Functional Materials Letters

Morphology evolution of inorganic/organic crystals during crystallization is a universal growth phenomenon. In this work, we have developed a capping agent-assisted strategy to clearly identify the whole process of morphology evolution in solution growth system. One kind of morphology evolution trend with three types of morphologies of cuprous oxide (Cu₂O) was kinetically observed at varying the molar ratio of EDTA/Cu(II) under three different pH values. Two kinds of morphology evolution trends of zinc oxide (ZnO) were also kinetically observed in the presence of H₂O₂ and CH₃COOH (HAc), respectively. Simulation results show that the morphology evolution of nano- to micro-scale crystals is strongly dependent on the bonding characteristics of a growth system. The present strategy positively explores the interesting principles of morphology evolution of functional materials, and can be widely extended to nano- to micro-scale devices research.

Nitrogen-doped titania (TiO_2-xN_y) nanoparticles with excellent visible-light-reactive photocatalytic activity were successfully prepared by a microwave-assisted hydrothermal process. The obtained TiO_2-xN_y powders showed high specific surface area, fine particle size and excellent photocatalytic ability for the oxidative destruction of nitrogen monoxide under irradiation of both visible-light (λ > 510 nm) and UV light (λ > 290 nm).

This paper presents the work undertaken to understand chemical reactivity on LiNbO₃ surface induced by thermal change and optical irradiation to the crystal. Kelvin probe-based, electrostatic force microscopy is applied to map potential distribution on the surface of the LiNbO₃ with nanodomain structures fabricated using piezoresponse force microscopy. In-vacuum observation shows that the potential contrast and polarity of the nanodomain structures can be controlled by shifting temperature between 298 and 393 K. The measured potential difference between domains of opposite polarity suggests that surface adsorption plays a key role in potential formation. Upon ultraviolet irradiation, a preferential deposition of Ag particles on the +Z surfaces of LiNbO₃ becomes possible. We have inferred a mechanism by which photovoltaic effect causes the Ag particles to be deposited selectively on the +Z surfaces. The photovoltaic current density and ferroelectric crystal thickness appear to be fundamental parameters of the photocatalytic deposition.

The effects of Hf substitution and C content on the magnetic properties, phase evolution and microstructure of melt spun (Sm, Pr)Co_7-xHf_xC_y (x = 0–0.5; y = 0–0.14) ribbons have been studied. A proper Hf substitution is helpful not only in stabilizing 1:7 phase but also in enhancing its magnetic anisotropy field, as a result, magnetic properties of B_{r = 6.4 kG}, _iH_c = 7.3 kOe and (BH)_max = 8.7 MGOe for SmCo_6.9Hf_0.1 ribbons are obtained. Besides, a small amount of C addition in the ribbons could slightly modify phase constitution and effectively refine their microstructure to strengthen the exchange coupling effect between magnetic grains. It leads to the improvement of the magnetic properties for SmCo_6.8Hf_0.2C_0.12 nanocomposites. Finally, a slight Pr substitution for Sm may further increase the magnetization and the magnetic properties, the optimal magnetic properties of B_r = 7.1 kG, _iH_c = 8.5 kOe and (BH)_max = 11.2 MGOe could be achieved for the directly quenched Sm_0.8Pr_0.2Co_6.9Hf_0.1C_0.12 ribbons.

Monodispersed magnetite (Fe₃O₄) nanoparticles can be synthesized by thermal decomposition of iron(III) acetylacetonate, Fe(acac)₃. High saturation magnetization M_S of the magnetite particles is extremely important to realize the full potential of magnetite materials in biomedical application. In this work, we have studied the different effects (time, temperature and surfactant) on structure and magnetic properties of Fe₃O₄ nanoparticles. The M_S of the particles are enhanced after the synthesis at a higher reaction temperature and/or a longer reaction time. However, the increase in reaction temperature and/or reaction time resulted in particle size increase and the broadening of the particle size distribution. In this work, high M_S value of the magnetite particles has been achieved through adopting surfactant or modification of solvent to overcome the temperature and time effects, while the smaller size particles with an acceptable size distribution has been maintained. Size and morphology of the particles were studied by TEM while magnetic properties of the particles were measured using VSM. The saturation magnetization M_S of the particles can be increased at higher reaction temperature and/or longer reaction time, while narrow size distribution of the particles can be maintained either by the selective adsorption of oleic acid to the particle surface or by synthesizing them using solvent free thermal decomposition reaction.

