Noncrystal ionic model for silica glass
1957; US government; Volume: 59; Issue: 2 Linguagem: Inglês
10.6028/jres.059.015
ISSN2376-5305
Autores Tópico(s)Material Science and Thermodynamics
ResumoA m odel was co nstru cted of p entagon al rin gs o f sy mm et ric al tet ra hedra so co nn ected t ha t all fiy e silico ns li e in one plane and all angles Si-O-Si app rox imate 180°.From an ini tial tet rahedro n t he rings extend in six direction s to include its six edges and in t he r es ul t in g t hree-dim ensional network ea ch rin g forms a commo n in te rface b e t,~ee n d od eca • hedral cages w hi ch necessar il}: ha ve fiv efold symmetry , Such sy mmetry is in co mpat ible WI th formatJOll of a cryst al lattIC e and t hu s t he stru cture can be exte nded in t hree dimensio ns only to a very limi ted deg ree and wi th unavoidable stress and distor tional s t rain , A stressed net work clu ste r of s uch dodecahedral cages is here called a Vl'I' R ON ,If t he scale in t his m od el for silica gla ss is t aken a s 1.6 A, fr om s ilico n t o oxyge n t he model is found to be in acco rd wi t h othe r radia l di stan ces compu ted fro m diffractio n 'data a nd t he d ens ity is correct wi t hin 10 p ercent .The apertures o f t he p entagonal in terfaces a re of s ui table size to explain data on the diff usion of noble ga ses t hrough sili ca , Sha red pent a fa ces a nd in termi ttent oxyge n b ridges connect neig hb oring vi t ron s a nd co ns t it ute a n in t erst itial t iss ue of r ela tively wea kened st ru ctu re t ha t a cco un ts for t he obser ved low te nsil e st rengths of s ili ca glass a nd p rov ides cha nn els fo r viscous fio\\', Th e prop osed cages Il'o ul d be less plia bl e at low te mperat llres a nd so t he in terst it ia l t iss ues betwee n " it rons s houl d expa nd in I'olume on coo li ng, This " negative" ex pa nsion wo uld cO ll nte r t he norma l ex pa nsio n a nd acco un t fo r t he know n ext remely s ma ll (ne t) e x p a n s i vit ~' o f sili ca glass , The acco mpan y in g chan ges i n st re ngth a nd volum e of t he gla ss at its \I'eak es t places, t he i nte rst it ial t iss ues, ca n acco un t for a number of kn own " a nomalies" i n t herma l be havior of s ilica glass \I' it h re,pecL to i ts volume, co mpressibi lity, clast ic mod uli , a nd l'isco us fl ol\'.. The v itron co ncep L r eco ncil es t he crystaJli te an d t he network theories of glass by p rop o s in ~ llu clei t ha t cannot g rOll' extell sil'ely and a dist ri b u t io n of locali zed st resses t hat m a y co nstit u te a n a ccep tabl e degree o f randomn ess a nd prov ide effective modul ations in co nt inui ty, This co ncept o f d e fi nite bu t limi ted microreg ulal'ity i n stru cture s u g~ests t hat other noncr,vs tal s Y lllmet ri e~ sho uld be s tud ied for p oss ibl e inLe res L in ot her fi eld s s uc h a s li q u ids a nd hi g h p ol,v mers, ------.-~
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