Superionic Conductivity
738 Superionic conductivity
883 946 884 947 885 948 886 244Pu (98.6%) 949 detection 887 system 950 888 951 889 hydrogen 952 890 gas medium 953 891 954 18 48 beam dose892 of 5•10 Ca ions 955 893 956 894 projectiles 957 895 958 896 recoils 959 897 960 quadrupole focusing position-sensitive 898 frontal detector 961 beam veto detector 899 quadrupole 962 focusing 900 e-of-flight 963 4 m (13 ft) tim 901 detector 964 magnetic 902 965 deflection side 903rotating 966 detector 904 target 967 905 stop 968 906 evaporationresidues 969 907 start 970 908 971 909 972 Fig. 2. Example of a gas-filled separator used for producing superheavy elements: the Dubna Gas-Filled Recoil Separator 910 973 (DGFRS) at the Joint Institute for Nuclear Research. (JINR, Dubna, Russia) 911 974 912 975 913 for the heaviest nuclei. Position correlations make yettobeconfirmed. Another isotope of element 113 976 914 it possible to determine genetically related events produced in the cold-fusion reaction 70Zn + 209Bi was 977 915 in a decay chain occurring within the detector, and recently reported by a Japanese group at RIKEN, but 978 916 the times between such events can be measured to is also unconfirmed. See TRANSURANIUM ELEMENTS. 979 917 obtain lifetime or half-life information about the iso- Mark A. Stoyer 980 918 topes observed. Rapid identification and character- Bibliography. D. C. Hoffman, A. Ghiorso, and G. T. 981 919 ization of interesting coincidence events involving Seaborg, The Transuranium People: The inside 982 920 evaporation residues and alphaparticles sometimes Story, ICP, London, 2000; Yu. Ts. Oganessian, V. K. 983 921 allows the beam to be interrupted for a short time Utyonkov, and K. J. Moody, Voyage to the Super- 984 922 so that subsequent alpha decays or fissions occur in heavy Island, Sci. Amer., 282(1):45–49, January 2000; 985 923 the detectors under lower background conditions. M. Sch¨adel (ed.), The Chemistry of Superheavy Ele- 986 924 See PARTICLE DETECTOR. ments, Kluwer Academic, Dordrecht, 2003. 987 925 An example of the setup for a typical superheavy 988 926 experiment is shown in Fig. 2. 989 927 Because some of the more recently synthesized 990 928 superheavy elements have longer half-lives, experi- Superionic conductivity 991 929 ments to study the chemical properties of those ele- The electrical conductivity exhibited by a small 992 930 ments can be designed. Several experiments aimed group of solids with high ionic conductivity and neg- 993 931 at chemically identifying the decay products of some ligible electronic conductivity. In general, ionic con- 994 932 of the heaviest elements have observed evidence for ductivity is due to the motion of ions, whereas the 995 933 (1) dubnium (Db), a great great great granddaughter electronic conductivity results from the flow of elec- 996 934 of element 115, and (2) element 112, the daughter trons. For superionic conductors, also called fast ion 997 935 of element 114. conductors or solid electrolytes, the specific conduc- 998 936 Current status. Elements up to Z = 112 have been tivity (σ ) is usually within the range from about 10−3 999 937 confirmed. Once confirmed, element names are then to 10 siemens per centimeter. These values are very 1000 938 suggested by the scientists first synthesizing them. high for a crystalline ionic solid, but are still lower 1001 939 Scientists at GSI have been credited with discov- than many electronic conductors such as metals, 1002 940 ering elements 107–112. A name for element 112 which have typical values ranging from 10 to 105 S 1003 941 has not been proposed yet, but scientists at GSI cm−1. See CONDUCTANCE; CONDUCTION (ELECTRIC- 1004 942 have proposed roentgenium (chemical symbol Rg) ITY); ELECTRICAL CONDUCTIVITY OF METALS; ELEC- 1005 943 for element 111. Elements with Z = 113–116 and TROLYTIC CONDUCTANCE; SOLID-STATE CHEMISTRY. 1006 944 118 were synthesized by a team from Dubna and Since ionic conductivity increases with increasing 1007 945 Lawrence Livermore National Laboratory but have temperature, many superionic conductors (such as 1008 Superionic conductivity 739
