The influence of mechanical activation on the structure and properties of strontium titanate ceramics

Cilj ove doktorske disertacije je bio da se analizira i sagleda uticaj mehaničke aktivacije na strukturu i svojstva nedopiranog i dopiranog SrTiO3 praha, kao i uticaj na strukturu i svojstva keramike dobijene sinterovanjem ispresaka pomenutih prahova. U slučaju sinterovanih nedopiranih SrTiO3 uzoraka dodatno su razmatrane promene u brzini densifikacije i kinetici sinterovanja. Uticaj mehaničke aktivacije i dopanta na električna svojstva stroncijum-titanatne keramike je analiziran merenjem frekventne zavisnosti relativne dielektrične permitivnosti i tangensa ugla dielektričnih gubitaka na sobnoj temperaturi. Magnetnim merenjima, kod dopiranih SrTiO3 uzoraka dobijenih dodavanjem mangan(IV)-oksida, praćen je uticaj mehaničke aktivacije, koncentracije dopanta i jačine magnetnog polja na vrednost specifične magnetizacije. Mehanička aktivacija SrTiO3 praha je vršena u visoko-energetskom planetarnom mlinu sa kuglama, pri čemu je vreme aktivacije nedopiranog praha iznosilo: 5, 10, 30, 60, 90 i 120 minuta, dok je efekat dopiranja razmatran za aktivaciju od 10, 30 i 120 minuta. U cilju dobijanja SrTiO3:Mn sistema, tipa Sr1-xMnxTiO3 (SMnT) ili SrTi1-xMnxO3 (STMn), u polazni SrTiO3 prah je dodat mangan(IV)-oksid (MnO2), pri čemu su izabrane vrednosti za x bile: 0,03, 0,06 i 0,12. Raspodela veličina čestica je ukazala da je mehanička aktivacija dovela do usitnjavanja čestica polaznog praha, uz istovremenu pojavu šire raspodele veličina čestica pri dužim vremenima aktivacije. Skenirajućom i transmisionom elektronskom mikroskopijom (SEM i TEM) analizirana je mikrostruktura i morfologija polaznog i mehanički aktiviranih prahova. BET metodom je utvrđeno da se najviša vrednost specifične površine kod nedopiranih prahova postiže pri aktivaciji u trajanju od 60 minuta, dok pri daljoj aktivaciji specifična površina ostaje približno konstantne vrednosti, usled povećane sekundarne aglomerizacije. Pokazano je da je gustina ispreska kod praha aktiviranog tokom 10-30 minuta najveća. Zapaženo je sniženje intenziteta XRD pikova sa porastom vremena aktivacije, kao i širenje difrakcionih linija, usled smanjenja veličine kristalita i porasta mikronaprezanja. Primenjena mehanička aktivacija je omogućila formiranje kubnog nanokristalnog SrTiO3 praha, pri čemu veličina kristalita opada i do ~20 nm sa uvećanjem vremena aktivacije. Uočena promena u vrednosti parametra kristalne rešetke a je posredno ukazala i na moguće promene u koncentraciji kiseonikovih vakancija. Analizom Ramanovih spektara je praćen uticaj mehaničke aktivacije na promene u fononskom spektru prahova, sa posebnim akcentom na polarne TO mode, čiji oblik i intenzitet bitno zavise od defekata kao što su kiseonikove vakancije. Analiza optičkih svojstava kristalnih materijala pomoću UV-Vis spektroskopije je ukazala na sniženje energije zabranjene zone sa uvećanjem vremena aktivacije. Primenom dilatometrije je ispitivan uticaj mehaničke aktivacije na početni stadijum sinterovanja dvostrano presovanih SrTiO3 prahova. Uočeno je značajno smanjenje temperature početka skupljanja ispreska, kao i uticaj na brzinu skupljanja i na konačne gustine sinterovanih uzoraka. Primena Dornove metode je ukazala na značajno smanjenje efektivne energije aktivacije transporta mase u početnom stadijumu sinterovanja, sa porastom vremena mehaničke aktivacije polaznog praha. Za uzorke dobijene sinterovanjem do 1300 oC, uz izotermsko zadržavanje na maksimalnoj temperaturi u trajanju od 2h, je izvršena korelacija između zapaženih strukturnih promena i promena u dielektričnim svojstvima, nastalim usled mehaničke aktivacije. Posebna pažnja je posvećena uticaju promena: gustine, poroznosti, veličine zrna i kristalita, kao i uticaju promena parametra kristalne rešetke, na dielektrična svojstva. Promene vrednosti relativne dielektrične permitivnosti su razmatrane i sa stanovišta promena u oblasti granice zrna, posebno imajući u vidu rezultate koji su ukazali na promenu koncentracije kiseonikovih vakancija...The aim of this doctoral