Hanliang Zhu

Effects of Ageing and Seeding on the Formation of Zeolite Y from Coal Fly Ash

Zeolite Y was selectively synthesised by treating Tarong fly ash in a hydrothermal system. The effects of ageing and seeding on the formation of the resultant phases, crystallisation kinetics, and gel chemistry of Si and Al were investigated. Most of the Si and Al components in the Tarong fly ash could be effectively transformed into zeolite Y in the presence of seeds but not the mineral phase, like mullite. The maximum crystallinity of zeolite Y obtained was 72%. The cation-exchange capacity (CEC) of the fly ash was 0.08 mmol/g but increased to 3.2 mmol/g after a proper treatment. Crystallisation of zeolite materials from fly ash is quite different from that of normal zeolite synthesis because the sources of Si and Al are relatively less reactive and other cation ions (e.g., K+, Mg2+) are present in fly ash. Zeolite P is a competitive phase present in the resulting products that could be eliminated by employing the seeding method. NMR study demonstrated that ageing plays an important role in enhancing the hydrothermal condition during which both Si and Al in fly ash dissolved into a basic solution and reacted to form ring-like structures, and further to zeolite materials. Seeding can selectively induce the formation of zeolite Y and eliminate the processes of induction and nucleation.

Interfacial strengthening by soft phase in lamellar microstructure of TiAl alloys

The authors report a mechanism of interfacial strengthening during creep deformation of fully lamellar TiAl alloys. In the lamellar microstructure of TiAl alloys, the α2 phase or the β phase is softer than the γ phase at elevated temperature. Conventionally, the creep resistance increases with decreasing the volume fraction of the soft phases. However, the lamellar interfaces of γ∕α2 or γ∕β retard the motion of dislocations during creep, causing a great strengthening, and thus the creep resistance increases with increasing the amount of the phase interfaces. In this study, a physically based model is developed to explain the strengthening behavior of the soft phases in lamellar TiAl alloys.

Pore Structure Tailoring of Pillared Clays with Cation Doping Techniques

Techniques and mechanism of doping controlled amounts of various cations into pillared clays without causing precipitation or damages to the pillared layered structures are reviewed and discussed. Transition metals of great interest in catalysis can be doped in the micropores of pillared clay in ionic forms by a two-step process. The micropore structures and surface nature of pillared clays are altered by the introduced cations, and this results in a significant improvement in adsorption properties of the clays. Adsorption of water, air components and organic vapors on cation-doped pillared clays were studied. The effects of the amount and species of cations on the pore structure and adsorption behavior are discussed. It is demonstrated that the presence of doped Ca2+ ions can effectively aides the control of modification of the pillared clays of large pore openings. Controlled cation doping is a simple and powerful tool for improving the adsorption properties of pillared clay.

Classification of Streaking Defects on Anodized Aluminium Extrusions

Streaking is a common problem on anodised extrusions of 6xxx series soft alloys. This paper presents various types of streaking defects on the basis of industry practice and experimental results. The streaking defects are classified according to their root causes. This provides a basis for developing effective methods for preventing the formation of these defects for the extrusion.

Development of Oxide Dispersion Strengthened Steels for High Temperature Nuclear Structural Applications

Oxide dispersion strengthened (ODS) steels are the most promising candidate materials for high temperature nuclear applications. Mechanical alloying and subsequent thermomechanical treatments are applied to manufacture the ODS steels. Recently improved chemical composition and manufacturing processes have been developed to produce ultrafine grain size with high number-density of nanoscale oxide particles and high dislocation density in the microstructure. Usually, fine grains degrade creep resistance at elevated temperatures. However, the fine-grained ODS steels exhibit not only good radiation resistance, but also superior creep properties. The present paper reviews the chemical compositions, manufacturing processing, microstructural features, thermal creep properties and radiation resistance of recently developed ODS steels. Special attention is paid to the effects of the fine-scale microstructural features on thermal creep and radiation resistance.

Effect of process variables on the formation of streak defects on anodized aluminum extrusions: An overview

Abstract Streak defects are often present on anodized extrusions of 6xxx series aluminum alloys, increasing the fabrication cost of these products. Moreover, streaking often only becomes visible after etching and anodizing treatments, rather than in the as-extruded condition, making it difficult to identify the original causes and influencing factors of these defects. In this paper, various process variables that influence the formation of streak defects on anodized aluminium extrusions are reviewed on the basis of a literature review, industrial practice and experimental results. The influencing factors involved in various processing steps such as billet quality, extrusion process, die design and etching process are considered. Effective measures for preventing the formation of streak defects in industrial extrusion products are discussed.

