Superior Thermal Conductivity Research Articles

SPEED: an integrated, smartphone-operated, handheld digital PCR Device for point-of-care testing

This study elaborates on the design, fabrication, and data analysis details of SPEED, a recently proposed smartphone-based digital polymerase chain reaction (dPCR) device. The dPCR chips incorporate partition diameters ranging from 50 μm to 5 μm, and these partitions are organized into six distinct blocks to facilitate image processing. Due to the superior thermal conductivity of Si and its potential for mass production, the dPCR chips were fabricated on a Si substrate. A temperature control system based on a high-power density Peltier element and a preheating/cooling PCR protocol user interface shortening the thermal cycle time. The optical design employs four 470 nm light-emitting diodes as light sources, with filters and mirrors effectively managing the light emitted during PCR. An algorithm is utilized for image processing and illumination nonuniformity correction including conversion to a monochromatic format, partition identification, skew correction, and the generation of an image correction mask. We validated the device using a range of deoxyribonucleic acid targets, demonstrating its potential applicability across multiple fields. Therefore, we provide guidance and verification of the design and testing of the recently proposed SPEED device.

Mass-Based Hybrid Nanofluid Model for Thermal Radiation Analysis of MHD Flow over a Wedge Embedded in Porous Medium

This study addresses the intricate interplay of magnetohydrodynamics (MHD), thermal radiation, and porous media effects, which are crucial in numerous engineering applications, including aerospace, energy systems, and environmental processes. The development of a mass-based hybrid nanofluid model signifies a novel approach, potentially yielding more accurate predictions and insights into the thermal behavior of fluids in diverse scenarios. Thus, the current research explores the heat transfer characteristics of a unique nanofluid known as TiO2 (titania)-CuO (copper oxide)/H2O (water) hybrid nanofluid. This nanofluid flows past a static or moving wedge considering the impact of thermal radiation and magnetic field in the appearance of porous medium. To calculate the effective thermophysical attributions of the hybrid (TiO2-CuO) nanofluid, a mass-based strategy is employed. This approach involves analyzing the masses of both the first and second nanoparticles, along with the mass of the base fluid, as essential input parameters. The proposed mathematical model is modified to a dimensionless form by applying similarity transformations. The numerical solution is obtained by utilizing the bvp4c built-in function within the MATLAB environment. Graphs illustrate the influence of various parameters on temperature and velocity trends, including the magnetic field parameter and heat absorption/generation parameter as well as the thermal radiation parameter. It is noted that along with the enhancement in the values of parameters related to porous medium or magnetic field, the velocity of the hybrid nanofluid improves. This occurs when the moving wedge parameter’s value is below 1. Conversely, when the moving wedge parameter’s value exceeds 1, the velocity of the hybrid nanofluid decreases. The shape factor is more effective in the temperature profile for developed inputs of heat absorption/generation parameter. A juxtaposition of enhancement in heat transfer rate due to nanofluid (TiO2/H2O) and hybrid nanofluid (TiO2-CuO/H2O) is likewise presented. The main outcome indicates that the hybrid nanofluid exhibits superior thermal conductivity relative to the conventional nanofluid.

Zr as an Alternative Grain Refiner in the Novel AlSi5Cu2Mg Alloy

Al-Si-Cu-Mg alloys are among the most significant types of aluminum alloys, accounting for 85–90% of all castings used in the automotive sector. These alloys are used, for example, in the manufacturing of engine blocks and cylinder heads due to their excellent specific strength (ratio of strength to specific weight) and superior castability and thermal conductivity. This study investigated the effect of using Zr as an alternative grain refiner in the novel AlSi5Cu2Mg cylinder head alloy. The microstructure of this alloy could not be refined via common Al-Ti-B grain refiners due to its specifically designed chemical composition, which limits the maximum Ti content to 0.03 wt.%. The results showed that the addition of Zr via the AlZr20 master alloy led to a gradual increase in the solidus temperature and to the grain refinement of the microstructure with the addition of as little as 0.05 wt.% Zr. The addition of more Zr (0.10, 0.15, and 0.20 wt.%) led to a gradual grain refinement effect for the alloy. The presence of Zr in the AlSi5Cu2Mg alloy was reflected in the formation of Zr-rich intermetallic phases with acicular morphology. Such phases acted as potent nucleants for the α-Al grain.