dissertation is to analyze and evaluate the effect of mechanical activation on the structure and properties of undoped and doped SrTiO3 powders, as well as the effect on the structure and properties of the ceramics obtained by sintering these powders. For the sintered undoped SrTiO3 samples, changes in the densification rate were also analyzed. The effect of mechanical activation and dopant on the electrical properties of strontium titanate ceramics was analyzed by measuring the frequency-dependent relative dielectric permittivity and the loss tangent at room temperature. Magnetic measurements performed on the doped SrTiO3 samples obtained by the addition of manganese(IV) oxide were used to monitor the influence of mechanical activation, dopant concentration and magnetic field strength on magnetization. The mechanical activation of the SrTiO3 powder was performed in a high-energy planetary ball mill; the activation times of the undoped powder were: 5, 10, 30, 60, 90, and 120 minutes, while the doping effect was analyzed for the activation periods of 10, 30, and 120 minutes. In order to obtain SrTiO3:Mn systems belonging to Sr1-xMnxTiO3 (SMnT) or SrTi1-xMnxO3 (STMn) compound types, manganese(IV) oxide (MnO2) was added to the starting SrTiO3 powder, where the values selected for x were: 0.03, 0.06 and 0.12. The particle size distribution indicated that mechanical activation resulted in the comminution of the starting powder particles, accompanied with a wider particle size distribution at longer activation times. The microstructural and morphological analysis of the starting and mechanically activated powders was performed using scanning and transmission electron microscopy (SEM and TEM). The BET method revealed that 60-minute activation resulted in the highest specific surface area in undoped powders, while further activation led to a relatively constant specific surface due to increased secondary agglomeration. The density of the powder activated for 10-30 minutes was the highest. The intensity of XRD peaks decreased with longer activation times and diffraction lines broadened due to decreased crystallite sizes and increased microstrain. Mechanical activation led to the formation of cubic nanocrystalline SrTiO3 powder, with the minimum crystallite size of 20 nm. The observed change in the value of the crystal lattice parameter indirectly indicated possible changes in the concentration of oxygen vacancies. The analysis of Raman spectra revealed the effect of mechanical activation on the phonon spectrum of powders, with particular emphasis on polar TO modes which shape and intensity generally depend on defects such as oxygen vacancies. The analysis of the optical properties of crystalline materials by UV-Vis spectroscopy indicated a correlation between decreased band gap energy and increased activation times. The effect of mechanical activation on the initial stage of sintering of bilaterally pressed SrTiO3 powders was investigated by dilatometry. A significant decrease in the temperature of the onset of densification was observed, as well as the effect of mechanical activation on the densification rate and on the final density of the sintered samples. The application of the Dorn method indicated a significant decrease in the effective activation energy of mass transport in the initial sintering stage, with a longer time of the mechanical activation of the starting powder. For the samples obtained by sintering up to 1300 °C, with isothermal retention at the maximum temperature for 2 h, a correlation between the structural changes and the changes in dielectric properties resulting from mechanical activation was observed. Particular attention was paid to the effect of changes: density, porosity, grain and crystallite size, as well as the effect of crystal lattice parameter changes on the dielectric properties. The changed values of the relative dielectric permittivity were analyzed in the context of the changes in the grainboundary region, with special respect to the results indicating changes in the oxygen vacancy concentration

