Space Charge Segregation at Grain Boundaries in Titanium Dioxide: I, Relationship between Lattice Defect Chemistry and Space Charge Potential

The electrical potential difference between the interface and the bulk in TiO₂ is obtained as a function of temperature, oxygen pressure, and acceptor or donor doping from a space charge model that explicitly includes the high-temperature lattice defect chemistry. Using defect equilibrium constants for TiO₂ from previous literature studies, it is shown that for a space charge determined by ionic defect equilibration with the interface, the potential is negative in undoped and acceptor-doped TiO₂ and positive at high donor concentrations. The isoelectric point lies in the donor-doped regime at high temperatures due to the contribution of defects from reduction, even for fairly oxidizing ambients.     

Interactions and Chemistry of Defects at the Grain Boundaries of Ceramics

A thermodynamic approach is used to derive a defect chemistry formulation at the grain boundary of an ionic solid. Considering the elastic and/or electrostatic interactions between charged defects, the equilibrium electrostatic potential and the concentration of charged defects at the grain boundaries can be quantitatively predicted. The obtained result has general applicability at all levels of defect concentrations. Defect concentration at the boundary has been determined to critically depend on the interactions between defects, electrostatic potential, and segregation. These interactions, either acting individually or coupling with each other, lead to a nonuniform defect distribution near the grain boundary. Calculations also show that the grain-boundary segregation of an aliovalent solute is greatly affected by doping through the electrostatic or elastic interactions. The variation of the grain-boundary characteristics with the processing parameters, such as dopant concentration and temperature, is discussed in relation with the proposed model.     

Role of solid state chemistry in high temperature corrosion

Most of the reactions of solid materials in high temperature environments are controlled by crystal chemistry. In other words, chemical reactivity is determined by the crystal structure and defect structure of the solid materials in these aggressive media. In this paper, we shall discuss some corrosion reactions of solid materials that have gained considerable importance recently, in the field of high temperature corrosion, pointing out how solid state chemistry plays a role in this crucial branch of chemical engineering and technology.     

Nonstoichiometry in Undoped BaTiO3

Equilibrium electrical conductivity data for large-grained, poly crystalline, undoped BaTiO₃, as a function of temperature, 750° to 1000°C, and oxygen partial pressure, 10⁻²⁰< P _O2<10⁻¹ MPa, were quantitatively fit to a defect model involving only doubly ionized oxygen vacancies, electrons, holes, and accidental acceptor impurities. The latter are invariably present in sufficient excess to control the defect concentrations through the compensating oxygen vacancies, except under the most severely reducing conditions. Singly ionized oxygen vacancies play no discernible role in the defect chemistry of BaTiO₃ within this experimental range. The derived accidental acceptor content has a slight temperature dependence which may reflect some small amount of defect association. Deviation of the conductivity minima from the ideal shape yields a small P _O2-independent conductivity contribution, which is tentatively identified as oxygen vacancy conduction.     

Temperature‐ and Atmosphere‐Dependent Defect Chemistry Model of SnO2 Nanocrystalline Film

By combining the concept of defect chemistry and the small‐polaron hopping conduction model, the present work takes an intensively considering of the electron conduction mechanism in the nonstoichiometric SnO₂ nanocrystalline film. The temperature‐dependent and atmosphere‐dependent relationship between the electrical conductivity and the defect reaction is outlined. To investigate the influence of temperature and atmosphere on the electrical properties of the SnO₂ nanocrystalline film, a temperature‐programmed system integrated with the high‐throughput screening platform of gas‐sensing materials (HTSP‐GM) is developed as the test tool in this work. With this platform, the temperature‐dependent conductivity of SnO₂ nanocrystalline film in different atmosphere (dry air, nitrogen, and formaldehyde) was conducted. A good fit between the theoretical deductions and experimental results is achieved.     

Towards low temperature thermal exfoliation of graphite oxide for graphene production

Realizing mass production of graphene materials at low cost and high quality is urgently required for their real applications. Thermal exfoliation of graphite oxide (GO) is considered as a promising strategy though it normally requires a high exfoliation temperature together with a fast heating rate, making the produced graphenes suffer from high cost and concentrated topological defects. A mild exfoliation of GO at a far lower temperature than the predicted minimum temperature, has been demonstrated by introducing a high vacuum to exert an outward drawing force which helps effective exfoliation of the stacked graphene layers. In this contribution, together with a discussion on the foundation of thermal exfoliation and the general principle for low-temperature exfoliation, we review current strategies and indicate possible novel approaches. Low cost and easy operability are highlighted for the low-temperature exfoliation and the resulting graphene materials are characterized by low defect concentration, and unique and tunable surface chemistry to promote potential mass applications in energy-related areas.     

