Thermal Analysis of Nanocrystallization of Anatase TiO[sub 2] (original) (raw)

Tailoring thermal conduction in anatase TiO2

Communications Physics, 2019

Thermal conductivity (κ) plays an essential role in functional devices. It is advantageous to design materials where one can tune κ in a wide range according to its function: single-crystals and nanowires of anatase polymorph of titanium dioxide, broadly used in applications ranging from photovoltaics, reflective coatings to memristors, have been synthesized in large quantities. Here we identify a new, strong diffusion mechanism of heat by polaronic structures due to oxygen vacancies, which considerably influences both the absolute value and the temperature dependence of κ. The additional decrease of κ is achieved in anatase nanowires organized into foam, where porosity and the quasi-one-dimensional size-effect dramatically hinder the propagation of heat, resulting in an extremely low κ = 0.014 W/Km at room-temperature. Doping this anatase foam could herald promising applications, in particular in thermoelectricity.

Thermal conductivity and secondary porosity of single anatase TiO 2 nanowire

Nanotechnology, 2012

Single anatase TiO 2 nanowire is synthesized using the electrospinning technique with the sol-gel method and is suspended over a pre-processed 100 µm-wide TEM grid for further characterization. The diameters of the nanowires fall in the range of 250-400 nm. The transient electrothermal (TET) method is adopted to acquire the voltage-time (U-t) profile of the Ir-coated nanowire under step Joule heating. The intrinsic thermal diffusivity of single anatase TiO 2 nanowires varies from 1.3 to 4.6 × 10 −6 m 2 s −1 , and the thermal conductivity changes distinctly from 1.3 to 5.6 W m −1 K −1 , much lower than the value of the bulk counterpart: 8.5 W m −1 K −1 . The density and thermal conductivity increase significantly with the diameter, largely because at larger diameters less secondary porosity is left by decomposition of organic composites and their escape from the wire during calcination. The density of TiO 2 nanowires is found to be much lower than that of the bulk counterpart. This is supported by the SEM image of the secondary porous surface. High secondary porosity is observed for TiO 2 nanowires, ranging from 18% to 63%. This very high secondary porosity confirms that the decomposition of PVP content may distort the fibrous matrix and leave vacancies. In addition, the transition from amorphous to anatase phase could also create a porous state due to crystal particle aggregation.

Thermal Analysis of Nanocrystallization of Anatase TiO2

2010

In the present paper, nanosized titanium dioxide (TiO2) is synthesized by wet-chemical technique, and characterized by x-ray diffraction (XRD), Fourier transform infrared reflectance (FTIR) and differential scanning calorimeter (DSC) thermograms. DSC thermogram shows a definite exothermic peak indicating the transformation from amorphous to crystalline phase. The FTIR spectra shows main peak around 1384 cm-1, which is attributed to the Ti-O bond in both the i.e. amorphous and crystalline samples.

Thermoelectric power of mixed electronic-ionic conductors II. Case of titanium dioxide

Ionics, 2004

The purpose of the present work is the determination of the thermopower components corresponding to different charge carriers (electrons, electron holes and ions) for TiO 2 and the use of these data for evaluation of the effect of symmetry between these two properties. The procedure of the determination of these components was based on the following two approximations: 9 The first approximation is based on a symmetrical model assuming a consistency between thermopower and electrical conductivity within the n-p transition (minimum of electronic component of the electrical conductivity corresponds to zero value of the electronic component of thermopower). 9 The second approximation is based on the apparent asymmetry between thermopower and electrical conductivity within the n-p transition as determined from the first approximation. The analysis, based on the data of the electronic components of thermopower and electrical conductivity for TiO2 single crystal, results in the band gap (using the Jonker formalism). The determined band gap is equal to 2.77 eV and 2.57 eV at the first and the second approximations, respectively, while the band gap determined from the experimentally measured data is equal 3.35 eV. These values are consistent with the band gap determined from the data of electrical conductivity corresponding to the n-p transition point (Eg = 3.16 eV) and for the data measured experimentally and those free of the ionic conductivity component (Eg = 2.79 eV). The obtained results indicate that thermopower and electrical conductivity most likely exhibit the effect of symmetry.

