Search

The paper presents nonlinear analysis of square CFT columns using distributed plasticity fiber elements. Behaviour of CFT columns is nonlinear and it is necessary to include different nonlinear effects into the numerical model in order to simulate their behaviour properly. Model proposed in this work considers: nonlinear behaviour of concrete and steel using nonlinear stress-strain models, confinement effect and geometrical nonlinearities. Tests exposed to static loading with different slenderness, different D/t ratio (where D is the total dimension of a cross section and t is the thickness of steel tube) and different loading conditions are analyzed. Stress-strain models that best approximate the behaviour of CFT columns are determined from a detailed parametric study. The proposed numerical model is validated by comparing numerical with experimental results available in the literature. U radu je prikazana nelinearna analiza kvadratnih CFT stubova, pomoću fiber konačnog elemenata raspodeljene plastičnosti. CFT stubove karakteriše nelinearno ponašanje, što je potrebno imati u vidu u numeričkom modelu. Model predložen u ovom radu uzima u obzir sledeće nelinearne uticaje: nelinearno ponašanje betona i čelika primenom odgovarajućih konstitutivnih relacija, efekat utezanja betona i geometrijske nelinearnosti. Analizirani su uzorci izloženi delovanju statičkog opterećenja koji imaju različitu vitkost, odnos D/t (odnos ukupne dimenzije čeličnog profila [D] i debljine čeličnog profila [t]), kao i uzorci izloženi različitim tipovima naprezanja. Na osnovu parametarske analize, određeni su modeli materijala za čelik i beton, kao i njihovi parametri koji najbolje aproksimiraju ponašanje CFT stubova. Tačnost modela proverena je zatim i upoređivanjem sa eksperimentalnim podacima dostupnim u literaturi.

The nucleation of K2TiGe3O9 crystals in an undercooled melt of stoichiometric composition was studied. Volume nucleation with spherical morphology of the growing crystals was determined. The nucleation range is in the temperature interval 540-630 degrees C. A maximum of the steady state nucleation rate of I-max = 1.4 x 10(15) m(-3).s(-1) was determined at the temperature T-n(max) = 582 degrees C. The temperature and time dependence of the nucleation rate was analyzed and it was shown that such a nucleation behavior was the result of a low thermodynamic and kinetic barrier. The results showed that the temperature intervals of nucleation and crystal growth of this undercooled melt partly overlapped.

The crystallization of K2O center dot TiO2 center dot 3GeO(2) glass under non-isothermal condition was studied. In powdered glass with particle sizes less than 0.15 mm, surface crystallization was dominant and an activation energy of crystal growth of E-a,E-s = 327 +/- 50 kJ mol(-1) was calculated. In the size range 0.15 to 0.45 mm, both surface and volume crystallization occurred. For particle sizes gt 0.45 mm, volume crystallization dominated with spherulitic morphology of the crystals growth and E-a,E-v = 359 +/- 64 kJ mol(-1) was calculated.

The crystallisation behaviour of calcium phosphate glass containing vanadium oxide was studied. Glass with a molar ratio of [CaO]/[P2O5] = 1.13 and a V2O5 content of 8 mol.% was prepared for the experiments. The crystallisation of bulk glass samples was performed tinder isothermal conditions at T = 650-820 degrees C. The surface crystallisation mechanism of studied glass with dendritic growth of the primary alpha-Ca2P2O7 phase was observed. In all crystallised samples, beta-Ca2P2O7 and VP2O7 appeared as the secondary phases. In the investigated temperature range, crystal growth rates 9 x 10(-9) - 2.14 x 10(-6) m/s were determined. The temperature dependence of the crystal growth rate makes possible to calculate activation energy E-a = 407 +/- 30 kJ/mol of crystallization. The experimental data were compared with those estimated by using a theoretical model of the crystal growth rate and good agreement between the results was found.