1 64 ZrO2) exhibit high ionic conductivity only at tem- 2 peratures substantially higher than room tempera- 65 3 ture. High temperatures provide the thermal energy 66 4 needed to overcome the activation energy for ion 67 5 hopping (from site to site) and increase the num- 68 6 ber of defect sites needed for ion migration. For 69 7 70 some crystalline solids (such as Li2SO4 and AgI), high 8 temperatures lead to polymorphic phase transitions, 71 9 which cause an abrupt increase in the ionic conduc- 72 10 tivity. See IONIC CRYSTALS; POLYMORPHISM (CRYSTAL- 73 11 LOGRAPHY). 74 12 While ionic conductivity is a common property of 75 13 liquid electrolyte solutions or molten salts, a typical 76 14 ionic solid (such as NaCl) has negligible ionic con- 77 15 ductivity, often below 10−9 Scm−1. Special structural 78 16 features are required in order for a crystalline solid 79 17 to have superionic conductivity. One common fea- 80 18 ture is that such solids have a rigid framework with 81 19 open channels or layers along which ions can mi- 82 20 grate. Other factors that contribute to the high ionic 83 21 conductivity include small ion size, low ion charge, 84 22 low coordination number for mobile ions, high con- 85 23 centration of mobile ions, a large number of vacant 86 Fig. 1. Structural diagram of sodium β-alumina showing 24 sites, and high polarizability of anionic frameworks. open conduction layers. Color spheres are oxygen sites and 87 25 See CRYSTAL DEFECTS; CRYSTAL STRUCTURE; ION. dark gray spheres represent mobile Na+ ions. Aluminum 88 26 Superionic conductors. Superionic conductors can sites are located at the center of light gray polyhedra. 89 27 be classified according to the type of mobile ions, 90 28 the dimensionality of conduction pathways, or the named NASICON (from sodium superionic conduc- 91 29 structure type of the nonmobile portion of the crys- tor). It is a member in a family of materials with 92 30 93 tal structure. the general formula of Na1+xZr2(P3−xSix)O12 (0 < 31 Cationic conductors. Common cationic conductors x < 3), and its crystal structure contains channels 94 32 usually contain ions such as Ag+,Na+,Li+,orH+. within a three-dimensional framework built from 95 33 96 Silver iodide and its derivatives are among the earli- ZrO6 octahedra and (P,Si)O4 tetrahedra. See ION 34 est studied ionic conductors. At room temperature, EXCHANGE. 97 35 the Ag+ conductivity in AgI is low because I− ions Li+ superionic conductors are highly desirable for 98 36 adopt either hexagonal or cubic-close-packed struc- all-solid-state lithium batteries. However, few crys- 99 ◦ ◦ 37 ture. At temperatures above 146 C (295 F), AgI trans- talline Li+ compounds exhibit high ionic conduc- 100 38 forms into a polymorph in which I− ions adopt an tivity, particularly at room temperature. Some Li+ 101 39 102 open body-centered-cubic packed structure contain- conductors are based on structures of Li4SiO4 and 40 + 4+ + 103 ing disordered Ag ions and the conductivity of AgI Li4GeO4. Substitutions of Si and/or Li ions with 41 increases sharply to about 1 S cm−1. Many other sim- other cations such as P5+ or Zn2+ could lead to 104 42 ilar Ag+ conductors have been found. One of them, substantial improvement in conductivity. One com- 105 43 −1 106 RbAg4I5, has a specific conductivity of 0.26 S cm , pound, Li14ZnGe4O16, has a specific conductivity of ◦ ◦ 44 even at room temperature. about 10−1 Scm−1 at 300 C (572 F) and is named 107 + 45 Sodium β-alumina is a well-known Na conductor LISICON. See SOLID-STATE BATTERY. 