Corrosion effect of allylthiourea on bulk nanocrystalline ingot iron in diluted acidic sulphate solution

The effect of allylthiourea (ATU) on the corrosion behaviors of bulk nanocrystalline ingot iron (BNII) fabricated from conventional polycrystalline ingot iron (CPII) by severe rolling technique was studied. In the corrosive solution-0.1 mol L⁻¹ H₂SO₄ + 0.25 mol L⁻¹ Na₂SO₄ inhibited by ATU at ambient temperature, for CPII, when immersed for as short as 5 min, an inductive loop appears at different concentrations at the Nyquist complex planes; for BNII, only a simple capacitance loop appears in the Nyqusit plot at the same period. This difference on the time effect is attributed to the higher volume fractions of defects, which are caused by severe rolling technique compared with its coarse counterpart-CPII. With the elongation of the immersion time, for BNII, two time constants appear in the Nyquist plot; for CPII, the impedance spectrum shows a slightly depressed semicircular shape, and a critical concentration is observed. The corrosion effect of ATU for BNII is really negative in comparison with CPII.     

Effects of sintering atmosphere on the densification and microstructure of yttrium aluminum garnet fibers prepared by sol-gel process

Yttrium aluminum garnet (YAG) fibers were prepared by a sol-gel method, and then sintered in air or nitrogen atmosphere, respectively. The effects of sintering atmosphere on the densification process and microstructure of YAG fibers were investigated. No obvious difference can be found in the fibers sintered below 800 °C. At 1100 °C, the grain size of YAG fibers sintered in nitrogen is much smaller than in air. This difference is much clearer at the higher temperature of 1200 °C. The fine grains are explained by the existence of residual carbon in YAG fibers, which can be trapped at the grain boundaries to hinder the movement of grain boundary. Meanwhile, the densification degree of fibers sintered in nitrogen is higher than in air at 1200 °C, due to the smaller grain size and higher oxygen vacancy concentration generated in the nitrogen atmosphere, which leads to a higher fiber densification rate.     

Study of local structure around zirconium ions in grain boundaries of polycrystalline α-alumina by X-ray absorption spectroscopy and chemical analysis of thin foils

The chemical state and local structure around zirconium ions in doped α-polycrystalline alumina (300 wt ppm ZrO₂) was studied by X-ray absorption spectroscopy measurements at the zirconium K edge and by scanning transmission electron microscopy on thin foils. The zirconium segregation in alumina grain boundaries depends on alumina grain size: for a mean grain size of about ∼3·5 μm, the grain boundaries are saturated with zirconium. The zirconium grain boundaries to bulk concentration ratio, calculated from energy dispersive X-ray spectrometry measurement on thin foils is equal to ∼(5±3)×10³. The zirconium located in grain boundaries forms nanocrystalline tetragonal zirconia particles. This allotropic zirconia form is stabilised at room temperature because of the important contribution of the surface free energy. ©     

Effects of humidity on the electrical behaviour of Sr0·97Ti0·97Fe0·03O3-δ

It is widely recognised that the electrical behaviour of acceptor-doped strontium titanate ceramics is often controlled by resistive grain boundaries. This work shows that the electrical behaviour of Sr_0·97Ti_1-xFe_xO_3−δ materials may also be affected by other factors such as the humidity in the atmosphere, especially for materials with open porosity. For example, the impedance of dense samples with large grain size may exceed the impedance of samples with residual porosity. The bulk and grain boundary resistivities both increase with the humidity in the atmosphere.     

Low temperature sintering of LaNbO4 proton conductors from freeze-dried precursors

A synthesis method based on freeze-dried precursors was used to obtain nanocrystalline powders of pure and Ca-doped LaNbO₄ at 800 °C. Dense ceramics were prepared at temperature as low as 1100 °C. The LaNbO₄ ceramics were examined by scanning electron microscopy (SEM) to study the microstructure evolution with the sintering temperature. The densification and grain growth rate are lower in Ca-doped samples. The bulk and grain boundary contributions to the overall conductivity were studied by impedance spectroscopy under different gases. Samples sintered at low temperature and with smaller grain size exhibit higher grain boundary resistance and consequently lower total conductivity.     

Grain growth and segregation in Fe-doped SrTiO3: Experimental evidence for solute drag

In functional ceramics, the impact of dopants on bulk crystals is generally well understood. Their impact on grain boundaries is less well known. The present study investigates the impact of acceptor dopants on grain growth in strontium titanate. Scanning electron microscopy and analytical (scanning) transmission electron microscopy have been used to gain knowledge on Fe segregation behavior, grain sizes, and grain size distributions of SrTiO₃. While undoped microstructures show normal grain growth at low temperatures (<1350 °C), doped microstructures evolve bimodally. With increasing acceptor dopant concentration, an increasing population of small grains develops. It is shown that Fe segregates to the grain boundaries due to its negative charge and a positive boundary potential. Thus, the experimental findings seem to be well explained by the theory of solute drag: The diffusion of segregated defects (‘solutes’) at grain boundaries can retard grain boundary migration.     