Structure and crystallization of K 2 O·TiO 2 ·3GeO 2 glass have been investigated by means of Fourier transform infrared spectroscopy (FTIR), differential thermal analysis (DTA), differential scanning calorimetry (DSC), x-ray diffraction analysis (XRD), and scanning electron microscopy (SEM). XRD analysis of crystallized glass reveals the presence of crystalline K 2 O TiO 2 3GeO 2 indicating the same composition of crystalline and parent glass. For glass particle size > 0.15 mm volume crystallization is dominant crystallization mechanism. SEM investigation of the crystallized glass confirmed volume crystallization of spherical nuclei. Struktura a vlastnosti K2O·TiO2·3GeO2 skel byly zkoumány infraèervenou spektrometrií s Fourierovou transformací (FTIR), diferenèní termickou analýzou (DTA), diferenèní skenovací kalorimetrií (DSC), RTG difrakèní analýzou (XRD) a skenovací elektronovou mikroskopií (SEM). RTG analýza zkrystalovaných skel odhalila pøítomnost krystalického K2O·TiO2·3GeO2 ukazující totožné složení krystalické a skelné fáze. Dominujícím mechanismem krystalizace èástic skla vìtších než 0,15 mm je objemová krystalizace sférických nukleí, která byla rovnìž potvrzena SEM analýzou.

Low temperature processes during the synthesis of PbO-B 2 O 3 -SiO 2 -La 2 O 3 glasses were studied by thermal analysis of the batch mixture of glasses and X-ray powder diffraction analysis of samples that crystallized during invesrtigations of the crystallization ability of the synthesized glasses. The obtained results and available thermodynamic data indicate the formation of PbO·B 2 O 3 , PbO·2B 2 O 3 and La 2 O 3 ·B 2 O 3 ·SiO 2 in the temperature interval 400-700°C.

Razvoj novih materijala, kao i nastojanja za iskorišćenjem njihovih tehnološki atraktivnih karakteristika doveli su do intenzivnije sinteze novih ultrafinih struktura i nanofaznih prahova. Istraživanja obuhvaćena radom fokusirana su na dirigovanu sintezu nanostrukturnih oksidnih prahova iz sistema koje odlikuje izrazita jonska (ZnCr2O4) i elektronska provodnost(Bi1.8Pb0.2Sr2Ca2Cu3Ox) primenom metode reakcionog raspršivanja. Akcenat je stavljen na nastanak faza koje su potencijalni nosioci svojstava značajnih za tehničku primenu materijala. Primenjena strategija rezultat je razvoja metode sa stanovišta definisanja odnosa u trijadi sinteza-struktura-svojstvo i izvodjenja procesa kontrolisane sinteze. Proces sinteze ZnCr2O4 praha izveden je u uslovima ultrazvučnog reakcionog raspršivanja rastvora nitrata koncentracije 0.03mol/dm3 pri unapred definisanim parametrima procesa i za različito vreme zadržavanja kapi/čestice na Tmax uz dodatan boravak u reakcionim zonama sa nižom temperaturom. Čestice nastaju procesom zapreminskom precipitacije, nukleacije, rasta kristala i obrazovanja primarnih nanočestica koje nisu u slobodnom stanju vec izgradjuju agregate sferičnog oblika i srednje veličine 470nm. Za prah iz filtra karakterističan je nastanak čestica ZnCr2O4 na čijoj se površini jasno uočavaju primarne čestice veličine nekoliko desetina nanometra. Strukturna analiza ovog praha pokazuje prisustvo primarnih kristalita veličine 44nm, i najveću utvrdjenu vrednost mikronaprezanja, 0.638%. Produženje vremena boravka čestica u reakcionoj zoni dovodi do promene morfologije prahova, pri čemu je za čestice praha zone I karakteristično nastajanja punih čestica, glatke površine i homogenog sastava. Veličina primarnih kristalita odredjena na bazi strukturne