Application of machine learning in predicting the thermal conductivity of single-filler polymer composites

Polymer composites with superior thermal conductivity and low electrical conductivity are pivotal in the cooling of electronic devices. Despite their prevalence, the accurate prediction of the thermal conductivity of these composites remains a challenge. The emergence of machine learning (ML) provides a groundbreaking approach to solving this issue. In this study, we constructed a comprehensive dataset collected from previous experimental papers and successfully predicted the thermal conductivity of single-filler polymer composites using four ML regression algorithms: random forest regression (RFR), gradient boosting decision tree (GBDT), extreme gradient boosting (XGBoost), and Gaussian process regression (GPR). By employing feature engineering to select pertinent features from the original datasets, the accuracy of the four models on the test set was improved, among which GBDT exhibited the highest accuracy with the Pearson correlation coefficient value of 0.981. Factors such as filler volume fraction and matrix thermal conductivity significantly influence the thermal conductivity of composite materials, while the thermal conductivity of the fillers has a relatively minor impact. Additionally, we identified the topological polar surface area (TPSA) as a crucial descriptor for surface modifications, quantifying diverse surface-modifying agents. Due to the incorporation of more descriptors, ML models exhibit higher precision and broader applicability compared to empirical formulas. Our study provides an effective tool for predicting the thermal conductivity of polymer composites with single fillers and underscores the potential of machine learning in accelerating materials design.

Lateral Heterostructure Formed by Highly Thermally Conductive Fluorinated Graphene for Efficient Device Thermal Management.

The continued miniaturization of chips demands highly thermally conductive materials and effective thermal management strategies. Particularly, the high-field transport of the devices built with 2D materials is limited by self-heating. Here a systematic control of heat flow in single-side fluorinated graphene (FG) with varying degrees of fluorination is reported, revealing a superior room-temperature thermal conductivity as high as 128W m-1 K-1. Monolayer graphene/FG lateral heterostructures with seamless junctions are approached for device fabrication. Efficient in-plane heat removal paths from graphene channel to side FG are created, contributing significant reduction of the channel peak temperature and improvement in the current-carrying capability and power density. Molecular dynamics simulations indicate that the interfacial thermal conductance of the heterostructure is facilitated by the high degree of overlap in the phonon vibrational spectra. The findings offer novel design insights for efficient heat dissipation in micro- and nanoelectronic devices.

Enhancing Thermal Conductivity in Polymer Composites through Molding-Assisted Orientation of Boron Nitride.

Incrementing thermal conductivity in polymer composites through the incorporation of inorganic thermally conductive fillers is typically constrained by the requirement of high filler content. This necessity often complicates processing and adversely affects mechanical properties. This study presents the fabrication of a polystyrene (PS)/boron nitride (BN) composite exhibiting elevated thermal conductivity with a modest 10 wt% BN content, achieved through optimized compression molding. Adjustments to molding parameters, including molding-cycle numbers, temperature, and pressure, were explored. The molding process, conducted above the glass transition temperature of PS, facilitated orientational alignment of BN within the PS matrix predominantly in the in-plane direction. This orientation, achieved at low filler loading, resulted in a threefold enhancement of thermal conductivity following a single molding time. Furthermore, the in-plane alignment of BN within the PS matrix was found to intensify with increased molding time and pressure, markedly boosting the in-plane thermal conductivity of the PS/BN molded composites. Within the range of molding parameters examined, the highest thermal conductivity (1.6 W/m·K) was observed in PS/BN composites subjected to five molding cycles at 140 °C and 10 MPa, without compromising mechanical properties. This study suggests that compression molding, which allows low filler content and straightforward operation, offers a viable approach for the mass production of polymer composites with superior thermal conductivity.

