific read that I strongly recommend. The entirety is available online. $\endgroup$ – Iwillnotexist Idonotexist May 4 '17 at 20:25
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    $\begingroup$ @IwillnotexistIdonotexist Make sure to read the foreword. Now it is clear that anyone working with rocket fuels is outstandingly mad. I don’t mean garden-variety crazy or a merely raving lunatic. I mean a record-shattering exponent of far-out insanity. There are, after all, some chemicals that explode shatteringly, some that flame ravenously, some that corrode hellishly, some that poison sneakily, and some that stink stenchily. As far as I know, though, only liquid rocket fuels have all these delightful properties combined into one delectable whole. space.stackexchange.com/q/3805/415 $\endgroup$ – a CVn May 5 '17 at 14:26
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    $\begingroup$ @MichaelKjörling Oh, I've read the book cover-to-cover; There are other amazing quotes in it, like "[ClF3] is, of course, extremely toxic, but that's the least of the problem. It is hypergolic with every known fuel, and so rapidly hypergolic that no ignition delay has ever been measured. It is also hypergolic with such things as cloth, wood, and test engineers, not to mention asbestos, sand, and water — with which it reacts explosively." $\endgroup$ – Iwillnotexist Idonotexist May 6 '17 at 1:19
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    $\begingroup$ Hydrazine (though not in the combination mentioned) is used in real world production rockets a lot - it is also a nasty poison, judging by the warnings that have been given to people to stay away from rocket accident debris because of the toxicity of even the trace amounts that could remain on the debris. $\endgroup$ – rackandboneman May 7 '17 at 3:16
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Mainly for the sake of morbid curiosity I'll mention one of my favorites. Steel balls that are used in grinding mills (SAG mills) apparently are heated and their surfaces hardened while grinding, and stresses induced in the steel ball itself. When taken out of the mill and allowed to cool, they sometimes explode (and I'd imagine a large, exploding ball of steel would be a tad dangerous). Such explosions may have even killed people. Here's a discussion of exploding SAG mill steel balls... (NOTE: Side benefit is you get to keep a straight face while discussing exploding balls).

ruble3 (Mining)30 Mar 05 08:23 Glad to be here! That particular incident happened in the early nineties-I hadn't worked in process plants before & was skeptical until I heard it from the horses mouth -still not sure why it happens - only refernce I could find was a case on a legal website where a family sued the mill after a worker was killed by an exploding ball - they blamed a 'manufacturing defect' ?? -still looking

tomrivet (Chemical)1 Jun 05 00:59 We have exploding balls quite a bit. The current thoughts are that the outside of the ball gets work hardend at a high temp (say 40C). Then the ball shrinks as it cools...

arunmrao (Materials)1 Jun 05 13:01 The balls explode due to builtup internal stresses. There is a volumetric expansion as some of the austenite has not completely transformed into martensite during heat treatment. It is this residual austenite which causes explosion of the balls. I have seen crazy things happening,imagine on a dark night!!

TurinShroud (Mining)1 Jun 05 16:59 I work at a copper mine that has a SAG mill with 5" balls. When we go in the mill for liner inspections, you can feel balls exploding underneath your feet. Last year my boss got cut in the ear by one.

I always thought that the balls exploded due to the internal stress generated by the outside cooling (and shrinking) faster than the inside. If we wait long enough before entering the SAG mill, the balls are no longer cooling and stop exploding.

Just yesterday, one of the operators said the balls can explode even after a long time. I had never heard of the balls exploding after they have cooled down. Has anyone else out there ever heard of the balls exploding after a long time?

If the exlosion is due to differential cooling, I just don't see how they could explode after they have been out the SAG mill for more than a few hours

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There's one option that comes to mind that might interest you: highly saturated solutions.

Have you ever seen a sodium acetate heat pack? They're sold in various outdoorsy stores, and they just look like a little plastic packet filled with fluid, with a small disc inside. When you activate the disc, you see this:

Sodium heat pack crystal activation

This surprising action is due to a natural phenomenon called supersaturation. Supersaturation is a term used to describe a solution that has more dissolved solute in it than should be possible. One way to achieve this is with heating and cooling, because solvents tend to be able to dissolve more solute at higher temperatures, due to some quirk of chemistry that I don't currently understand. A supersaturated solution is in an unstable state; certain impulses will knock it out of its careful equilibrium and return it to a more usual state. The little disc in the pack contains one of those impulses; a small crystal of sodium acetate that can start a crystallization cascade.