difraktometrijske analize iznosi 33nm. Na bazi faktora okupiranosti katjonskih položaja u jediničnoj ćeliji spinela utvrdjeni odnos katjona Zn i Cr blizak je zadatoj vrednosti (0.5) i iznosi: 0.496 (prah zone I), 0.481 (prah zone II) i 0.518 (prah iz filtra), dok su nešto više vrednosti ovog odnosa dobijene na bazi EDS analize. Dodatni termički tretman prahova (2h,1000 0C), dovodi do stabilisanja spinelne faze sa odnosom Zn/Cr=0.52 uz izdvajanje ZnO (1,9 %mas.). Na osnovu analize termodinamički ostvarivih defekata u spinelu pokazano je da je favorizovani nastanak faze u kojoj egzistira višak Zn2+ jona posledica obrazovanja Cr4+ grupe defekata. Ostvarena morfologija čestica spinelne faze odgovara kristalnim formama za koje se modelom predvidjaju poboljšana senzorska svojstva. Razvoj faza i obrazovanje Bi1.8Pb0.2Sr2Ca2Cu3Ox (2223 faza) u postupku reakcionog raspršivanja rastvora nitrata ukupne koncentracije 1mol/dm3 praćen je sa aspekta modifikacije prekursora (2 %mas. uree), odnosno iniciranja egzotermne reakcije u kapi/ čestici. Pokazano je da proces kontrolisane hidrolize uz intenzivno oslobadjanje toplote dovodi do procesa termičke eksplozije kapi/čestice, što prividno ubrzava proces razlaganja nitrata zbog smanjenja zapremine u kojoj se reakcija odvija što ima za posledicu i promenu faznog sastava čestica. Morfologija čestica ostaje nepromenjena, te prah karakteriše prisustvo sferičnih čestica srednje veličine 140nm. Difraktometrijska analiza ukazuje na dobru kristaliničnost prahova, uz karakterističnu koegzistenziju ortorombične 2212 i tetragonalne 2223 faze, kao i prisustvo malih količina Sr14Cu24O41, Ca2PbO4 i Ca2CuO3. Sastav praha odredjen je na bazi semi-kvantitativne analize površine i definisan je stehiometrijom Bi1.9Sr1.56Ca2.23Cu3.18Ox. Za prah dobijen reakcionim raspršivanjem osnovnog prekursorskog rastvora nitrata karakteristično je prisustvo dve različite morfološke forme čestica, i to: submikronskih (400 nm) sferičnih čestica glatke površine, karakterističnih za proces reakcionog raspršivanja i čestica poligonalnog oblika čija dimenzija iznosi i do 15 m u pravcu duže ose. Po svom hemijskom sastavu čestice poligonalnog oblika odlikuje izuzetno visok sardžaj stroncijuma i kiseonika, i njihovo prisustvo u prahu je verovatno posledica nepotpunog rastvaranja polazne stroncijumove soli koja konverzijom u procesu sinteze daje čestice stroncijum oksida. Difraktometrijska analiza je potvrdila prisustvo SrO a nađeno je i par refleksija Sr(NO3)2. Po svom faznom sastavu prah je okarakterisan maksimalnim udelom 2212 faze, dok se 2223 faza i Ca2PbO4 u uzorku nalaze kao prateće faze. Na bazi semi-kvantitativne analize ukupnog uzorka definisana je stehiometrija Bi2.02Sr1.71Ca2.23Cu3.01Ox. Karakteristično in-situ obrazovanje 2223 faze u procesu sinteze reakcionim raspršivanjem kao i izostajanje stvaranja bizumutata tipa (Ca,Sr)-Bi-O omogućava olakšanu konverziju pratećih faza u 2223 fazu tokom dodatnog termičkog tretmana sintetisanih prahova. Dvočasovni tretman u kiseoniku pri 845 °C dovodi do promene stehiometrije čestica i ukazuje na značaj funikularnih i pendularnih stupnjeva aglomeracije u daljem procesu obrazovanja koherentnih struktura 2223 faze. Development of advanced materials, together with the efforts invested in the optimization of their current properties result today in the opening of a new field: design of ultra-fine structure materials. Nanostructured materials with new and improved