Strength of Partly Sintered Alumina Compacts

Pressed compacts of sized alumina powders were studied to determine the dependence of strength of compacts as sintering begins and proceeds through the initial stage. By carefully controlling the time and temperature of heat treatment in a helium atmosphere, it was determined that the strength-controlling feature of these porous compacts is the area of the interparticle boundaries. Under isothermal conditions the strength was approximately proportional to time of sintering to the 2/7 power as described by theory. From these relations, it is possible to develop the semi-empirical relation between strength and porosity that predicted zero strength at a porosity equivalent to the unfired condition of the powder compact. Although not a confirmation of the grain boundary sintering model, the strength and porosity data are more consistent with the grain boundary diffusion model for initial sintering of alumina than with bulk diffusion models.     

Microstructural and high-temperature impedance spectroscopy study of Ba6MNb9O30 (M=Ga,Sc, In) relaxor dielectric ceramics with tetragonal tungsten bronze structure

This work reports on the microstructural and high-temperature impedance spectroscopy study of a family of dielectric ceramics Ba₆MNb₉O₃₀ (M=Ga, Sc, In) of tetragonal tungsten bronze (TTB) structure with relaxor properties. For Ba₆GaNb₉O₃₀ and Ba₆InNb₉O₃₀ pellets, the SEM images have revealed good, dense internal microstructures, with well-bonded grains and only discrete porosity; in contrast Ba₆ScNb₉O₃₀ pellets had a poorer microstructure, with many small and poorly-bonded grains gathered in agglomerates, resulting in significant continuous porosity and poorly defined grain boundary regions. The electroactive regions were characterised by the bulk and grain boundaries capacitances and resistances, while their contribution to the electrical conduction process was estimated by determining activation energies from the temperature (Arrhenius) dependence of both electric conductivities and time constants. For Ga and In analogues the electronic conductivity are dominated by the bulk response, while for Sc analogue, the poorly defined grain boundaries give a bulk-like response, mixing with the main bulk contribution.     

Combustion synthesis and electromagnetic properties of nanocrystalline Pb(Zr0.52Ti0.48)O3

The properties of nanocrystalline lead zirconate titanate [PZT, Pb(Zr_0.52Ti_0.48)O₃] thick films and pellets synthesized by self-propagating auto combustion route using sucrose as a fuel are reported. The X-ray diffraction reveals the formation of nanocrystalline tetragonal PZT having a particle size of 5.52 nm in thick films and 7.08 nm in bulk material. SEM images show the grain size of 300–400 nm with some agglomeration, as well as modified open grain boundaries and grain growth in the PZT thick film as compared to the bulk material. The DC electric resistivity of PZT shows semiconducting behavior. We also characterized the 8–18 GHz microwave insertion loss, absorption, complex permittivity, and microwave conductivity of synthesized materials. The real part of permittivity and dielectric loss were found to decrease with increasing frequency.     

蜡石粒度组成对铁水包用ASC材料性能的影响

在铁水包用常规Al2O3-SiC-C砖的配料组成中引入不同粒度组成的蜡石,研究了蜡石粒度组成对Al2O3-SiC-C材料加热永久线变化率、显气孔率、体积密度、耐压强度、抗氧化性和抗渣性的影响。结果表明:随着处理温度的升高,各试样的显气孔率先增大后减小,体积密度减小;随着所添加蜡石粒度的减小,试样在1 000和1 400℃埋炭热处理后的永久线变化率减小,常温耐压强度增大,1 400℃时的抗氧化性增强,但1 500℃时的抗侵蚀性降低。     

Consolidation of ceramic nanopowders

Abstract

This review is dedicated to the problems of nanostructured ceramics consolidation.

Laboratory practice in recent years has displayed several potentially important technologies for consolidation of ceramic nanopowders, such as spark plasma sintering, high pressure sintering and rate controlled sintering. The grain growth factor in these processes was found to be less than 10. These advanced technologies have to be adapted to consolidation of nanopowders and require nanopowders specifically designed for consolidation purposes. When adapted to nanopowders, these techniques must be accomplished in rapid rate mode to eliminate residual porosity and retain nanosize grains. Practical verification, however, has exposed problems such as large residual porosity, stable pores in triple junctions, defective grain boundaries and intensive grain growth. All these problems can be avoided when the temperature–pressure–time schedule of sintering is optimised with respect to minimal grain growth.     

