Carbon Fiber Surface Research Articles

Toward High‐Capacity Carbon Fiber Cathodes for Structural Batteries using Electrophoretic Deposition: Effects of Oxidative Surface Treatment on Carbon Fibers

Structural batteries possess multifunctional capability to store electrochemical energy and carry mechanical load concurrently. Carbon fiber cathodes (CFC), one of the main components in structural batteries, can be fabricated by depositing cathode active materials on carbon fibers using techniques such as electrophoretic deposition (EPD). However, intrinsically inert surface of carbon fibers may result in weak adhesion. In this study, different oxidative surface treatments (acid, electrochemical, and heat) are evaluated based on their ability to activate surfaces of carbon fibers. The mechanical and electrochemical performance of resultant CFC fabricated with lithium nickel manganese cobalt oxide (NMC 111) via EPD are analyzed. The best‐performing CFC are achieved using acid‐oxidized carbon fibers due to their improved interfacial adhesion. Acid‐oxidized AS4C 3k CFC yield a high specific capacity of 151 mAh g−1 after 100 cycles at 1 C and are stable over 100 cycles at 1 C with capacity retention close to 100% and give a stiffness of 25 GPa and ultimate tensile strength of 260 MPa. Acid‐oxidized 12k CFC show higher mechanical performance with stiffness of 53 GPa and ultimate tensile strength of more than 500 MPa, which make them more favorable to be used for structural batteries.

Large scale uniform Ni-P plated carbon fiber for boosting urea electro-oxidation and electro-detection.

Seeking an excellent electrocatalyst is the trickiest issue for the application of urea electro-oxidation and electro-detection. Phosphorus-doped nickel plating on carbon fibers (Ni-P/CF) is synthesized by simple electroless plating. SEM results exhibit that the Ni-P densely and uniformly grows onto the surface of carbon fibers (CF), forming carbon fibers-like nanoarchitectures. Benefiting from the carbon fibers-like nano architectures with abundant exposed active sites on the surface of CF, electron transfer can be synchronously facilitated, and Ni-P/CF displays superior urea electrooxidation (UOR) performance with potentials of 1.40V to reach 100mAcm-2. Impressively, it can maintain at 20mAcm-2 for 48h without evident activity attenuation, demonstrating robust durability. Cycle stability shows that the voltage has only increased by 10mV at 300mAcm-2 from the 10th to 20000th cycles. Most importantly, Ni-P/CF at a length of 100cm with good reproducibility was successfully synthesized, denoting great potential for large-scale industrial production. Therefore, this work not only affords cost-effective tactics for urea-rich wastewater degradation but also can achieve practical medical applications.

Degradation of sertraline using a continuous-flow filtration/photocatalysis system based on carbon fiber (PWB-3) modified with WO3 under LED chip irradiation

This work describes a filtration/photocatalytic system based on carbon fiber (CF) PWB-3 modified with WO3, to obtain a system capable of simultaneously filtering and degrading sertraline (SER) in simulated and real effluents. The CF was prepared by chemical electrodeposition and subsequent thermal treatment at 450 °C. X-ray diffraction patterns of CF/WO3/450 showed characteristic peaks for C and WO3 in monoclinic phase (more photoactive). The material presented broad absorption in UV–Vis range (300–700 nm), and a band gap of 3.6 eV. Scanning electron micrographs showed a thin film (WO3) formation on the CF surface. XPS analysis assigned the species C 1s, O 1s, W (4f, 4f7/2, 4d5/2 4d3/2), in addition to an intense peak at 530 eV attributed to metallic bonding (W-O). Modification with WO3 increased BET surface area from 0.20 to 3.1 m2 g−1, indicative of mesoporous-type material. Evaluation of the filtration/photocatalysis system under experimental conditions (100 mL, 2.0 mg L-1 SER, pH 5.6, and UV LED irradiation) enabled the optimization of reactor operating parameters. Use of an ideal flow rate of 180 mL min−1 and a membrane stabilization time of 30 min provided adequate contact between the solid and liquid phases and good mass transfer, achieving concentrations below the limit of detection for simulated and real effluent samples, at 90 and 30 min, respectively. It was shown that HO•, 1O2, O2•-, H2O2, and h+ played an important role in SER degradation, with some degradation products being identified and the main route being hydroxylation, demethylation, and dechlorination reactions. The combined system showed excellent potential for filtration applications, reducing problems associated with encrustation, providing rapid retention, and simplifying the treatment of contaminated water.