properties could be prepared through different techniques. Investigations presented here are focused on conducted synthesis of nanostructured oxide powders in ZnCr2O4 and Bi1.8Pb0.2Sr2Ca2Cu3Ox systems using the spray pyrolysis method. The main emphasis is on controlled evolution of specific phases characterized with properties suitable for potential technical application. The strategy used is the result of methodology development from the viewpoint of establishing synthesis-structure-properties relations. Because of that, investigation of phase thermal stability, particle morphology - shape, size and agglomeration, crystal structure - grain boundary and defects concentration, homogeneity of chemical composition are of special interest. As a result of ZnCr2O4 powder conduction synthesis using spray pyrolysis of nitrate salts, spherical particles with uniform size around 470 nm are obtained. Synthesized dense particles are uniform in shape and posses small differences in crystallinity and stoichiometry. Effect of droplet/particle residence time on final powder morphology is established through investigation of precursor decomposition, nucleation and growth of primary crystallites in nanoporous precipitated particles. It was shown that increase of the residence time indicates growth of nanocrystallites with mean sizes around 40nm leading to dense particle evolution. Also, formation of a spinel phase with high homogeneity and uniformity of chemical composition in all produced samples are characterized with the cation ratio of Zn/Cr=0.68. Constant mismatching in stoichiometry from the assigned value, as well as additional changes in particle morphology after their thermal treatment at 1000 °C (2h) is the consequence of ZnO dissolution in a spinel cell and is explained by a model, which is based on computing of crystal surface formation energy and surfaces attachment energy. It was shown that the achieved particle morphology is predicted by a model as an unique surface structure which posses improved sensor and catalytic properties. Specific phase development in the Bi1.8Pb0.2Sr2Ca2Cu3Ox system synthesized through conducted spray pyrolysis process is estimated from the viewpoint of nitrate precursor modification due to in situ existence of a thermal source in every droplet. The effect of controlled hydrolysis as well as thermal explosion of droplets on 2223 phase formation is defined based on the investigation of salt precipitation and the decomposition process. Additional reduction in particle size from 400 to 140 nm and maintenance of structural and compositional homogeneity result in intensification of different phase conversions and orthorhombic 2223 phase formation. Coexistence of both, 2212 and 2223 phase is determined for all samples. Their further conversion is favorized due to the presence of liquid phase rich in Ca2+ and Cu3+ ions during additional thermal treatment in oxygen. Shortening of the time needed for vacancy distribution during orthogonal-tetragonal transition results in phase intercalation and formation of a 2223 phase richer in strontium content. Initial sintering and coherent phase development in thermally treated samples are related to the presence of funicular and pendulum agglomeration states. Establishing of conducted nanostructured oxide synthesis through spray pyrolysis process classify this method as one that is successful in production of advanced functional materials.