108 46 whose discovery in 1960s led to an active field of re- Many proton (H+) conductors are based on hy- 109 47 search on ionic conductors. β-alumina was originally drated materials and are usable close to ambient 110 48 111 thought to be a polymorph of Al2O3, but it was later temperature. Proton conductors can be prepared 49 found to have complex compositions. Other oxides through ion exchange of sodium β-alumina with 112 50 + 113 suchasNa2O are also present to stabilize its crystal H3O . Other examples include hydrogen uranyl 51 β 114 structure. The general formula of sodium -alumina phosphate (HUO2PO4 · 4H2O) and some hydrated 52 · = 2− 115 is Na2O nAl2O3 (n 5–11). Closed-packed O lay- heteropolyacids such as H3(PMo12O40) · nH2O. High- 53 116 ers can adopt different stacking sequences, leading temperature proton conductors are based on SrCeO3 54 β β 117 to two structural variations labeled as and .In and BaCeO3 with the perovskite-type structure. After 55 both structures, 75% of the oxygen ions are miss- partial substitution of Ce4+ sites with trivalent ions, 118 56 ing within every fifth O2− layer, creating open layers suchasY3+, these oxides exhibit high H+ conductiv- 119 + 57 along which Na ions can migrate (Fig. 1). The con- ity in a moist atmosphere. See PEROVSKITE. 120 58 ductivity of sodium β-alumina is high even at room Anionic conductors. Common anionic conductors are 121 59 temperature and is on the order of 10−2 Scm−1.Na+ usually oxide (O2−)orfluoride (F−) conductors, such 122 60 123 ions can be readily exchanged with other cations as stabilized zirconias and PbF2, which require high 61 such as Li+,Cu+, and Ag+, giving rise to other types temperatures for fast ion conductivity. These con- 124 62 β 125 of ionic conductors. In addition to sodium -alumina, ductors usually have the fluorite (CaF2) type struc- 63 + 2− − 126 there exists another Na conductor (Na3Zr2PSi2O12) ture in which anions (O or F ) are arranged at cor- 740 Superluminal motion
127 are essential for the development of all-solid-state 190 128 electrochemical devices, which have many advan- 191 129 tages over those based on liquid electrolytes includ- 192 130 ing ease of miniaturization and high-temperature 193 131 stability. Oxygen gas sensors based on oxide con- 194 132 ductors are being widely used to monitor automo- 195 133 bile exhaust gases. Oxide conductors are also being 196 134 studied for the construction of solid-oxide fuel cells 197 135 (SOFC). Nafion and related sulfonated polymers are 198 136 under investigation as proton conductors in polymer 199 137 electrolyte membrane (PEM) fuel cells. See BATTERY; 200 138 ELECTROCHROMIC DEVICES; ENERGY STORAGE; FUEL 201 139 CELL; MICROSENSOR. Xianhui Bu; Pingyun Feng 202 140 Bibliography. G. Alberti and M. Casciola, Solid state 203 141 protonic conductors, present main applications and 204 142 future prospects, Solid State Ionics, 145:3–16, 2001; 205 143 P.G. Bruce (ed.), Solid State Electrochemistry, Cam- 206 144 207 Fig. 2. Diagram of the CaF2 type structure. Common anionic conductors, such as bridge University Press, 1995; P. Knauth and H. stabilized zirconia, adopt145 this structure type. Oxygen anions shown in color spheres form L. Tuller, Solid-state ionics: Roots, status, and fu- 208 the cubic arrangement. Not all oxygen sites are occupied in stabilized zirconia. Cations 146 209 such as Zr4+ occupy the center of every other cube represented by solid cubes. ture prospects, J. Amer. Ceram. Soc., 85:1654–1680, 147 2002; B. B. Owens, Solid-state electrolytes: Overview 210 148 of materials and applications during the last third 211 149 ners of a cube and cations occupy body centers of of the twentieth century, J. Power Sources, 90:2–8, 212 150 alternative cubes. Stabilized zirconias are solid solu- 2000; A. R. West, Basic Solid State Chemistry, Wiley, 213 151 214 tions between zirconia (ZrO2) and CaO or Y2O3. The New York, 1999. 