Current Smart Materials - Volume 2, Issue 1, 2017

Background: Electrorheological (ER) fluids are composed of polarizable particles dispersed in insulating oil. The creep-recovery test can be applied to ER fluids to reveal the viscoelastic response of the dispersed particles which depend on conductivity of particles, amount of applied stress, magnitude of electric field strength, and particle volume fraction. In this study, ER properties of the Polyaniline/bentonite (PAni/BNT) particles, dispersed in silicone oil (SO), were investigated as functions of electric field strength (E), shear rate, shear stress, temperature, and frequency. Further, to determine the viscoelastic deformations of the ER fluid, creep and creep-recovery tests were also applied to the dispersion under external electric field strength. Methods: PAni/BNT conducting composite was synthesized by in situ chemical oxidative polymerization method and characterized by FTIR, UV-vis, TGA, SEM, apparent density, particle size, temperature- dependent conductivity (85-400K) and zeta (ξ)-potential measurements. To determine the ER and viscoelastic behaviors of the ER fluid, Thermo-Haake RS600 Rheometer (Germany) was used. Results: PAni/BNT conducting composite was successfully synthesized and its' structure and morphology were confirmed by FTIR and SEM analyzes. Thermal stability of PAni/BNT was observed to increase with the inclusion of BNT clay. Conductivity of the composite was found to be in semiconducting range which is suitable for ER measurements. ξ-potential results in SO revealed that the colloidal stability of PAni/BNT was enhanced to +44 mV which is high enough for an ER material. PAni/BNT was found to be sensitive to the external electric field strength, exhibiting a typical shearthinning non-Newtonian viscoelastic behavior. The changes of G´ and G´´ with frequency showed that PAni/BNT composite behaves as viscoelastic material having vibration damping capability under the influence of electric field strength at elevated frequencies. Creep and creep-recovery tests revealed that PAni/BNT shows viscoelastic deformation with 55% recovery under E = 2 kV/mm condition. Conclusion: The structure of PAni/BNT composite was characterized by various technics. The ξ- potential value of PAni/BNT particles dispersed in SO medium was determined to be in colloidally stable range with +44 mV. PAni/BNT/SO dispersion exhibited phase transition from a liquid-like to a solid-like with applied electric field strength, and yield stress value observed to reach to 200 Pa under E = 2 kV/mm. Finally, creep and creep-recovery tests were applied to PAni/BNT/SO dispersion and reversible non-linear viscoelastic deformations were observed with 55% recovery under E.

Background: Electrorheological fluids, when subjected to an external electric field, show radical and reversible changes in their physical properties: Rheological, thermal, electrical - as a result of the structuring of the dispersed phase particles. This article presents the results of experimental studies of the viscoelastic properties of high-concentrated electrorheological fluids with complex dispersed phase in a pre-yield area under shear stresses below the yield stress in different deformation modes. The components of the dispersed phase were bentonite clay and chromium phosphate or aerosil and chromium phosphate. Objective: The objective of this work is to determine dependencies of the viscoelastic parameters of high-concentrated electrorheological fluids with complex dispersed phase on electric field strength and on dispersed phase composition in different regimes of mechanical tests, to establish general regularities of electrorheological fluids deformation in non-stationary conditions. Methods: The investigation of the rheological properties in a pre-yield area under shear stresses below the yield stress were carried out in the following shear regimes: the forced tangential sinusoidal oscillations with constant frequency, linear increase of shear stress, linear increase of normal stress (compression mode), creep mode at constant shear stress and its cessation. Results: It was found that the sample with dispersed phase based on bentonite clay and chromium phosphate shows the biggest increase in the values of the components of the complex shear modulus among the studied compositions. The most robust static structure is observed in the sample containing aerosil and chromium phosphate. Preliminary holding in electric field (5 min) leads to hardening of the structure: in creep mode and under linear increase of shear stress the deformation decreases by 2-3 times. In the compression mode fluids exhibit linear elasticity for high field strengths in the investigated range of normal strains. At lower field strengths there is weakening of the structure and nonlinear deformation development with shear strain increasing. Conclusion: Variation of the regimes of mechanical tests and of electric field strength in combination with purposefully changed composition of electrorheological fluids makes it possible to vary their mechanical behavior. This allows one to create purposefully materials that have such characteristics as rigid, elastic, viscous, plastic properties and their combinations. The experimental dependences obtained in the work allowed the offering of a generalized non-stationary rheological model of elastoviscoplastic behavior of electrorheological fluids.