Experimental Analysis of Nanofluids in PEMFC Cooling Plate Integrated with Thermoelectric Generator

A thermoelectric generator (TEG) is considered a feasible option to recover waste heat from a Proton exchange membrane fuel cell (PEMFC) into electrical energy. However, its application is limited due to its low efficiency. Meanwhile, nanofluids have emerged as an alternative coolant in heat transfer due to their superior thermal conductivity characteristics. Thus, this study aims to improve the efficiency of the TEG using nanofluids. The experimental study was conducted on a test bench that coupled a PEMFC cooling plate and TEG which was subjected to a 0.5% volume concentration of 𝐴𝑙2𝑂3∶𝑆𝑖𝑂2 hybrid nanofluids at a flow rate of Re 300 to 1000. The mixture ratio of 𝐴𝑙2𝑂3∶𝑆𝑖𝑂2 hybrid nanofluids studied was 10:90 ratio, single nanofluids of 𝐴𝑙2𝑂3 and 𝑆𝑖𝑂2 and also the base fluid of water. Upon completion, the improvement in power output due to an improved temperature difference of the thermoelectric generator (TEG) is observed. The highest TEG performance was shown by hybrid nanofluids of 10:90 (𝐴𝑙2𝑂3∶𝑆𝑖𝑂2) with a 17% improvement as compared to the base fluid of water. This is due to the bigger temperature difference which is caused by better thermal conductivity of hybrid nanofluids as compared to the base fluid.

Radiation-Induced Defect Formation Kinetics in Inconel–Cu Multimetallic Layered Composites

This study investigates the stability of Inconel–Cu Multimetallic Layered Composites (MMLCs) in nuclear reactor applications using Molecular Dynamics simulations. The focus is on understanding the underlying mechanisms governing the properties of MMLCs for advanced nuclear reactors, specifically, the mechanochemistry of the interface between Inconel and copper alloys. The selection of Inconel–Cu MMLCs is primarily due to copper’s superior thermal conductivity, enhancing heat management within reactors by preventing hotspots and ensuring uniform temperature distribution. This research examines Incoloy 800H and two Inconel variants (718 and 625), assessing their stability at 1000 K after exposure to 10 keV collision cascades up to 0.12 dpa. Notable findings include defect clustering on the {1 2 0} family of planes of Inconel and Cu, with Stacking Faults and Lomer–Cottrell locks on the Inconel side.

Preparation and thermal performance enhancement of roasted iron tailings based shape-stabilized phase change materials for thermal storage

A new type of shape-stabilized phase change material (SSPCM) was fabricated for thermal energy storage (TES) using a facile synthetic strategy. Lauric acid (LA) was synchronously incorporated into the structural framework of roasted iron tailings (RIT) and graphene nanoparticles (GNP), in which RIT and GNP served as an ideal compatible supporting matrix, meanwhile, GNP was employed as a superior thermal conductivity additive, and LA acted as the working medium. The leakage results of the prepared composites were qualitatively and quantitatively analyzed. The surface morphology, chemical structure, thermal properties, heat transfer efficiency, thermal reliability, and thermal stability of the prepared SSPCMs were examined systematically. The prepared LA/RIT/GNP (5%) SSPCM exhibited relatively excellent thermal performance, which can meet the requirement of the future applications. Particularly, compared to pure LA, the measured thermal conductivity of LA/RIT/GNP (5%) with the addition of GNP was 9.03 times higher. The heat transfer efficiency of SSPCM was augmented by 91.67%. The mechanism of the improvement in thermal performance was revealed by the analysis of the microstructure characteristic parameters. In conclusion, the prepared SSPCM not only has great value in the low-temperature field of TES but also provides a new approach for high-value use of solid waste tailings, which expands the conventional utilization of iron tailings with a high-value approach.

Comparing mechanism response and thermal conductivity of Ti3C2 and Ti3C2O2

This study uses molecular dynamics to investigate the effect of various temperatures and sample sizes on the mechanical mechanism and thermal conductivity of Ti3C2 and Ti3C2O2 Mxenes. The size of the Mxenes decides the severity of the crack and the von Mises stress clustering. The elastic phase trend of Ti3C2 materials in different sizes follows Hooke’s law, while the complex elastic trend is for the Ti3C2O2 models. The material toughness of Ti3C2 is relatively high, and the material’s response to the force is relatively stable and linear during the process of being subjected to pressure. The Ti3C2O2 Mxene presents a low toughness, low stability, and easier breakage during stress due to the complex structure and the formation of anatase and rutile TiO2 phases. The thermal conductivity decreases when the temperature increases or the material sizes decrease for both materials. Notably, Ti3C2 shows superior thermal conductivity in comparison to the Ti3C2O2 Mxene.