Analysis of defect mechanisms in nonstoichiometric ceria–zirconia by the microwave cavity perturbation method

In this microwave study, the defect chemistry of ceria–zirconia solid solutions (CZO, Ce_1−yZr_yO_2−δ) was investigated at high temperatures by a resonant microwave method. Specifically, the effects of temperature and Zr content on the dielectric properties and defect chemistry mechanisms in CZO were analyzed. Experiments were performed on a series of different CZO powders (y = 0.2, 0.33, 0.50, 0.67). Measurements at 600°C and different oxygen partial pressures (p_O2 = 10⁻²⁶–0.2 bar) confirm a dominant n-type conduction of small-polarons in CZO due to the preferred formation of oxygen vacancies, which is also supported by a multimodal analysis. Polarization losses were found to be negligible in the GHz range. Furthermore, an increased relative permittivity was observed in CZO, which correlates with the concentration of oxygen vacancies in CZO. Our microwave study is the first to provide a comprehensive data set for the dielectric properties of CZO powder sample in a wide range of different conditions. In addition, the connection of dielectric properties to CZO defect chemistry mechanisms is presented. The results are in good agreement with findings in the literature and may contribute to a better understanding of microwave-based state diagnosis of CZO-based materials, as it discussed for three-way catalysts.     

A green chemical method for synthesis of ZnO nanoparticles from solid-state decomposition of Schiff-bases derived from amino acid alanine complexes

As a part of the desire to save the environment via green chemistry practices, we report a novel method to synthesize ZnO nanoparticles from nontoxic and biocompatible chemicals where no pollutant or combustible side product is produced. In this recipe, a binary Zn(II) Schiff-base complex is obtained from alanine where water is used as solvent and a biologically compatible amino acid instead of toxic amines is used as a nitrogen source. The Schiff-base complex is subsequently heat treated to synthesize ZnO particles via a solid-state decomposition process. The effect of post heat treatment temperature (400, 500, and 600 °C) on microstructure and defect content of ZnO nanoparticles is investigated. The formation of single phase ZnO particles is confirmed by XRD θ–2θ patterns and FTIR spectra. TEM and SEM micrographs indicate the formation of nanoparticles with a particle size of 50–110 nm for different heat treatment temperatures. Combing XRD, FTIR, and PL results, it is revealed that the samples heat treated at intermediate temperatures (500 °C) possess the lowest defect concentration and a favorable crystallinity. This study emphasizes on green chemistry and synthesis of nanomaterials through ecofriendly methods to save our planet and its reservoirs for future and next generations.     

The significance of defect chemistry for the rate of gas–solid reactions: three examples

Three examples are revisited in which the reaction rate could be reliably correlated with point defect chemistry highlighting the role of point defects as acid–base active centers. In the case of dehydrohalogenation of tertiary butyl chloride, AgCl becomes increasingly active as heterogeneous catalyst, if AgCl is homogeneously or heterogeneously doped. By such a procedure the silver vacancy concentration is adequately increased. The oxygen incorporation into SrTiO₃ offers an example in which the surface mechanism in terms of adsorbed species, oxygen vacancies and electronic centers has been elucidated. Appropriate surface coatings give rise to significant catalytic effects. Increasing iron (acceptor) doping not only changes the point defect chemistry but also the nature of the rate determining step. Lastly, the electrocatalytic function of Sr-doped LaMnO₃ is considered as regards oxygen reduction reaction and O²⁻ incorporation into Y-doped ZrO₂ in the context of solid oxide fuel cells. Again the defect chemistry is of prime importance for the reaction rate.     

Defect Chemistry of Undoped and Acceptor-Doped BaPbO3

The equilibrium defect chemistry of polycrystalline, undoped, and acceptor-doped BaPbO₃ was studied by measurement of the equilibrium electrical conductivity as a function of temperature, 800°–900°C, and oxygen activity, 10⁻¹⁸–1 atm. Both equilibrium electrical conductivity data of undoped and acceptor-doped samples were quantitatively fit to a defect model involving only doubly ionized oxygen vacancies, lead vacancies, holes, and acceptor impurities. The results in low and midrange of oxygen activity are dominated by acceptor impurities, whether deliberately added or not. Only in the highly oxidized condition is the conductivity independent of impurity content, confirming that this region represents the intrinsic behavior of BaPbO₃.     

dc Electrical Degradation of Perovskite-Type Titanates: II, Single Crystals

The resistance degradation of Al-doped, Ni-doped, and Fedoped SrTiO₃ single crystals is studied as a function of the temperature and the applied dc voltage. The individual influence of the acceptor dopants is discussed. Data of the mobility of oxygen vacancies are obtained from the determination of the time evolution of the electric field distribution between the electrodes, from electrocoloration studies, and from an examination of the thermal annealing after degradation. In a numerical calculation based on the defect chemistry and transport properties of titanates, the electrocoloration process is simulated and a qualitative explanation of the degradation process is presented.     