Synergistically enhancing weavability and interface behavior by applying PDMS/MXene on carbon fiber surface through ultrasound assistance

The poor interfacial properties and weavability of carbon fiber reinforced polymer (CFRP) composites stem from the surface chemical inertness and low wear resistance of carbon fiber (CF). Herein, we developed an effective approach to fabricate the CF@Polydimethylsiloxane/MXene (CF@PDMS/MXene) using ultrasound-assisted techniques, to improve both wear resistance and interfacial properties. The effectiveness of the modified treatment was assessed by Fourier transform infrared spectroscopy (FT-IR), microscopic confocal laser Raman spectrometer (Raman), X-ray photoelectron spectroscopy (XPS), and thermogravimetric analysis (TGA). The success of the graft was evaluated using field emission scanning electron microscopes (FE-SEM) and energy dispersive spectrometry (EDS). The interfacial shear strength (IFSS) of the composite was evaluated through fiber pull-out experiments. Different weight ratios of MXene nanoparticles were employed during the grafting process to investigate their impact on surface morphology, wear resistance, and interfacial properties. The results demonstrated that CF@PDMS/MXene-1.5 wt% exhibited a residual fracture tensile strength 14.8 % higher than desized CF when the MXene concentration was increased to 1.5 wt%. Additionally, the IFSS of CF@PDMS/MXene-1.5 wt% was observed to be 113 % greater than desized CF. Consequently, this novel modification strategy holds great promise for enhancing the mechanical performance of CFRP composites.

The Effect of Chemical and Thermal Treatment for Desizing on the Properties and Chemical Functional Groups of Carbon Fiber.

In this paper, in order to upcycle carbon fibers (CF), the changes in their mechanical and chemical properties in accordance with time and temperature were investigated, in addition to the oxygen functional group mechanism. When acetone as a chemical desizing agent was used, treatment with acetone for 0.5 h at 60 °C was the optimal condition for the complete removal of the sizing agent, and there was no deterioration in tensile strength. At 25 °C, the carbonyl group (C=O) and hydroxyl group (C-O) declined in comparison to commercial CF, but a novel lactone group (O=C-O) was created. At 60 °C, the oxygen present in the sizing agent was removed and C=O, C-O, and O=C-O decreased. On the contrary, in the case of thermal desizing in an inert gas nitrogen atmosphere, by increasing the temperature, functional groups combining carbon and oxygen were reduced, because nitrogen and oxygen atoms combined with C=O and C-O on the CF surface were eliminated in the form of CO, NO, CO2, NO2, and O2. When desizing via chemical and thermal methods, the amount of functional groups combining carbon and oxygen on the CF surface decreased. Desizing was performed as a pretreatment for surface treatment, so the methods and conditions were different, and related research is insufficient. In this study, we attempted to derive the optimal conditions for desizing treatment by identifying the surface characteristics and mechanisms according to chemical and thermal desizing treatment methods.

Ti3C2Tx MXene coated carbon fibre electrodes for high performance structural supercapacitors

MXenes, while excellent for electrical energy storage, face challenges in translating their nanoscale properties to macroscale structures. To address this, MXene-coated carbon fibers (CF) were developed as electrode materials for structural supercapacitor composites. CF surface chemistry was tailored using aryl diazonium salts (–NO2, –NH2, -SH, –COOH) to optimize MXene adhesion. Electrodes were characterized for surface chemistry, morphology, and properties including electrochemical, mechanical, and interfacial aspects. Electrodes developed from MXene coated on CF functionalized with a poly(o-phenyelenediamine) coating exhibited a specific capacitance of 157 F/g−1 at 5 mV s−1, a ∼725-fold improvement in capacitance compared to pristine unfunctionalized CF, with minimal to no compromise in the underlying strength and stiffness of the CF. The MXene coating also improved the interfacial adhesion of the electrode to an epoxy-based resin by 54 %. The process was scaled up to functionalize and coat woven CF mats and were used to fabricate a structural supercapacitor device. The device exhibited a specific capacitance of 908 mF/g−1 at 0.5 mA g−1, representing a remarkable ∼42-fold improvement compared to the control. This lightweight multifunctional supercapacitor composite holds great potential in aerospace, automotive, and renewable energy storage sectors, addressing global challenges tied to fossil fuel combustion and associated environmental and socio-economic issues.

Low temperature multi-catalytic growth and growth mechanism of carbon nanotubes on carbon fiber surfaces

In order to reduce the etching effect of the catalysts to carbon fibers caused by high temperature during the chemical vapor deposition (CVD) process, four multi-element catalysts, Fe–Co, Fe–Ni, Co–Ni and Fe–Co–Ni, were used to realize the low temperature growth of carbon nanotubes (CNTs) on carbon fibers at 350 °C–400 °C. The results show that the growth state of CNTs has a great relationship with the type of catalysts. The catalytic efficiency of Fe–Co catalysts is low, but the graphitization degree of CNTs is relatively high. The Fe–Co–Ni catalysts has high catalytic efficiency but low graphitization degree of CNTs. The tensile strength of carbon fiber/CNTs reinforcements prepared by Fe–Ni catalysts at 400 °C is the highest, reaching 3.99 GPa, which is 11.14% higher than that of desized fiber. The melt drop phenomenon of the catalysts was found by TEM, indicating the formation of the liquid phase catalysts during the growth of CNTs. This phenomenon can change the diffusion mode of carbon atoms in the catalyst and significantly reduce the growth activation energy of CNTs, so that CNTs can grow at lower temperatures. Based on the detailed analysis of the CVD process, a low temperature growth model of CNTs on carbon fibers was proposed.