Charge–discharge characteristics of electrical double layer capacitors (EDLCs) using κ-carrageenan–chitosan based electrolyte is the focus of the present work. Chitosan and κ-carrageenan were blended to obtain films with high mechanical strength. The room temperature conductivity of 0.5 g chitosan + 0.5 g κ-carrageenan (CCPA) film and 0.5 g chitosan + 0.5 g κ-carrageenan + 0.1765 g ammonium nitrate (CCPAAN) film are (1.38 ± 0.36) × 10^-6S cm^-1 and (2.39 ± 0.83) × 10^-4S cm^-1, respectively. The conducting species is H⁺ and conduction occurs via a Grotthuss mechanism. The resistance of the phenol resin based activated carbon electrode film is between 15 to 20 Ω. The capacitance of the EDLCs is stable over 20 cycles. The average discharge capacitance of the EDLC employing CCPAAN electrolyte and activated carbon electrodes is between 13 to 18.5 F g^-1.

The shape memory alloy strips of Ti₅₀Ni₃₀Cu₂₀ and Ti₅₀Ni₂₅Cu₂₅ have been fabricated by arc melt overflow technique. Their microstructures and shape memory characteristics were investigated by means of X-ray diffraction (XRD), optical microscopy and differential scanning calorimetry (DSC). The microstructure of the as-cast strips exhibited columnar grains normal to the strip surface. XRD analysis showed that the martensitic transformation of B2–B19 occurred in the alloy strips. During thermal cyclic deformation with the applied stress of 60 MPa, transformation hysteresis and elongation associated with the B2–B19 transformation were observed to be 4.9°C and 1.8% in Ti₅₀Ni₃₀Cu₂₀ alloy strip and 3.5°C and 1.7% in Ti₅₀Ni₂₅Cu₂₅ alloy strip. The as-cast strip of Ti₅₀Ni₂₅Cu₂₅ alloy also showed a perfect superelasticity and its stress hysteresis was as small as 14 MPa. These mechanical properties and shape memory characteristics of the alloy strips were ascribed to B2–B19 transformation and the controlled microstructures produced by rapid solidification of the arc melt overflow process.

In order to clarify the effect of annealing on the shape memory behavior of Ti-50.85at.%Ni alloy, the deformation and transformation behavior were investigated using tensile tests and differential scanning calorimeter (DSC). The martensitic transformation temperature increases with increasing annealing temperature until it reach as a maximum, and then decreases with further increasing annealing temperature. This can be rationalized by interaction between the distribution of Ti₃Ni₄ precipitates and recovery of cold-worked structure. The R-phase transformation temperature increases with increasing annealing temperature until reaching a maximum, and then decreases with a further increase of annealing temperature. This is attributed to the change of Ni content in the matrix caused by precipitation of Ti₃Ni₄. The critical stress for slip decreases rapidly with increasing annealing temperature, influenced by interaction between the distribution of Ti₃Ni₄ precipitates and recovery of cold-worked structure.

Porous Ti-50.5at.%Ni shape memory alloy (SMA) samples with a range of porosities were prepared by spacer sintering. The porous structure of the alloy was examined using scanning electron microscopy (SEM). The phase constituents of the porous TiNi alloy were determined by X-ray diffraction (XRD). The shape memory behavior of the porous TiNi alloy was investigated using loading–unloading compression tests. Results indicate that the porous TiNi alloy exhibits superelasticity and the recoverable strain by the superelasticity decreases with the increase of porosity. After a prestrain of 7%, the superelastically recovered strains for the porous TiNi alloy samples with porosities of 46%, 59%, 69% and 77% are 2.0%, 1.8%, 1.5% and 1.3%, respectively. The pores in the TiNi alloy samples cause stress/strain concentration, as well as crack initiation, which adversely affect the shape memory behavior of the porous TiNi alloy.