Advanced ceramics with integrated structures and functions: Machine learning prediction and experimental verification

Improving the thermal conductivities and bending strengths of ceramic substrates is crucial for their applicability as key components of integrated chips. Herein, we report the latest study in which the thermal conductivity and bending strength of aluminum nitride (AlN) ceramics were systematically predicted using high-precision prediction model approached with a machine learning (ML) method based on extreme gradient enhancement method (XGBoost). The ML model was used to rank the effects of the process parameters, and SHapley Additive exPlanations (SHAP) was employed to quantify the contributions of different factors. Sintering additives were found to have the predominant influence on the thermal conductivity and bending strength; particularly, those with cationic radii within 0.085–0.105 nm enhance both properties. By using Pr2O3 as the sintering additive and adopting the preparation conditions recommended by the ML model, we prepared AlN ceramics with thermal conductivity as high as 195.63 W m−1 K−1 and bending strength of 371.60 ± 9.31 MPa, thus satisfying the application requirement of high thermal conductivity. The proposed model is also applicable to alumina and silicon nitride ceramics. This study provides a practicable and once-for-all strategy to realize the entire process from the ML prediction design to the preparation of ceramic materials exhibiting superior thermal conductivity and high strength.

Impact of fin material properties and the inclination angle on the thermal efficiency of evacuated tube solar water heater: An experimental study

In recent years, evacuated tube solar water heaters (ETSWH) have become popular due to their efficiency and low maintenance based on trends in India's solar thermal market. Numerous studies have focused on improving system performance. In the scientific literature, there is a lack of information on the selection of appropriate fin material and the benefits associated with it. The thermal efficiency (ηt) of ETSWH is directly impacted by the choice of fin material. In this context, the goal of this study was to assess the impact of copper and aluminum using fin material as well as the inclination angle on ETSWH's ηt. The experiments are conducted for mass flow rate (mwt) varying from 2 to 5l/h. With a mwt of 2l/h, the highest outlet temperature of 55°C was recorded, while the highest ηt was found at 41.2°C with a mwt of 5l/h. At all mwt, copper fin is more efficient than aluminum fin due to their superior thermal conductivity. Among the considered inclination angles, at an inclination angle of 30°, the efficiency of ETSWH is found to be optimal. ETSWH with a copper fin attains (ηt = 58.57%), which is more significant than the case when the aluminum fin is used (ηt = 51.50%).

Janus polyurethane composite film with highly aligned graphene fluoride and liquid metal: A versatile technique for thermal management and electromagnetic interference shielding

Flexible and multi-functional thermoconductive polymer composites have significant potential for application in advanced electronics. However, achieving a composite with necessary requirements such as flexibility, thermal conductivity, electromagnetic interference (EMI) shielding, and electrical insulation are challenging. The concept of Janus materials has provided the requirements of advanced electronics by forming a layered structure in polyurethane via sedimentation of liquid metal (LM) and graphene fluoride (GF). The fillers in the polyurethane were elongated and aligned along the draw direction using a stretching-induced filler alignment approach, which enhanced in-plane thermal conductivity and EMI shielding performance simultaneously. The Janus composite film exhibited electrical conductivity on one side and insulation on the other side, demonstrating the true nature of the Janus structure, while the film showed superior thermal conductivity (9.62 W·m−1·K−1) and EMI shielding effectiveness (55 dB) at a draw ratio of 2. These findings provide a novel approach to fabricating multifunctional composites for electronics applications.

Surface modification of spherical silica micro‐powder using silane coupling agents and their application in epoxy resin‐based composite materials

AbstractIn recent years, the packaging industry has utilized electronic packaging materials with epoxy resin (EP) and high‐performance fillers, exhibiting superior mechanical properties and thermal conductivity. Spherical silica micro‐powder (SSP) is a critical thermal conductive filler but faces challenges due to its inorganic nature. Surface modification is essential to improve SSP's compatibility and dispersion in organic matrices for effective use in EP‐based composites. In this paper, the effects of the following four modification methods on SSP/EP composites were studied: (1) the processes include co‐modification with dimethyldimethoxysilane (DMDMS) and trimethylethoxysilane (MTES), (2) modification with NaOH, (3) co‐modification with DMDMS and MTES in an alkaline environment after NaOH modification, and (4) co‐modification with methyltrimethoxysilane (MTMS) and propyltrimethoxysilane (PTMOS). Through comprehensive characterization and evaluation, it is evident that modification schemes 1, 3, and 4 significantly enhance SSP's hydrophobicity, dispersibility, and compatibility with EP. This led to the improvement of thermal conductivity and stability of the composite, and modification scheme 4 showed the highest enhancement, with a 42.6% increase in thermal conductivity. Moreover, the mechanical properties of modified composite materials, such as bending strength, impact resistance, and tensile strength, surpass those of unmodified SSP/EP composite materials. The study provides valuable insights for reducing manufacturing costs and enhancing productivity in the electronic packaging sector.

