High-performance Design Research Articles

Fe@Fe3C Core-Shell Nanoparticles Embedded in Polyvinyl Alcohol-Derived Porous Carbon toward Light and High-Performance Microwave Absorption

High-performance microwave absorbents play a key role in information technology field. In this work, excellent microwave absorption property was achieved via embedded Fe@Fe3C core–shell nanoparticles in polyvinyl alcohol-derived porous carbon matrix. The results indicate that the microwave absorption performance will be greatly enhanced due to the interface between the nanoparticles and matrix, as well as the core–shell and porous structure, increase the energy consumption capacity via dipole polarization, interface loss and the multiple reflection/scattering. The maximum reflection loss (RLmax) reaches −56.88 dB at 11.3 GHz with the thickness of 2.11 mm, and the effective absorption bandwidth (EAB, RL<−10 dB) can get up to 5.2 GHz (from 12.9 to 18 GHz) with the thickness of 1.64 mm. Moreover, due to the low density of the porous carbon matrix, the total weight of the absorbents can be greatly induced, which provides an efficient way for lightweight and high-performance microwave absorption materials design.

A High-Performance Low Complex Design and Implementation of QRS Detector Using Modified MaMeMi Filter Optimized with Mayfly Optimization Algorithm

Electrocardiogram (ECG) is considered as the important diagnostic tests in medical field for detecting the cardiac anomalies. But, the ECG signals are polluted with numerous noise from power line intrusion, muscle noise, baseline wander, motion artifacts, low frequency noise signals, high frequency noise signals and T-wave, which automatically affects the QRS profile. The existing method provides the result in lesser accuracy with higher rate of error detection. To overcome these issues, QRS detector using modified maximum mean minimum (MoMaMeMi) filter optimized with mayfly optimization algorithm (QRS-MoMaMeMi-MOA) is proposed in this paper for less computational cost along with resource requirements. The proposed filter design consists of two phases for detecting QRS detector, such as filtering process associated to the enhancement and detection phase. Initially, the ECG data are taken from MIT/BIH arrhythmia dataset (MIT-AD). For eradicating the baseline wander in ECG data, MaMeMi filter is used. For expanding the performance of the modified MaMeMi filter, filter parameters, such as [Formula: see text] and [Formula: see text] are optimized by MOA to accomplish the best values and measure the performance of the whole QRS detector. For high frequency noise suppression in ECG data, the range function, noise subtractors, modified triangular detector are used. Then, heart beat detection can be done with the help of adaptive thresholding technique. The proposed filter design is carried out in MATLAB and implemented on field programmable gate arrays (FPGAs). The proposed QRS-MoMaMeMi-MOA filter design had 0.93%, 0.12% and 0.19% higher accuracy and 89.32%, 50% and 62% low detection error rate, compared to the existing filters, like Kalman filtering based adaptive threshold algorithm for QRS complex detection (QRS-KF-ATA), QRS detection of ECG signal utilizing hybrid derivative with MaMeMi filter by efficiently removing the baseline wander (QRS-HD-MaMeMi), and knowledge-based QRS detection operated by cascade of moving average filters (QRS-CAF). Then, the device utilization of the proposed FPGA implementation of the QRS-MoMaMeMi-MOA filter provides 95.556% and 71.428% lower power usage compared with the existing algorithms, like Kalman filtering based adaptive threshold algorithm for QRS complex detection in FPGA (FPGA-QRS-KF-ATA), and efficient architecture for QRS detection in FPGA utilizing integer Haar wavelet transform (FPGA-QRS-IHWT).

Meta-Optics-Empowered Switchable Integrated Mode Converter Based on the Adjoint Method.

Monolithic integrated mode converters with high integration are essential to photonic integrated circuits (PICs), and they are widely used in next-generation optical communications and complex quantum systems. It is expected that PICs will become more miniaturized, multifunctional, and intelligent with the development of micro/nano-technology. The increase in design space makes it difficult to realize high-performance device design based on traditional parameter sweeping or heuristic design, especially in the optimal design of reconfigurable PIC devices. Combining the mode coupling theory and adjoint calculation method, we proposed a design method for a switchable mode converter. The device could realize the transmission of TE0 mode and the conversion from TE0 to TE1 mode with a footprint of 0.9 × 7.5 μm2 based on the phase change materials (PCMs). We also found that the mode purity could reach 78.2% in both states at the working wavelength of 1.55 μm. The designed method will provide a new impetus for programmable photonic integrated devices and find broad application prospects in communication, optical neural networks, and sensing.

