# Tantalum–Hafnium: Optical Hydrogen Sensing Materials for High-Temperature Applications

## TL;DR

Tantalum-hafnium alloys improve hydrogen sensing at high temperatures by expanding detection ranges and enhancing sensitivity.

## Contribution

Alloying tantalum with hafnium creates a stable material with improved hydrogen sensing at high temperatures.

## Key findings

- Tantalum-hafnium alloys maintain a stable structure with up to 21% hafnium.
- Alloying improves sensitivity at low hydrogen concentrations above 200 °C by two times compared to pure tantalum.
- The alloy preserves the large, hysteresis-free sensing range and stability of tantalum.

## Abstract

Thin film metal hydride optical sensors, especially those
made
from tantalum, offer a large, hysteresis-free hydrogen sensing range,
fast response times and great stability. However, due to the shift
in tantalum’s hydrogen sensing ranges with rising temperatures,
tantalum becomes inadequate for the detection of low hydrogen concentrations
(<10+3 ppm) above 200 °C, making it unsuitable
for high-temperature applications. We show that the properties of
tantalum can be tailored by alloying tantalum with hafnium. Optical
transmission measurements, ex situ and in situ X-ray diffraction and
X-ray and neutron reflectometry are used to show that the introduction
of Hf in Ta results in a solid solution with a stable structure with
up to 21% Hf. Alloying Ta with Hf expands the unit cell, which alters
the enthalpy of hydrogenation and shifts the sensing range to lower
concentrations. Moreover, alloying Ta with Hf improves the sensitivity
at low hydrogen concentrations (<10+3 ppm) and for temperatures
exceeding 200 °C by about two times compared to pure Ta while
preserving its large, hysteresis-free sensing range and excellent
stability.