Application Of Energy Technologies Research Articles

The deposition characteristics of micron particles in heat exchange pipelines

Comprehensive understanding of particles deposition in heat exchange pipelines is significant for environment and energy engineering applications. In this study, a numerical simulation method was used to investigate the deposition characteristics of micron particles in heat exchange pipelines based on Reynolds stress model and discrete particle model. Based on previous research, an improved particle deposition model combined with particle rebound was presented. Moreover, the improved model was used to study the effect of thermophoretic force (hot wall and cold wall), flow velocity, and particle diameter on the non-dimensional deposition rate of particles. The results showed that the non-dimensional deposition rate increased with the increase of non-dimensional relaxation time and then approached a certain value. After introducing the particle rebound, the non-dimensional deposition rate exhibited different trend, i.e., first increasing and then decreasing. When the gas had significant temperature gradient in the boundary layer, the effect of thermophoretic force on small particles was found to be significantly stronger than that of large particles. Moreover, the higher the temperature gradient, the more obvious the thermophoretic force of the particles. The results indicated that the thermophoretic forces prevented the deposition of particles on the hot wall, and promoted this process on the cold wall. Furthermore, the flow velocity also significantly affected the particle deposition rate. At the same flow velocity, the particles non-dimensional deposition rate first increased and then decreased with the increase in the particle diameter, and it showed a different maximum at different flow velocities.

The Analytical Transport Network Model for Energy Materials Design and Characterization: Extensions for the Modeling of Coupled Flows and Structural Mechanics

The design and characterization of next-generation, heterogeneous materials for application in energy technologies (batteries, fuel cells, etc.) increasingly relies on a fundamental understanding of how the 3-D properties of a material’s microstructure influences its performance. The Analytical Transport Network (ATN) Model was developed to offer a computationally quick and inexpensive means for analytically examining the link between 3-D structure and performance in such materials [1], [2]. In this talk, we present recent extensions of the model to constant-property, coupled, flow phenomena that conform to Onsager reciprocity (e.g., thermoelectric or electrokinetic phenomena). In addition, we discuss the development of a mechanics of materials model written in ATN nomenclature, which makes it possible to characterize a network’s flow and mechanical properties using the same analytical formalism. Finally, we present refinements and extensions to our software for applying the model to 3-D, voxel-based images, which can be artificially generated or obtained by 3-D imaging techniques, such as x-ray tomography. The ATN model is a lumped-element approach for modeling potential flow (and recently structural mechanics) in microstructural networks, e.g., in porous materials, heterogeneous materials, or, generally, in any composite whose components comprise multiple, interwoven networks of channels [1], [2]. The central question that ATN aims to address is: How does the relative arrangement and the individual geometric characteristics of a network’s channels, i.e., their topology and morphology, impact the network’s flow behavior? The question is motivated by the longer-term goal of rationally designing microstructural networks to obtain desired material properties. In [1]and [2], steady-state, diffusive potential flow of a single-entity was considered, e.g., electrical/heat conduction or trace species diffusion. In [2], the model additionally accounted for the loss or gain of flow across channel boundaries, e.g., heat loss due to surface convection or species consumption due to 1storder surface reactions. ATN predictions of effective network resistance and surface exchange were calculated, with minimal additional error, up to 6 orders of magnitude faster than Finite Element Analysis (FEA). Furthermore, because ATN’s fundamental modeling element is a physical channel, as opposed to sub-channel-scale element, graph theory could be used to mathematically examine how the interplay of channel morphology and topology impact network flow. These advantages carry into the model’s extensions presented in this talk. [1] A. P. Cocco, A. Nakajo, and W. K. S. Chiu, “Analytical transport network theory to guide the design of 3-D microstructural networks in energy materials: Part 1. Flow without reactions,” J. Power Sources, vol. 372, 2017. [2] A. P. Cocco and W. K. S. Chiu, “Analytical transport network theory to guide the design of 3-D microstructural networks in energy materials: Part 2. Flow with reactions,” J. Power Sources, 2017.