Achieving Efficient Neutron and Gamma Discrimination in a Highly Stable 6Li-Loaded Cs3Cu2I5 Perovskite Scintillator.

Low-dimensional Cu(I) perovskite halides with efficient exciton emissions have recently emerged as promising scintillation materials for X-ray and gamma-ray detection applications. Here, we demonstrate the possibility of using zero-dimensional Cs3Cu2I5 for the sensitive detection of thermal neutrons and neutron-gamma discrimination enabled by Li doping. Single crystals of Cs3Cu2I5 doped with 95% enriched 6Li were grown by the Bridgman method. Cs3Cu2I5:6Li offers a compelling combination of high stability against moisture and oxygen, a decent energy resolution of 4.8% for 662 keV 137Cs gamma-rays, a high light yield of 30 000 photons/MeV for gamma-rays, and 96 000 photons/neutron for thermal neutron, and a good neutron-gamma pulse shape discrimination figure of merit of 2.27. Our discovery of 6Li-doped low-dimensional perovskite halides opens up a new horizon for stable and high-performance neutron-gamma scintillator design.

Versatile optical beam routers based on inversely designed supercell metagratings

Metagratings manipulating the wavefront with a subwavelength volume have been widely explored in recent years for their potential applications in optical beam routers, such as laser beam splitters and combiners. Current metagratings are hampered from practical beam rerouting due to inhibitive computational load of engineering design and expensive nanofabrication cost. In this work, we apply the emerging concept of supercell metagratings, which arranges random binary gratings of a unit cell in a one-dimensional periodicity, to simplify such design and fabrication processes. A high-performance inverse design framework for the supercell gratings is proposed. In such a framework, we use coupled wave algorithm as a fast-converging electromagnetic solver for the forward calculation and particle swarm optimization algorithm for the backward optimization. By manipulating the building blocks of a unit cell, specific diffraction orders of supercell gratings can be intentionally strengthened or eliminated. We finally realize high-efficiency, broadband and multi-functionality optical beam splitters and combiners both in visible and near-infrared regimes. We envision that the proposed method offers an avenue towards the mass production realization of flat optics.

A Comprehensive Study of Ceramic Matrix Composites for Space Applications

Ceramic matrix composites (CMCs) have grown in popularity as a material for a range of high as well as protection components, increasing the need to better understand the impacts of multiple machining methods. It is primarily composed of ceramic fibers embedded in the matrix. Ceramic materials, especially carbon fibers and carbon were used to create the matrix and fibers. These ceramics include a huge variety of non-metallic inorganic materials that are regularly utilized under high temperatures. The aircraft industry became revolutionized by this unique combination of materials, which made parts better resistant under extreme conditions as well as lighter than the earlier technology. The development, properties, and production of ceramic matrix composites, as well as space applications, are discussed in this article. Ceramic materials have an interesting set of properties, including great strength and stiffness under extremely high temperatures, chemical inertness, low density, etc. In CMC, ceramics are used in the matrix as well as reinforcement. The matrix material keeps things running smoothly while the reinforcement delivers unique special properties. Ceramic matrix composites are developed for applications that required high thermal and mechanical characteristics, which include nuclear power plants, aircraft, chemical plants, space structures, and transportation services. Even though advanced aircraft relies on high-performance propulsion systems, improving the total impulses over the total mass ratio for rocket engines becomes essential for improving their performance that demands reduced engine structural weight as well as higher component heat resistance. The evolution of new ultra-high-temperature composites having high-temperature resistance as well as low density that a substitute super alloy and refractory metal material has become so essential and laid the foundation for high-performance engine design. The benefits of continuous fiber- reinforced CMC with high-temperature engine designs have long been recognized as a better measure of a country’s ability to design and produce spacecraft, modern aircraft, and weapons. Ceramic matrix composites materials are used in various aircraft type engines, aircraft brake disks, high-temperature gas turbines components, slide bearing components, hot gas duct, flame holders and components for burners are made by using oxide CMCs.