Fundamentals and practical dielectric implications of stoichiometry and chemical design in a high-performance ferroelectric oxide: BaTiO3

The processing science and fundamental understanding of defect chemistry of BaTiO₃ is a model example of how material science is used to guide the materials engineering of capacitive devices. The fundamentals are discussed from the phase equilibria, defect chemistry, and the impact on intrinsic properties. We reviewed the phenomenological defect chemistry approaches and considered the importance of doping strategies and the formation of associated point defect complexes. First principles calculations have proven to be a most informative strategy towards understanding these complexes and implications conduction mechanism. The nature of mixed conduction is considered with various dopants in the BaTiO₃ and conditions that can arise with different oxygen vacancy concentrations over a wide range of conditions. Defect dynamics are considered experimentally in terms of associations and dissociations kinetics of oxygen vacancies from these various complexes. The deleterious impact on time-dependent properties is reviewed, including time-dependent dielectric breakdown, fatigue, and aging.     

Characterization of Defect Structure in Acceptor-Modified Piezoelectric Ceramics by Multifrequency and Multipulse Electron Paramagnetic Resonance Spectroscopy

The defect chemistry in the vicinity aliovalent acceptor-type transition-metal functional centers in piezoelectric perovskite oxides is characterized by means of multifrequency and multipulse electron paramagnetic resonance spectroscopy, assisted by density-functional theory calculations. The focus is on the formation of charged     and neutral     defect dipoles, which are discussed causing internal bias fields, as well as isovalent manganese substitutes. Based on this nanoscale characterization of the defect structure, its impact on macroscopic material properties is discussed.     

Catalytic oxidation over lanthanum-transition metal perovskite materials

The thrust of this work is to follow the defect chemistry of the simple LaCoO₃ system in an attempt to probe if there is a relationship between the defect chemistry and the activity of this perovskite-type material to catalytic methane combustion. A simple flow-through reactor has been used to study the combustion of methane between room temperature and 1100°C. Using a gel-type precipitation method it has been proved possible to synthesise a single phase perovskite material with a bulk La:Co metal atom ratio of 1:1.1. PXRD, Rietveld analysis and density measurements show that the perovskite phase is non-stoichiometric with a deficiency of lanthanum ions in the lattice. This material is rather ineffective as a catalytic material. A more active form can be prepared at a La:Co metal atom ratio of 1:1 when a mixed phase (perovskite/lanthana) is produced. This material exhibits both higher activity to methane combustion and the storage/evolution of oxygen (as measured using DSC and TPD techniques).

The results of activity tests have been rationalised using XPS where large amounts of O⁻ species are seen at the surface. It is proposed that these ions occupy anion vacancies created to compensate for the reduced cation charge in the lattice. This is not possible for the single phase material where vacancies are compensated for by the presence of valency changes of cobalt and/or oxygen.

Further work has been carried by doping of the perovskite with cations of valencies +2, +3 and +4 in an attempt to control the non-stoichiometry. In this way, it has been proved possible to provide simple synthetic routes to active methane combustion catalysts.     

A computational study of the role of chlorine in the partial oxidation of methane by MgO and Li/MgO

We investigate the surface defect chemistry of MgO and Li/MgO paying special attention to the effects of added chloride ions on the active sites in the materials when used as partial oxidation catalysts. We calculate that chloride ions will segregate to the surface of these materials and will be preferentially sited at low coordinate sites. The [LiCl] defect cluster is also calculated to be bound, both of which effects will have a major influence on catalytic properties. From our calculations we also comment on the siting and selectivities of the active sites in MgO and Li/MgO.     

Calculation of the e.m.f. of solid oxide fuel cells

The electromotive force (e.m.f.) of a solid oxide fuel cell (SOFC) based on fluorite structure electrolyte is calculated by defect chemistry analysis. This is strongly influenced by the characteristic oxygen pressure, P _O2 ^* , at which the ionic transference number of the electrolyte is 0.5. It is concluded that e.m.f. decreases with increasing temperature to that of a purely ionic electrolyte-based fuel cell. The low anodic oxygen partial pressure has a smaller influence on the SOFC based on an electrolyte with mixed conduction than on the one based on a purely ionic electrolyte.     