Nanostructured metallic transition metal carbides, nitrides, phosphides, and borides for energy storage and conversion

Abstract Metallic-like transition metal-based nanostructures (MLTMNs) has recently arisen as robust and highly efficient materials for energy storage and conversion. Owning to extraordinary advantages over the semiconducting/insulating ones (in terms of fast reaction kinetics, rapid electrical transport, and intrinsically high activity) combined with the high natural abundance, this class of materials is progressively developed towards commercial applications in real energy technologies. This review summarizes and discusses the progress in energy storages and conversions that employ MLTMNs. After the introduction and fundamental characteristics, developments in synthetic methodologies of MLTMNs and its application in energy storage and conversion are provided with more attention on strategies to improve electrochemical performances. Personal outlook on the challenges and opportunities of MLTMNs for industrial applications in real energy technologies are proposed and discussed in the conclusion.

Metal organic frameworks derived single atom catalysts for electrocatalytic energy conversion

The development of efficient and cost-effective catalysts to catalyze a wide variety of electrochemical reactions is key to realize the large-scale application of renewable and clean energy technologies. Owing to the maximum atom-utilization efficiency and unique electronic and geometric structures, single atom catalysts (SACs) have exhibited superior performance in various catalytic systems. Recently, assembled from the functionalized organic linkers and metal nodes, metal-organic frameworks (MOFs) with ultrafine porosity have received tremendous attention as precursors or self-sacrificing templates for preparing porous SACs. Here, the recent advances toward the synthesis strategies for using MOF precursors/templates to construct SACs are systematically summarized with special emphasis on the types of central metal sites. The electrochemical applications of these recently emerged MOF-derived SACs for various energy-conversion processes, such as oxygen reduction/evolution reaction (ORR/OER), hydrogen evolution reaction (HER), and CO2 reduction reaction (CO2RR), are also discussed and reviewed. Finally, the current challenges and prospects regarding the development of MOF-derived SACs are proposed.

Large Sample Neutron Activation Analysis of Ceramic Matrix Composites

Carbon fiber reinforced SiC ceramic matrix composites (Cf/SiC) are promising structural materials for a variety of high-temperature aerospace and energy applications. Since the matrix elements are carbon and silicon, they present low activation after neutron irradiation and therefore are materials of particular interest for fusion energy technology applications. Large Sample Neutron Activation Analysis (LSNAA) was applied to determine the elemental composition of Cf/SiC specimens joined together using a high temperature graphite based adhesive. The neutron irradiations and gamma ray measurements were performed at the BISNIS facility of the Hoger Onderwijs Reactor, TU Delft. The results of this study demonstrate the feasibility of application of LSNAA as a cost-effective method for non-destructive elemental composition analysis of whole ceramic specimens enabling the calculation of induced activity and dose rate after irradiation in different neutron spectra.

Prospects and challenges of graphene based fuel cells

Abstract Novel characteristics of graphene have captured great attention of researchers for energy technology applications. Incorporation of graphene related hybrid and composite materials have demonstrated high performance and durability for fuel cell energy conversion devices. This article overviews graphene based materials for fuel cell technology applications such as electrodes additives, bipolar plates and proton conducting electrolyte membrane. The graphene dispersion over electrodes has revealed enhanced exposure of electrochemically active surface area for improved electro-catalytic activity towards fuel oxidation and oxidant reduction reactions. The issue of device stack durability and degraded performance due to corrosion of bipolar plates is discussed by incorporating graphene based materials. In proton exchange membrane devices, graphene as an electrolyte has shown an excellent performance towards high ionic conductivity and power density. The graphene incorporation in fuel cell devices has exhibited commendable performance and has bright future for commercial applications.

Deep elastic strain engineering of bandgap through machine learning

Nanoscale specimens of semiconductor materials as diverse as silicon and diamond are now known to be deformable to large elastic strains without inelastic relaxation. These discoveries harbinger a new age of deep elastic strain engineering of the band structure and device performance of electronic materials. Many possibilities remain to be investigated as to what pure silicon can do as the most versatile electronic material and what an ultrawide bandgap material such as diamond, with many appealing functional figures of merit, can offer after overcoming its present commercial immaturity. Deep elastic strain engineering explores full six-dimensional space of admissible nonlinear elastic strain and its effects on physical properties. Here we present a general method that combines machine learning and ab initio calculations to guide strain engineering whereby material properties and performance could be designed. This method invokes recent advances in the field of artificial intelligence by utilizing a limited amount of ab initio data for the training of a surrogate model, predicting electronic bandgap within an accuracy of 8 meV. Our model is capable of discovering the indirect-to-direct bandgap transition and semiconductor-to-metal transition in silicon by scanning the entire strain space. It is also able to identify the most energy-efficient strain pathways that would transform diamond from an ultrawide-bandgap material to a smaller-bandgap semiconductor. A broad framework is presented to tailor any target figure of merit by recourse to deep elastic strain engineering and machine learning for a variety of applications in microelectronics, optoelectronics, photonics, and energy technologies.