3D Printed “Kirigami”-Inspired Deployable Bi-Focal Beam-Scanning Dielectric Reflectarray Antenna for mm-Wave Applications

This article presents a state-of-the-art 3-D-printed dielectric reflectarray antenna featuring one-shot deployability and wide-angle beam-scanning ability. The design is enabled by a series of a “kirigami”-inspired two-stage snapping-like element structure that can be retracted by 66% to save space. The prototype is fabricated with a stereolithography (SLA) 3-D printing method with the Formlabs Flexible 80A photopolymer. A bifocal phase distribution method is utilized to optimize the RF performance of the array, realized −30° to −10° and 10° to 30° beam-scanning ability. The outcome of this work demonstrated a high-performance dielectric reflectarray design with ease of fabrication for 5G and satellite communication applications.

A thinning algorithm based approach to controlling structural complexity in topology optimization

Shape and topology optimization techniques aim to maximize structural performance through material redistribution. Effectively controlling structural complexity during the form-finding process remains a challenging issue. Structural complexity is usually characterized by the number of connected components (e.g., beams and bars), tunnels, and cavities in the structure. Existing structural complexity control approaches often prescribe the number of existing cavities. However, for three-dimensional problems, it is highly desirable to control the number of tunnels during the optimization process. Inspired by the topology-preserving feature of a thinning algorithm, this paper presents a direct approach to controlling the topology of continuum structures under the framework of the bi-directional evolutionary structural optimization (BESO) method. The new approach can explicitly control the number of tunnels and cavities for both two- and three-dimensional problems. In addition to the structural topology, the minimum length scale of structural components can be easily controlled. Numerical results demonstrate that, for a given set of loading and boundary conditions, the proposed methodology may produce multiple high-performance designs with distinct topologies. The techniques developed from this study will be useful for practical applications in architecture and engineering, where the structural complexity usually needs to be controlled to balance the aesthetic, functional, economical, and other considerations. • A structural complexity control approach is proposed for topology optimization. • Efficient designs with controllable topologies can be easily obtained. • This approach is practically useful for creating diverse and competitive designs.

Recent Advances in Woven Spacer Fabric Sandwich Composite Panels: A Review.

Because of the advantageous characteristics of strong integrity, lightweight, high performance, and various designs, woven spacer fabric (WSF) and its composite are extensively used in construction, traffic, and aerospace, among other fields. This paper first describes the WSF structure, including core yarns and cross-linking, and then discusses the influence of the processing parameters, among angle of the wall decisive the failure mode on the plate properties. Moreover, we summarize the molding and filling technology of WSF composite sandwich panels and discuss the process order, resulting in a significant effect on the stiffness of the sandwich composite plate; the current processing is mostly hand lay-up technology. In addition, we introduce the core and matrix material of the sandwich composite plate, which are mainly polyurethane (PU) foam and epoxy resin (70% of matrix material), respectively. Finally, the mechanical properties of WSF composite sandwich panels are summarized, including bending, compression, impact, shear, and peel properties. Factors influencing the mechanical properties are analyzed to provide a theoretical basis for future plate design and preparation.

A framework for obtaining a BIM-compatible design solution based on quantitative decisions on building performance

The direction towards high-performance buildings soars the complexity of the design process. Therefore, Building Performance Simulation (BPS) tools are used to aid designers to realize the implications of early design decisions. Coupling BPS tools and geometrical tools featured with Building Information Modeling (BIM) has recently received much attention. Yet, Computer-Aided Drafting (CAD-based) geometrical tools are still the preferred choice for a large portion of designers. Therefore, this paper aims to develop a Performance-based Design Framework (PbDF) to aid designers in obtaining an optimal design solution compatible with BIM software based on a quantitative decision that satisfies both daylighting, energy, and cost of buildings at the early stage of design. The PbDF included 3 D modeling, daylight and energy simulations, cost calculations, decision-making using Technique for Order by Similarity to Ideal Solution (TOPSIS), and BIM generation. The framework was tested through two stages and validated by face validity approach. The test results demonstrated that the proposed framework can assist designers to efficiently propose high-performance designs and send them to BIM software to complete the rest of the tasks required for design and construction. The PbDF was accepted as reasonable for its intended purpose by people who are knowledgeable about the performance-based design.

High-performance compact athermal panoramic annular lens design with separated radial optical power.

