Synthesis, structure, characterization and photoelectrochemical application of pseudobrookite Fe2TiO5, thick layers

U poređenju sa fotonaponskim ćelijama, fotoelektrohemijske ćelije predstavljajuefikasan vid pretvaranja sunčeve energije u električnu, jer se pored energije dobija ivodonik koji se može iskoristiti kao gorivo. Fotoelektrohemijske ćelije sastoje se odpoluprovodničke anode (p- ili n-tipa) koja apsorbuje svetlost, pri čemu se konverzijaenergije odigrava na granici faza poluprovodnik-elektrolit.U ovoj doktorskoj disertaciji primenjena su tri istraživačka pravca za dobijanjefilmova u svojstvu fotoaktivnih anoda. Prvi istraživački pravac bio je pripremananočestičnog poluprovodnika - pseudobrukita, Fe2TiO5, reakcijom u čvrstom stanjuizmeđu anatasa, TiO2 i hematita, α-Fe2O3. Analiza dobijanja najpre je praćena natabletama radi optimizacije sastava, temperature i vremena sinterovanja. Ispitan je uticajpromene temperature sinterovanja (750 – 1250 C) na fazni sastav, temperaturu faznogprelaza, morfologiju, kao i električna svojstva sinterovanih uzoraka. Izabrane su dveoksidne smeše maseni odnos prahova 40%α-Fe2O3/60%TiO2 (4F6T) i60%α-Fe2O3/40%TiO2 (6F4T) što približno odgovara molskom odnosu početnihprahova hematita i anatasa od 1:3 i 1,5:2, sa ciljem da se dobije pseudobrukit i analizirauticaj viška anatasa. Uzorci sinterovani na temperaturi 850 C sadržali su rutil ipseudobrukit sa rombičnom strukturom čime je potvrđeno da gvožđe ubrzava faznutransformaciju anatasa u rutil pri čemu se formira i pseudobrukitna faza. Dalje povišenjetemperature uticalo je na povećanje gustine uzoraka, promene u veličini zrna ismanjenje električne otpornosti.Drugi istraživački pravac bio je priprema elektrodnih filmova sitoštampanjempasti koje su dobijene mešanjem homogenizovanih prahova sa organskom smolom(butil celuloza), rastvaračem (terpinol) i malom količinom veziva – ROSiO2B2O3staklo (RO-oksid retkih zemalja). Dobijene paste deponovane su sitoštampom nasupstrat od alumine i potom sinterovane u hibridnoj peći 60 minuta u temperaturnomopsegu 800 – 950 C. Rendgenskom difrakcionom analizom utvrđeno je da se na800 C formira monoklinični pseudobrukit i da je završena fazna transformacija anatasau rutil, kao i da je dalje povišenje temperature utiče na prelaz monokliničnog u rombičnipseudobrukit. Uzorci sinterovani na 850 C imali su malu veličinu zrna sa homogenomstrukturom, dok je dalje povišenje temperature uticalo na formiranje pseudobrukita savećim zrnima štapićastog oblika. Strujno-naponska analiza pokazala je da uzorak60%α-Fe2O3/40%TiO2 sinterovan na 850 C ima mogućnost za primenu ufotoelektrohemijskim ćelijama...Compared to photovoltaic cells, photoelectrochemical cells represent an efficientway of converting solar into electrical energy, because besides energy, hydrogen is alsoavailable that could be used as a fuel. A photoelectrochemical cell consists of asemiconductor anode (p- or n-type) that absorbs light, whereby energy conversion takesplace at the boundary of the semiconductor-electrolyte phase.This doctoral dissertation is divided into three main research directions, relatedto the preparation of films in the form of photoactive anodes. The first one describespreparation of the nanoparticle semiconductor - pseudobrookite, Fe2TiO5, by a solidstate reaction between anatase, TiO2, and hematite, α-Fe2O3. Pseudobrookite formationwas first monitored on tablets to optimize the composition, temperature and time ofsintering. The influence of the sintering temperature (750 –1250C) on the phasecomposition, phase transition temperature, morphology and electrical properties of bulksintered powder mixtures composed of starting anatase and hematite nanopowders wasinvestigated. Two oxide mixtures were first selected - nanopowders of -Fe2O3(hematite) and TiO2 (anatase 99.7%) were mixed in the weight ratios 40:60 and 60:40,respectively which approximately corresponds to the molar ratios of starting hematiteand anatase powders of 1:3 and 1.5:2, with the aim of obtaining pseudobrookite andanalyzing the effect of excess anatase. Samples sintered at 850 °C contained rutile andpseudobrookite with an orthorhombic structure, which confirmed that iron acceleratesthe phase transformation of anatase to rutile, thereby forming a pseudobrookite phase.Higher sintering temperatures lead to increased sample density, changes in grain sizeand decreased electric resistivity.The second research direction was the preparation of films starting fromprepared oxide mixtures with an organic vehicle (butyl cellulose), solvent (terpinol) anda small amount of binding lead boron silicone oxide glass frit (ROSiO2B2O3). Pasteswere deposited onto alumina substrate using the screen printing technique and thensintered 60 minutes at 800 950 C. X-ray diffraction analysis determined formation ofmonoclinic pseudobrookite at 800 C and completion of the phase transformation ofanatase into rutile. Further increase in temperature leads to the transition of monoclinicpseudobrookite into orthorhombic pseudobrookite. Samples sintered at 850 C still hada small grain size, with a relatively homogenous distribution, while further increase intemperature lead to the formation of larger rod-shaped grains. Analysis of currentvoltage measurements of thick film samples sintered at 850 C showed that the60%α-Fe2O3/40%TiO2 sample had potential for application as a photoanode for lightdriven water splitting..

