127 research outputs found
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Deciphering the Effects of Decentralization on Water Rights: State to Urban Inconsistencies in Bolivia
In a country that is internationally praised for its recent economic success, how has the decentralized government of Bolivia left the responsibility to the local department in securing their own basic human needs? Is the disconnection beneficial in that it provides communities with a propitious autonomy? Or does a decentralized government provide benefits for both national and local sectors? This project demonstrates the benefits and drawbacks of a decentralized Bolivian government that has intended to rely on the role of small-scale grassroots organizations and NGOâs to fill the sobering gap between state and local governments.
Using an urban political-ecology approach, this study investigates the socio-hydroscape in Cochabamba, Bolivia. By tracing the metabolic patterns of water in and out of the city, revealing the tensions in power-relations regarding an uneven distribution of resources, a very concentrated conflict in the city becomes increasingly apparent. I argue that this conflict is posited by larger, overarching state policies that have allowed them to persist. Major political reform implemented during the height of Boliviaâs neoliberal era has given historically oppressed communities their long-sought autonomy from the state, while simultaneously producing a market-based system converting water into a commodity. These state policies have jointly manifested a seemingly perfect scenario for improving the lives of marginalized groups, but in reality continues to hinder communities dependent on private networks and grassroots organizations known as water-committees to acquire potable water. The state-to-urban inconsistencies create a paradox forcing all Bolivianos to become subjects in the central government\u27s political discourse
Macroscopic constitutive model for ergodic and non-ergodic lead-free relaxors
A fully electromechanically coupled, three dimensional phenomenological constitutive model for relaxor ferroelectric materials was developed for the use in a finite-element-method (FEM) solution procedure. This macroscopic model was used to simulate the macroscopic electromechanical response of lead-free ergodic 0.94Na1/2Bi1/2TiO3â0.06BaTiO3 and non-ergodic 0.90Na1/2Bi1/2TiO3â0.06BaTiO3â0.04K0.5Na0.5NbO3 relaxor materials. The presented constitutive model is capable of accounting for the observed pinched hysteretic response as well as non-deviatoric polarization induced strain and internal order transitions. Time integration of the history dependent internal variables is done with a predictor-corrector integration scheme. The adaptability of the constitutive model regarding the pinching of the hystereses is shown. Simulations are compared to experimental observations
Influence of Oxygen Vacancies on the Impedance Spectrum of Al2O3âNa0.5K0.5NbO3 Composites
Composites based on ferroelectric Na0.5K0.5NbO3 (NKN) and paraelectric Al2O3 were observed using impedance spectroscopy at different temperatures. This allowed for the evaluation of the conduction processes of the bulk and grain boundary, revealing the occurrence of interdiffusion and changes in the conductive properties. The effective conductivity decreased with the increase in Al2O3, which is due to the highly resistive nature of Al2O3. Interestingly, the activation energy for the bulk response increased from 0.87 ± 0.06 eV to approximately 1.12 ± 0.03 eV, and the activation energy of the grain boundary decreased from 1.26 ± 0.09 eV to 0.99 ± 0.09 eV. These observations of the activation energy revealed the interdiffusion of Al3+ ions, leading to the formation of oxygen vacancies in close vicinity to the grain boundary. As a result, the conduction path for charge carriers shifted to take place predominantly along the grain boundary
Tailoring of unipolar strain in lead-free piezoelectrics using the ceramic/ceramic composite approach
The electric-field-induced strain response mechanism in a polycrystalline ceramic/ceramic composite of relaxor and ferroelectric materials has been studied using in situ high-energy x-ray diffraction. The addition of ferroelectric phase material in the relaxor matrix has produced a system where a small volume fraction behaves independently of the bulk under an applied electric field. Inter- and intra-grain models of the strain mechanism in the composite material consistent with the diffraction data have been proposed. The results show that such ceramic/ceramic composite microstructure has the potential for tailoring properties of future piezoelectric materials over a wider range than is possible in uniform compositions.open1
Fabrication of porous thick films using roomâtemperature aerosol deposition
Abstract
A novel technique for the rapid roomâtemperature deposition of porous ceramic, glass, or metal thick films using the aerosol deposition (AD) method is presented. The process is based on the coâdeposition of the desired film material and a second waterâsoluble constituent, resulting in a ceramicâceramic composite. Following the subsequent removal of waterâsoluble end member, a network of pores is retained. To demonstrate the process, porous BaTiO3 thick films were fabricated through coâdeposition with NaCl. Microstructural images show the clear development of a porous structure, which was found to enhance the dielectric properties over dense thick films, possibly related to the lower extent of internal residual stress. This simple but highly effective porous structure fabrication can be applied to any film and substrate material stable in water and is promising for the application of ADâprocessed films in gas sensors, solid oxide fuel cells, and humidity sensors