Fe2O3 nanoparticles immobilized on N and S codoped C as an efficient multifunctional catalyst for oxygen reduction reaction and overall water electrolysis

Currently, multifunctional electrocatalysts with superior performance are very vital for developing various clean and regenerated energy systems. Herein, an effective multifunctional electrocatalyst comprising Fe2O3 nanoparticles immobilized on N and S codoped C has been synthesized via heat-treatment of Fe(II) complex at 800 °C (denoted as Fe2O3/NS-C-800). Favorable features including the introduction of maghemite nanoparticles, N/S-codoping effect, and close contact between the Fe2O3 nanoparticles and NS-C ender the Fe2O3/NS-C-800 with high multifunctional catalytic performance. The onset potential (0.97 V) and half-wave potential (0.81 V) of the Fe2O3/NS-C-800 towards oxygen reduction reaction (ORR) are comparable to Pt/C (0.99 and 0.82 V). The Fe2O3/NS-C-800 also exhibits high oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) activity with low OER and HER overpotentials of 0.37 and −0.27 V at 10 mA cm−2, respectively. In addition, higher ORR, OER and HER stabilities than Pt/C are observed for the Fe2O3/NS-C-800. More importantly, the assembled water electrolyzer using the Fe2O3/NS-C-800 as the anode and cathode exhibits a high stability at a water electrolysis current density of 10 mA cm−2. The present study offers a new promising non-noble multifunctional catalyst for future application in renewable energy technologies.

Effect of nano chitosan polymer blends for energy engineering applications

ABSTRACT In the present investigation biocomposites of Nanochitosan(NC) and Polyvinyl pyrollidone were prepared in the proportion 2:1 with and without consideration of cross-linking agent glutarldehyde which enhances thermal properties, tensile strength, decreases elongation of flims. The prepared blend crosslinks in the presence and absence of glutaraldehyde were observed by various spectral studies such as FTIR, XRD and SEM. The charaecterisation studies reveal that that the proposed binary blends can act as promising biomaterials for energy engineering applications.

Controlling the Activity and Stability of Electrochemical Interfaces Using Atom-by-Atom Metal Substitution of Redox Species.

Understanding the molecular-level properties of electrochemically active ions at operating electrode-electrolyte interfaces (EEI) is key to the rational development of high-performance nanostructured surfaces for applications in energy technology. Herein, an electrochemical cell coupled with ion soft landing is employed to examine the effect of "atom-by-atom" metal substitution on the activity and stability of well-defined redox-active anions, PMo xW12- xO403- ( x = 0, 1, 2, 3, 6, 9, or 12) at nanostructured ionic liquid EEI. A striking observation made by in situ electrochemical measurements and further supported by theoretical calculations is that the substitution of only one to three tungsten atoms by molybdenum atoms in the PW12O403- anions results in a substantial spike in their first reduction potential. Specifically, PMo3W9O403- showed the highest redox activity in both in situ electrochemical measurements and as part of a functional redox supercapacitor device, making it a "super-active redox anion" compared with all other PMo xW12- xO403- species. Electronic structure calculations showed that metal substitution in PMo xW12- xO403- causes the lowest unoccupied molecular orbital (LUMO) to protrude locally, making it the "active site" for reduction of the anion. Several critical factors contribute to the observed trend in redox activity including (i) multiple isomeric structures populated at room temperature, which affect the experimentally determined reduction potential; (ii) substantial decrease of the LUMO energy upon replacement of W atoms with more-electronegative Mo atoms; and (iii) structural relaxation of the reduced species produced after the first reduction step. Our results illustrate a path to achieving superior performance of technologically relevant EEIs in functional nanoscale devices through understanding of the molecular-level electronic properties of specific electroactive species with "atom-by-atom" precision.