Efficient preparation of polydimethylsiloxane‐based phase change composites with sandwich structure

AbstractWith the rapid development of portable electronic devices, there is an urgent need for the multifunctional composites like excellent electromagnetic interference (EMI) shielding performance, mechanical property, and thermal management capabilities. Here, due to the low thermal conductivity of phase change materials (PCMs), a sandwich structure carbon nanotube/phase change microcapsule/carbon nanotube (CNT/PMC/CNT (CPC)) film was prepared by vacuum‐assisted filtration (VAF) method. CNT, PMC, and cellulose nanocrystal (CNC) were used as functional fillers. The addition of CNCs was used to improve the mechanical performance of CPC film. Compared with PMC‐CNC (PC) film, CPC film with sandwich structure prevented leakage phenomenon efficiently. Subsequently, the use of ultrasonic‐assisted forced infiltration (UAFI) successfully infiltrated polydimethylsiloxane (PDMS) matrix in the filler skeleton to obtain CNT/PMC/CNT/PDMS (CPCP) composites. When filler mass ratio (CNT: CNC and PMC: CNC) reached 10:1, the thermal conductivity of CPCP composites was 2.32 W/(m·K) and total EMI shielding effectiveness (EMI SET) of it with 0.38 mm thickness reached 37.46 dB at 12.4 GHz. Besides, these sandwich composites exhibited the good mechanical properties and heat storage capacity. And we systematically observed the impact of different addition amounts of PC homogeneous dispersing solution on phase change ability of CPCP10:1 composites. In summary, CPCP phase change composites with sandwich structure shows potential application prospect in thermal management materials with EMI shielding of electronic devices.Highlight The sandwich structure CPCP composites prepared by the combination of VAF and UAFI method, showing superior thermal conductivity and phase change ability. CPCP has excellent EMI shielding performance, which can meet the need of TMMs with EMI shielding performance. Compared with same thickness PC film, sandwich CPC film can prevent leakage phenomenon efficiently.

Enhanced thermal conductivity and mechanical properties of extruded Mg-4La-2Al-0.3Mn alloy via hot forging

The Mg-4La-2Al-0.3Mn (LA42) alloy, known for its remarkable thermal conductivity of 110 W/(m⋅K), faces limitations owing to its inherently low strength. In this study, a composite deformation process, involving hot forging and extrusion, was investigated to enhance both the thermal conductivity and mechanical properties of the LA42 alloy. The findings revealed that the composite deformation process effectively precipitated an increased number of solute atoms from the matrix, thereby mitigating the lattice distortion effect and resulting in a superior thermal conductivity of 148.2 W/(m⋅K). Additionally, the combination of hot forging and extrusion proved to be efficient in achieving more pronounced grain refinement with increased dislocation density. Consequently, the yield strength of LA42 alloy was substantially enhanced while maintaining a high elongation, reaching values of 211.1 MPa and 12.5 %, respectively. These exceptional overall properties establish a competitive edge when compared to other state-of-the-art magnesium alloys reported previously.

Synergistic effect of positive temperature coefficient and thermally conductive micro‐nano fillers on electrical and thermal properties of epoxy resin

