Processes of structural defect creation in pure and doped MgO and NaCl single crystals under condition of low or super high density of electronic excitations
Date
2013-02-07
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Abstract
Paljude teaduse, tehnoloogia ja energeetika valdkondade areng sõltub materjalide (k.a. laia keelutsooniga materjalid – LKM) kiirituskindluse parendamisest footonite, elektronide, neutronite ja ioonide kahjuliku mõju vastu. Doktoritöö jätkab kolme konkureeriva protsessi – luminestsentsi, struktuursete defektide tekkimise ja soojuse eraldumise – uurimist LKM kiiritamisel footonitega või osakestega, mis tekitavad erinevat elektroonsete ergastuste (EE) tihedust. Uurimistöö oli teostatud puhastel või erinevate lisanditega dopeeritud MgO ja NaCl monokristallidel, millel on sama tahkkeskendatud kuubiline kristallvõre ja lähedased keelutsooni laiused, kuid väga erinevad sulamistemperatuurid (NaCl – 801 °C, MgO – 2947 °C) ja EE struktuur. Oli näidatud, et väga madalatel temperatuuridel Frenkeli defektide tekkeenergia NaCl kristallis on keelutsooni laiusest suurem, nii et kiiritusdefektid moodustuvad ainult energeetiliste (“kuumade”) elektronide rekombineerumisel autolokaliseeritud aukudega. “Luminestsentskaitse” selle protsessi vastu on võimalik kristalli luminestseeruvate ioonide lisamise teel. Niisugused ioonid interakteeruvad kuumade elektronidega ja teisendavad osa nende energiast luminestsentsiks, seega konkureerides defektide tekkimise protsessiga. MgO monokristallid, kus ei eksitonid, ega elektronid või augud ei autolokaliseeru regulaarses võres, on väga kiirituskindlad röntgen- ja gamma-kiirte vastu. Kuid kiiritamisel tekkinud augud võivad autolokaliseeruda lisandioonide (berüllium, kaltsium, kroom) kõrval, mis suurendab defektide moodustumise efektiivsust selliste aukude ja kuumade elektronide rekombineerumisel. Näidati, et MgO kristalli kiiritamine raskete kulla või uraani ioonidega energiaga ligikaudu 2 GeV tekitab ülikõrget EE tihedust ja pika elueaga keerulisi struktuuridefekte. Lihtsad anioonsed defektid, bivakantsid ja keerulisemad defektide kompleksid olid uuritud, kasutades mitu erinevat spektroskoopia meetodit laias energiate ja temperatuuride vahemikus. Erilist tähelepanu osutati kroomiga dopeeritud MgO kristallidele, kuna kroomioon, sõltuvalt kontsentratsioonist, võib kas käituda luminestsentskaitsena või suurendada defektide teke efektiivsust.
The development of many fields of science, technology and energetics depends on the improvement of radiation resistance of materials (incl. wide-gap materials – WGM) against destructive influence of photons, electrons, neutrons, and ions. This thesis continues the research of three competing processes – luminescence, formation of structural defects, and heat release – at the irradiation of WGM with photons and particles, which provide various density of electronic excitations (EE). The research was conducted on pure and doped with different impurities MgO and NaCl single crystals, which have the same face-centered cubic structure and close values of a band gap, but very different melting points (NaCl – 801 °C, MgO – 2947 °C) and the structure of EE. It was shown, that in NaCl at very low temperatures, the energy of Frenkel defect formation is higher than the band gap, so the radiation defects are formed only at the recombination of energetic (“hot”) conduction electrons with self-trapped holes. The “luminescent protection” against such process is possible by introducing luminescent ions into the crystal. These ions interact with hot electrons and partly convert their energy to luminescence, therefore, competing with the process of defect creation. MgO single crystals, where neither excitons nor electrons and holes undergo self-trapping in a regular lattice, are highly resistant against X- and gamma-rays. However, the holes produced by irradiation may be localized near impurity ions (beryllium, calcium, chromium), which increases the efficiency of defect formation at hole recombination with hot electrons. It was demonstrated, that the irradiation of MgO with heavy gold or uranium ions with the energies about 2 GeV creates an extremely high density of EE and long-lived complex structural defects. Simple anion defects, bivacancies and more complex associates were investigated using several spectroscopy methods in a wide range of energies and temperatures. Special attention was given to MgO crystals doped with chromium ions, which, depending on concentration, can provide either luminescent protection or the increase of the defect formation efficiency.
The development of many fields of science, technology and energetics depends on the improvement of radiation resistance of materials (incl. wide-gap materials – WGM) against destructive influence of photons, electrons, neutrons, and ions. This thesis continues the research of three competing processes – luminescence, formation of structural defects, and heat release – at the irradiation of WGM with photons and particles, which provide various density of electronic excitations (EE). The research was conducted on pure and doped with different impurities MgO and NaCl single crystals, which have the same face-centered cubic structure and close values of a band gap, but very different melting points (NaCl – 801 °C, MgO – 2947 °C) and the structure of EE. It was shown, that in NaCl at very low temperatures, the energy of Frenkel defect formation is higher than the band gap, so the radiation defects are formed only at the recombination of energetic (“hot”) conduction electrons with self-trapped holes. The “luminescent protection” against such process is possible by introducing luminescent ions into the crystal. These ions interact with hot electrons and partly convert their energy to luminescence, therefore, competing with the process of defect creation. MgO single crystals, where neither excitons nor electrons and holes undergo self-trapping in a regular lattice, are highly resistant against X- and gamma-rays. However, the holes produced by irradiation may be localized near impurity ions (beryllium, calcium, chromium), which increases the efficiency of defect formation at hole recombination with hot electrons. It was demonstrated, that the irradiation of MgO with heavy gold or uranium ions with the energies about 2 GeV creates an extremely high density of EE and long-lived complex structural defects. Simple anion defects, bivacancies and more complex associates were investigated using several spectroscopy methods in a wide range of energies and temperatures. Special attention was given to MgO crystals doped with chromium ions, which, depending on concentration, can provide either luminescent protection or the increase of the defect formation efficiency.
Description
Väitekirja elektrooniline versioon ei sisalda publikatsioone.
Keywords
materjalifüüsika, monokristallid, kristallivõre defektid, elektronergastused, materials physics, monocrystals, crystal lattice defects, electronic excitations