skip to main content

Title: Mechanical, thermal, and electrochemical properties of Pr doped ceria from wafer curvature measurements
This work demonstrates, for the first time, that a variety of disparate and technologically-relevent thermal, mechanical, and electrochemical oxygen-exchange material properties can all be obtained from in situ , current-collector-free wafer curvature measurements. Specifically, temperature or oxygen partial pressure induced changes in the curvature of 230 nm thick (100)-oriented Pr 0.1 Ce 0.9 O 1.95−x (10PCO) films atop 200 μm thick single crystal yttria stabilized zirconia or magnesium oxide substrates were used to measure the biaxial modulus, Young's modulus, thermal expansion coefficient, thermo-chemical expansion coefficient, oxygen nonstoichiometry, chemical oxygen surface exchange coefficient, oxygen surface exchange resistance, thermal stress, chemical stress, thermal strain, and chemical strain of the model mixed ionic electronic conducting material 10PCO. The (100)-oriented thin film 10PCO thermal expansion coefficient, thermo-chemical expansion coefficient, oxygen nonstoichiometry, and Young's modulus (which is essentially constant, at ∼200 MPa, over the entire 280–700 °C temperature range in air) measured here were similar to those from other bulk and thin film 10PCO studies. In addition, the measured PCO10 oxygen surface coefficients were in agreement with those reported by other in situ , current-collector-free techniques. Taken together, this work highlights the advantages of using a sample's mechanical response, instead of the more traditional electrical more » response, to probe the electrochemical properties of the ion-exchange materials used in solid oxide fuel cell, solid oxide electrolysis cell, gas-sensing, battery, emission control, water splitting, water purification, and other electrochemically-active devices. « less
Authors:
;
Award ID(s):
1254453
Publication Date:
NSF-PAR ID:
10198063
Journal Name:
Physical Chemistry Chemical Physics
Volume:
20
Issue:
43
Page Range or eLocation-ID:
27350 to 27360
ISSN:
1463-9076
Sponsoring Org:
National Science Foundation
More Like this
  1. Sr(Ti 1−x Fe x )O 3−δ (STF) has recently been explored as an oxygen electrode for solid oxide electrochemical cells (SOCs). Model thin film electrode studies show oxygen surface exchange rates that generally improve with increasing Fe content when x < 0.5, and are comparable to the best Co-containing perovskite electrode materials. Recent results on porous electrodes with the specific composition Sr(Ti 0.3 Fe 0.7 )O 3−δ show excellent electrode performance and stability, but other compositions have not been tested. Here we report results for porous electrodes with a range of compositions from x = 0.5 to 0.9. The polarization resistance decreases with increasing Fe content up to x = 0.7, but increases for further increases in x . This results from the interaction of two effects – the oxygen solid state diffusion coefficient increases with increasing x , but the electrode surface area and surface oxygen exchange rate decrease due to increased sinterability and Sr surface segregation for the Fe-rich compositions. Symmetric cells showed no degradation during 1000 h life tests at 700 °C even at a current density of 1.5 A cm −2 , showing that all the STF electrode compositions worked stably in both fuel cell modemore »and electrolysis modes. The excellent stability may be explained by X-ray Photoelectron Spectroscopy (XPS) results showing that the amount of surface segregated Sr did not change during the long-term testing, and by relatively low polarization resistances that help avoid electrode delamination.« less
  2. Heterogeneous bonding between metals and ceramics is of significant relevance to a wide range of applications in the fields of industry, defense, and aerospace. Metal/ceramic bonding can be used in various specific part applications such as vacuum tubes, automotive use of ceramic rotors, and rocket igniter bodies. However, the bonding of ceramic to metal has been challenging mainly due to (1) the low wettability of ceramics, on which the adhesion of molten adhesive bonders is limited and (2) the large difference between the coefficients of thermal expansion (CTE) of the two dissimilar bonded materials, which develops significant mechanical stresses at the interface and potentially leads to mechanical failures. Vapor-phase deposition is a widely used thin film processing technique in both academic research laboratories and manufacturing industries. Since vapor phase coatings do not require wettability or hydrophobicity, a uniform and strongly adherent layer is deposited over virtually any substrate, including ceramics. In this presentation, we report on the effect of vapor phase-deposited interfacial metal layers on the mechanical properties of bonding between stainless steel and Zerodur (lithium aluminosilicate-based glass ceramic). Direct-current magnetron sputtering was utilized to deposit various thin interfacial layers containing Ti, Cu, or Sn. In addition, to minimize themore »unfavorable stress at the bonded interface due to the large CTE difference, a low temperature allow solder, that can be chemically and mechanically activated at temperatures of approximately 200 °C, was used. The solder is made from a composite of Ti-Sn-Ce-In. A custom-built fixture and universal testing machine were used to evaluate the bonding strength in shear, which was monitored in-situ with LabView throughout the measurement. The shear strength of the bonding between stainless steel and Zerodur was systematically characterized as a function of interfacial metal and metal processing temperature during sputter depositions. Maximum shear strength of the bonding of 4.36 MPa was obtained with Cu interfacial layers, compared to 3.53 MPa from Sn and 3.42 MPa from Ti adhesion promoting layers. These bonding strengths are significantly higher than those (~0.05 MPa) of contacts without interfacial reactive thin metals. The fracture surface microstructures are presented as well. It was found that the point of failure, when Cu interfacial layers were used, was between the coated Cu film and alloy bonder. This varied from the Sn and Ti interfacial layers where the main point of failure was between the interfacial film and Zerodur interface. The findings of the effect of thin adhesion promoting metal layers and failure behaviors may be of importance to some metal/ceramic heterogeneous bonding studies that require high bonding strength and low residual stresses at the bonding interface. The authors gratefully acknowledge the financial support of the Improvement of Measurement Standards and Technology for Mechanical Metrology (Grant No. 20011028) by KRISS.« less
  3. Abstract