Local structure change evidenced by temperature-dependent elastic measurements: Case study on Bi1/2Na1/2TiO3-based lead-free relaxor piezoceramics
The temperature-dependent Young's modulus Y(T) of the lead-free piezoceramics of 0.8Bi(1/2)Na(1/2)TiO(3)-0.2Bi(1/2)K(1/2)TiO(3) (20BKT) and 0.96(0.8Bi(1/2)Na(1/2)TiO(3)-0.2Bi(1/2)K(1/2)TiO(3))-0.04 BiZn1/2Ti1/2O3 (4BZT) is measured with the impulse excitation technique and contrasted with corresponding dielectric and structural data. While the dielectric properties suggest a phase transition, the high resolution XRD patterns remain virtually unchanged from room temperature up to high temperatures, confirming no change in their long-range order. In contrast, the elastic properties indicate a broad and diffuse ferroelastic transition denoted by a minimum in Y(T). By analogy to the elastic and dielectric data of PbZrxTi1-xO3 and PLZT, it is concluded that 20BKT and 4BZT are relaxors with polar nanoregions embedded in a metrically cubic matrix. Interestingly, no indication for the freezing temperature was reflected in any of the employed measurement techniques. From the saturation of Y(T), it is suggested that the Burns temperature may be approximated as 700 degrees C. Moreover, it is found that the modification with the ternary end-member BiZn1/2Ti1/2O3 results in an increase in Young's modulus. A comparison with the Bi1/2Na1/2TiO3-BaTiO3-K0.5Na0.5NbO3 yields the same results.open0
Stress-modulated relaxor-to-ferroelectric transition in lead-free (Na1/2Bi1/2)TiO3-BaTiO3 ferroelectrics
The effect of external mechanical fields on relaxor 0.94 (Na1/2Bi1/2)TiO3−0.06 BaTiO3 was investigated by means of temperature- and stress-dependent dielectric constant measurements between 223 and 673 K. Analogous to previous investigations that showed an electric-field-induced ferroelectric long-range order in relaxor ferroelectrics, we show that compressive stress can also result in the transition to the long-range ferroelectric order, marked by the formation of an anomaly in the permittivity-temperature curves and a nonlinear, remanent change in permittivity during mechanical loading. In situ stress-dependent high-energy x-ray diffraction experiments were performed at room temperature and reveal an apparent phase transition during mechanical loading, consistent with previous macroscopic electrical measurements. The transition lines between the relaxor states and the stress-induced ferroelectric state were determined at constant temperatures with stress-dependent dielectric constant measurements, providing a stress-temperature phase diagram
Electric-field-induced paraelectric to ferroelectric phase transformation in prototypical polycrystalline BaTiOâ
An electric-field-induced paraelectric cubic to ferroelectric tetragonal phase transformation has been directly observed in prototypical polycrystalline BaTiO3 at temperatures above the Curie point (TC) using in situ high-energy synchrotron X-ray diffraction. The transformation persisted to a maximum temperature of 4-°C above TC. The nature of the observed field-induced transformation and the resulting development of domain texture within the induced phase were dependent on the proximity to the transition temperature, corresponding well to previous macroscopic measurements. The transition electric field increased with increasing temperature above TC, while the magnitude of the resultant tetragonal domain texture at the maximum electric field (4-kV mm-1) decreased at higher temperatures. These results provide insights into the phase transformation behavior of a prototypical ferroelectric and have important implications for the development of future large-strain phase-change actuator materials
Designing properties of (Na1/2Bix) TiO3-based materials through A-site non-stoichiometry
Point defects largely determine the properties of functional oxides. So far, limited knowledge exists on
the impact of cation vacancies on electroceramics, especially in (Na1/2Bi1/2)TiO3 (NBT)-based materials.
Here, we report on the drastic effect of A-site non-stoichiometry on the cation diffusion and functional
properties in the representative ferroelectric (Na1/2Bi1/2)TiO3âSrTiO3 (NBTâST). Experiments on NBT/ST
bilayers and NBTâST with Bi non-stoichiometry reveal that Sr2+-diffusion is enhanced by up to six orders
of magnitude along the grain boundaries in Bi-deficient material as compared to Bi-excess material with
values of grain boundary diffusion B108 cm2 s
1 and B1013 cm2 s
1 in the bulk. This also means
a nine orders of magnitude higher diffusion coefficient compared to reports from other Sr-diffusion
coefficients in ceramics. Bi-excess leads to the formation of a material with a coreâshell microstructure.
This results in 38% higher strain and one order of magnitude lower remanent polarization. In contrast,
Bi-deficiency leads to a ceramic with a grain size six times larger than in the Bi-excess material and
homogeneous distribution of compounds. Thus, the work sheds light on the rich opportunities that
A-site stoichiometry offers to tailor NBT-based materials microstructure, transport properties, and
electromechanical properties.T. F., A. A., and K. G. W. gratefully acknowledge financial support
by the Deutsche Forschungsgemeinschaft under WE 4972/2 and
FR 3718/1-1. T. F. thanks Dr Edvinas Navickas for his help with the ToF-SIMS measurements. M. A. acknowledges the support of the
Feodor Lynen Research Fellowship Program of the Alexander von
Humboldt Foundation. M. D. and L. M.-L. acknowledge financial
support from the Hessen State Ministry of Higher Education,
Research and the Arts via LOEWE RESPONSE. L. M.-L. acknowledges financial support from DFG Grant MO 3010/3-1
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