AbstractThis study investigates the effect of positive temperature coefficient (PTC) fillers, i.e., barium strontium titanate (BST‐60) and nano hexagonal boron nitride (h‐BN) fillers, on thermal and dielectric insulation properties of epoxy (EP). The electrical resistivity, DC breakdown strength, space charge, thermal conductivity, thermogravimetric analysis, and glass transition temperature (Tg) of EP composites were measured. Results indicate that, below curie temperature, the EP composites containing 0.5 and 1 wt% of BST and 5 wt% of h‐BN have superior electrical resistivity, breakdown strength, and thermal conductivity. However, above curie temperature, the EP composites containing 2 and 2.5 wt% of BST and 5 wt% of h‐BN have superior insulation properties than the pure EP and other composites. While the thermal properties of EP improve with an increase in h‐BN loading to 10 wt%, its dielectric properties are slightly compromised due to the presence of tiny defects in the composite material. It is hypothesized that the BST‐60 suppresses the negative temperature coefficient effect (NTC) of EP by regulating the electrical resistivity above the curie point, whereas nano h‐BN enhances the thermal conductivity of EP by constructing a heat dissipation path. The proposed mechanism for charge dynamics along with heat conduction‐dissipation explains the fillers' behavior at different temperatures.Highlights Analysis of epoxy composites with polydopamine‐modified BST and h‐BN fillers. PTC fillers are used to mitigate the negative temperature effect of epoxy. Optimal fillers wt% improves dielectric and thermal properties. This study shows that optimized epoxy composites can be used for packaging.

Wettability Gradient-Induced Diode: MXene-Engineered Membrane for Passive-Evaporative Cooling

HighlightsEngineering MXene into electrospun nanofibers can effectively enhance its thermal emissivity and conductance, and the unidirectional water transport of the wettability-gradient-induced-diode (WGID) membrane displayed diode-like properties with wettability gradient by tailoring the water contact angle of each single layer.The WGID membrane could achieve a cooling temperature of 1.5 °C in the “dry” state, and 7.1 °C in the “wet” state, with high emissivity of 96.40% in the MIR range, superior thermal conductivity of 0.3349 W m−1 K−1.Zero-energy-consumption for personal cooling management via multiple heat dissipation pathways, including thermal radiation, conduction, and evaporation.

Mechanism of enhanced thermal conductivity of hybrid nanofluids by adjusting mixing ratio of nanoparticles

At certain mixing ratios, hybrid nanofluids exhibit superior thermal conductivity performance compared with mono nanofluids, without significant enhancements in viscosity. Studying the microstructures formed by nanoparticles in the base fluid is key to revealing the mechanism of enhanced heat transfer in nanofluidic systems. Here, effects of varying the nanoparticle volume fraction (0.5–2.0 vol%) and Cu:Al volume mixing ratio (20:80, 40:60, 50:50, 60:40, and 80:20) of Cu-Al/Ar hybrid nanofluids were investigated using non-equilibrium molecular dynamics (NEMD) simulations at a temperature of 85 K. The mechanism of thermal conductivity enhancement was elucidated through quantitative analysis of the nanolayer structure and atom trajectory. The results showed that the thermal conductivities of Cu-Al/Ar hybrid nanofluids with a mixing ratio of 20:80 were lower than those of the corresponding Cu/Ar mono nanofluids with volume fraction below 1.3 vol% due to the low quantity of Cu nanoparticles. However, when the quantity of Cu nanoparticles was increased by using mixing ratios from 40:60 to 80:20, the thermal conductivities were higher at all volume fractions. Compared to Cu/Ar nanofluids, the thermal conductivity of Cu-Al/Ar hybrid nanofluids improved from 0.149 to 0.166 W/(m·K) which was higher 2.76 % to 14.48 % as the mixing ratio increased from 20:80 to 80:20 at 2.0 vol%. Radial distribution function (g(r)), the number density and diffusion rate calculations indicated that the density and velocity of Ar atoms around Cu atoms were higher than those of Al atoms due to stronger interactions between Cu-Ar atoms. The main reason for the enhanced thermal conductivity of hybrid nanofluids with increasing mixing ratio was the higher unit nanolayer density and Brownian motion velocity.

Fabrication of high thermal conductivity and wear resistance graphite/copper composites by microwave hot press sintering

Graphite/copper composites have great potential for tribological applications because of their high thermal and abrasion resistance. In this study, we successfully manufactured graphite/copper composites that demonstrated superior thermal conductivity and wear resistance by employing microwave hot press sintering (MHPS). The results demonstrated an enhancement in the density and hardness of the composites as the graphite particle size increased, owing to the influence of MHPS. Meanwhile, the graphite/copper composite with a graphite particle size of 300 µm has excellent thermal conductivity and friction properties, with a thermal conductivity of up to 679.5 W/mK and a wear rate of only 3.019 10−6 cm3/(N·m). The work is expected to provide a new route for the developing highly thermally conductive and abrasion-resistant materials.