Gas sensing properties of nanocrystalline metal oxide powders produced by thermal decomposition and mechanochemical processing

The objective of this research, was the synthesis of LaFeO3 and SnO2 fine powders for the subsequent preparation of thick film gas sensors. On producing fine metal oxide powders, often it is not possible to ensure separation of the particles during the synthesis, resulting in the formation of highly agglomerated material. In addition, there are often high synthetic costs associated with the powders obtained by these methods. Thermal decomposition and mechanochemical processing methods were selected to produce fine metal oxide powders. Thermal decomposition of a heteronuclear complex is a simple and relatively cheap method. Heat treatment of La[Fe(CN)6] · 4H2O leads to single-phase perovskite-type LaFeO3 fine powders. Heating in the temperature range 600-750 °C causes fast crystallite growth of slightly agglomerated particles and X-ray diffraction analysis showed only the pattern of orthorhombic transition phase of LaFeO3 particles. A paste for the preparation of the LaFeO3 thick film coating was obtained by mixing of polyvinyl alcohol solution and decomposed powder in a ball mill for 1 h. It was determined that there are two factors important for gas sensing, concentration of surface metal ions [Fe3+], and the concentration of oxygen adsorptive sites [Vo(..)]. LaFeO3−δ thick film with small crystallites, promotes a more rapid NO2 gas reaction at the surface and allows an equilibrium state to be obtained at 350 °C. Mechanochemical processing (MCP) is selected as the second, low cost method of manufacturing of fine powders in a conventional ball mill. During milling, deformation, fracture, and welding of powder particles continuously occur. The chemical reactions are activated by the repeated ball-powder collisions. Most of the reports on MCP that have appeared to date, concern the use of high-energy mills. It is shown that it may be possible to produce fine powder particles using a centrifugal mill of the conventional type instead of high-energy one. Nanocrystalline SnO2 powder was produced by two different chemical reactions. The first reaction, initiated by ball milling, produces water and the second reaction does not produce water. It should be noted that water, produced by the chemical reaction during milling, has a considerable influence on the reactivity of surface. Milling of predetermined stoichiometric amounts of SnCl2 with Ca(OH)2 and K2CO3 in an excess of CaCl2 and KCl respectively, resulted in the formation of the desired mass of SnO. After heat treatment and removal of the salt, slightly agglomerated SnO2 particles were produced with a tetragonal phase, confirmed by X-ray diffraction pattern. A very narrow particle size distribution of the powder is observed. The response of the LaFeO3 thick film to NO2 gas is investigated in the temperature range 250-350 °C, where the surface reactions are moderately fast. On exposure to low concentrations of H2S gas in air in the range 20-50 ppm the SnO2 film, prepared from anhydrous powder has higher gas response than the film prepared from hydrated powder.reviewe

Comparison of electrophysical properties of PZT-Type ceramics obtained by conventional and mechanochemical methods

In the paper, the multicomponent PZT-type ceramics with Pb(Zr0.49Ti0.51).94Mn0.015Sb0.01W0.015Ni0.03O3 composition have been obtained by conventional and mechanochemical methods. With conventional ceramic technology, PZT-type ceramics have been synthesized by the method of calcination powder (850 C/4 h). Instead of this step, the mechanochemical synthesis process for di erent milling periods (15 h, 25 h, 50 h, 75 h) has been applied for a second batch of samples. To obtain the dense PZT-type ceramic samples, powders have been sintered by free sintering method at conditions of 1150 C/2 h. Studies have shown that the perovskite structure of the PZT-type material is formed during mechanochemical activation of powders during the technological process at low temperature. The application of the mechanochemical synthesis to obtain the PZT-type materials also allows shortening of the technological process, and the useful electrophysical properties of ceramic samples are not reduced at the same time. The presented results have confirmed that the investigated materials can be used in microelectronic applications, especially as elements of actuators and piezoelectric transducers

Review of Artificial Muscle Based on Contractile Polymers

An artificial muscle with strength and speed equal to that of a human muscle may soon be possible. Polymer gels exhibit abrubt volume changes in response to variations in their external conditions -- shrinking or swelling up to 1000 times their original volume. Through the conversion of chemical or electrical energy into mechanical work, a number of devices have already been constructed which produce forces up to 100N/cm2 and contraction rates on the order of a second. Through the promise of an artificial muscle is real, many fundamental physical and engineering questions remain before the extent or limit of these devices is known

Modeling strategy for dynamic-modal mechanophore in double-network hydrogel composites with self-growing and tailorable mechanical strength

Smart materials with self-growing and tailorable mechanical strength have wide-range potential applications in self-healing, self-repairing, self-assembly, artificial muscle, soft robots and intelligent devices. However, their working mechanisms and principles are not fully understood yet and mathematically and physical modeling is a huge challenge, as traditionally synthesized materials cannot self-grow and reconstruct themselves once formed or deformed. In this study, a phenomenological constitutive model was developed to investigate the working mechanisms of self-growing and tailorable mechanical strength in double-network (DN) hydrogel composites, induced by mechanochemical transduction of dynamic-modal mechanophore. An extended Maxwell model was firstly employed to characterize the mechanical unzipping of hydrogel composites, and then mechanochemically induced destruction and reconstruction processes of brittle network in the hydrogel composite were formulated. The enhanced mechanical strength of brittle network has been identified as the key driving force to generate self-growing and tailorable mechanical strength in the hydrogel composite. Finally, a stress-strain constitutive relationship was developed for the dynamic-modal mechanophorein the hydrogel composite. Simulation results obtained from the proposed model were compared with the experimental data, and a good agreement has been achieved. This study provides an effective strategy for modelling and exploring the working mechanism in the mechanoresponsive DN hydrogel composites with self-growing and tailorable mechanical strength

Structural and dielectric properties of Ba(Ti0.96Sn0.01Zr0.03)O3 perovskite nanoparticles fabricated by mechanochemical synthesis route

Lead free Nanocrystalline BaTiO3 doped with Sn and Zr is prepared by a combination of solid-state reaction and high energy ball milling (HBM) technique in a temperature range 30–150 °C, over a frequency range 40 Hz – 1 MHz. A single-phase nanocrystalline sample with ABO3 type of perovskite structure with cubic symmetry was confirms by XRD diffraction. The crystallite and grain size determined from Scherrer equation and intercept method are 38.2 nm and 46.13 nm respectively. FE-SEM images show samples are dense and have different microstructures with certain amount of porosity. A grain size of 46.13 nm is obtained by using linear intercept method. Room temperature (RT) variation of ′and tan as a function of frequency of the modified BT system has also been studied. Variation of dielectric properties with frequency shows the usual behaviour of dielectric materials i.e decrease of the value of ′ with the increase of frequency. A dielectric anomalies is observed corresponding to phase transitions viz tetragonal to cubic (− ) at 70oC. These effect can guide to design the nanostructure for various practical applications of MLCC