Those heat packs work because they contain more sodium acetate than the surrounding water can dissolve. In order to prepare them, you heat them up, which allows the water to absorb all of the sodium acetate, and then when you cool the pack back down the water and sodium form a supersaturated solution. When the crystallization process is triggered (by the little disc or simply by a hard shock), the sodium acetate crystallizes out of the water, which releases enough energy to comfortably warm your hands for the next half hour.

I think this could apply quite well to your particular idea. A ship with a hold full of some kind of concentrated solution could be quite safe so long as the solution is kept warm (perhaps it was produced on a volcanic planet, originally), but were its temperature to fall, the now-supersaturated solution could pose quite a problem once its energy is released.

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  • $\begingroup$ There is a nucleation cost preventing the solute from crystallizing out. The solute needs to form seed crystals before it can precipitate out, that is a minimal number of solute atoms/molecules must pack together into a large enough crystal. Basically the number of particles packing together must exceed the number being redissolved. Below a certain seed size dissolution wins, above that size precipitation wins. One can throw the balance by disturbing the solution e.g bending the metal in the packet provides a surface for precipitation, adding seed crystals to the solution, even a good thump. $\endgroup$ – Carel May 4 '17 at 23:30
  • $\begingroup$ Oh, that makes sense. Cool, now I can explain this properly next time. $\endgroup$ – Elia May 5 '17 at 19:05
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    $\begingroup$ Could the effects be more dramatic with dissolved gases? I think it might be possible to cool a saturated solution of CO2 in water below 0C if there are no nucleation sites for either CO2 bubbles or ice crystals, but CO2 starts bubbling out or ice crystals for, it wouldn't be long before all the CO2 was pushed out of solution. $\endgroup$ – supercat May 5 '17 at 20:17
  • $\begingroup$ @supercat Will's answer shows what happens. CO2 is largely a gas at the same temperatures and pressures that water is a liquid and the governing mechanism is it's dissolution as "A gas in a liquid" i.e. Henry's law. For it to explode really requires a rapid transition of sorts from one state to another. Now you can drive many a system away from it's equilibrium state such that it would rapidly return towards it. But rapid is a relative term. It may not be quick enough to cause an explosion. Will's bottles probably cracked quickly but the water oozed out slowly(his fridge/town is still intact) $\endgroup$ – Carel May 8 '17 at 3:07
  • $\begingroup$ @Carel: If the CO2-water mixture is in a container that won't allow it to expand, then it could explode in the same sense that a dry-ice bomb does. When the container fails, the pressure in the immediate vicinity will increase very rapidly. $\endgroup$ – supercat May 8 '17 at 3:46
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According to thermodynamics, more ordered states are favoured at lower temperatures, so essentially what you are looking for is a system which increases in volume as it becomes more ordered, which is unusual. As mentioned in another answer, this happens when beer turns to ice, but the volume change is quite small. Another process is as follows:

Hydrogen sulphide generated in oil refinery desulphurisation processes is converted to sulphur by the Claus process:

H2S + 1.5O2 (air) -> SO2 + H2O
2H2S + SO2 -> 3S2 + 2H2O

Sulphur made in this way contains short chain like molecules of the form H-S-..-S-H. As it cools, these decompose to form ordered stable S8 rings with the release of H2S gas. Vessels containing molten sulphur made by this process must be properly vented to ensure they do not overpressure and explode as the sulphur cools. Ihe H2S (hydrogen sulphide, rotten egg gas) presents additional hazards as it is smelly and highly toxic.

A quick google search found this document discussing the handling of this type of sulphur, see pages 4-5. http://www.trimeric.com/assets/15v07-mcintush-molten-sulfur-storage-tank-loading-and-vapor-ejection-systems-review.pdf

EDIT: a better reference http://pubs.acs.org/doi/abs/10.1021/ie50477a047?journalCode=iechad

The other source of sulphur on Earth is via mining, including surface deposits in the craters of semi active volcanoes. I think it quite plausible that sulphur mined on a planet with a high atmospheric pressure such as Venus would contain these H2Sx molecules. As an aside, note that the best place in the solar system for mining sulphur is probably be https://en.wikipedia.org/wiki/Io_(moon) though it has only a trace atmosphere so I would expect the sulphur there to be fully degassed.

