DTU Energy — the Department of Energy Conversion and Storage at Technical University of Denmark — has purchased high-flux CT (computed tomography) equipment to undertake the non-destructive investigation of energy-related materials — including those used in batteries, electrolysis cells and fuel cells.

The high-energy (225kV) ‘micro-focus source’ penetrates dense samples in the search for suitable materials to develop sustainable ‘green energy’ systems.

Non-renewables currently generate about 85% of the world’s energy, but the issues with these finite resources have been known for a long time: fossil fuels will one day be depleted, but in the meantime greenhouse gases such as CO2 (created by burning these fuels) are contributing to global warming.

In the search to find alternative resources to power the world, DTU Energy’s primary objective is to develop technologies for the conversion and storage of energy derived from ‘fluctuating sources’ such as wind power and solar power.

Wind power, for example, can be stored by converting the electricity produced into a fuel such as hydrogen or a hydrocarbon.

CT provides insight into material structures that cannot be obtained from 2-D images. DTU Energy’s pre-existing imaging methods include electron microscopy (transmission electron microscopes and scanning electron microscopes — TEM and SEM), FIB-tomography, TOF-SIMS, XPS, neutron imaging, scanning-probe microscopy and diffraction tomography.

However, for material research, DTU required a CT system capable of delivering insight into the complex 3-D structures characteristic of most energy conversion and storage devices.

The structure of batteries and electro-chemical cells can be difficult to analyse from 2-D images alone. Indeed, for most ‘characterisation methods’, it is necessary to break or cut the materials to investigate the internal parts.

With the CT facility, it is possible to investigate the devices in 3-D without prior preparation, giving information on internal surfaces and material structure such as the connectivity of pores.

Large capacity

The equipment selected by DTU Energy was an XT H 225 ST system from Nikon Metrology (www.nikonmetrology.com). This is one of the company’s ‘extended’ models, and it allows samples up to 50kg in weight and 300mm in diameter to be scanned.

This capacity is what DTU Energy needs to inspect large, heavy samples such as solar, fuel and electrolysis cells (the system’s 16-bit Perkin Elmer 1620 flat panel allowing high-definition images to be obtained).

The CT scanner is also equipped with a multi-metal target that generates specific X-ray spectra. As well as the standard tungsten target, operators can select three other materials — silver, molybdenum and copper — thereby allowing the use of X-ray emissions at lower energies for the improved analysis of some materials.

Furthermore, the flexibility provided by a wide range of options allows the Nikon Metrology system to be used by quality laboratories, production facilities and research departments; this is useful, as other departments at the university will use the equipment (these include DTU Compute, DTU Mechanics and DTU Physics).

The system has so far provided useful information on pore fraction, pore shape and connectivity in cells, as well as on interface areas between materials and particle sizes. It has also been used to investigate the internal 3-D structure of batteries and track changes during and after ageing cycles.

With over 25 years of expertise, CT specialists at Nikon Metrology have developed and manufactured complete systems incorporating proprietary micro-focus X-ray sources and high-precision manipulators that are programmable in five axes — plus fast acquisition and reconstruction software.

The XT H 225 ST comes with a motorised FID (Focal spot to Imager Distance), which allows the detector to be moved closer to the source. A shorter FID means that the X-ray flux is increased, reducing the imaging exposure and scan time (DTU Energy has made use of the high flux of the ST model to facilitate faster scans for in-situ measurements).

DTU Energy’s Søren Bredmose Simonsen said: “The motivation for buying the Nikon system was the 225kV micro-source with a minimum spot size of 3µm, which allows the scanning of dense materials containing heavy elements and larger samples — such as batteries, solar cells and fuel/electrolysis cells.”