Determinant factors in site selection for photovoltaic projects: A systematic review

The choice of great places for installation of solar power plants has become a key issue in terms of project planning because of the increased number of investments in the photovoltaic sector. This study is a systematic review of the literature that seeks to identify the determining factors in choosing the best location for solar photovoltaic power plants, through previous research on the application of renewable energy technologies in great contexts of location. Among a total of 130 academic studies filtered by the keywords “photovoltaic energy,” “power plants,” “location,” and “factor” on the bases ScienceDirect, Scopus, Web of Science, and IEEE, a total of 27 studies were identified. These articles were carefully explored, including years of publication, countries of origin, and identification of factors that each author demonstrated. It has been extracted 28 factors, organized in six points of view: socioenvironmental, location, economic, political, climatic, and orographic. It was verified that the determining factors for choosing the best locations are solar irradiation, substation distance, slope, distance of roads, distance from urban areas, and land use. The results of this research may assist academic students and investors in identifying factors that they should consider in their decision making and may also assist in the efficient planning of renewable energy management to ensure the sustainable development of power generation through the photovoltaic source.

The Influence of Admixtures on the Hydration Process of Soundless Cracking Demolition Agents (SCDA) for Fragmentation of Saturated Deep Geological Reservoir Rock Formations

Alternative fragmentation technologies such as soundless cracking demolition agents (SCDAs) can minimize adverse environmental impacts associated with conventional rock fracturing methods used in mining and energy industries. However, application of SCDA in deep underground environments is limited due to (1) inability of SCDA to react in saturated rock masses as a result of dilution and mass washout effects, and (2) slow expansive pressure generation in SCDA, which delays post-fracturing operations. This study addresses the first issue by modifying a generic SCDA using a viscosity-enhancing admixture (VEA), namely welan gum, to produce a hydrophobic SCDA for direct application in submerged conditions. The effect of the VEA, on the mechanical, microstructural and mineralogical morphology of hydrating SCDA was also investigated. According to the findings, adding just 0.1% of VEA by weight to the SCDA in combination with a water-reducing admixture significantly improves the washout resistance without compromising the fluidity of SCDA, however, at the expense of rapid expansive pressure generation rates. The reduction in expansive pressure, which is unfavourable for mining and energy engineering applications is caused by the interaction of VEA with the hydrating SCDA. This is evident in the SEM and XRD results observed. This urges the consideration of both positive and negative effects of welan gum in SCDA: enhancement of washout resistance and reduction of expansive pressure development prior to any field application.

Thermal Optimization of Heat Sink for Inverter Applications

Power inverters enable efficient conversion of DC voltage and current to AC voltage and current and are intermediate device in renewable energy technology application especially solar, wind and thermoelectric energy. The key technologies behind power inverters switching operation are the semiconductor devices such as the IRF 3205 MOSFET chip. During operation the switches dissipate heat and if the dissipated heat is not properly managed, thermally induced failure may occur. The research work involves generation of a heat sink model and its optimization for a 24VDC/2.5K.V.A inverter. The thermal resistance, fval, of the optimized heat sink was found to be equal to 0.0345 °C/W which is less than the thermal resistance of the heat sink to ambient of the IRF 3205 MOSFET switch of 0.5063 °C/W. The thermal management of the power inverter package yields experimental and modeled package temperatures of 34.7°C and 36.5°C respectively. The optimized heat sink using the model created leads to reduction in size, weight and reliable operation of the power inverter.

An empirical analysis on awareness and intention adoption of residential ground source heat pump systems in Greece

Residential heating, cooling and Domestic Hot Water (DHW) production are responsible for a significant part of the residential energy consumption in Greece, which is currently based on the consumption of fossil fuels. A solution for the reduction of fossil fuel use -and all the negative effects that their use implies- is the application of renewable energy technologies, among which Ground Source Heat Pump (GSHP) systems are included. The present study examined awareness and adoption intention issues concerning this technology in Greece, through the conduction of a questionnaire survey. Specifically, it investigated public's knowledge on issues involving this technology, intention of installing it in households, main information sources and installation barriers. Socioeconomic and residence characteristics, as well as consumers’ preferences and attitudes affecting the knowledge and adoption issues were examined. Factors affecting the subjects under investigation are gender, age, education level, environmentally friendly behavior and awareness, as well as having an occupation, studies or interests related to environment, technology or engineering. Additionally, the intention of installing a GSHP system is affected by specific household characteristics, infrastructure and consumers’ preferences on characteristics of heating systems. In order to promote GSHPs, suitable financial incentives, regulatory improvements and awareness activities are required.