Polycrystalline BFO thin films of similar thickness but subjected to different annealing durations (2 hr and 4 hr) were deposited on SRO/Pt/TiO₂/SiO₂/Si substrates via RF sputtering. Phase identification by using X-ray diffractions confirms the pure BFO phase of the thin films fabricated. Polarization–Electric Field (P–E) loop study measured at different frequencies shows that a longer annealing duration led to the formation of space charges in the BFO thin films and thus caused a poor ferroelectric property observed in the hysteresis loop. The results of leakage current measurement for the two BFO films are consistent with the ferroelectric measurement, where a higher leakage current is demonstrated by the BFO film annealed for 4 hr.

This paper presents study of a novel impedance pumping induced by high frequency actuation. PZT (Lead Zirconate Titanate) cantilever beam was being integrated as a form of high frequency actuator to induce flow. The drive test microchannel has the dimension 15 × 3 × 0.4 mm³, and a significant flow rate of 36 μL/s has being measured. Furthermore, flow behavior as functions of actuation amplitude and frequency are presented. It is found that flows were in positive correlations with compression and frequency. The present novel PZT driven micro impedance pump may offer an alternate solution in microfludic systems.

Relaxor perovskite lead nickel niobate (PNN) ceramics were fabricated by using B-site precursor method, in which both columbite-NiNb₂O₆ and corundum-Ni₄Nb₂O₉ were separately employed as nickel niobate B-site precursors. It has been found that optimization of sintering condition can successfully lead to highly dense and pure PNN ceramics in both routes. Between the two B-site precursor routes, pure perovskite PNN ceramics with slightly better densification and dielectric properties can be produced by using the columbite-route.

Flexoelectric effect and its influence on the application of multifunctional ferroelectrics have been investigated. Theory of flexoelectric coupling has indicated that mechanical strain gradient can impact polarization in a way analogous to electric field. Experimentally, magnitudes of the flexoelectric coefficients have been measured in ferroelectric, incipient ferroelectric and relaxor ferroelectric perovskites. Present data of flexoelectricity suggests that such unconventional electromechanical coupling could make unique contribution to properly engineered ferroelectric thin films and nanostructures. Flexoelectric effect is expected to intensify at small dimensions and get large enough at nanoscale to significantly impact phase transition and functional response in ferroelectrics.

Titanium oxide nanotube is a promising material for versatile applications. However, its structure and formation mechanism were quite confusedly reported. Therefore, it is essential to fully understand its structure, formation mechanism and stability. In this study, the structure, formation and stability of titanium oxide nanotube were studied in detail. New experimental findings on the morphology of titanium oxide nanotubes were reported, demonstrating close-opened nanotubes, cone-shaped nanotubes, twin and ternary nanotubes etc. A structural model of the titanium oxide nanotubes is proposed on the basis of cylinders of smoothly curved sheets of TiO₆ octahedra, among which H⁺ and H₃O⁺ are intercalated. The formation mechanism of titanium oxide nanotubes was deduced and discussed on the basis of their structural characteristics and chemical bonding behaviors of constituent atoms. The stability of the nanotubes was also studied under different conditions.

The vertical Bridgman technique was an effective technique to grow congruent oxide crystals. However, it is difficult to grow incongruent crystals due to the erosion of the fluxes on seeds which was usually put at the bottom of the crucible. Recently, we developed a flux Bridgman growth process for some difficult-grown functional crystals, such as PZNT, ZnO and SLN. The properties of as-grown crystals were characterized and the crystal quality was evaluated. The results show that the flux Bridgman method is a potential approach to grow incongruent oxide crystals for industrial applications.

We have grown epitaxial SRO thin films of thickness of about 50 nm on the STO (001) substrates with a small miscut angle of 0.04° by step-flow mode. A mode transition from two-dimensional mixed growth of layer-by-layer and step-flow growth to complete step-flow growth was observed, which is associated with the change in the surface mobility of adatoms and the rate of edge diffusion along the steps.