Razvoj novih materijala, kao i nastojanja za iskorišćenjem njihovih tehnološki atraktivnih karakteristika doveli su do intenzivnije sinteze novih ultrafinih struktura i nanofaznih prahova. Istraživanja obuhvaćena radom fokusirana su na dirigovanu sintezu nanostrukturnih oksidnih prahova iz sistema koje odlikuje izrazita jonska (ZnCr2O4) i elektronska provodnost(Bi1.8Pb0.2Sr2Ca2Cu3Ox) primenom metode reakcionog raspršivanja. Akcenat je stavljen na nastanak faza koje su potencijalni nosioci svojstava značajnih za tehničku primenu materijala. Primenjena strategija rezultat je razvoja metode sa stanovišta definisanja odnosa u trijadi sinteza-struktura-svojstvo i izvodjenja procesa kontrolisane sinteze. Proces sinteze ZnCr2O4 praha izveden je u uslovima ultrazvučnog reakcionog raspršivanja rastvora nitrata koncentracije 0.03mol/dm3 pri unapred definisanim parametrima procesa i za različito vreme zadržavanja kapi/čestice na Tmax uz dodatan boravak u reakcionim zonama sa nižom temperaturom. Čestice nastaju procesom zapreminskom precipitacije, nukleacije, rasta kristala i obrazovanja primarnih nanočestica koje nisu u slobodnom stanju vec izgradjuju agregate sferičnog oblika i srednje veličine 470nm. Za prah iz filtra karakterističan je nastanak čestica ZnCr2O4 na čijoj se površini jasno uočavaju primarne čestice veličine nekoliko desetina nanometra. Strukturna analiza ovog praha pokazuje prisustvo primarnih kristalita veličine 44nm, i najveću utvrdjenu vrednost mikronaprezanja, 0.638%. Produženje vremena boravka čestica u reakcionoj zoni dovodi do promene morfologije prahova, pri čemu je za čestice praha zone I karakteristično nastajanja punih čestica, glatke površine i homogenog sastava. Veličina primarnih kristalita odredjena na bazi strukturne difraktometrijske analize iznosi 33nm. Na bazi faktora okupiranosti katjonskih položaja u jediničnoj ćeliji spinela utvrdjeni odnos katjona Zn i Cr blizak je zadatoj vrednosti (0.5) i iznosi: 0.496 (prah zone I), 0.481 (prah zone II) i 0.518 (prah iz filtra), dok su nešto više vrednosti ovog odnosa dobijene na bazi EDS analize. Dodatni termički tretman prahova (2h,1000 0C), dovodi do stabilisanja spinelne faze sa odnosom Zn/Cr=0.52 uz izdvajanje ZnO (1,9 %mas.). Na osnovu analize termodinamički ostvarivih defekata u spinelu pokazano je da je favorizovani nastanak faze u kojoj egzistira višak Zn2+ jona posledica obrazovanja Cr4+ grupe defekata. Ostvarena morfologija čestica spinelne faze odgovara kristalnim formama za koje se modelom predvidjaju poboljšana senzorska svojstva. Razvoj faza i obrazovanje Bi1.8Pb0.2Sr2Ca2Cu3Ox (2223 faza) u postupku reakcionog raspršivanja rastvora nitrata ukupne koncentracije 1mol/dm3 praćen je sa aspekta modifikacije prekursora (2 %mas. uree), odnosno iniciranja egzotermne reakcije u kapi/ čestici. Pokazano je da proces kontrolisane hidrolize uz intenzivno oslobadjanje toplote dovodi do procesa termičke eksplozije kapi/čestice, što prividno ubrzava proces razlaganja nitrata zbog smanjenja zapremine u kojoj se reakcija odvija što ima za posledicu i promenu faznog sastava čestica. Morfologija čestica ostaje nepromenjena, te prah karakteriše prisustvo sferičnih čestica srednje veličine 140nm. Difraktometrijska analiza ukazuje na dobru kristaliničnost prahova, uz karakterističnu koegzistenziju ortorombične 2212 i tetragonalne 2223 faze, kao i prisustvo malih količina Sr14Cu24O41, Ca2PbO4 i Ca2CuO3. Sastav praha odredjen je na bazi semi-kvantitativne analize površine i definisan je stehiometrijom Bi1.9Sr1.56Ca2.23Cu3.18Ox. Za prah dobijen reakcionim raspršivanjem osnovnog prekursorskog rastvora nitrata karakteristično je prisustvo dve različite morfološke forme čestica, i to: submikronskih (400 nm) sferičnih čestica glatke površine, karakterističnih za proces reakcionog raspršivanja i čestica poligonalnog oblika čija dimenzija iznosi i do 15 m u pravcu duže ose. Po svom hemijskom sastavu čestice poligonalnog oblika odlikuje izuzetno visok sardžaj stroncijuma i kiseonika, i njihovo prisustvo u prahu je verovatno posledica nepotpunog rastvaranja polazne stroncijumove soli koja konverzijom u procesu sinteze daje čestice stroncijum oksida. Difraktometrijska analiza je potvrdila prisustvo SrO a nađeno je i par refleksija Sr(NO3)2. Po svom faznom sastavu prah je okarakterisan maksimalnim udelom 2212 faze, dok