152 215 doping of ZrO2 also helps to prevent phase transi- 153 tions to noncubic forms, when temperatures are low- 216 154 ered. The replacement of some Zr4+ sites with lower 217 155 charged cations (such as Ca2+ or Y3+) helps create Superluminal motion 218 156 vacant oxygen sites, which contribute to the high- Proper motion of an astronomical object apparently 219 157 temperature ionic conductivity (Fig. 2). See FLUO- exceeding the velocity of light, c. This phenomenon 220 158 RITE. is relatively common in the nuclei of quasars, many of 221 159 Amorphous conductors. In addition to crystalline in- which exhibit systematic changes in images of their 222 160 organic solids, superionic conductivity can also be radio-frequency emission over periods of months to 223 161 found in amorphous inorganic materials. In gen- years. In some cases, features in the image appear 224 162 eral, glassy electrolytes based on sulfides and halides to separate at a speed inferred to be more than 225 163 have much higher conductivity than oxides at am- 10 times the speed of light, given the great distance 226 164 bient temperatures. Many Li+ and Ag+ conducting of the quasars from Earth. 227 165 glasses are known. One Li+ conducting glass with the Superluminal motion was one of the most ex- 228 166 229 composition 0.7Li2S–0.3P2S5 has a conductivity of citing discoveries to emerge from a technique in ◦ ◦ 167 0.16 S cm−1 at 25 C (77 F). Even though oxide-based radio astronomy first developed in the late 1960s and 230 168 glasses have low conductivity, they are more stable called very long baseline interferometry (VLBI). This 231 169 than sulfides or halides at high temperatures. See method involves the tape recording of radio signals 232 170 AMORPHOUS SOLID. from large antennas at up to 10–15 locations across 233 171 Polymer-based conductors. Superionic conductivity is the Earth, and the combination of these signals in a 234 172 also known in polymer-based solids. These poly- computer to form a radio image of the quasar at ex- 235 173 mer electrolytes have two general types: polymer-salt tremely high resolution (less than 0.001 arcsecond). 236 174 complexes and polyelectrolytes. Polymer-salt com- A sequence of images of quasar 3C 279, one of the 237 175 plexes are generally made by dissolving a salt (such as best-studied examples, illustrates the phenomenon 238 176 239 LiClO4) in a polymer [such as poly(ethylene oxide)]. of superluminal motion (Fig. 1). See QUASAR; RADIO 177 Both cations and anions in polymer-salt electrolytes ASTRONOMY; RADIO TELESCOPE. 240 178 can be mobile. In polyelectrolytes, the polymer back- Objects involved. Superluminal motion is seen 241 179 bone contains covalently attached charged (positive mostly in quasars but also in some other active galac- 242 180 or negative) groups and the charge-balancing coun- tic nuclei. This rapid motion is confined to within 243 181 terions are able to make long-range migration. The a few tens of parsecs of the nucleus, whose power 244 182 best-known polyelectrolyte is Nafion perfluorocar- source is believed to be a massive black hole. Data on 245 183 bon sulfonic acid polymer, which is a proton con- variability are now available on more than 60 objects 246 184 ductor under wet conditions. of which at least 30 examples of superluminal mo- 247 185 Applications. Superionic conductors are an impor- tion are now known, and the list continues to grow. 248 186 tant group of materials that have large-scale tech- Most show apparent speeds less than 10c, but ex- 249 187 nological applications in areas such as energy stor- amples of speeds above 20c have been found. A few 250 188 age and generation (electrolyzers, batteries, and fuel quasars and galaxies with good data exhibit motion 251 189 cells), gas sensors, and electrochromic devices. They that is subluminal (that is, with speeds less than c). 252