High P–T Nano-Mechanics of Polycrystalline Nickel

We have conducted high P–T synchrotron X-ray and time-of-flight neutron diffraction experiments as well as indentation measurements to study equation of state, constitutive properties, and hardness of nanocrystalline and bulk nickel. Our lattice volume–pressure data present a clear evidence of elastic softening in nanocrystalline Ni as compared with the bulk nickel. We show that the enhanced overall compressibility of nanocrystalline Ni is a consequence of the higher compressibility of the surface shell of Ni nanocrystals, which supports the results of molecular dynamics simulation and a generalized model of a nanocrystal with expanded surface layer. The analytical methods we developed based on the peak-profile of diffraction data allow us to identify “micro/local” yield due to high stress concentration at the grain-to-grain contacts and “macro/bulk” yield due to deviatoric stress over the entire sample. The graphic approach of our strain/stress analyses can also reveal the corresponding yield strength, grain crushing/growth, work hardening/softening, and thermal relaxation under high P–T conditions, as well as the intrinsic residual/surface strains in the polycrystalline bulks. From micro-indentation measurements, we found that a low-temperature annealing (T < 0.4 T_m) hardens nanocrystalline Ni, leading to an inverse Hall–Petch relationship. We explain this abnormal Hall–Petch effect in terms of impurity segregation to the grain boundaries of the nanocrystalline Ni.     

Flash Spark Plasma Sintering of 3YSZ: Modified sintering pathway and impact on grain boundary formation

Yttria stabilized zirconia (3 mol% YSZ) ceramics were prepared by Flash-SPS, while allowing high heating rates up to 200 °C/s, which led to the extremely fast densification within a few seconds. The high heating rates had strong impact on sintering mechanisms, in terms of densification and grain growth. While the specimens ended with 5–15 vol% porosity and limited grain growth (< 350 nm), their hardness is higher than fully dense counterpart SPSed ceramics. Using the sintering trajectories, microstructural observations, and impedance spectroscopy, we highlight altered sintering mechanism which resulted in very thin grain boundaries compared to SPS. It appears that densification is largely advanced at grain boundary interfaces, with no residual nano-pores at the grain junctions, where some pores with size comparable to grain size were present. This opens up opportunities for the fabrication of porous lightweight ceramics with good mechanical properties.     

Influence of microstructure on oxygen transport in perovskite type membranes

Abstract

The influence of bulk microstructure (grain size distribution, grain boundary length) on the oxygen transport properties of permeation membranes has been investigated. For this purpose, La_0.5Sr_0.5FeO_3-δ samples with different microstructures were prepared by varying sintering time and temperature. Average grain sizes, which ranged from 0.20 to 1.43 μm, were determined by SEM analysis. The oxygen transport properties of the samples were characterised by permeation measurements as a function of temperature in an air/argon oxygen partial pressure gradient. The fluxes presented a change in activation energy, which was attributed to a change in the rate limiting step, from bulk diffusion at lower temperature (< 850°C) to surface limitation at higher temperature (> 900°C). Only transport through the bulk was influenced by the microstructure, with the highest flux for the smallest grains. At 800°C, the fluxes were respectively 0.06, 0.03 and 0.01 μmol cm^-2 s^-1 through ≈ 1 mm thick samples with average grain sizes of 0.20, 0.63 and 1.43 μm respectively. This would imply that oxygen transport occurs more rapidly along grain boundaries than through the bulk. Grain boundary structure and composition were analysed by TEM.     

Characterisation of BaZr0.9Y0.1O3‐δ Prepared by Three Different Synthesis Methods: Study of the Sinterability and the Conductivity

BaZr_0.9Y_0.1O_3‐δ has been synthesised by three different methods: the solid‐state reaction, the spray pyrolysis and the spray drying. Significantly different apparent lattice parameters (between 0.4192 nm for the sample prepared by the solid‐state reaction method and sintered at 1,500 °C and 0.4206 nm for the sample prepared by the solid‐state reaction method and sintered at 1,720 °C) are observed after calcination and sintering, depending on the synthesis method and the sintering temperature. The bulk conductivity values also vary over several orders of magnitude (–7.2< log σ_b <–3.6 at 300 °C) depending on the synthesis method and the sintering temperature. These variations of the bulk conductivity and also the activation energy are correlated with variations of the apparent lattice parameter. The influence of the preparation method on the electrical properties is discussed. The grain boundaries are more resistive than the bulk. The variation of the grain boundary conductivity could be correlated to the microstructure in terms of the grain size.     