se 2223 faza i Ca2PbO4 u uzorku nalaze kao prateće faze. Na bazi semi-kvantitativne analize ukupnog uzorka definisana je stehiometrija Bi2.02Sr1.71Ca2.23Cu3.01Ox. Karakteristično in-situ obrazovanje 2223 faze u procesu sinteze reakcionim raspršivanjem kao i izostajanje stvaranja bizumutata tipa (Ca,Sr)-Bi-O omogućava olakšanu konverziju pratećih faza u 2223 fazu tokom dodatnog termičkog tretmana sintetisanih prahova. Dvočasovni tretman u kiseoniku pri 845 °C dovodi do promene stehiometrije čestica i ukazuje na značaj funikularnih i pendularnih stupnjeva aglomeracije u daljem procesu obrazovanja koherentnih struktura 2223 faze. Development of advanced materials, together with the efforts invested in the optimization of their current properties result today in the opening of a new field: design of ultra-fine structure materials. Nanostructured materials with new and improved properties could be prepared through different techniques. Investigations presented here are focused on conducted synthesis of nanostructured oxide powders in ZnCr2O4 and Bi1.8Pb0.2Sr2Ca2Cu3Ox systems using the spray pyrolysis method. The main emphasis is on controlled evolution of specific phases characterized with properties suitable for potential technical application. The strategy used is the result of methodology development from the viewpoint of establishing synthesis-structure-properties relations. Because of that, investigation of phase thermal stability, particle morphology - shape, size and agglomeration, crystal structure - grain boundary and defects concentration, homogeneity of chemical composition are of special interest. As a result of ZnCr2O4 powder conduction synthesis using spray pyrolysis of nitrate salts, spherical particles with uniform size around 470 nm are obtained. Synthesized dense particles are uniform in shape and posses small differences in crystallinity and stoichiometry. Effect of droplet/particle residence time on final powder morphology is established through investigation of precursor decomposition, nucleation and growth of primary crystallites in nanoporous precipitated particles. It was shown that increase of the residence time indicates growth of nanocrystallites with mean sizes around 40nm leading to dense particle evolution. Also, formation of a spinel phase with high homogeneity and uniformity of chemical composition in all produced samples are characterized with the cation ratio of Zn/Cr=0.68. Constant mismatching in stoichiometry from the assigned value, as well as additional changes in particle morphology after their thermal treatment at 1000 °C (2h) is the consequence of ZnO dissolution in a spinel cell and is explained by a model, which is based on computing of crystal surface formation energy and surfaces attachment energy. It was shown that the achieved particle morphology is predicted by a model as an unique surface structure which posses improved sensor and catalytic properties. Specific phase development in the Bi1.8Pb0.2Sr2Ca2Cu3Ox system synthesized through conducted spray pyrolysis process is estimated from the viewpoint of nitrate precursor modification due to in situ existence of a thermal source in every droplet. The effect of controlled hydrolysis as well as thermal explosion of droplets on 2223 phase formation is defined based on the investigation of salt precipitation and the decomposition process. Additional reduction in particle size from 400 to 140 nm and maintenance of structural and compositional homogeneity result in intensification of different phase conversions and orthorhombic 2223 phase formation. Coexistence of both, 2212 and 2223 phase is determined for all samples. Their further conversion is favorized due to the presence of liquid phase rich in Ca2+ and Cu3+ ions during additional thermal treatment in oxygen. Shortening of the time needed for vacancy distribution during orthogonal-tetragonal transition results in phase intercalation and formation of a 2223 phase richer in strontium content. Initial sintering and coherent phase development in thermally treated samples are related to the presence of funicular and pendulum agglomeration states. Establishing of conducted nanostructured oxide synthesis through spray pyrolysis process classify this method as one that is successful in production of advanced functional materials.