Background: Efficiency of the individual electrorheological materials is evaluated through their electrorheological characteristics. However, these characteristics are obtained using not only various geometries (plate-plate or concentric cylinders) but also completely different generation of an applied electric field. Objective: Responsibility (comparison) of an electrorheological characterization by the individual measurements. Methods: This study summarizes and compares the results for one identical material (suspension of polyaniline powders mixed (10 wt%) in silicone oil) using both geometries housed in two commercial rotational rheometers: MCR 501 (Anton Paar Co.) and Gemini CVOR 150 (Malvern Instruments) in which the electric circuit loops are completed by a metal spring wire leaning against a central shaft and through an electrolyte filled in the circumferential gutter, respectively. Results: In the case of using the parallel-plate geometry, the data obtained by Physica MCR 501 provides higher values than those obtained by the Gemini CVOR 150. For concentric cylinders the data obtained by both devices are comparable. Conclusion: For both geometries and devices the functional behaviour of the studied rheological characteristics is in full correspondence.

Background: Recently, core-shell typed electrorheological particles have been developed and become a research focus. In this work, monodisperse polystyrene/poly(2-methylaniline) core-shell microspheres were prepared and the shell morphology were adjusted to either smooth or urchin-like. Therefore, the influence of surface morphology on electrorheological effect was studied by comparing the electrorheological properties of the two kinds of core-shell microspheres. Methods: The surface morphology of microspheres was adjusted by using different initiators and the products were observed by scanning electron microscopy. Their electrorheological properties were tested by a rotational rheometer. Results: Sea urchin-like shells can be obtained by using iron (III) nitrate nonahydrate as an initiator. While ammonium persulphate could generate a high density of polymerisation centres, and smooth surfaces can be readily formed. In the shear flow curves, the sea urchin-like particles showed much higher shear stress than the particles with smooth surface. In the dynamic sweep tests, the sea urchinlike particles exhibited broader linear viscoelastic region and higher storage moduli under electric field. Conclusion: The low oxygen potential of the ferric salt leads to the growth of poly(2-methylaniline) nanofibres, while ammonium persulphate with a higher oxygen potential tends to form a smoother surface. The sea urchin-like polystyrene/poly(2-methylaniline) particles showed higher electrorheological properties. This indicates that the sea urchin-like particles can generate larger drag force and interparticle friction due to the rough surface and larger surface area of the sea urchin-like particles.

Background: Layered inorganic materials are important candidates as smart electrorheological (ER) materials because of their low cost and anisotropic morphology. However, the ER effect of most of layered materials is too weak to satisfy the requirement of applications. This paper reported a new kind of layered ER material based on polyaniline-intercalated titanate nanoplates (PANITNP). We presented the preparation, structure characterization, and rheological measurement. And finally we conducted dielectric spectra to analyze the mechanism of enhanced ER effect after intercalation of conducting PANI into TNP. Methods: An intercalation chemical process was used to prepare the aniline-intercalated titanate nanoplates and then an oxidative polymerization was further employed to prepare PANI-TNP. Different techniques including scanning electron microscopy, X-ray powder diffraction, Fourier transform infrared spectrometer, thermogravimetric analysis were used to analyze the morphology, structure and thermal properties of samples. A stress-controlled electro-rheolometer was used to measure the rheological curve of PANI-TNP ER suspensions. Two-point method was used to measure the DC conductivity. An impedance analyzer was used to analyze the dielectric spectra. Results: The structure characterization showed that the PANI-TNP possessed 100 nm thickness and 500-1000 nm lateral size, and PANI was intercalated into the interlayer of layered titanate with weight content of ~ 9%. Under electric fields, the investigation about rheological properties of suspension containing PANI-TNP showed that the PANI-TNP suspension exhibited significantly higher ER effect compared to the suspensions containing pure TNP. At the same electric field, the shear stress and ER efficiency of PANI-TNP suspension was 2.5 times and 7 times as high as those of pure TNP suspension. The dielectric measurements indicated that the intercalation of conducting PANI into TNP significantly improved particle polarization, which was in accordance with its enhanced ER effect. Conclusion: To enhance ER effect of layered materials, this paper developed a kind of polyanilineintercalated titanate nanoplates (PANI-TNP). The intercalation of conducting polyaniline into TNP could significantly improve the particle polarization and this results in the enhancement of ER effect of TNP.

