Cracking solution

A ceramic which is claimed to withstand extreme heat, while remaining non-brittle, could hold the key to a new generation of nuclear reactors. Stuart Nathan explains.

Russian scientists claim to have developed a ceramic able to withstand multiple heating and cooling cycles, while remaining non-brittle. One application could be the vital components for a new generation of nuclear reactors.

The team, from the Leipunsky Physics-Energy Institute in Obninsk, constructed the ceramic from a wide variety of differently-sized granules. ‘A distinguishing feature of our ceramic is its structure,’ said project manager Irina Kurina. ‘Generally speaking, there are three types of component in the structure: large grains of oxide material (from 50–100µm), fine grains (from one to 10µm) and a little emptiness. In other words there are pores, located in a special way, predominantly around the boundaries of the grains.’

In conventional ceramics, the grains that make up the bulk of the structure are anchored firmly in place. Unlike metals — whose atoms have a degree of freedom to vibrate, allowing the material to conduct heat or move around each other, imparting ductility and plasticity — ceramic materials are rigid. When heated, this energy cannot easily propagate through the material.

This makes them good thermal insulators, but as the heat does not spread, the ceramic cannot expand evenly, and as it cools it can crack or crumble. Ceramics are rated according to the number of heating/cooling cycles they can withstand while retaining their integrity.

Kurina’s ceramic is claimed to combine heat conductivity that exceeds reference data, enhanced plasticity and thermal stability. It gains its properties largely because of pores in the structure. ‘Such pores are ideal for plastic deformation, and fine grains help to soften mechanical or thermal impacts,’ she said.

Kurina compares the structure as being like ‘cobblestones in sand’, with the large grains ‘stuck’ in the matrix of the smaller ones. ‘The crystalline lattice of such a ceramic is very mobile; it has many defects,’ she said.

This gives the material some of the properties of a metal. It even gives it an unusually high thermal conductivity, stemming from the structure’s ability to support quantum tunnelling, where under certain circumstances electrons can pass through energy barriers which would normally be impassible.

The ceramic is a mixture of metal oxides — mainly aluminium, magnesium and zirconium, with thorium and uranium also possible constituents for nuclear purposes. The oxides are obtained by precipitation from soluble salts of the metals; the chemical constituents of the solutions and the reagents helps to determine the size of particles and the number of the crucial defects in them. The oxide powders are then annealed, pressed and sintered.

One possible application for the ceramic is in fuel pellets for nuclear reactors. The team has investigated the properties of a uranium dioxide-containing ceramic for nuclear fuel, and found that at the operating temperature of a reactor, 600–700°C, the temperature gradient from the centre of the pellet to its outer surface ‘is considerably less than when using UO2 pellets made according to standard production processes.’ The pellets’ plasticity is also improved, the team said, by adding minute amounts of tin and titanium oxides.

The increased thermal conductivity appears to be an electronic phenomenon. X-ray photoelectron spectroscopy reveals that there are two types of uranium in the ceramic — one has four unpaired electrons, which allows it to bind to two oxygen atoms, and one has less than four, meaning it bonds to one oxygen and has electrons ‘spare’. These can tunnel through the structure, allowing energy to propagate through the ceramic. Reducing the temperature gradient through the pellet makes for more uniform thermal expansion, and allows the structure to withstand heating and cooling, said Kurina.