What is Pykrete?
For a brief period during World War II, the hopes of Allied defense efforts were pinned on ice. In 1942, journalist-turned-scientist Geoffrey Pyke, working for the British military, thought that massive ships made of ice could serve as bulletproof aircraft carriers to protect trans-Atlantic shipping lines between the United States and Great Britain. Shrouded under the codename Habbakuk (a misspelling of the Biblical book Habakkuk), Pyke's idea got the green light for funding from Winston Churchill. Drawing on Pyke's initial concept, Habbakuk researchers, including Nobel Prize winner Max Perutz, created an icy — and incredibly strong — material called pykrete.
Pure ice has a peculiar set of characteristics, being both soft and brittle. When water freezes, the hydrogen and oxygen atoms form six-sided crystalline structures. The open space in those crystals explains why water expands as it freezes and why it's susceptible to pressure changes. Pykrete takes advantage of the strength of that crystalline structure and improves upon it, somewhat like reinforcing concrete with steel wiring. It's a simple mixture of 14 percent wood pulp and 86 percent water. The cellulose fibers from the wood pulp enhance the frozen water's strength and durability. In fact, upon freezing the sludge, pykrete is up to 14 times stronger than regular ice. It also outperforms the power of concrete.
High seas were riddled with stealthy German U-boats during Word War II. So the British sought to create an indestructible fleet of their own.
The iceberg ships that George Pyke first envisioned would've had incredibly thick walls to resist damage. Thinner blocks of ice couldn't withstand bullets and torpedoes. Pykrete, on the other hand, is bulletproof. A material's resistance to bullets depends on how well it absorbs energy across it to minimize force of impact. Brittle substances like ice shatter more readily than, say, bullet-resistant glass treated with energy-absorbing plastic. According to Max Perutz, the wood pulp fibers in pykrete remedy that brittleness problem while remaining ductile. That slight tweak gives pykrete its remarkable crush resistance.
Melting Ship: Pykrete's Thermal Conductivity
Although pykrete is highly durable, there's still the potential problem of it melting. The rate at which heat can move through a substance is referred to as thermal conductivity. In regard to ice, thermal conductivity measures the speed at which it melts. The physical process of ice melting happens when the chemical bonds between water molecules in the crystalline structure of ice are broken due to heat activity. Adding the wood fiber particles lowers the overall thermal conductivity, therefore slowing melting. However, the wood pulp can't completely halt pykrete from melting, especially in warmer waters.
In the end, cost barriers and resource depletion had chilling effects on George Pyke's icy fleets. For instance, it would've required more steel to house the frozen ice than it would've taken to build an entirely new ship. Also, by 1943, technology had caught up with necessity, with better equipped aircraft carriers in use.
The end result? Although physics proved that pykrete had impressive potential, it was mostly left to thaw out after the British government shut down project Habbakuk.