A Multi-beam Optical Stress Sensor (MOSS) is a curvature measurement platform which is commonly used to measure the film stress in bilayer samples. It has been widely used as an in-situ technique to measure the film stress during deposition.¹ However, when combined with the dual substrate method proposed by Zhao et al,² in situ curvature measurements can be used to measure Young’s moduli and thermo-chemical expansion coefficients simultaneously as a function of temperature. Using the curvature relaxation (κR) technique developed recently,^3-5 oxygen surface exchange coefficients (k_chem) can also be measured as a function of temperature using in situ curvature measurements. In this work, the Young’s moduli, thermo-chemical expansion coefficients and k_chem values of praseodymium doped ceria (PCO) were measured as a function of temperature using a MOSS.

First, phase pure Pr_0.1Ce_0.9O_1.95 (PCO) powder was prepared through glycine nitrate combustion and subsequent calcination at 1100^oC in air. This powder was then pressed in a stainless-steel die and fired to 1450^oC to produce a pulsed laser deposition (PLD) target. In preparation for PLD, (001) oriented 9.5% yttria doped zirconia (YSZ) and (001) oriented magnesium oxide (MgO) substrates (Crystec, GmbH) were pre-annealed at 1450^oC for 20 hours to remove residual stress within them. PCO PLD was then conducted at 680^oC for 20 min, with a 10^-2 torr oxygen partial pressure and 350 mJ power density. After deposition, the PCO bilayers were re-equilibrated with air by firing them in air at 1000^oC for 1 hour.

For dual substrates measurements, stress vs. temperature data for PCO|YSZ and PCO|MgO were collected with a 1^oC/min heating rate and a 0.2^oC/min cooling rate. The slopes of the stress vs. temperature curves can be expressed by:

dσ_PCO|YSZ/dT = M_PCO(α_YSZ-α_PCO) (1)

dσ_PCO|MgO/dT = M_PCO(α_MgO-α_PCO) (2)

where is the stress of bilayer sample, T is the temperature, M is the biaxial modulus of the film, is the thermo-chemical expansion coefficient. With two unknowns and two equations, and were then extracted as a function of temperature. The Young’s moduli were then calculated from assuming a Poisson’s ratio of 0.33 as has been done previously for ⁶.

For κR measurements, relaxation data were recorded at 650~725^oC with 25^oC increments. The oxygen partial pressure was switched between synthetic air (20%O₂-80%Ar) and 10% diluted synthetic air (10% synthetic air-90%Ar).

Figure 1 shows the Young’s modulus and thermo-chemical expansion coefficients measured here compared to other literature studies.^6-8 In contrast to other studies the present study produced PCO Young’s moduli over a complete range of temperatures. In addition, the PCO Young’s moduli started to decrease significantly once the PCO started to become nonstoichiometric (as indicated by an uptick in chemical expansion in Figure 1b). The PCO k_chem values (not shown) were in good agreement with the k_chem values measured by other electrode-free techniques, such as optical relaxation.⁹