A panoramic annular lens can provide abundant spatial information in real time with its compact and scalable structure. To deal with extreme temperature environments and to improve the utilization of an image plane, we propose a high-performance compact athermal panoramic annular lens with separated radial optical power. By introducing multiple free-form surfaces, the system achieves a field of view of (40∘∼100∘)×360∘, an f-number of 2.2, and a total length of 40 mm. The modulation transfer function of the system's full fields of view is greater than 0.5 at 200 lp/mm, and the distortion is less than 2.5%. It can work stably in the range of -30∘C to 70°C. The proposed system is compact, athermal, and high-performing and has broad application prospects in the fields of visual positioning and biomedicine.

Effects of gear flexibility on the dynamic characteristics of spur and helical gear system

The gear foundation is commonly assumed to be a rigid body in analyzing the dynamic characteristics of the gear system. This modeling approach may not be appropriate for building the gear model with lightweight features. Therefore, this paper proposes a gear dynamic model considering the flexible gear. The shell element with gyroscopic effect is employed to establish the gear finite element model, retaining the gear foundation and teeth structure. The fixed interface modal synthesis method is used to improve computational efficiency. The proposed model is verified by comparing it with the results calculated by Ansys. Based on the proposed model, the effects of gear flexibility on the dynamic characteristics of the spur and helical gear system are investigated. Numerical results show that the developed model can degenerate into the classical gear dynamic model. Due to the assumption of the gear as rigid discs, the predicted resonance speed by the conventional gear dynamic model is larger than the proposed model. Gear flexibility affects the dynamic characteristics of the spur and helical gear system differently. It's worth noting that the dynamic axial meshing force can excite the nodal diameter vibration in the helical gear system. The proposed model provides the theoretical basis and optimization tools for the high-performance design and vibration and noise reduction of high-speed lightweight gear transmission systems.

Molecular mechanics-based design of high-modulus epoxy to enhance composite compressive properties

The performance of carbon fiber reinforced composites has been shown to be constrained by the low compression to tensile strength ratio, which blocks its lightweight application. Designing and preparing high-strength, high-modulus resins has become a popular direction in the field of synthetic materials and increasing resin modulus has become one of the key goals for improving composite compression properties. This study improves the interactions between molecular segments and reduces chain motility by introducing polar groups into resin molecules and increasing the reactive functionality. Four typical polar epoxy resins with high purity were synthesized, with a modulus of 6500 MPa and a tensile strength of 86 MPa when cured. Hydrogen bonds are detected and the complex process of group changes as a function of temperature is investigated using in-situ IR. The compression properties of the composites made with the newly synthesized resins were significantly improved, which suggests the significance of high-performance resin design in lightweight carbon fiber composite materials.

Flow direction algorithm-based machine learning approaches for the prediction of high-performance concrete strength property

The main significance of utilizing high-performance concrete as an effective item in the construction industry is the compressive strength assessment which requires a vast investigation of the design mix with calculated relevant compressive strength. Through the intelligence approaches, planning an accurate relationship between high-performance concrete different mix designs and their compressive strength is obtainable with the lowest cost of time and finance. Two models based on support vector regression methods are developed in this regard. The optimal output is calculated by tuning support vector regression key constraints by flow direction and biography-based optimization algorithm. The data set collected from the literature is divided into the training, and the testing phase, where the training data is used to develop the models, and the testing data is utilized to validate the accuracy of the models. The results showed a higher accuracy of the FDA_SVR method than the BBO_SVR method, with R2 values of 0.9939 and 0.9755, respectively. moreover, the confidence level obtained about 4.0273 and 7.367 for the FDA-SVR and BBO-SVR, respectively, demonstrating the accurate prediction capacity of the FDA-SVR model for high-performance concrete compressive strength.

A compact, low-profile, and high-performance monopulse antenna array for missile seeker

In this paper, the authors presented a compact, low-profile, and high-performance ku-band monopulse antenna array (MAA) for missile seekers with reduced complexity involved in fabrication, assembly, and installation. They investigated a multilayered microstrip patch Antenna technology by systematically developing an efficient synthesis process, high-performance design of radiating elements as well a feed network integrated with monopulse circuitry, and innovative multiple layered joining processes, which overcome limitations of the existing MAA system. Optimum aperture distribution corresponding to desired radiation characteristics is synthesized using a hybrid synthesis technique in which collapse distribution (CD) is employed to initialize the Genetic Algorithm (CD-AvGA). The authors designed the proposed MAA with computed coefficient and carried out fabrication, assembly, and functional validation. The obtained experimental results are in close agreement with simulated ones and satisfy the missile seeker’s stringent functional and platform requirements.