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As indicated by Thorsten S.’s answer, there are solids that have different phases.

I'd like to add examples where a phase-change can be induced by cooling.

Some typee of steel undergo cryogenic hardening treatments. At about −185 °C more austenite transforms into martensite, the phase change propigating at the speed of sound once it starts somewhere in the metal.

Now alloys designed to make use of this will not change size significantly, warping and exploding. But other steels could have that problem.

Then there is the remarkable metal nitinol. It does undergo profound changes, and can have two shapes “memorized”, one for hot a d one for cold. This works cia the same kind of crystallization change, triggered by temperature and pressure inducing austenite-to-martensite and martensite-to-austenite transformations.

A totally different mechanism might be caused by the separation of a mixture. Atoms of type 1 might slip inside the crystal lattice of 2, making it take up no space. But shrink the lattice a bit as it cools, and atom 1 is forced out. These can work together, with different crystal phases permitting other atoms to move in or not, as with carbon and iron.

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This answer is slightly off-topic as it is not about a crystal or other chemical substance that explodes when it's allowed to cool down too much. However, this answer is about an object that does explode really violently if it's allowed to radiate away too much energy. And it's a really useful thing for a scifi setting: A miniature black hole.

If you have a miniature black hole of around 600000 tons of mass, it will explode within 3.5 years. You must constantly feed such a black hole to keep its energy output at manageable levels. If you fail to do so, it will blast you to pieces: If my calculations are correct, it will evaporate an energy equivalent to 233 tons of mass within its last second of life. I have my doubts that our planet would survive such a huge explosion, it's really far beyond anything the largest nuclear weapons can do.

Why would a space transporter use a black hole? Well, for propulsion! (See the wikipedia article on the black hole starship for reference.) Turns out that small black holes make for quite efficient, high power reactors that can be fed with any fuel.

Trouble is, the smaller the "black" hole, the brighter it gets. Now, the wikipedia article on the black-hole-starship puts the size of a useful black hole around 606000 tons which would evaporate within 3.5 years. Such a black hole would need regular feeding to keep it at manageable power levels. If you fail to feed it fast enough, it will blow up, destroying anything in the vicinity. Especially the transporter that used the black hole for propulsion won't survive this.

As I said, this answer bends the rules of the question slightly: While the black hole does loose energy, it does not get colder. On the contrary, it gets hotter. Nevertheless, it does fit perfectly into the idea of having a space transporter that just explodes when it's unattended for too long. Especially with providing a really good reason for the dangerous object being within the transporter.

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A two or three part mineral might do it

In real life, such a mineral on Earth wouldn't last long because there's so much regular temperature changes. Also, highly reactive elements tend to have reacted a long long time ago (ex. fluorine is always always found bound to some other element.)

Temperature changes got me thinking about phase changes.

  • Plasma to Gas: Nope.
  • Gas to Liquid: Probably not.
  • Liquid to Solid: Quite possibly.

All of these transitions are endothermic. Most ignition sources for explosives are exothermic; electric current, fire, lower speed explosions (and resulting shock). Fire as an initiator won't work since it too is an exothermic reaction which could push the temperature of the mineral back above the danger temperature. Let's see if we can find an ignition source that can still work at lower temperatures but gives a nice boom too.

Piezo-electric

Piezoelectric materials generate a small charge when they physically deform. This property could be leveraged to generate an ignition current.

Mineral Matrix

Piezoelectric crystals + explosive + compression material = Boom!

General Process:

  1. Above critical temperature, the compression material doesn't squeeze the piezoelectric crystals enough for them to develop a charge.

  2. At critical temperature, the compression material begins to condense from a liquid or gel into a crystal. These crystals exert pressure on the piezoelectric crystals embedded in the explosive.

  3. As freezing continues, the piezos snap into smaller pieces or snap back into their original shape thereby releasing their charge into the surrounding matrix. If enough of these piezos snap back at the same time, they might generate enough current to detonate the explosive.

The compression material actually makes this mineral more dangerous as explosives in confined spaces tend to react more powerfully than if detonated in the open air.