Estimating Pore Size Distribution from the Differential Curves of Comparison Plots

Comparison plots, t- or alpha (s)-plot were constructed from the data of nitrogen adsorption on porous solids and on a nonporous reference. The variations in adsorption amount V with respect to t or alpha (s), represented by the tangent of the plots, dV/dt (or dV/d alpha (s)), are indicatives of the pore filling and thickening of the adsorbed layer. The shape of the differential plots, dV/dt (or dV/d alpha (s)), versus t (or alpha (s)) thus provide reliable information of the pore size distribution (PSD) of the adsorbents. Sharp peaks were observed on the differential plots for the samples of a uniform pore structure, such as alumina pillared clay and MCM-41 samples, because the filling of the uniform pores occurs in a very narrow range of relative pressure, and thus a very narrow range of t or alpha (s). For the samples of irregular pores, a broad peak on the differential curve is observed, reflecting a wide PSD. Besides, the position of the peak on t axis is intrinsically correlated to the pore size. An empirical expression for such a relation is suggested to be D = 6.347t, using experimental data of MCM-41 samples of various pore sizes. With this simple relation we can convert the differential curve to a PSD curve for microporous and mesoporous solids. This approach involves no mechanism of adsorption, concerning only the phenomenal process of adsorption by porous solids. It avoids theoretical assumption land thus the accompanying limitations), and complicated calculations. For the purpose of deriving PSD information from adsorption isotherm it is reliable and ready to be applied over a wide range of pore sizes.

A reliable and highly efficient exfoliation method for water-dispersible MoS2 nanosheet

Since the pioneering work on exfoliated single-layer graphene, layered inorganic nanosheet materials have been widely explored due to their unusual properties with potential applications in energy devices and optical electronics. Among these layered inorganic nanosheets, two-dimensional thin MoS2nanosheets show extraordinary properties such as the presence of a direct bandgap, magnetism, superconductivity and ferroelectricity. Over the past few years, solution-processed exfoliation methods of layered materials have been extensively studied; most of the exfoliation processes employ organic solvents or use surfactants as well as other functionalization agents. Although pure water is considered as an ideal solvent, however, it is generally believed stable dispersions of water could not be achieved due to poor solubility MoS2 in water. Thus, there are very limited studies for developing of water based MoS2 dispersions. Here we introduce a facile, green and reliable exfoliation method for producing water-dispersible MoS2 nanosheet without surfactant. Pure water was used as a solvent and this exfoliation process was achieved by thinning the bulk MoS2 by mechanical force between sandpapers and dispersing it through probe sonication in water. The exfoliated single or few-layered MoS2 nanosheets were characterized by TEM and SEM images. The lateral dimensions of the nanosheets were around 500 nm to 5 µm, the same range as obtained in the organic solvents as reported. Zeta potential measurements indicated that electrical charges may be responsible for the stabilization of the dispersions. Overall, it is concluded that with this exfoliation strategy, water can be used as a useful dispersible solvent for MoS2 nanosheets. Although the stability of the dispersions may not be as high as in organic solvents, the present method could be employed for a number of applications where the dispersions can be produced on site and organic solvents are not desirable.

The Effect of Milling Time on the Microstructural Characteristics and Strengthening Mechanisms of NiMo-SiC Alloys Prepared via Powder Metallurgy

A new generation of alloys, which rely on a combination of various strengthening mechanisms, has been developed for application in molten salt nuclear reactors. In the current study, a battery of dispersion and precipitation-strengthened (DPS) NiMo-based alloys containing varying amounts of SiC (0.5–2.5 wt %) were prepared from Ni-Mo-SiC powder mixture via a mechanical alloying (MA) route followed by spark plasma sintering (SPS) and rapid cooling. Neutron Powder Diffraction (NPD), Electron Back Scattering Diffraction (EBSD), and Transmission Electron Microscopy (TEM) were employed in the characterization of the microstructural properties of these in-house prepared NiMo-SiC DPS alloys. The study showed that uniformly-dispersed SiC particles provide dispersion strengthening, the precipitation of nano-scale Ni3Si particles provides precipitation strengthening, and the solid-solution of Mo in the Ni matrix provides solid-solution strengthening. It was further shown that the milling time has significant effects on the microstructural characteristics of these alloys. Increased milling time seems to limit the grain growth of the NiMo matrix by producing well-dispersed Mo2C particles during sintering. The amount of grain boundaries greatly increases the Hall–Petch strengthening, resulting in significantly higher strength in the case of 48-h-milled NiMo-SiC DPS alloys compared with the 8-h-milled alloys. However, it was also shown that the total elongation is considerably reduced in the 48-h-milled NiMo-SiC DPS alloy due to high porosity. The porosity is a result of cold welding of the powder mixture during the extended milling process.

The Role of Interfacial Precipitates on Creep Behaviour of Power Metallurgy (PM) Ti-48Al-2Cr-2Nb+1W Alloy

Pre-alloyed powders with a nominal composition of Ti-48Al-2Cr-2Nb+1W were consolidated by hot isostatic pressing (HIP). After the HIP process, a step cooled heat treatment (SCHT) with a carefully controlled cooling rate was applied to homogenize the HIP’ed microstructure and produce a fully lamellar microstructure. Following the SCHT, various isothermal aging at 950 °C and step aging processes form interfacial precipitates at the lamellar interfaces. The morphology, size, and distribution of the precipitates are dependent on the aging condition. Creep tests were carried out in air at 760 °C and 276 MPa to investigate the effect of interfacial precipitates. Primary creep resistance and creep life of the 8 and 144 hr aged conditions are improved substantially compared to the unaged condition due to the existence of the interfacial precipitates. However, the step aging process improves the creep resistance only slightly, probably because of the size and distribution differences of the interfacial precipitates compared to the 144 hr aged condition. Microstructure control is important since it has a substantial influence on creep behavior, especially primary creep resistance.