Nanoscale Stoichiometric Analysis of a High-Temperature Superconductor 415
Examples are the reduction of samples in a hydrogen flow (Graf et al., 1990), iodometric titration (Benzi et al., 2004), or spectrophotometry—which involves dissolution of YBCO in acid solvents then wavelength absorbance measurements (Nedeltcheva, 1995; Nedeltcheva&Vladimirova, 2001). These methods provide an average value of oxygen content over large sample volumes, giving neither an indication of localized segregation effects nor the distribution of point defects, and are likely insufficient for property predictions unless the sample is microscopically and macroscopically uniform. Localized, nanoscale transmission electron microscopy
The present study makes use of all these advances, along
with a systematic study of experimental conditions to provide accuratemeasurements of 3Dnanoscale oxygen stoichiometry on bulk samples of the perovskite superconductor YBCO, and discusses the wider implications of this work for experiments on a larger range of functional oxide materials.
EXPERIMENTAL METHODS
(TEM)-based observations have always been significantly challenging due to the low scattering factor of O atoms. In 2003, there was a breakthrough by Jia et al. (2003) who used aberration corrected TEM on YBCO and STO samples to observe the individual atoms in each lattice site, including oxygen. In recent years, developments in instrumentation and analytical techniques have made high resolution transmission electronmicroscopy studiesmore common on YBCOsamples (Suvorova et al., 2014). Such measurements allow oxygen col- umns to be resolved on an atomic scale in two-dimensional (2D) projections down various zone axes, but oxygen vacancies remain hard to detect within columns of oxygen atoms. One of the only techniques which, so far, has the
capability to detect nanosegregation in3Dand local variations in oxygen content in YBCO is atom probe tomography (APT). APT is based on the concept of field evaporation, and combines time-of-flight mass spectrometry with a position sensitive detector to reconstruct a 3D image of samples with near-atomic resolution. Previous studies can be found in the scientific literature pertaining to nanoscale analysis of YBCO through field emission techniques (Kellogg & Brenner, 1987; Cerezo et al., 1988; Melmed et al., 1988; Nishikawa & Nagai, 1988). These studies were undertaken in the 1980s using either a 1D atom probe or a field ion microscope (FIM). FIM was used to attempt superconductivity measurements by comparing the evidence of preferential evaporation in super- conducting samples compared with insulating samples at 50K (Melmed et al., 1988). Localized chemical composition measurements were restricted to very small 1D analysis volumes. The detected compositions were lower than the nominal oxygen content of the fully oxygenated perovskite, the closest measurement being deficient by >3 at% O (Cerezo et al., 1988). More recent developments in APT instrumenta- tion and analytical tools include reflectron lenses (Clifton et al., 2008) that allow for substantial improvements in the precision of chemical analysis, the implementation of laser pulsing (Bunton et al., 2007; Cerezo et al., 2007; Deconihout et al., 2007) and micro-electrodes (Kelly & Larson, 2000), which allow the analysis of a much greater sample volume from a wider range of compound materials and the use of a focused ion beam (FIB) microscope for sample preparation (Kelly & Larson, 2000; Thompson et al., 2007), which enables the site-specific analysis of individual selected features. In addition, sophisticated post-acquisition analytical tools such as multiple hit correlation histograms (Saxey, 2011) can now be used to correct for aberrations and artifacts.
Single crystal YBCO samples, provided by the bulk super- conductivity group in the Cambridge University Engineering Department, were produced by the Top Seeded Melt Growth (TSMG) process (Babu et al., 2012). Powders of the required compositions were pressed into a compact pellet, followed by partialmelting and peritectic regrowth of the Y1Ba2Cu3O7 −δ phase (Y-123). A chemically stable GdBa2Cu3O7 seed was used as a heterogeneous nucleation site for epitaxial growth of a large Y-123 single crystal. An excess of 20–30% of Y2Ba1Cu1O5 phase (Y-211) particles was included in the starting powder as inclusions to produce pinning sites for magnetic flux lines to enable superconductivity (Murakami et al., 1991). 1wt% of CeO2 was also added to refine the Y-211 particles during the melt process, which is known to result in a significant increase in flux pinning strength and consequent improvement in superconducting properties (Kim et al., 1992; Pinol et al., 1994). The mechanism responsible for this refining effect is not fully understood, but may involve the solid-state formation of Y2O3 nano- particles that act as nucleation sites for small Y-211 particles (Vilalta et al., 1997). Superconductivity temperature measurements were