γ-Ester-Functionalized 1,1-Dicyanomethylene-3-indanone End-Capped Nonfullerene Acceptors for High-Performance, Annealing-Free Organic Solar Cells.

Modifying the end-capping groups in nonfullerene acceptors (NFAs) is an effective strategy for modulating their properties and that of the entire NFAs. This study reports the synthesis of a novel γ-ester-functionalized IC end-capping group (IC-γe) and its incorporation into the benzothiadiazole-fused central core, yielding isomer-free IC-γe end-capped NFAs, such as Y-IC-γe, Y-FIC-γe, and Y-ClIC-γe. The resultant NFAs exhibited similar absorption profiles but upshifted the lowest unoccupied molecular orbital energy level compared with those of the ester-free analogues, such as Y6 and Y7. Without thermal annealing, an excellent power conversion efficiency (PCE) of 16.4% is realized in the annealing-free OSC based on Y-FIC-γe with the PM6 donor polymer, which outperforms the OSCs based on Y-IC-γe and Y-ClIC-γe. In addition, the OSCs based on asymmetric Y-FIC-γe and Y-ClIC-γe have higher thermal stability with more than 83% PCE retention at an elevated temperature after 456 h than the symmetric Y-IC-γe case. In this study, we not only establish the structure-property relationship regarding the ester functionality and symmetricity tuning on the NFAs but also diagnose the reasons for the best-performing Y-FIC-γe-based OSCs, providing useful information for a novel high-performing NFA design strategy.

Fabrication of hierarchical core–shell carbon microspheres@ layered double hydroxide@ polyphosphazene architecture in flame‐retarding polypropylene

Developing advanced performance polypropylene (PP) composites with less smoke, eco-friendly and efficient flame retardants (FRs) has been a growing focus of research. In particular, it was significant to reduce the emissions of smoke and toxic gases generated during the burning of PP so that it meets green and safe industrial requirements. In this work, a CMSs@LDH@PZN hierarchical core–shell framework was designed and synthesized by using carbon microspheres (CMSs) and layered double hydroxides (LDHs) nanosheets connected to CMSs by electrostatic forces with polyphosphazene (PZN). The structure, composition and morphology of CMSs@LDH@PZN were characterized by SEM, TEM, FT-IR, XRD and TG, and the results were shown the triumphant preparation of the well-designed hierarchical core–shell structure CMSs@LDH@PZN. Additionally, the total smoke production, peak heat release rate and total heat release rate of PP/CMSs@LDH@PZN composites were decreased by 56.2%, 41% and 43.4%, respectively. With the addition of 20 wt% CMSs@LDH@PZN alone, the PP/CMSs@LDH@PZN composites reached a V-0 rating in the Vertical combustion tester (UL-94), and had a limiting oxygen index (LOI) value of 29.6%. Moreover, the heat release and toxic smoke emission were significantly lessened, which can be own to the adsorption of core–shell hybrid materials and the barrier effect of the P and N element crosslinked carbon layer. Herein, this work extends the idea of preparing multipurpose high-performance composites and designs and synthesizes an eco-friendly and efficient flame retardant with a double shell structure.

(Invited) Evaluation of Steam Splitting (OER) Kinetics in Praseodymium-Based Perovskite Thin Film Electrodes for Efficient Intermediate-Temperature Water Electrolysis