Thermal Conductivity in Nanocrystalline Ceria Thin Films

The thermal conductivity of nanocrystalline ceria films grown by unbalanced magnetron sputtering is determined as a function of temperature using laser‐based modulated thermoreflectance. The films exhibit significantly reduced conductivity compared with stoichiometric bulk CeO₂. A variety of microstructure imaging techniques including X‐ray diffraction, scanning and transmission electron microscopy, X‐ray photoelectron analysis, and electron energy loss spectroscopy indicate that the thermal conductivity is influenced by grain boundaries, dislocations, and oxygen vacancies. The temperature dependence of the thermal conductivity is analyzed using an analytical solution of the Boltzmann transport equation. The conclusion of this study is that oxygen vacancies pose a smaller impediment to thermal transport when they segregate along grain boundaries.     

Effect of lead content on the performance of niobium‐doped {100} textured lead zirconate titanate films

Niobium‐doped, {100} textured, gradient‐free, lead zirconate titanate (PZT) films were fabricated from solutions with different lead contents. Film lead content was controlled through changes in the average solution lead excess from 14.7% to 17 at.%. The low field dielectric response as well as the polarization‐electric field hysteresis loops were not a strong function of lead content. However, films with lower lead contents in the precursor tended to withstand higher poling fields than films prepared from more lead‐rich precursors. Although no residual PbO was observed at the grain boundaries, films prepared from more lead‐rich solutions had higher levels of grain‐boundary porosity, lower breakdown strengths, and lower threshold electric fields at which cracking was observed.     

Mechanism of "Solid-State" Single-Crystal Conversion in Alumina

Single crystals of Al₂O₃ were reproducibly grown from an MgO-doped polycrystalline precursor. The single crystals were grown through controlled abnormal grain growth at temperatures between 1670° and 1945°C. It was observed that CaO impurities segregated to the boundary between the single crystal and the polycrystalline region, and formed a wetting intergranular film. This type of film is required to produce the highly mobile grain boundaries that facilitate single-crystal conversion. The measured grain boundary mobilities correspond reasonably well with the mobilities calculated from data for a grain boundary containing a film with properties of the bulk glass, although some deviation from bulk behavior is indicated by the difference in activation energy. The grain boundaries are the most highly mobile alumina grain boundaries measured to date. This suggests that extrinsic effects produce the highest grain boundary mobility, rather than intrinsic behavior, which has conventionally been assumed to be the fastest.     

Ionic conduction,colossal permittivity and dielectric relaxation behavior of solid electrolyte Li3xLa2/3-xTiO3 ceramics

Perovskite-type solid electrolyte lanthanum lithium titanate (LLTO), exhibiting high intrinsic ionic conductivity, has been attracting interests because of its potential use in all solid-state lithium-ion batteries. In this work, we prepared LLTO ceramics by solid state reaction method and studied their conductivity and dielectric properties systematically. It is found that the bulk conductivity of LLTO is several orders of magnitude higher than the grain boundary conductivity. In addition, colossal permittivity was observed in LLTO ceramics in wide frequency/temperature ranges. Two non-Debye type relaxation peaks were observed in the imaginary part of permittivity, resulting from Li⁺ ions motion and accumulation near interfaces of grains/grain boundaries/electrodes. It is suggested that colossal permittivity may originate from the lithium ion dipoles inside the samples and the interfacial polarization of lithium ion accumulation near the grain boundaries. These results clarify the relations among colossal permittivity, relaxation behavior and ionic conduction in solid ion conductor ceramics.     

Dense Iodoapatite Ceramics Consolidated by Low‐Temperature Spark Plasma Sintering

Pb_9.85(VO₄)₆I_1.7, a potential waste form for long‐lived I‐129 immobilization, experiences phase decomposition and thus iodine loss at an elevated temperature above 400°C, presenting a significant challenge for effective management of radioactive iodine. In this work, we report low‐temperature consolidation of dense iodoapatite pellets with above 95% theoretical density by spark plasma sintering (SPS) at temperatures as low as 350°C for 20 min without iodine loss. Microstructure analysis indicates a nanocrystalline ceramic with an average grain size less than 100 nm. Grain growth dominates the sintered microstructure at higher temperatures and longer durations. The dense nanoceramics have significantly‐improved fracture toughness as compared with bulk coarsened grain structures. The effects of sintering temperatures (350°C, 400°C, 500°C, and 700°C) and durations (0–20 min) on microstructure, density, fracture morphology, and mechanical properties including Young's modulus and hardness of bulk samples were investigated. Low temperature densified iodoapatites suggest immense potential of SPS as an advanced materials fabrication technology for the development of waste forms for immobilization of volatile radionuclides including radioactive iodine.