Arash Vaghef-KoodehiDepartment of Electrical Engineering, University of Guilan, Iranarashvam@gmail.comAbstractThe performance of ZnO-based varistors—the frontline defenders against electrical surges—ultimately comes down to what happens at their grain boundaries. In this work, we explore how the dance of charges across these interfaces, combined with careful microstructural tuning, reshapes the electrical behavior of multicomponent ZnO ceramics. Guided by the Double-Schottky Barrier model, we examined varistors made via both conventional sintering and the rapid, high-pressure route of Spark Plasma Sintering (SPS). The SPS process delivered near-perfect densification (98.7–99.6%), ultra-fine grains just 0.5–2 μm across, and taller barrier heights (0.95 eV), all contributing to dramatic improvements: nonlinearity coefficients climbing above 50, breakdown voltages reaching 4.8 kV/mm, leakage currents almost vanishing (< 10 μA/cm²), and energy absorption soaring beyond 300 J/cm³.Keywords: ZnO varistors, Interface charge dynamics, Spark Plasma Sintering, Double-Schottky Barrier, Microstructural optimization, Surge protection devices

AbstractRecent years have witnessed a paradigm shift in metal oxide varistor (MOV) technology, moving away from traditional ZnO-Bi₂O₃ systems towards alternative non-bismuth oxide materials and emerging nanostructured composites. This review critically examines developments in praseodymium-, barium-, tin oxide (SnO₂)-, tungsten oxide (WO₃)-based varistors, as well as the transformative impact of nanomaterial additives and advanced processing methods such as spark plasma sintering. The distinct microstructural and electrical features of each system are systematically compared, highlighting their unique operating ranges, stability, and surge protection capabilities. Special attention is given to the role of nanoengineering in achieving unprecedented control over nonlinearity, breakdown voltage, and energy absorption. The review concludes by outlining future directions, including environmentally friendly chemistries, composite varistor architectures, and self-healing mechanisms, positioning MOVs as critical components in tomorrow’s high-performance and sustainable electrical protection systems.Keywords : Metal oxide varistor (MOV), Non-bismuth systems, Praseodymium-based varistor, Barium titanate (BaTiO₃), Tin oxide (SnO₂), Tungsten oxide (WO₃), Nonlinear electrical properties.1. IntroductionMetal oxide varistors (MOVs) have been dominated by ZnO-Bi₂O₃ systems since their commercial introduction in the early 1970s [1]. While these traditional systems offer excellent nonlinear electrical characteristics, they present several limitations including bismuth volatilization during processing, thermal instability, and limited upper-temperature operation [2]. These constraints have driven extensive research into alternative material systems that can either complement or replace bismuth-based formulations for specific applications.As power systems evolve and electronic devices become more sophisticated, the demand for varistors with specialized characteristics has grown. Vaghef Kodehi’s recent work demonstrates that “specially designed nanostructured varistors can achieve nonlinearity coefficients significantly higher than commercial versions, while maintaining precise control over breakdown voltage thresholds across an extensive operating range (330V-11kV)” [3]. This finding underscores the importance of exploring novel material compositions and processing techniques.This review examines the state of the art in alternative varistor materials, with special attention to non-bismuth systems and their unique properties. By analyzing the fundamental mechanisms that govern varistor behavior in these systems, we aim to provide a comprehensive resource for researchers and engineers developing specialized surge protection solutions.2. Traditional Bismuth-Based ZnO Varistor Systems2.1 Composition and MicrostructureThe classic ZnO-Bi₂O₃ varistor typically contains 90-97 mol% ZnO with Bi₂O₃ (0.5-3 mol%) acting as the primary sintering aid [4]. Additional minor dopants such as Sb₂O₃, MnO₂, Co₃O₄, and rare earth oxides are added to control grain growth and enhance electrical properties [5].

Arash Vaghef-Koodehi1,2*1 Nano Science and Nanotechnology Center, University of Kashan, Kashan 8731573153, Iran2 Guilan Province Electricity Distribution Company, Guilan, Rasht, Iran*Corresponding author: a.vaghef@gilanpdc.irAbstractWe present a highly flexible multilayer graphene-based Schottky photodetector optimized for photovoltaic applications with exceptional mechanical stability, ultra-wide spectral responsivity (300-2500 nm), and high sensitivity (0.87 A/W at 600-700 nm wavelength range). By integrating a precisely controlled multilayer graphene structure (3-5 layers) with an optimized waveguide design on a flexible polyimide substrate, we demonstrate a photodetection mechanism that maintains performance even under severe bending conditions (radii as small as 5 mm with over 10,000 bending cycles). The mechanical flexibility is characterized through comprehensive strain analysis showing less than 4% responsivity degradation under extreme deformation. Unlike conventional rigid photodetectors, our device exhibits actively tunable photonic and electrical characteristics through gate voltage modulation, allowing for spectral sensitivity optimization across the solar spectrum. Through epsilon-near-zero condition engineering, the graphene-based photodetector achieves enhanced responsivity of 1.76 A/W at telecommunication wavelengths. The unique combination of high quantum efficiency (>70%), ultralow dark current (10-15 A), and remarkable mechanical flexibility enables applications in curved surfaces, wearable photovoltaics, and next-generation solar cells with enhanced harvesting capabilities. Computational modeling and experimental verification confirm the superior performance metrics compared to existing flexible photovoltaic technologies, with significantly improved energy harvesting efficiency under variable illumination conditions and mechanical states. This technology demonstrates a path forward for integrating high-performance photodetection with structural adaptability for diverse photovoltaic applications.Keywords: Flexible Graphene Photodetector, Multilayer Graphene, Schottky Junction, Broadband Spectral Responsivity, Mechanical Stability, Wearable Photovoltaics1. IntroductionThe photovoltaic industry faces critical challenges in adapting to varied installation environments, including non-planar surfaces, mobility requirements, and integration with existing infrastructure [1,2]. Conventional silicon-based solar cells, while efficient, lack the mechanical flexibility required for these emerging applications [3]. Graphene, with its remarkable electrical, optical, and mechanical properties, offers a promising platform for addressing these limitations [4,5].Recent advances in graphene-based optoelectronic devices have demonstrated their potential for photodetection [6,7], but challenges remain in optimizing these devices for practical photovoltaic applications, particularly regarding flexibility, broad spectral response, and stability under mechanical strain [8]. While several studies have explored graphene photodetectors for optical communication applications [9,10], their adaptation for solar energy harvesting under varying mechanical conditions remains underexplored.Vaghef-Koodehi et al. [45,46] have previously demonstrated the potential of graphene-InP Schottky photodetectors for telecommunication wavelengths, showing high responsivity through careful waveguide integration. Building on this foundation, we have developed a novel flexible platform that extends these capabilities to a much broader spectral range while introducing exceptional mechanical flexibility.In this paper, we present a novel flexible photodetector based on a multilayer graphene Schottky junction that maintains exceptional performance under mechanical deformation. The device leverages the unique properties of graphene, including its mechanical flexibility, tunable optical absorption, and carrier dynamics, to create a photodetection platform specifically optimized for solar energy harvesting across diverse installation environments.2. Structure and Mechanism of Operation2.1. Device Architecture and FabricationThe photodetector employs a multi-layered structure (Fig. 1) consisting of:A flexible polyimide substrate (25 μm thickness) providing mechanical support and flexibilityA waveguide structure optimized for light trapping and propagationA trilayer graphene film serving as the active photodetection mediumMetal contacts (Ti/Au) for electrical connectionsA transparent top encapsulation layer protecting against environmental factors