Influence of mechanical activation on mechanical properties of PVDF-nanoparticle composites

The influence of mechanically activated fillers (ZnO, BaTiO3 and SrTiO3 ultra-fine powders) on mechanical properties of poly(vinylidene) fluoride (PVDF) and oxide nanoparticle composite was investigated using molecular simulations. Mechanical activation leads to the creation of new surfaces and the comminution of the initial powder particles, which affects the crystallization of PVDF matrix. In addition, prolonged mechanical activation leads to agglomeration of nanoparticles into “soft” and “hard” agglomerates of different sizes. All of this has a significant effect on mechanical properties of PVDF-nanoparticle composites. Microstructural changes due to mechanical activation in ZnO, BaTiO3 and SrTiO3 powders were investigated using SEM and XRD, while dependence of mechanical properties on nanoparticle size was investigated using molecular simulation. These show that smaller nanoparticles significantly enhance the mechanical properties of PVDF-nanoparticle composite and allow use of mechanical activation as a means of reducing the amount of nanoparticle filler in the composite, while achieving the same of superior mechanical properties.Conference poster: [https://hdl.handle.net/21.15107/rcub_dais_861

Influence of mechanical activation on mechanical properties of PVDF-nanoparticle composites

Mechanical activation leads to the creation of new surfaces and decrease in particle size of the initial powder particles, which affects the crystallization of PVDF matrix. In addition, prolonged mechanical activation leads to agglomeration of nanoparticles agglomerates of different sizes. Microstructural changes due to mechanical activation in ZnO, BaTiO3, SrTiO3 and TiO2 powders were investigated using electron microscopy (TEM and SEM), x-ray diffraction (XRD) and particle size analysis (PSA). These were correlated with molecular simulations of mechanical properties of PVDF-nanocluster composites with different oxide nanocluster sizes (1, 1.5 and 2 nm) Each system was simulated using a periodic system of metal oxide nanocrystal inside β-PVDF matrix. β-PVDF was chosen for its superior properties over α- and γ- phases. Geometry of each system was optimized prior to the calculation of mechanical properties using GULP v4.3 software package. Molecular simulations show that, in general, mechanical properties (Young modulus as an indication of elasticity, Shear modulus as an indication of hardness) for all systems improve with decrease in nanocluster size. This suggests that mechanical activation of fillers in PVDF-metal oxide nanoparticle composites should have a positive effect on the mechanical properties of composite systems. This suggests mechanical activation as an easy an inexpensive method of improving mechanical properties of polymer-nanocrystal composite materials.Conference abstract: [https://hdl.handle.net/21.15107/rcub_dais_882

Analysis of the Initial-Stage Sintering of Mechanically Activated SrTiO3

The initial-stage of sintering plays a significant role in determining the final microstructure that defines the main characteristics of electroceramics materials such as functional properties. In this article non-isothermal sintering of non-activated and mechanically activated SrTiO3 samples was investigated up to 1300 degrees C. Dilatometric curves indicate that mechanical activation leads to an earlier onset of sintering, suggesting that it should lead to a more homogenous and denser sintered product. Analysis of the initial stage of sintering reveals that the sintering process of all examinated samples consists of two or three overlapping single-step processes, with a change in the dominant mass transport mechanism. The values of apparent activation energy of the considered single-step process exhibit a significant decrease with an increase in mechanical activation time. The values of the density of samples after isothermal sintering indicate that the final stage of sintering has not been reached by 1300 degrees C

Effects of Mechanical Activation on the Formation and Sintering Kinetics of Barium Strontium Titanate Ceramics

The influence of mechanical activation on the formation, sintering kinetics and morphology was investigated in sintered barium strontium titanate (BST) ceramics with different Ba-to-Sr ratios. Initial powders were mechanically activated for 20 and 120 min, leading to mechano-chemical reaction and formation of BaxSr1-xTiO3 phases. Agglomeration was found to represent an important factor in the process of formation of BaxSr1-xTiO3 phases around 800 oC and during sintering. It reduces the effectiveness of mechanical activation on formation of BaxSr1-xTiO3 phases beyond the short period (20 min), while in the process of sintering, prolonged mechanical activation (120 min) leads to a significant reduction in sintering temperature and the corresponding value of activation energy. In addition, all three systems show a phase transformation around 1100 oC, attributed to the hexagonal-to-cubic phase transition. Morphology of the final sintered ceramics can be correlated primarily with the state of the pre-sintered powder, where mechanically activated powders with smaller particle size produced more compact and less porous final product

The influence of mechanical activation on microstructure and dielectric properties of SrTiO3 ceramics

In recent years, a lot of interest has been shown in obtaining materials with predetermined properties. The aim is to establish a functional dependence between the synthesis parameters, structural characteristics, and properties of the material. Ceramic materials based on strontium titanate (SrTiO3) are of special interest due to their unique physical-chemical properties. Having in mind the importance of examining the influence of synthesis parameters on the process of obtaining and properties of functional electroceramic materials, and the importance of SrTiO3 as a perovskite material, the motive was to analyze and consider the influence of mechanical activation. It has been established that the time of mechanical activation (0, 10, 30, 60, 90, and 120 minutes) of SrTiO3 powders indirectly affects on electrical properties of SrTiO3 ceramics. It was noticed that in SrTiO3 ceramics the values of relative dielectric permittivity in the radio frequency range (0,3 MHz - 3 GHz) are stable, which is important for the fabrication of electronic components. Microstructural SEM analysis showed that the increase in mechanical activation time results in less porous samples. It was found that the value of the relative dielectric permittivity of ceramic samples at room temperature changes following the combined effect of changes in sample density, grain size, as well as changes in the grain boundary region. The maximum value of dielectric permittivity was observed in the sample activated for 10 minutes. Also, the sample activated for 10 min exhibits relatively low values of loss tangent, compared to the other mechanically activated samples, providing the best overall dielectric performance compared to other samples

