A Multi-Beam Optical Stress Sensor (MOSS) provides a contact-free, in-situ platform to measure thin film stress based on the curvature of a bilayer sample. Originally, this instrument was used to measure thin film stress during deposition. However, when combined with a specially designed chemical reactor, it can be used to measure the oxygen surface exchange coefficient of thin film materials [1,2, 3]. In this work, the oxygen exchange coefficient (k_chem), stress, biaxial modulus, thermal expansion coefficient (CTE) and chemical expansion coefficient (CCE) were measured as a function of temperature via the wafer curvature technique.

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 30 min, with a 10^-5 torr oxygen partial pressure. After deposition, the PCO bilayers were re-equilibrated with air by firing them in air at 1000^oC for 1 hour.

 To measure k_chem, a solution of Fick’s second law was fit to the bilayer curvature response of PCO|YSZ samples responding to abrupt oxygen partial pressure changes (0.21 – 0.021), as shown in Fig. 1 (a). Then the data was fitted to to get k_chem values. To measure the biaxial modulus, CTE and CCE, curvature values for PCO|YSZ and PCO|MgO were measured from 25^oC to 700^oC with a 0.5^oC/min ramping rate, converted to stress using Stoney’s equation, and determined using the dual substrate method [4].

As shown in Fig. 1 (b), the PCO thin film k_chem values measured here were comparable with other literature PCO thin film k_chem values measured using electrode-free techniques such optical relaxation [5, 6]. However, the PCO k_chem values measured here were 1-2 orders lower than those measured via impedance spectroscopy (which uses electrodes) [5, 7, 8]. Additional experiments to measure the PCO biaxial modulus, CTE and CCE are underway.

Acknowledgements

This material is based upon work supported by the Department of Energy under Award Number DE-FE0023315.

References

[1] Yang Q, Burye T E, Lunt R R, et al. Solid State Ionics, 2013, 249: 123-128

[2] Yang Q, Nicholas J D. Journal of the Electrochemical Society, 2014, 161(11): F3025-F3031.

[3] Nicholas, Jason D. Extreme Mechanics Letters, 9, (2016): 405-421.

[4] Zhao J H, Du Y, Morgen M, et al. Journal of Applied Physics, 2000, 87(3): 1575-1577.

[5] Kim J J, Bishop S R, Thompson N J, et al. ECS transactions, 2013, 57(1): 1979-1984.

[6]. S. R. Bishop, J. J. Kim, N. Thompson and H. L. Tuller, ECS Trans., 45, 491 (2012).

[7] Dalslet, Bjarke Thomas, Martin Søgaard, and Peter Vang Hendriksen. Journal of The Electrochemical Society 154.12 (2007): B1276-B1287.

[8] Chen D, Bishop S R, Tuller H L. Journal of Electroceramics, 2012, 28(1): 62-69.

Fig 1. (a) Raw and fitted PCO-YSZ curvature relaxation data and (b) a PCO k_chem comparison