Background: Materials like ZnO, TiOx, ZnS have wide applications in energy technology especially in device and sensor applications. Determination of mechanical properties like hardness, toughness and adhesion of these materials deposited in the form of thin films is possible through nanoindentation and scratch test. The assessment of these mechanical properties improves the quality of application of the oxide and sulphide thin films in energy devices. Methods: Nanoindentation is a type of indentation where the indenter size is of nanometric order. It is a depth sensing indentation based upon continuous stiffness mode (CSM). The analysis includes fitting the load depth curve obtained with a power law equation developed by Oliver and Pharr. The area under the load depth curve gives the plastic energy. Indentation at higher loads leads to crack formations which are used in studying the toughness of the sample. The initiation and propagation of cracks are the two most important aspects of indentation fracture mechanics. The nature of the cracks varied from radial, lateral to ring like. Scratch tests with a ramping load are used to assess the adhesion of the coating with the substrate. The load at which the coating fails is called the critical load (Lc). The nature of failure occurring in the scratch track is again related to a combination of coating and substrate hardness. Results: We approached the failure in terms of deconvoluting the failure region into geometrical cardioid shapes with delaminated region deconvoluted into three geometric cardioids with cusp meeting with indenter corner. The crack and delaminated area dimensions give us the deamination aspect ratio, shape factor and crack growth rate. The penetration depth during scratch and after load removal indicates elastic recovery of the sample. The optical picture of the scratched region indicates chevron cracks. Conclusion: A novel method of deconvoluting the failure region surrounding nanoindentation with geometrical shapes was introduced. The nature of film and substrate was found to have effect over sink in and pile up effect during nanoindentation. The chevron cracks during scratch tests are through thickness cracks 45° to the scratch direction and indicate cohesive failure of the coating before adhesive failure.

Background: Emission of Secondary electron from material surface is the key parameter which defines all properties of plasma bounded by that surface. In dc glow discharge plasmas, these emitted electrons from the cathode provide the necessary feedback to sustain the discharge. There are various processes responsible for emission of electron from the cathode surface. The most dominant among these processes are bombardment of energetic neutrals, excited atoms or molecules, photons, surface reactions etc. in addition to the ion induced secondary electron emission. Further, among these various governing processes contribution of each process, to secondary electron yield vary according to the operating discharge condition. The secondary electrons emitted due to all the above mentioned processes are called Effective Secondary Electron Emission Coefficient (ESEEC). Measurement of ESEEC is highly desired in theoretical as well as experimental gas discharge physics. In this work, we propose a novel model for estimation of ESEEC and the results were obtained using this model for cathode material made up of Brass. Methods: A self-consistent model for measurement of ESEEC is developed under glow discharge plasma condition. It requires knowledge of only two experimental parameters namely total discharge power across the electrode (PT) which is product of discharge current (I) and applied voltage (V) and the fraction of power carried by ion to the cathode (Pi) to estimate the value of ESEEC. The power carried by ion to the cathode has been estimated using power balance at cathode. Results: The measured value of ESEEC varies from 1.65 - 0.81 for the constant cathode bias of -600 V and pressure range of 0.08 - 0.4 mbar. The obtained experimental results are verified by two methods of power influx measurement namely, from the slope of temporal temperature profile of cathode while cooling and heating cycles and from the Stefan's Boltzmann Law at saturation temperature of the cathode. The obtained values of power influx from both the methods are in good agreement with each other. Conclusion: A self consistent model for measurement of ESEEC has been proposed. Using this model, the value of ESEEC for Brass has been successfully obtained, under glow discharge plasma condition. The proposed model is based on power balance in a discharge process and requires measurement of only two discharge parameters, namely, total discharge power (PT) and the fraction of discharge power carried by ions i.e. (Pi). This model may be applied over wide range of operating parameters to obtain the value of ESEEC for different materials.