Room Temperature Synthesis and Thermal Evolution of Porous Nanocrystalline TiO2 Anatase

Chemistry of Materials, 2012

TiO 2 nanoparticles are a major component in many areas, and especially for dye-sensitized solar cells (DSSC) as a result of their electronic structure that allows them to collect the electrons transferred from the dye molecules after sunlight irradiation, as well as of their semiconducting properties, which provide the surface transport of these electrons up to the collecting electrode. However, for this application or others, the optimization of both structural and electronic properties of titanium oxide is still a challenge because it depends on both crystalline structure and material nano/mesostructure. We report how small (<6 nm) titanium oxide nanoparticles were synthesized by a single step method, with the anatase crystalline phase obtained at room temperature, and an opened nanostructure. A mixture design method was required to identify the precise composition that led to the suitable material. Mesoporous materials made of pure anatase nanocrystals were obtained with the suitable porosity (5 nm pore diameter, 190 m 2 /g, 0.3 mL/g porous volume) without any surfactant agent. Both the evolution of the crystal size and nature of the phases were studied as a function of heating temperatures ranging from 20 to 800°C. These materials display a good thermal stability up to 400°C, in term of crystal size, and up to 700°C, regarding the crystalline phase. Finally, the study of their semiconducting properties as a function of the crystal size allowed us to confirm the previous theoretical models regarding the crystal size-dependence of band gap and to set the limit of the size quantum confinement effect around 7 nm.

Electrical properties of polycrystalline TiO2· Thermoelectric power

Ionics, 2007

The present work determined thermoelectric power for high-purity polycrystalline TiO 2 at elevated temperatures (1,123-1,323 K) and in the gas phase of controlled oxygen activity, 10 −13 Pa < p(O 2 ) < 10 5 Pa. The slope of the thermoelectric power vs log p(O 2 ) is 1/10, instead of 1/6 expected by the theory and observed for TiO 2 single crystal. The discrepancy between the two is considered in terms of the effect of the local grain boundary structure on thermoelectric power. A comparison between the electrical conductivity and thermoelectric power data indicates that the oxygen activity values related to the n-p transition point determined by thermoelectric power are lower than those determined by electrical conductivity.

Size effect on thermal stability of nanocrystalline anatase TiO 2

Journal of Physics D: Applied Physics, 2013

Thermal stability of nanocrystalline anatase TiO 2 against coarsening and anatase-rutile phase transformation was studied using both a pyroprobe heater and a conventional furnace. The pyroprobe heater, because of the programmable control and the ultra-fast heating rate (up to 20 000 • C s −1 ), for the first time, allows us to access the very early stage of the sintering and phase-transformation processes. Our short time (0-30 s) heat treatments reveal that rapid grain growth takes place first in anatase nanoparticles (NPs) upon the initial heating due to the lower activation energy compared with that for the anatase-rutile phase transformation. Meanwhile, rutile-like structural elements develop at the interface between anatase NPs during the fast grain growth period, which evolve into rutile nuclei with time, followed by nuclei growth, to convert nanocrystalline anatase into rutile rapidly in the temperature range where the phase transformation does not occur in coarse anatase TiO 2 . Overall, both grain growth and phase transformation in smaller anatase NPs happen at lower temperatures and faster than in bigger ones. The coupled sintering-phase-transformation mechanism can be exploited to design thermally stable nanocrystalline anatase TiO 2 by reducing the sintering kinetics, for example, via minority additives.

Nonlinear effects in transient electrothermal characterization of anatase TiO2 nanowires

Review of Scientific Instruments, 2012

As an effective transient thermal characterization technique, the transient electrothermal (TET) technique features a capability of measuring micro/nanoscale samples of diverse electrical conducting natures. In this work, single anatase titanium dioxide (TiO 2 ) nanowires fabricated using the electrospinning method are characterized using the TET technique. Time-dependent nonlinear effect is observed for both rise and fall stages in the voltage-time (U-t) response profile. The coated iridium film and soldered platinum pads possibly compromise the linear Ohmic effect and introduce undesired effects into the whole system. Two quantitative methods: generalized function analysis and direct capacitance derivation, are developed to suppress the nonlinear effect based on U-t profiles. Data processing is performed to determine the thermal diffusivity using global fitting under nonconstant electrical heating. The effective thermal diffusivities from modified analysis processes stay in the range from 2 to 6 × 10 −6 m 2 /s. The results from both methods agree well with each other. The general function analysis method is also applicable for samples of short time thermal transport or for an experimental instrument that has relatively long rise time.