Two step sintering of the ZnTiO3 nanopowder

Metastabile nanopowder ZnTiO3 pressed into cylindrical compacts at 200 MPa was submitted to conventional heating with isothermal holding at 931oC for 10 minutes, 25 minutes and 40 minutes. Same compacts were heated with two-step sintering schedule with maximal 912oC and isothermal holding at 896oC, for approximately the same holding times as with isothermal. Shrinkage during heating was monitored with dilatometric device, while microstructure was determined with atomic force microscopy. XRD patterns were collected for the most prominent samples. Sintered specimens microstructure showed differences introduced during last sintering stage by two heating schedules

Two step sintering of the ZnTiO3 nanopowder

Metastabile nanopowder ZnTiO3 pressed into cylindrical compacts at 200 MPa was submitted to conventional heating with isothermal holding at 931oC for 10 minutes, 25 minutes and 40 minutes. Same compacts were heated with two-step sintering schedule with maximal 912oC and isothermal holding at 896oC, for approximately the same holding times as with isothermal. Shrinkage during heating was monitored with dilatometric device, while microstructure was determined with atomic force microscopy. XRD patterns were collected for the most prominent samples. Sintered specimens microstructure showed differences introduced during last sintering stage by two heating schedules

Two step sintering of the ZnTiO3 nanopowder

Metastabile nanopowder ZnTiO3 pressed into cylindrical compacts at 200 MPa was submitted to conventional heating with isothermal holding at 931oC for 10 minutes, 25 minutes and 40 minutes. Same compacts were heated with two-step sintering schedule with maximal 912oC and isothermal holding at 896oC, for approximately the same holding times as with isothermal. Shrinkage during heating was monitored with dilatometric device, while microstructure was determined with atomic force microscopy. XRD patterns were collected for the most prominent samples. Sintered specimens microstructure showed differences introduced during last sintering stage by two heating schedules

Application of Nanocrystalline Pseudobrookite (Fe2TiO5) Thick Films for Humidity Sensing

