xov xwm

Taw qhia
Cristobalite yog qhov tsis tshua muaj siab SiO2 homomorphous variant, thiab nws cov thermodynamic stability ntau yog 1470 ℃ ~ 1728 ℃ (nyob rau hauv ib txwm siab). β Cristobalite yog nws cov theem kub kub, tab sis nws tuaj yeem khaws cia rau hauv daim ntawv metastable mus rau qhov kub qis heev kom txog rau thaum kev hloov pauv hom hloov pauv tshwm sim ntawm 250 ℃ α Cristobalite. Txawm hais tias cristobalite tuaj yeem crystallized los ntawm SiO2 yaj hauv nws thaj chaw thermodynamic stability, feem ntau cristobalite nyob rau hauv cov xwm txheej yog tsim nyob rau hauv metastable tej yam kev mob. Piv txwv li, diatomite hloov mus rau cristobalite chert los yog microcrystalline opal (opal CT, opal C) thaum lub sij hawm diagenesis, thiab lawv lub ntsiab mineral theem yog α Cristobalite), uas nws hloov kub nyob rau hauv lub ruaj khov cheeb tsam ntawm quartz; Nyob rau hauv cov xwm txheej ntawm granulite facies metamorphism, cristobalite precipitated los ntawm cov nplua nuj Na Al Si yaj, muaj nyob rau hauv garnet raws li ib tug inclusion thiab coexisted nrog albite, tsim ib tug kub thiab siab mob ntawm 800 ℃, 01GPa, kuj nyob rau hauv lub ruaj khov cheeb tsam ntawm quartz. Tsis tas li ntawd, metastable cristobalite kuj tseem tsim nyob rau hauv ntau cov ntaub ntawv uas tsis yog xim hlau thaum lub sij hawm kho cua sov, thiab qhov tsim kub yog nyob rau hauv lub thermodynamic stability ntawm tridymite.
Formative mechanism
Diatomite hloov mus ua cristobalite ntawm 900 ℃ ~ 1300 ℃; Opal hloov mus ua cristobalite ntawm 1200 ℃; Quartz kuj tsim nyob rau hauv kaolinite ntawm 1260 ℃; Cov hluavtaws hluavtaws MCM-41 mesoporous SiO2 molecular sieve tau hloov mus ua cristobalite ntawm 1000 ℃. Metastable cristobalite kuj tsim nyob rau hauv lwm yam txheej txheem xws li ceramic sintering thiab mullite npaj. Rau kev piav qhia ntawm lub metastable tsim mechanism ntawm cristobalite, nws tau pom zoo tias nws yog ib tug tsis-equilibrium thermodynamic txheej txheem, mas tswj los ntawm cov tshuaj tiv thaiv kinetics mechanism. Raws li cov metastable tsim hom ntawm cristobalite tau hais los saum toj no, nws yuav luag ib qho kev ntseeg tias cristobalite hloov los ntawm amorphous SiO2, txawm nyob rau hauv cov txheej txheem ntawm kaolinite tshav kub kho, mullite npaj thiab ceramic sintering, cristobalite kuj hloov los ntawm amorphous SiO2.
Lub hom phiaj
Txij li thaum muaj kev lag luam hauv xyoo 1940s, cov khoom siv roj carbon dub tau siv dav los ua cov neeg ua haujlwm txhawb zog hauv cov khoom siv roj hmab. Tsis tas li ntawd, lawv tseem tuaj yeem siv rau hauv kev lag luam tshuaj, tshuaj tua kab, number case, xim, xim, tshuaj txhuam hniav, ntawv, zaub mov, pub, tshuaj pleev ib ce, roj teeb thiab lwm yam lag luam.
Cov tshuaj formula ntawm dawb carbon dub nyob rau hauv txoj kev tsim yog SiO2nH2O. Vim tias nws siv zoo ib yam li cov pa roj carbon dub thiab yog dawb, nws yog hu ua dawb carbon dub. Raws li kev tsim khoom sib txawv, cov pa roj carbon dub tuaj yeem muab faib ua cov pa roj carbon dub (precipitated hydrated silica) thiab fumed dawb carbon dub (fumed silica). Ob yam khoom muaj ntau txoj kev tsim, khoom thiab siv. Txoj kev roj theem feem ntau yog siv silicon tetrachloride thiab silicon dioxide tau los ntawm huab cua combustion. Cov khoom yog zoo, thiab qhov nruab nrab particle loj tuaj yeem tsawg dua 5 microns. Txoj kev los nag yog los nag silica los ntawm kev ntxiv sulfuric acid rau sodium silicate. Qhov nruab nrab particle loj yog li 7-12 microns. Cov fumed silica yog kim thiab tsis yooj yim rau nqus dej noo, yog li nws feem ntau siv los ua tus neeg sawv cev matting hauv txheej txheej.
Cov iav dej ntawm txoj kev nitric acid reacts nrog nitric acid los tsim silicon dioxide, uas yog tom qab ntawd npaj rau hauv hluav taws xob qib silicon dioxide los ntawm kev yaug, pickling, deionized dej yaug thiab lub cev qhuav dej.