HT-XRD allows the molecular structure of materials to be determined in situ up to 1,600°C and under dynamic reactive/inert gas environment. Structural changes and inter-material interactions at temperature often cannot readily be gleaned from post-mortem analysis. The system is equipped with multiple gas inputs, each controlled by electronic mass-flow controller, with optional O2 getters. Gases exiting the sample chamber may be interrogated by gas chromatography (e.g., to determine H2 production rate) and O2 meter (e.g., to monitor thermal reduction). An optical pyrometer observes the sample surface to monitor surface temperature as well as indicate changes in the sample’s emissivity brought about by reaction. The HT-XRD apparatus is depicted in Figure 1.

Thermal analysis is often run in parallel with HT-XRD to quantify levels and rates of reaction (reduction, oxidation, decomposition, etc.) as a complimentary investigative technique, and is used to benchmark material performance against a common standard.  Several thermal analyzers are available to the Node, including thermogravimetric analyzers and differential scanning calorimeters, including the ability to run under controlled humidity (steam) at temperatures up to 1,550°C. The thermal analyzer incorporates a close-coupled quadrupole mass spectrometer capable of measuring up to mass/charge = 1024, and with furnace temperatures up to 2,000°C. An example of recent thermal analysis work to determine oxygen vacancy formation in perovskites as a function of temperature and partial pressure of oxygen is given in Figure 2 [Ref. 1]. Thermal conductivity and diffusivity measurements are also available.

The combination of HT-XRD and thermal analysis provides a clear understanding of materials’ performance under realistic operating conditions [Refs. 2, 4]. In the thermal analysis laboratory, we also have the ability to thermally process model sample geometries up to 1,700 °C and conduct isotopic labeling for coupling with techniques such as Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS, available in a separate lab at Sandia). This technique enables, for example, oxygen permeation within monolithic reactive material specimens to be imaged at microscopic scale to identify high resistance oxygen diffusion pathways [Ref. 3]. Representative SIMS maps are shown in Figure 3, exemplifying the poor utilization of bulk iron oxide phases in a thermochemical reduction-oxidation process compared to iron that is dissolved into an oxide-conducting matrix of yttria-stabilized zirconia (YSZ).

Rapid thermal cycling of materials can be achieved via two approaches. One is the use of a special high-rate furnace on a thermal analyzer which can achieve controlled ramp rates of up to 1,000°C min-1 up to 1,200°C, and cool at several 100°C min-1. Gases can be switched between inert and oxidizing as required. Thus, a specimen can be subjected to many thermal or thermochemical cycles in a short period of time, while simultaneously monitoring mass changes. The second approach uses a home-built furnace with mechanical actuator that can move a platform containing multiple specimens in and out of the hot zone of a tube furnace at a programmed rate. Gas switching is also automated to enable thermochemical cycles to be performed. The maximum sample temperature is ca. 1,500°C, and the ΔT is dependent on the distance the sample is moved from the hot zone. Cycle times on the order of a minute are easily achievable. Off-gas can be analyzed via mass spec, GC, or gas-phase FTIR.