Pseudobrookite nanocrystalline thick films were screen printed on alumina substrate with small interdigitated PdAg electrodes (width 6 mm, length 12 mm, electrode spacing 0.2 mm) and fired at 600°C for 30 minutes. Scanning electron microscopy (SEM) of the thick film surface confirmed the formation of a porous structure consisting of agglomerated nanocrystalline grains of pseudobrookite. Impedance response of pseudobrookite thick film samples was measured in a humidity chamber at operating temperatures of 25 and 50°C in the relative humidity (RH) range 40-90% and frequency range 42 Hz-1 MHz. At the lowest frequency of 42 Hz at 25°C the impedance reduced 7 times (from 35.74 MΩ for RH 40% to 4.91 MΩ for RH 90%) and at 50°C 33 times (from 30.98 MΩ for RH 40% to 0.944MΩ for RH 90%). Low hysteresis (1.82 and 3.65%) was obtained at 25 and 50°C, respectively. Complex impedance was analyzed using an equivalent circuit consisting of parallel impedance and constant phase (CPE) element showing the dominant influence of grain boundaries. © 2018 IEEE

Dilatometer as a scientific tool

Dilatometry is defined as the dimensional change of a solid specimen recorded during temperature schedule. Set of data obtained in such a manner are known as dilatogram. Dilatograms for different sort of materials represent an important trace for deduction of other materials properties than volume. Property parameters directly appointable from the dilatogram graph are: thermal expansion coefficient for the temperature interval, glass transition temperature, phasetransition temperature, sintering shrinkage and sintering temperature for maximal shrinkage rate, crystallization point temperature for amorphous bulk metallic glasses, defect annealing temperature. From dilatometric data we can also calculate more complex values such as the sintering activation energy, deduce sintering kinetic mechanisms, for the phase transition kinetic parameters and phase composition, defect concentration, materials thermal expansion coefficient at a particular temperature, solid state reaction kinetic parameters. Dilatometric devices regarding the construction are divided into contact and non contact ones, for they physically exert force on the specimen or not. Furthermore, contact dilatometric devices can be ascribed due to their construction as vertical and horizontal. This categorization leads to different and changeable contact force on the specimen. Vertical dilatometers usually use higher and temporarily changeable forces applied on the specimen. They can be, with suitable equipment, used for other mechanical properties determination than expansion, such as compressibility, tension or inflection. Non contact devices are divided into interferometric and optical. Interferometric ones use a two laser beams construction where for the length change measuring they count the number of wave lengths that are formed as a path difference between two beams. Optical devices, however, uses monochromatic light projected on the specimen that forms shadow recorded on an optical sensor. Obtained images are then analyzed for the specimen`s dimensional change

Nanocrystalline SnO2-Zn2SnO4 composite thick films applied as humidity sensors

Starting ZnO and SnO2 nanopowders (<100 nm) were mixed in a suitable ratio and calcined at 1050 °C for 2 hours to obtain nanocrystalline SnO2-Zn2SnO4 composite powder. Structural characterization performed by X-ray diffraction (XRD) and Scanning Electron Microscopy (SEM) showed that a nanocrystalline composite SnO2-Zn2SnO4 powder was obtained. Thick film paste was made by adding organic vehicles to the powder. Screen printing of four and five layers of thick film paste was performed on two interdigitated test electrode configurations. They were calcined at 500 and 600 °C for 30 minutes. Impedance response was analyzed at several working temperatures (20-60 °C) in the relative humidity range 30-90% and frequency 42 Hz to 1 MHz. Increase in relative humidity lead to a decrease in impedance, especially at lower frequencies. The sensor time delay between absorption and desorption processes was low and the response and recovery times fast showing that the nanocystalline SnO2-Zn2SnO4 composite has potential for application in humidity sensing