Background: As a kind of intelligent materials, electrorheological (ER) fluids have been researched increasingly deeply and widely. Dynamic viscoelastic property is an innate and important property of EF fluids, which can be controlled by external electric field strength (E). However, viscoelasticity of zeolite-based ER fluids has not been researched deeply in small strain under enhanced E. Methods: This work is focused on the dynamic viscoelasticity under enhanced E with small oscillation amplitude. In this work, zeolite-based ER fluids were prepared with five mass fractions, and their dynamic viscoelastic properties were measured by a commercial rheometer under small amplitude oscillation in E sweep mode. Results: Two singular points including the inflected point and the crossover point under different angular frequencies were observed for the first time in various concentrations of ER fluids when chains or clusters formed with E increasing. The inflected E increased with mass fraction of ER fluid increasing, while loss tangent (tan) decreased slightly. The crossover E decreased with mass fraction of ER fluid increasing, while tan increased. Conclusion: In this work, ER fluids in five mass fractions showed concentration-dependent inflection of tan, in which inflected E increased and inflected tan decreased slightly with the concentration increasing. The inflection was ascribed to similar elastic properties and viscous force under applied electric field in different angular frequencies here. Besides, the crossover of tan showed frequencyindependent and concentration-dependent properties which were explained by a viscoelastic model.

Background: We explored the profiles of binding energy (BE) and interband emission energy (IEE) of impurity doped quantum dots (QDs) in presence and absence of Gaussian white noise. In this context, change of BE and IEE by varying several pertinent parameters such as electric field, magnetic field, confinement potential, dopant location, dopant potential and aluminium concentration had also been carried out. Methods: Time-independent Schrödinger equation has been solved variationally by expressing the wave function as a linear combination of product of harmonic oscillator eigenfunctions. Results: Introduction of noise caused enhancement and depletion of BE and IEE which used to depend upon its pathway of application. The effective confinement potential and the difference between the impurity-free ground state energy and the binding energy played anchoring roles in designing the BE and IEE profiles, respectively. Conclusion: The findings highlighted the governing role delineated by noise in tuning the BE and IEE of QD systems containing impurity.

Background: The Thermoelectric (TE) materials and devices have been receiving much attention over past few decades due to their capacity and conversion between electrical energy and heat energy. The performance of thermoelectric materials can be estimated by the dimensionless figure of merit (ZT) and it is expressed as, ZT = σ S2T /Κ. So researchers are seeking for TE materials with high figure of merit. It is well known that density functional theory calculations and Boltzmann transport theory are suitable methods for predicting TE properties. Strontium titanate (SrTiO3) is the model system for thermoelectric enhancement on Perovskite based oxides. It contains nontoxic and environment friendly substances as compared with other thermoelectric systems. Methods: The theoretical investigations were done using the DFT with the plane wave pseudopotential formalism in Quantum Espresso package. Ultrasoft pseudopotential with the Perdew Burke Ernzerhof (PBE) exchange correlation function under GGA was used for calculations. In addition, theoretical investigations of transport properties for SrTiO3 were conducted within the semi classical Boltzmann transport theory, using a maximally localized Wannier function (MLWF). Results: The band structure clearly shows that valence and conduction bands are present at R and Γ respectively at Brillouin Zone. It reveals that, SrTiO3 exhibits an indirect band gap. The theoretical indirect band gap at Γ- R is 1.8616 eV. The transport properties, such as seebeck coefficient, thermal conductivity, electrical conductivity and figure of merit as a function of chemical potential, were calculated and compared with other available data. Conclusion: Density of states and electronic band structure are well matched with the available data. The semi classical Boltzmann transport theory was applied to study the transport properties of SrTiO3 using a maximally localized Wannier functions with constant relaxation time approximation. Present calculations suggest that this compound may be suitable candidate for thermoelectric applications.