Partial Conductivities in SrTiO3: Bulk Polarization Experiments, Oxygen Concentration Cell Measurements, and Defect-Chemical Modeling

Knowledge of the exchange kinetics of O₂ in SrTiO₃ allows us to design appropriate strategies to separate the ionic and the electronic conductivity. In the low-temperature range, where the overall surface reaction is very slow compared to bulk diffusion and measuring time, electrochemical cells of the type Pt|SrTiO₃|Pt are self-blocking and self-sealing and a Wagner–Hebb-type polarization succeeds without the necessity of using selectively blocking electrodes. In the present study the ionic conductivity data obtained for Feand Ni-doped SrTiO₃ in this way are compared to data obtained from the analysis of the oxygen partial pressure dependence of the total conductivity as well as to defect chemical calculations. In complete contrast to the low temperature situation, at high temperatures, where the surface reaction is fast, the emf technique is conveniently applicable. Results are presented for Pt, O₂|SrTiO₃|O₂, Pt cells. The conductivity behavior of SrTi(Fe)O₃ as a function of temperature (20°–1000°C) is complex, due to partially frozen-in equilibria, but even details can be quantitatively understood in terms of a simple defect chemistry. The turnover of the diffusion-controlled regime to the surface reaction-controlled regime can be shifted to significantly lower temperatures by using YBa₂Cu₃O_7–8 electrodes.     

Exploring point defects and trap states in undoped SrTiO3 single crystals

The defect chemistry and electronic trapping energies in undoped single crystalline SrTiO₃ were examined by electrochemical impedance spectroscopy at low (25–160 °C) and intermediate (500–700 °C) temperatures. Electronic and ionic conductivity as well as chemical capacitance values were obtained with a transmission line equivalent circuit. Impedance spectroscopy at low temperatures was used to quantify trapping energies of main ionic defects. Particularly the chemical capacitance is shown to be a highly valuable, though hardly used tool for establishing a defect model based solely on electrochemical measurements. It is very sensitive for minority charge carriers and can thus unveil otherwise hardly accessible defect concentrations. The chemical capacitance analysis yields a valence dependent acceptor concentration in the ppm range for the investigated samples. Complementary positron annihilation lifetime spectroscopy (PALS) suggests existence of Ti vacancies and both methods (chemical capacitance and PALS) agree in their quantification of the corresponding vacancy concentration (6 ppm). Beyond successfully predicting acceptor defect concentrations in undoped SrTiO₃, the method is sensitive for electronically relevant defects in sub-ppm concentrations.     

Recovery of surface defects on epoxy coatings and implications for the use of accelerated weathering

Surface roughness, arising from photodegradation, increases overall during weathering but may relax and diminish during episodes when exposure is limited. Different ambient temperatures will change the balance between photodegradation defect size and recovery, depending on the value of the glass transition temperature of the polymer. Epoxy coatings were exposed to periods of ultraviolet irradiation, after which the recovery of the surface roughness was monitored at several temperatures, above and below their glass transition temperatures. Atomic force microscopy, as well as following the increase in roughness with exposure, showed that increased exposure made phase separated domains more distinct. Recovery of nanoindentation on un-damaged coatings produced a similar value of the glass transition temperature to that deduced from the degradation roughness recovery. This was significantly lower at the surface of the epoxy coatings than was measured for the bulk. Confocal Raman spectroscopy was unable to detect any chemical difference between the surface of any films and deeper in their bulk. This evidence suggests that the low glass transition temperature is not due to different curing chemistry at the surface of the coating, but hints that the surface of these crosslinked coatings may relax differently to the bulk or have a different physical structure. These results lead to questions about how to change accelerated testing to better serve the needs of coatings’ technology and how to make progress in the overall goal of service lifetime prediction.     

Thermionic emission of amorphous diamond and field emission of carbon nanotubes

Thermal-field emission characteristics from nano-tips of amorphous diamond and carbon nanotubes at various temperatures are reported in this study. Amorphous diamond emitted more than 13 times more electrons at a temperature of 300 °C than at room temperature. In contrast, CNTs exhibited no increase of emitted current upon heating to 300 °C. The thermally agitated emission of amorphous diamond is attributed to the presence of defect bands. The formation of these defect bands raises the Fermi level into the upper part of the band gap, and thus reduces the energy barrier that the electrons must tunnel through. From defect bands within the band gap, the conduction band electrons were significantly increased due to electron tunnels from defect bands. The enhanced thermal-field emission originating from defect bands was observed in this study. This thermally agitated behavior of field emission for amorphous diamond was highly reproducible as observed in this research.