Nickel Manganite-Sodium Alginate Nano-Biocomposite for Temperature Sensing

Nanocrystalline nickel manganite (NiMn2O4) powder with a pure cubic spinel phase structure was synthesized via sol-gel combustion and characterized with XRD, FT-IR, XPS and SEM. The powder was mixed with sodium alginate gel to form a nano-biocomposite gel, dried at room temperature to form a thick film and characterized with FT-IR and SEM. DC resistance and AC impedance of sensor test structures obtained by drop casting the nano-biocomposite gel onto test interdigitated PdAg electrodes on an alumina substrate were measured in the temperature range of 20-50 degrees C at a constant relative humidity (RH) of 50% and at room temperature (25 degrees C) in the RH range of 40-90%. The material constant obtained from the measured decrease in resistance with temperature was determined to be 4523 K, while the temperature sensitivity at room temperature (25 degrees C) was -5.09%/K. Analysis of the complex impedance plots showed a dominant influence of grains. The decrease in complex impedance with increase in temperature confirmed the negative temperature coefficient effect. The grain resistance and grain relaxation frequency were determined using an equivalent circuit. The activation energy for conduction was determined as 0.45 eV from the temperature dependence of the grain resistance according to the small polaron hopping model, while the activation energy for relaxation was 0.43 eV determined from the Arrhenius dependence of the grain relaxation frequency on temperature

Metal oxide nanoparticles as active food packaging components

Replacement of non-degradable food packaging materials with a biodegradable alternative enables reduction of environmental pollution. Metal oxides nanoparticles are good candidates for enhancing and ensuring good mechanical, thermal and barrier properties of biodegradable polymer packaging films. Their incorporation in biodegradable food packaging films has also lead to enhanced antioxidant, antifungal and antibacterial properties of the food packaging and also oxygen barrier properties, UV protection, oxygen and ethylene scavenging resulting in an active food packaging material. The synthesis method has a significant influence on the resulting properties of metal oxide nanoparticles. Green synthesis using plant extracts and extracts of plant bio-waste as reducing and capping agents are one direction for obtaining metal oxide nanoparticles with improved antioxidant and antimicrobial properties. Utilization of bio-waste materials both for metal oxide nanoparticle synthesis and as a source of biopolymers for packaging enables better environmental protection and ensures a circular bioeconomy. In selection of metal oxide nanoparticles suitable for application in active packaging bio-nano-composites special attention needs to be paid to nanoparticle migration and cytotoxic activity in order to produce safe, active and biodegradable food packaging materials for the future

Humidity sensing potential of iron manganite (FeMNO3)

Though different metal oxide systems have been investigated and applied in humidity sensing as resistive or capacitive ceramic humidity sensors new materials remain the subject of much research. Iron manganite (FeMnO3) has a bixybyite type structure with the cubic space group. Iron manganite powder was obtained by solid state synthesis (milling in a planetary ball mill, calcination at 1000°C for 2 hours, milling) of starting hematite (Fe2O3) and manganese carbonate (MnCO3) powders mixed in a suitable ratio. Bulk samples were obtained by sintering green samples of pressed powder 8 mm in diameter at 1000oC for 4 hours. Thick film paste was obtained by mixing the powder with organic vehicles. Four layers were screen printed on test interdigitated electrodes on alumina substrate and fired at 900oC for 6 h. XRD analysis of bulk and thick film samples confirmed the formation of iron manganite with a perovskite structure. Scanning electron microscopy (SEM) analysis of freshly cleaved bulk samples showed a network of interconnected grains and pores. A similar structure was observed for the thick film sample surface. Change of complex impedance was monitored in a humidity chamber in the relative humidity range 30-90% at the working temperature of 25°C and frequency range 42 Hz to 1 MHz. In bulk samples at 100 Hz the impedance decreased from 32 (RH 30%) to 3 MΩ (RH 90%), while in thick film samples on test interdigitated electrodes it decreased from 8.24 (RH 30%) to 0.87 MΩ ((RH 90%). The thick film sensor response and recovery was several seconds and a low hysteresis value of 2.78% was obtained showing that iron manganite can successfully be applied for humidity sensing applications