Water electrolysis at high temperatures (> 600 ˚C) provides both kinetic (no precious metals) and thermodynamic (lower voltage requirement) benefits for hydrogen generation. However, recently steam electrolysis at intermediate temperatures (300 - 600 ˚C) has attracted attention, given potentially faster startup times, ready access to waste heat, and mitigation of some degradation modes, while maintaining some of the high temperature benefits. Intermediate-temperature electrolysis can be accomplished with protonic ceramic electrolysis cells (PCECs), for which a primary challenge is the need for efficient, chemically stable, and long-term durable oxygen/steam electrodes. Electrode material candidates tend to be perovskite-based oxides that must be triple conductors, enabling transfer of oxide ions, protons, and electronic species for PCECs. While Barium (Ba) containing perovskite (ABO3) oxygen electrodes are an active focus due to their low hydration enthalpies enabled by the basicity of Ba, their poor long-term stability and non-optimized catalytic activity hinder the practical deployment of PCECs. Additionally, segregation of large cations (e.g., Ba, Sr) often takes place on electrode surfaces, which further hinders their catalytic performance and long-term stability. Thus, we have been investigating synthesis and performance of lower-Ba or Ba-free oxygen/steam electrodes and relating these to the evolution of the surface chemistry during operation.In this work, we fabricated Pr-based perovskite oxides, including Pr(Ga,Mg)O3-x, Ba(Pr,Y)O3-x, and others, as thin films with well-defined geometries and surface areas using pulsed laser deposition and compared their performance to the standard Ba(Co,Zr,Fe,Y)O3-x. Considering the multiple valence states (3+, 4+) of Pr and its large size, which may lead to surface segregation, the inclusion of Pr, or even replacement of Ba with Pr, is hypothesized to benefit catalytic activity and stability. We examined the surface water splitting/ hydrogenation coefficients (k-values) using electrical conductivity relaxation with pH2O steps, as a function of time and gas atmosphere. Further investigations of the role of crystalline quality, composition, and thickness have been made to understand the factors controlling the thin film electrode activity and stability. Our results of quantitative k-values on well-defined surfaces can guide design of high performance steam/oxygen electrodes and understanding of degradation mechanisms impacting performance of PCECs.

Dual functionalized Ni substitution in shuttle-like In2O3 enabling high sensitivity NH3 detection

Hetero-atom doping is an effective way to improve the sensing performance of metal oxide semiconductor gas sensors. However, the synergistic effects generated from hetero-atom substitution in In2O3 lattice and the relationship between the fine surface structure and sensing performance is still ambiguous. Here, Ni substitution in shuttle-like In2O3 nanoparticles have been successfully synthesized, and the materials are fabricated as gas sensor to detect ammonia, which is a toxic molecule that is harmful for human healthy. We find the 2 wt% Ni-doped In2O3 NPs exhibit high sensitivity (Ra/Rg = 2569.42 towards 50 ppm NH3 at 140 °C), achieving 34 folds improvement compared with pristine In2O3 NPs. The sensor also shows good long-term stability, high selectivity and fast response/recovery (23/10 s). Detail structural analysis illustrate the substitution of Ni in rh-In2O3 phase and surface O−(ad) species neighboring the substitution sites are recognized as the active sites, and the reactive oxygen species and surface Bronster acidity can be dramatically enhanced after Ni modification, which contribute to the improvement of sensing performance. Our work illustrate the synergistic effects of hetero-atom doping on the sensing performance and pave the way for design of high performance sensing materials.

Empowering Base Stations With Co-Site Intelligent Reflecting Surfaces: User Association, Channel Estimation and Reflection Optimization

Intelligent reflecting surface (IRS) has emerged as a promising technique to enhance wireless communication performance cost-effectively. The existing literature has mainly considered IRS being deployed near user terminals to improve their performance. However, this approach may incur a high cost if IRSs need to be densely deployed in the network to cater to random user locations. To avoid such high deployment cost, in this paper we consider a new IRS aided wireless network architecture, where IRSs are deployed in the vicinity of each base station (BS) to assist in its communications with distributed users regardless of their locations. Besides significantly enhancing IRSs’ signal coverage, this scheme helps reduce the IRS-associated channel estimation overhead as compared to conventional user-side IRSs, by exploiting the nearly static BS-IRS channels over short distance. For this scheme, we propose a new two-stage transmission protocol to achieve IRS channel estimation and reflection optimization for uplink data transmission efficiently. In addition, we propose effective methods for solving the user-IRS association problem based on long-term/statistical channel knowledge and the selected user-IRS-BS cascaded channel estimation problem. Finally, all IRSs’ passive reflections are jointly optimized with the BS’s multi-antenna receive combining to maximize the minimum achievable rate among all users for data transmission. Numerical results show that the proposed co-site-IRS empowered BS scheme can achieve significant performance gains over the conventional BS without co-site IRS and existing schemes for IRS channel estimation and reflection optimization, thus enabling an appealing low-cost and high-performance BS design for future wireless networks.