Search

The technologically important frequency range for the application of electrostrictors and piezoelectrics is tens of Hz to tens of kHz. Sm3+- and Gd3+-doped ceria ceramics, excellent intermediate-temperature ion conductors, have been shown to exhibit very large electrostriction below 1 Hz. Why this is so is still not understood. While optimal design of ceria-based devices requires an in-depth understanding of their mechanical and electromechanical properties, systematic investigation of the influence of dopant size on frequency response is lacking. In this report, the mechanical and electromechanical properties of dense ceria ceramics doped with trivalent lanthanides (RE0.1Ce0.9O1.95, RE = Lu, Yb, Er, Gd, Sm, and Nd) were investigated. Young's, shear, and bulk moduli were obtained from ultrasound pulse echo measurements. Nanoindentation measurements revealed room-temperature creep in all samples as well as the dependence of Young's modulus on the unloading rate. Both are evidence for viscoelastic behavior, in this case anelasticity. For all samples, within the frequency range f = 0.15-150 Hz and electric field E ≤ 0.7 MV/m, the longitudinal electrostriction strain coefficient (|M33|) was 102 to 104-fold larger than expected for classical (Newnham) electrostrictors. However, electrostrictive strain in Er-, Gd-, Sm-, and Nd-doped ceramics exhibited marked frequency relaxation, with the Debye-type characteristic relaxation time τ ≤ 1 s, while for the smallest dopants-Lu and Yb-little change in electrostrictive strain was detected over the complete frequency range studied. We find that only the small, less-studied dopants continue to produce useable electrostrictive strain at the higher frequencies. We suggest that this striking difference in frequency response may be explained by postulating that introduction of a dopant induces two types of polarizable elastic dipoles and that the dopant size determines which of the two will be dominant.

Electromechanically active ceramic materials, piezoelectrics and electrostrictors, provide the backbone of a variety of consumer technologies. Gd- and Sm-doped ceria are ion conducting ceramics, finding application in fuel cells, oxygen sensors, and, potentially, as memristor materials. While optimal design of ceria-based devices requires a thorough understanding of their mechanical and electromechanical properties, reports of systematic study of the effect of dopant concentration on the electromechanical behavior of ceria-based ceramics are lacking. Here we report the longitudinal electrostriction strain coefficient (M33) of dense RExCe1-xO2-x/2 (x ≤ 0.25) ceramic pellets, where RE = Gd or Sm, measured under ambient conditions as a function of dopant concentration within the frequency range f = 0.15-350 Hz and electric field amplitude E ≤ 0.5 MV/m. For >100 Hz, all ceramic pellets tested, independent of dopant concentration, exhibit longitudinal electrostriction strain coefficient with magnitude on the order of 10-18 m2/V2. The quasi-static (f < 1 Hz) electrostriction strain coefficient for undoped ceria is comparable in magnitude, while introducing 5 mol % Gd or 5 mol % Sm produces an increase in M33 by up to 2 orders of magnitude. For x ≤ 0.1 (Gd)-0.15 (Sm), the Debye-type relaxation time constant (τ) is in the range 60-300 ms. The inverse relationship between dopant concentration and quasi-static electrostrictive strain parallels the anelasticity and ionic conductivity of Gd- and Sm-doped ceria ceramics, indicating that electrostriction is partially governed by ordering of vacancies and changes in local symmetry.

In recent years, interface engineering of solid electrolytes has been explored to increase their ionic conductivity and improve the performance of solid oxide fuel cells and other electrochemical power sources. It has been observed that the ionic conductivity of epitaxially grown thin films of some electrolytes is dramatically enhanced, which is often attributed to effects (e.g. strain-induced mobility changes) at the heterophase boundary with the substrate. Still largely unexplored is the possibility of manipulation of grain boundary resistivity in polycrystalline solid electrolyte films, clearly a limiting factor in their ionic conductivity. Here we report that the ionic conductivity of yttria stabilized zirconia thin films with nano-columnar grains grown on a MgO substrate nearly reaches that of the corresponding single crystal when the thickness of the films becomes less than roughly 8 nm (smaller by a factor of three at 500 °C). Using impedance spectroscopy, the grain boundary resistivity was probed as a function of film thickness. The resistivity of the grain boundaries near the film-substrate interface and film surface (within 4 nm of each) was almost entirely eliminated. This minimization of grain boundary resistivity is attributed to Mg(2+) diffusion from the MgO substrate into the YSZ grain boundaries, which is supported by time of flight secondary ion mass spectroscopy measurements. We suggest grain boundary "design" as an attractive method to obtain highly conductive solid electrolyte thin films.

Abstract Automated database search engines are one of the fundamental engines of high-throughput proteomics enabling daily identifications of hundreds of thousands of peptides and proteins from tandem mass (MS/MS) spectrometry data. Nevertheless, this automation also makes it humanly impossible to manually validate the vast lists of resulting identifications from such high-throughput searches. This challenge is usually addressed by using a Target-Decoy Approach (TDA) to impose an empirical False Discovery Rate (FDR) at a pre-determined threshold x% with the expectation that at most x% of the returned identifications would be false positives. But despite the fundamental importance of FDR estimates in ensuring the utility of large lists of identifications, there is surprisingly little consensus on exactly how TDA should be applied to minimize the chances of biased FDR estimates. In fact, since less rigorous TDA/FDR estimates tend to result in more identifications (at higher 'true' FDR), there is often little incentive to enforce strict TDA/FDR procedures in studies where the major metric of success is the size of the list of identifications and there are no follow up studies imposing hard cost constraints on the number of reported false positives. Here we address the problem of the accuracy of TDA estimates of empirical FDR. Using MS/MS spectra from samples where we were able to define a factual FDR estimator of 'true' FDR we evaluate several popular variants of the TDA procedure in a variety of database search contexts. We show that the fraction of false identifications can sometimes be over 10× higher than reported and may be unavoidably high for certain types of searches. In addition, we further report that the two-pass search strategy seems the most promising database search strategy. While unavoidably constrained by the particulars of any specific evaluation dataset, our observations support a series of recommendations towards maximizing the number of resulting identifications while controlling database searches with robust and reproducible TDA estimation of empirical FDR.