Rast kristala natrijum hlorata u blago presićenim vodenim rastvorima

Abstract

U ovoj doktorskoj disertaciji su predstavljeni rezultati proučavanja ponašanja kristala natrijum hlorata u blago presićenim rastvorima. Eksperimentalni dio rada je izveden u Laboratoriji za rast kristala na Fizičkom fakultetu u Beogradu. Izvedena su dva tipa eksperimenata. Proučavano je ponašanje kristala natrijum hlorata u vremenski dugim eksperimentima, gdje su posmatrane promjene u brzinama rasta kristala, nezavisno o makroskopskim spoljašnjim uslovima. Izučavani su i diskutovani uzroci postojanja disperzije brzina rasta malih kristala natrijum hlorata, kao i njihova vremenska evolucija. Radi detaljnog proučavanja pojave disprzije brzina rasta kristala i njihove vremenske evolucije u vremenski dugim eksperimentima uvedeni su pojmovi početne, intermedijalne i stabilisane brzine rasta kristala. Uočeno je da većina proučavanih kristala doživljava značajne promjene u brzinama u prvih nekoliko sati rasta. Izdvojeno je pet tipičnih promjena u brzinama rasta kristala: 1) smanjivanje u početnom periodu rasta od 2 - 4 h, zadržavanje dostignute vrijednost (u granicama greške mjerenja) do kraja eksperimenata. 2) smanivanje velikih početnih brzina rasta u početnom periodu od 2 - 4 h do intermedijalnih vrijednosti i dostizanje, nakon nekoliko narednih časova, stabilisane vrijednosti brzine. 3) brzine koje se nisu mjenjale tokom cijelog eksperimenta (u granicama greške mjerenja). 4) porast brzina u prvih nekoliko sati eksperimenta do vrijednosti koju su zadržale do kraja. 5) prestanak rasta kristala bilo u nekom od posmatranih pravaca, bilo u svim pravcima. Da bi se opisala disperzija brzina rasta, rezultati su fitovani višestrukom normalnom raspodjelom. Statistički podaci omogućili su dobro definisanje nekoliko maksimuma distribucija početnih, intermedijalih i stabilisanih brzina rasta, po redu. Predloženi su i diskutovani mogući uzroci promjena brzinama rasta proučavanih kristala i to smanjivanje mas - transfer koeficijenta, prisustvo primjesa, prelaz dominantne dislokacione grupe sa navećom aktivnosti sa jedne na drugu pljosan, promjene na površini kristala, naprezanje kristalne rešetke. U drugom tipu eksperimenata proučavano je ponašanje kristala natrijum hlorata raslih iz vodenih rastvora na temperaturama bliskim temperaturi zasićenja rastvora. Uočeno je postojanje tri tipa kristala: kristali koji su rasli, kristali koji nisu rasli (brzina rasta 0 nm/s u granicama greške mjerenja) i kristali koji su se rastvarali. Detaljno su analizirani mogući uzroci za pojavu koegzistencije kristala tokom cijelog perioda rasta, na presićenjima nižim od 0,18 %. Istraživanja su bila usmjerena na kristale koji su se rastvarali u presićenim rastvorima. Analize rezultata su pokazale da postojanje kristala koji se rastvaraju u presićenim vodenim rastvorima ne zavisi od: 1) položaja kristala u ćeliji za kristalizaciju, 2) orijentacije kristala u odnosu na pravac proticanja rastvora kroz ćeliju, 3) međusobnu udaljenost kristala i 4) veličine i debljine kristala. Predloženi su i diskutovani mogući razlozi prisustva kristala natrijum hlorata koji se rastvaraju u presićenim vodenim rastvorima i to: Ostvald ripening, makroskopski uslovi u ćeliji za kristalizaciju, mikrostruktura ratućih pljosni kristala, naprezanje kristalne rešetke, Gibs - Tomsonov efekat i prisustvo primjesa. Analiza eksperimentalnih rezultata predstavljenih u disertaciji doprinosi objašnjavanju proučavanih pojava u fizici rasta kristala.

In this thesis the results of investigation of sodium chlorate crystals behavior in slightly supersaturated solutions will be presented. Experimental part of investigations was performed in Laboratory for crystal growth on Faculty of physics, University of Belgrade. Two types of experiments were performed. Behavior of sodium chlorate crystals in long time experiments in which growth rate changes inherent to crystals, independent of macroscopic external conditions, was investigated. The results of growth rate dispersions of small sodium chlorate crystals as well as their evolution in time were also discussed. For detailed study of phenomena of crystal growth rate dispersion and their time evolution in long - term experiments were introduced in terms of initial, intermedial and stabilized crystal growth rates. It was noticed that most of the studied crystals exhibited significant changes in their growth rates in the first few hours of growth. Five typical changes in crystal growth rate were noticed: 1) Measured growth rates decreased during the initial period of growth for 2 - 4 h. These growth rates did not change until the end of the experiments (within the measurement error limits). 2) Very high initial growth rates decreased and then, after 2 - 4 h, reached relatively high intermediate growth rates that were retained for the following few hours. After that, they reached stabilized growth rates. 3) Growth rates did not change during the whole growth run (within the measurement error limits). 4) Increase of growth rates during the first couple of hours of the experiment and reached the values which were retained until the end of the growth. 5) Crystals did not grow at all, or in some of the observed directions. To describe growth rate dispersion, results were fitted with finite mixture of normal distributions. Statistics enabled well definition of several distribution maxima of initial, intermediate and stabilized growth rates, respectively. In this thesis are proposed and discussed possible causes of changes in growth rates of observed crystals such as: changes in mass – transfer coefficient, presense of impurities, transition of the dominant dislocation source from one face to the neighboring one, changes on crystal surface and lattice strain. Behavior of small sodium chlorate crystals grown from aqueos solution on the temperatures near the saturation temperature of the solution was observed in the second type of experiments. Three types of crystals were noticed: growing, non - growing (growth rate 0 nm/s within the measurement error limits) and dissolving crystals. Possible reasons for simultaneous growth and dissolution of crystals on supersaturation below 0,18% are discussed in detail. The investigations were focused on crystals that dissolve in supersaturated solution. After the results analysis, the existence of dissolving crystals in a supersaturated solution is independent of: 1) the crystal position in the crystallization chamber, 2) the crystal orientation with respect to the solution flow direction, 3) the distance between the closest neighbors, 4) the crystal size and thickness Possible reasons for coexistence of growing and dissolving crystals, such as an Ostwald ripening, microscopic conditions in the crystallization chamber, microstructure of growing face,overall crystal lattice strain, Gibbs - Thompson effect and the presence of impurities, are discussed. Analysis of the experimental results presented in this dissertation contributes to explaining the phenomena studied in the physics of crystal growth.