Sustainable Synthesis of Bismuth-Based Core-Shell and De-Alloyed Nanoparticles As Electrocatalysts for Proton Exchange Membrane Fuel Cells

The dissolution of base metal cores made of Ni, Fe, and Co in the hot, acidic environment of proton exchange membrane fuel cells (PEMFCs) has impeded the use of novel core-shell nanoparticles (NPs) where the precious metals Pt and Ru are confined to surface. Since electrocatalysis occurs on surfaces, a substantial reduction in precious metal use could be achieved with core-shell NPs relative to pure systems, as long as the core remains stable during use. Bi is an abundant earth metal with a low environmental and human toxicity and water stability across a wide pH range, particularly under the acidic conditions of a fuel cell environment. These characteristics make Bi an ideal candidate for use as a durable and inexpensive core material to minimize precious metal use in highly dispersed core-shell Bi-Pt nanoparticle (NP) electrocatalyst in PEMFCs. Bi has been found to substantially promote direct alcohol fuel cell (DAFC) efficiency relative to pure Pd by mitigating catalyst poisoning due to irreversibly bound reaction intermediates such as CO and acetaldehyde. Unfortunately, the formation of Bi NPs is difficult and currently based on unsustainable methods employing toxic organic solvents, expensive organic precursors, and dangerous reducing agents such as hydrazine and lithium that frequently yield highly non-uniform structures. In this work, we describe an inorganic ligand metal redox synthesis approach which is used to synthesize a variety of well-defined core-shell as well as Pt-Bi alloy nanoparticle structures under sustainable, all-aqueous conditions. The approach uses SnCl3- as both a reducing agent and inorganic stabilizing ligand to provide unprecedented control over the size and structure of these nanoparticles. Furthermore, the ligand itself exists in a variety of structures that can be used to control electrostatic assembly onto various substrates. The materials to be discussed in this presentation have never been previously reported and their synthesis is unexpected given both that pure Bi has well-known instability in high salt, low pH environment and that Bi-Sn ligand formation has not been previously recognized. The implications of such metal NP synthesis are transformative with respect to the practical realization and wide application of alternative energy technologies given its simplicity and sustainability. We will provide data showing that such structures can double the electrocatalytic oxygen reduction reaction mass activity and increase the specific activity by nearly an order of magnitude relative to pure systems (see graphic below). As important, we observe simultaneous mitigation of carbon monoxide poisoning due to the persistence of Bi and Sn on their surface. These observations are discussed relative to the surface distribution of active sites as characterized utilizing electrochemical adatom stripping. Figure 1

MUTUAL BENEFIT AND COMPLEMENTARITY IN THE ENERGY COOPERATION BETWEEN CHINA AND RUSSIA

Energy cooperation is shown as an important component of the comprehensive strategic partnership between China and Russia, as well as the source of the driving force for the development and maintenance of sustainable development of China-Russia relations related to the coincidence of the strategic interests of the two states. The main attention is paid to the formation of a new concept of cooperation between China and Russia, caused by fundamental changes in the structure of world energy at present, leading to increased coordination and complementarity in the energy sector. The article notes that the excess of supply over demand led to a decline in oil prices; a significant share of oil and gas consumption moved to the East. The change in the structure of energy resources is reflected in the reduction in the use of traditional (non-renewable) fossil energy resources and in the increase in the use of non-traditional (renewable) energy resources. The article draws attention to the fact that in addition to strengthening the complementarity of the energy of the two countries, there is the application of new Sino-Russian energy technologies, which create great opportunities for cooperation. And despite the fact that in implementing specific energy cooperation projects, the parties have faced a number of problems, bilateral energy cooperation still has broad prospects.

Research on Application of Automobile New Energy and Energy Saving Technology

At the same time of rapid development of economy and science and technology, energy crisis and environmental pollution are becoming important issues affecting human survival and development. Energy conservation and environmental protection are becoming themes for the harmonious development of human society in the 21st century. Automobile fuel consumption and exhaust emissions have become the main cause of energy crisis and environmental pollution. For the sustainable development of human society, it is urgent to apply new energy and energy-saving technologies in the automotive industry to reduce energy consumption and environmental pollution. Researching new energy sources and energy-saving technologies for automobiles has become an important direction for the development of automobiles. The power for braking is shifting from gasoline to clean diesel, hybrid power, and fuel cells. This article briefly discusses the development and application of new automotive energy and energy-saving technologies.