Influence of MN doping on the evolution of microstructure and optical properties of mechanically activated SrTiO3 powders

Manganese doped SrTiO3 powders with various manganese dioxide weight percentages in the range of 1.5, 3 and 6 wp% were prepared by a solid-state method in the presence of mechanical activation (10, 30 and 120 minutes). A systematic investigation by X-ray diffraction (XRD), scanning electron microscopy (SEM), particle size analisys (PSA), Brunauer-Emmett-Teller (BET) methods and Raman spectroscopy has been undertaken to evaluate the role of dopants on the microstructural and morphological study of the perovskite oxide obtained. The optical properties of the different manganese doped and activated SrTiO3 powders have been also evaluated. Mn insertion in SrTiO3 is discussed considering the possibility for Mn ions to occupy both Ti4+ and Sr2+ sites as well as manganese segregation and Mn incorporation-related non-homogeneities. The results demonstrated that Mn has substituted into the lattice and surface layers of the particles of SrTiO3 powders and the absorption onset shifted to higher values of wavelengths with increasing time of activation and dopant concentration. The lowest value of the band gap (Eg=3.10 eV) was registered with the longest activation for 120 minutes and the highest concentration of dopant (6 wp%). Combining doping with mechanical activation lower values of band gap can be achieved and that fact could be used in subsequent studies to make Mn-SrTiO3 more suitable visible-light photocatalysts

The influence of mechanical activation on microstructure and dielectric properties of SrTiO3 ceramics

In recent years, a lot of interest has been shown in obtaining materials with predetermined properties. The aim is to establish a functional dependence between the synthesis parameters, structural characteristics, and properties of the material. Ceramic materials based on strontium titanate (SrTiO3) are of special interest due to their unique physical-chemical properties. Having in mind the importance of examining the influence of synthesis parameters on the process of obtaining and properties of functional electroceramic materials, and the importance of SrTiO3 as a perovskite material, the motive was to analyze and consider the influence of mechanical activation. It has been established that the time of mechanical activation (0, 10, 30, 60, 90, and 120 minutes) of SrTiO3 powders indirectly affects on electrical properties of SrTiO3 ceramics. It was noticed that in SrTiO3 ceramics the values of relative dielectric permittivity in the radio frequency range (0,3 MHz-3 GHz) are stable, which is important for the fabrication of electronic components. Microstructural SEM analysis showed that the increase in mechanical activation time results in less porous samples. It was found that the value of the relative dielectric permittivity of ceramic samples at room temperature changes following the combined effect of changes in sample density, grain size, as well as changes in the grain boundary region. The maximum value of dielectric permittivity was observed in the sample activated for 10 minutes. Also, the sample activated for 10 min exhibits relatively low values of loss tangent, compared to the other mechanically activated samples, providing the best overall dielectric performance compared to other samples

Dielectric Properties of Mechanically Activated Strontium Titanate Ceramics

In this study, microstructure evolution and dielectric properties of SrTiO3 ceramic have been investigated, whereby mechanical activation of SrTiO3 powders was used to modify the functional properties of ceramic materials. Microstructural SEM analysis of SrTiO3 ceramics showed that the increase in mechanical activation time results in less porous samples. Raman spectroscopy indicated changes in the broadening and asymmetry of the TO2 mode with a change in the time of mechanical activation. TO2 mode showed a Fano asymmetry due to its interaction with polarization fluctuations in polar micro-regions, which are a consequence of the presence of oxygen vacancies caused by activation. The maximum value of dielectric permittivity was observed in the sample activated for 10 min. Also, the sample activated for 10 min exhibits relatively low values of loss tangent, compared to the other mechanically activated samples, providing the best overall dielectric performance compared to other samples

Dielectric Properties of Mechanically Activated Strontium Titanate Ceramics

In this study, microstructure evolution and dielectric properties of SrTiO3 ceramic have been investigated, whereby mechanical activation of SrTiO3 powders was used to modify the functional properties of ceramic materials. Microstructural SEM analysis of SrTiO3 ceramics showed that the increase in mechanical activation time results in less porous samples. Raman spectroscopy indicated changes in the broadening and asymmetry of the TO2 mode with a change in the time of mechanical activation. TO2 mode showed a Fano asymmetry due to its interaction with polarization fluctuations in polar micro-regions, which are a consequence of the presence of oxygen vacancies caused by activation. The maximum value of dielectric permittivity was observed in the sample activated for 10 min. Also, the sample activated for 10 min exhibits relatively low values of loss tangent, compared to the other mechanically activated samples, providing the best overall dielectric performance compared to other samples