    This work demonstrates the processing, modeling, and characterization of nanocrystalline refractory metal tantalum (Ta) as a new structural material for microelectromechanical system (MEMS) thermal actuators (TAs). Nanocrystalline Ta films have a coefficient of thermal expansion (CTE) and Young’s modulus comparable to bulk Ta but an approximately ten times greater yield strength. The mechanical properties and grain size remain stable after annealing at temperatures as high as 1000 °C. Ta has a high melting temperature (Tm = 3017 °C) and a low resistivity (ρ = 20 µΩ cm). Compared to TAs made from the dominant MEMS material, polycrystalline silicon (polysilicon,Tm = 1414 °C,ρ = 2000 µΩ cm), Ta TAs theoretically require less than half the power input for the same force and displacement, and their temperature change is half that of polysilicon. Ta TAs operate at a voltage 16 times lower than that of other TAs, making them compatible with complementary metal oxide semiconductors (CMOS). We selectα-phase Ta and etch 2.5-μm-thick sputter-deposited films with a 1 μm width while maintaining a vertical sidewall profile to ensure in-plane movement of TA legs. This is 25 times thicker than the thickest reactive-ion-etchedα-Ta reported in the technical literature. Residual stress sensitivities to sputter parameters and to hydrogen incorporation are investigated and controlled. Subsequently, a V-shaped TA is fabricated andmore »tested in air. Both conventional actuation by Joule heating and passive self-actuation are as predicted by models.

    « less
  4. Graphene oxide (GO) films have great potential for aerospace, electronics, and renewable energy applications. GO sheets are low-cost and water-soluble and retain some of Graphene’s exceptional properties once reduced. GO or reduced GO (rGO) sheets within a film interact with each other via secondary bonds and cross-linkers. These interfacial interactions include non-covalent bonds such as hydrogen bonding, ionic bonding, and π-π stacking. Stress transfer and failure mechanisms in GO and rGO films, specifically how linkers affect them, are not well understood. The present study investigates the influence of inter-particle interactions and film structures, focusing on hydrogen bonds introduced via cellulose nanocrystals (CNC), on failure and stress-transfer of the GO and rGO films. To this end, GO films with CNC crosslinkers were made, followed by a chemical reduction. The few-micron thick films were characterized using tensile testing. All tested films exhibited a brittle failure and achieved tensile strengths and modulus in the ~40-85 MPa and ~3.5-9 GPa ranges, respectively. To reveal stress transfer mechanisms in each sample, tensile in-situ Raman spectroscopy testing was carried out. By monitoring the changes in bandwidth and position of Raman bands while stretching the film, useful information such as sheet slippage and cross-linker interactions were gathered.more »The addition of CNC enhanced modulus but degraded strength for both GO and rGO films. Interestingly, the Raman G-peak shift at failure, indicative of stress transfer to individual GO/rGO particles, is commensurate with the films’ strengths. Correlating these results with the structure and composition of different films reveals new understanding of stress transfer between GO/rGO particles, paving the way for the scalable manufacturing of strong and stiff GO-based films.« less
  5. Engineering applications of current thermoset shape memory polymers are limited by three critical issues: demanding fabrication conditions (from 70 to 300 °C temperatures for hours or days), lack of reprocessability or recyclability, and low recovery stress and energy output. To address these problems simultaneously, a new UV curable and vitrimer-based epoxy thermoset shape memory polymer (VSMP) has been synthesized. A 1.1 mm thick VSMP film can be readily cured at room temperature under UV-irradiation (61 mW cm −2 ) in just 80 s. It possesses 36.7 MPa tensile strength, 230 MPa compressive strength, and 3120 MPa modulus at room temperature. It still has a compressive strength of 187 MPa at 120 °C. The covalent adaptable network (CAN) imparts the VSMP with recyclability, as reflected by two effective recycling cycles (>60% recycling efficiency). In addition, the VSMP exhibits good shape memory properties for multiple shape recovery cycles. With 20% compression programming strain, up to 13.4 MPa stable recovery stress and 1.05 MJ m −3 energy output in the rubbery state are achieved. With good mechanical strength, thermal stability, recyclability, and excellent shape memory properties combined with in situ UV-curing capabilities, the new VSMP is a promising multifunctional thermoset for engineering applications.