Conclusion

Despite a semi-plausible explanation above, this highly lucky matrix of materials is unlikely to exist. It could be engineered by humans, possibly, but highly unlikely for Ma Nature to shake her chemistry set and have it go boom this way.

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You might also consider something like a stud blaster (if that's the correct tanslation of "Bolzensprenger"). The usual length contraction of one component due to cooling creates enormous forces on the overall structure until it fails in an explosion-like event.

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  • $\begingroup$ Apparently this is a "Bar Breaker" in english. $\endgroup$ – Jasen May 4 '17 at 7:50
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Others have alluded to this, but water can "explode" if it cools too fast. It is one of the few known molecules which expands as it transitions from liquid to solid. If it freezes fast enough, it will first form an outer shell of ice, which will eventually burst, sometimes violently, as pressure builds up from the center freezing.

While this probably does not provide enough of an energy output for the type of explosion you are envisioning, who is to say that there is not some other compound out there which exhibits a similar behavior on a greater scale? Playing in the realm of space travel gives enough room for any number of exotic materials to have been discovered.

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Nuclear criticality

You can have a pile (literally) of fissile material, with space amongst the material intended for a moderator, which will not have critical mass without that moderator present. This can even happen naturally.

It is considered excellent reactor design to use a moderator which doesn't work unless it's reasonably cool. That is how most non-Russian non-Canadian reactors govern themselves. If core power exceeds demand, the water coolant will boil more, increasing steam voids (bubbles), which are a poor moderator compared to the liquid water the reactor is designed for. Power is passively reduced.

We've barely scratched the surface on potential reactor designs. We have played with small modular reactors, which are factory-made and sealed, and not for the user to tamper with. Conceivably the transport instruction might be "keep the reactor hot enough for the coolant to be in gaseous state" otherwise it will go critical. Especially if tech is readily available that makes this easy.

It's not the reactor

You won't get a nuclear explosion from a runaway reactor. It might not even run away owing to the moderation method. But it would emit a heck of a lot of gamma radiation! (it would not be inside its massive biological shield; you wouldn't ship that, since it's plain concrete and easily made locally with indigenous materials.)

Anyway, your other cargo might not appreciate massive doses of gamma radiation none too much, and that might be what explodes.

And that is just the kind of dumb shipping mistakes that happen in the real world.

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Plasma of reactive molecules

This answer is as esoteric as my other beer answer is prosaic. Imagine two gases which combined and heated will react - let us say hydrogen and oxygen. Now convert the hydrogen and the oxygen to a completely ionized plasma and combine them. The ionized plasma state will prevent the typical chemical reaction and as long as the plasma is kept hot and charged the gases will not react. On cooling down the hot hydrogen and oxygen will react in the typical energetic way. I am trying hard to think of some advantage this mixed plasma would have over 2 reservoirs of gas...

Going even more out on a limb, consider intra-particle interactions in a highly charged hot plasma. These particles do not bump into each other any more as the electrons are all stripped away and moving independently. The positively charged nuclei repel each other. Imagine now a plasma composed of highly ionized hydrogen matter and antimatter. It is stable as long as everything is highly charged. If the plasma cools, hydrogen atoms will try to reform hydrogen gas, and if the 2 atoms are matter and antimatter there will be an explosion.

A plasma would be a nifty way to contain antimatter because you could contain it with magnetic fields. I hereby name this mixed hydrogen antihydrogen plasma "zoom juice" because you could release small quantities of it and use it as fuel.

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    $\begingroup$ antimatter nuclei would be negatively charged and attract the matter nuclei. $\endgroup$ – Chris Becke May 5 '17 at 16:22
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Uranium should do the trick.

Implosion type nukes work by compacting fissionable material that normally isn't critical. They do it with a non-nuclear explosion, but guess what would also shrink a hunk of uranium? That's right, thermal contraction.

Plus, it's easy to explain why a freighter would be transporting it if you make fission energy common in your universe

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Basically anything that forms crystal or amorphous structures upon solidifying which bear tension (think of a prince rupert drop...) AND become more brittle with cooling. Steel has been mentioned in other posts, and indeed quenched, untempered steel is known to potentially shatter or break quite energetically when provoked....

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