Boosting the electrocatalytic performance of Pt, Pd and Au embedded within mesoporous cobalt oxide for oxygen evolution reaction

High efficient and low-cost electrocatalysts for the oxygen evolution reaction (OER) are essential components of renewable energy technologies. However, the high onset potential and limited active sites restrict the catalytic activity of current electrocatalysts. Therefore, we are motivated by the development of cheap and efficient catalytic electrodes to promote the sluggish water-splitting systems associated with the large-scale application of clean and renewable energy technologies. Herein, a novel, simple, and efficient routine is presented by noble metal particles embedded within mesoporous metal oxide materials as high-efficiency anode catalysts for OER. Highly ordered mesoporous Co3O4 was prepared by a nanocasting method using the silica KIT-6 as hard template, showing an enhanced electrochemical performance. Then, M - Co3O4 (M = Pt, Pd, Au) nanomaterials were prepared by a simple but novel chemical reduction method. They show the high surface area of 112.3, 81.0 and 73.6 m2 g−1, which can provide more active surface area exposure leads to shorter paths of charges from electrolyte to electrode surface. Moreover, a three-dimensional highly ordered mesoporous structure can facilitate diffusion and penetration of electrolyte and oxygen, and can also keep catalyst nanoparticles in a well-dispersed condition with more active sites. Electrochemical measurements revealed that 20, 50, 25 wt% are the best weight contents for M (M = Pt, Pd, Au) in the Co3O4 with highest electrochemical activity (0.410, 0.415 and 0.422 V vs. SCE) and j0.7V reaching a maximum value. M-Co3O4(M = Pt, Pd, Au) materials exhibit superior activities and excellent long-duration stability in alkaline attributed to accelerating the formation of Co(IV) cations after being introduced M(M = Pt, Pd, Au) nanoparticles within mesoporous Co3O4. This kind of noble metal embedded within mesoporous oxide catalysts will hold a large potential as a highly promising electrocatalyst in the future.

Research on Energy Conservation Management Platform of Smart Grid Park

The energy conservation management platform of the park includes five parts: contract energy management, electric power substitution, new energy enterprises, local trading market, real-time monitoring of electricity consumption and enterprise energy efficiency analysis. It can help the enterprises improve the degree of mastery of their own electricity consumption while it can also improve the equipment of enterprises, offer new energy to enterprises, provide the electricity consumption guidance for the enterprises from all aspects, build demand response model, guide users to use the electricity scientifically, reduce the enterprise cost of electricity use, reduce the energy consumption of the enterprise effectively, improve the efficiency of energy use, guarantee the enterprise equipment to be operated safely, reliably, economically, promote energy conservation technology, energy efficiency improving technology, renewable energy technology development and application, and finally, promote the sustainable development of social energy conservation and emissions reduction at the same time.

Computational Fluid Dynamics Modeling of Flashing Flow in Convergent-Divergent Nozzle

In this paper, a computational fluid dynamics model of flashing flow, which considers the thermal nonequilibrium effect, has been proposed. In the proposed model, based on the two-phase mixture approach, the phase-change process depends on the difference between the vaporization pressure and the vapor partial pressure. The thermal nonequilibrium effect has been included by using ad hoc modeling of the boiling delay. The proposed model has been applied to the case of two-dimensional axisymmetric convergent-divergent nozzle, which is representative of well-known applications in nuclear and energy engineering applications (e.g., the primary flow in the motive nozzle of ejectors). The numerical results have been validated based on a benchmark case from the literature and have been compared with the numerical results previously obtained by different research groups. The proposed approach has shown a good level of agreement as regards the global and the local experimental fluid dynamic quantities. In addition, sensitivity analyses have been carried out concerning (a) grid independency, (b) turbulence modeling approaches, (c) near-wall treatment approaches, (d) turbulence inlet parameters, and (e) semi-empirical coefficients. In conclusion, the present paper aims to provide guidelines for the simulation of flash boiling flow in industrial applications.