performed using a Quantum Design SQUID (Super- conducting Quantum Interference Device), oriented with the applied field B//<a> axis. Measurements were performed at constant field, varying the temperature and measuring the corresponding variation in magnetic moment. Bulk samples were cut from these crystals along the
crystallographic<a> axis (the unit cell is shown in Fig. 1a) and polished to a 1μm diamond finish using ethanol as a lubricant to avoid decomposition of the YBCO. Secondary electron micrographs were acquired using a Zeiss NVision 40 dual beam scanning electron microscope (Carl Zeiss Ltd., Cambridge, Cambridgeshire, UK)—focused ion beam (SEM-FIB). Site-specific samples were prepared for APT analysis using the FIBlift-outtechniquefromaregion(Thompson et al., 2007) containing both Y-123 matrix and Y-211 particles. Triangular sections were mounted onto Cameca silicon sample holders (coupons) forAPTanalysis and sharpened until the tip diameter was <100nm. Gallium damage was minimized by reducing the voltage from 30 to 5kV for the final stages of polishing. A Cameca LEAP 3000X HR (Cameca SAS, Gennevillers
Cedex, France) was used for atom probe analysis. Samples were laser pulse evaporated, with a pulse frequency of 160 kHz, at a sample temperature of 50K. The choice of laser energy is well known to influence the analyzed stoichiometry of nonmetallic samples such as oxides
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57 |
Page 58 |
Page 59 |
Page 60 |
Page 61 |
Page 62 |
Page 63 |
Page 64 |
Page 65 |
Page 66 |
Page 67 |
Page 68 |
Page 69 |
Page 70 |
Page 71 |
Page 72 |
Page 73 |
Page 74 |
Page 75 |
Page 76 |
Page 77 |
Page 78 |
Page 79 |
Page 80 |
Page 81 |
Page 82 |
Page 83 |
Page 84 |
Page 85 |
Page 86 |
Page 87 |
Page 88 |
Page 89 |
Page 90 |
Page 91 |
Page 92 |
Page 93 |
Page 94 |
Page 95 |
Page 96 |
Page 97 |
Page 98 |
Page 99 |
Page 100 |
Page 101 |
Page 102 |
Page 103 |
Page 104 |
Page 105 |
Page 106 |
Page 107 |
Page 108 |
Page 109 |
Page 110 |
Page 111 |
Page 112 |
Page 113 |
Page 114 |
Page 115 |
Page 116 |
Page 117 |
Page 118 |
Page 119 |
Page 120 |
Page 121 |
Page 122 |
Page 123 |
Page 124 |
Page 125 |
Page 126 |
Page 127 |
Page 128 |
Page 129 |
Page 130 |
Page 131 |
Page 132 |
Page 133 |
Page 134 |
Page 135 |
Page 136 |
Page 137 |
Page 138 |
Page 139 |
Page 140 |
Page 141 |
Page 142 |
Page 143 |
Page 144 |
Page 145 |
Page 146 |
Page 147 |
Page 148 |
Page 149 |
Page 150 |
Page 151 |
Page 152 |
Page 153 |
Page 154 |
Page 155 |
Page 156 |
Page 157 |
Page 158 |
Page 159 |
Page 160 |
Page 161 |
Page 162 |
Page 163 |
Page 164 |
Page 165 |
Page 166 |
Page 167 |
Page 168 |
Page 169 |
Page 170 |
Page 171 |
Page 172 |
Page 173 |
Page 174 |
Page 175 |
Page 176 |
Page 177 |
Page 178 |
Page 179 |
Page 180 |
Page 181 |
Page 182 |
Page 183 |
Page 184 |
Page 185 |
Page 186 |
Page 187 |
Page 188 |
Page 189 |
Page 190 |
Page 191 |
Page 192 |
Page 193 |
Page 194 |
Page 195 |
Page 196 |
Page 197 |
Page 198 |
Page 199 |
Page 200 |
Page 201 |
Page 202 |
Page 203 |
Page 204 |
Page 205 |
Page 206 |
Page 207 |
Page 208 |
Page 209 |
Page 210 |
Page 211 |
Page 212 |
Page 213 |
Page 214 |
Page 215 |
Page 216 |
Page 217 |
Page 218 |
Page 219 |
Page 220 |
Page 221 |
Page 222 |
Page 223 |
Page 224 |
Page 225 |
Page 226 |
Page 227 |
Page 228 |
Page 229 |
Page 230 |
Page 231 |
Page 232 |
Page 233 |
Page 234 |
Page 235 |
Page 236 |
Page 237 |
Page 238 |
Page 239 |
Page 240 |
Page 241 |
Page 242 |
Page 243 |
Page 244 |
Page 245 |
Page 246 |
Page 247 |
Page 248 |
Page 249 |
Page 250 |
Page 251 |
Page 252 |
Page 253 |
Page 254 |
Page 255 |
Page 256 |
Page 257 |
Page 258 |
Page 259 |
Page 260 |
Page 261 |
Page 262 |
Page 263 |
Page 264 |
Page 265 |
Page 266 |
Page 267 |
Page 268 |
Page 269 |
Page 270 |
Page 271 |
Page 272 |
Page 273 |
Page 274 |
Page 275 |
Page 276 |
Page 277 |
Page 278 |
Page 279 |
Page 280 |
Page 281 |
Page 282 |
Page 283 |
Page 284