Selective Electrophoretic Deposition of Silicon Nanoparticles onto PAN‐Based Carbon Fiber as a Prospective Anode for Structural Li‐Ion Batteries

AbstractThe demand for revolutionizing the lightweight design of Li‐ion batteries has become inevitable due to the ever‐increasing development of electric transportation modes. Integration of structural and energy storage functionalities into a single structural battery device can be a smart way to improve the overall performance of electric vehicles. In this study, we propose a facile and cost‐effective approach to develop a prospective anode for structural Li‐ion batteries through electrophoretic deposition of silicon (Si) particles onto polyacrylonitrile (PAN)‐based carbon fiber. The synthesis method is able to selectively deposit small‐sized silicon particles on the surface of carbon fiber, producing a thin, continuous, and porous coating of silicon nanoparticles on commercial PAN‐based carbon fiber. The synthesized Si/PAN‐based carbon fiber electrode exhibits remarkable mechanical properties, delivering a tensile strength of 2.57 GPa and a tensile modulus of 118.2 GPa. Benefitting from the morphology of the deposited silicon, the discharge capacity of silicon/PAN‐based carbon fiber anode can reach 565 mAh g−1 with 81 % capacity retention after 50 cycles. This work highlights the potential of silicon‐modified carbon fiber electrodes obtained via a simple and cost‐effective deposition method for structural Li‐ion battery applications.

Interface engineering of carbon fiber composites using CNT: A review

AbstractThis paper aims to explore the potential of carbon nanotubes (CNTs) in enhancing the structural capability and multifunctionality of carbon fiber composites in aerospace applications, primarily by focusing on interfacial applications. The conventional method of dispersing CNTs in a matrix is not fully efficient in exploiting the mechanical and multifunctional performance of CNTs. Hence, the use of CNTs at the interface or as a coating on the surface of carbon fibers has been suggested as a means of achieving multifunctionality, in addition to enhanced mechanical performance. The paper presents an overview of the various processes for growing CNTs on carbon fiber surfaces and examines the effects of CNT geometry and growth parameters on the properties of grafted fibers and their composites. Furthermore, it discusses the potential improvements in thermal and electrical conductivity achievable by incorporating CNTs at the interface, as well as the benefits of using CNTs as a sizing layer for carbon fibers, including enhanced fracture toughness and resistance to delamination.Highlights Comprehensive study of interface engineering in carbon fibers and Carbon Fibre Reinforced Plastic (CFRPs) using carbon nanotubes (CNTs). Improved transverse mechanical properties and overall thermal and electrical properties. Multifunctional applications possible with the use of CNT. Both experimental and numerical studies reviewed.

Reinforced compressive and ablative properties of phenolic aerogel by in-situ polycarbosilane-derived ceramic

Lightweight ablative materials owing to their low density, high efficiency heat insulation and excellent designability, which are considered as the most promising candidate materials in the field of aerospace ablative thermal protection. In this work, a polycarbosilane (PCS) in-situ reinforced carbon felt/phenolic (PR/Cf) aerogel composite is reported. The surface of aerogel and carbon fiber was coated with PCS protective layer by sol gel method and secondary vacuum impregnation process. The aerogel composite material possesses low density (0.264–0.307 g/cm3), hydrophobic, low thermal conductivity (0.121–0.178 W/(m·K)) and significant high compressive strength (12.89 MPa). When the oxygen acetylene flame with high heat flow density ablates for 20 s, it shows high efficiency in heat protection and insulation, and the linear ablation rate is as low as 0.089 mm/s. The PL/PR/Cf aerogel composite shows competitive and broad application prospects in the field of ablation heat protection in the future.

Reversible surface modification of PAN-based carbon fibers by a ferrocene-based surfactant

The surface of carbon fibers (CFs) was modified by a surfactant (ferrocenemethyl)dodecyldimethylammonium bromide (FDDA) to enhance the interfacial ashesion between the CFs and surrounding matrix. Results showed that it could be electrochemically desorbed by a potentiostatic electro-oxidation method. The FDDA adsorption isotherm was attributed to the formation of multi-molecular layers mainly by non-electrostatic interactions. The adsorption and desorption of FDDA on the CFs have little effect on their tensile strength. The effects of FDDA modification on the interfacial properties of CF/epoxy composites were evaluated by a single-filament fragmentation test. Compared with the un-modified CFs, the FDDA-modified ones had significantly improved interfacial adhesion properties in the composites. This method provides a potential approach for preparing recyclable CF/resin composites.

Codeposition of Gallic Acid and Polyethylene Imine on Carbon Fiber Surfaces to Enhance Interfacial Properties of Epoxy Composites

Codeposition of Gallic Acid and Polyethylene Imine on Carbon Fiber Surfaces to Enhance Interfacial Properties of Epoxy Composites

Effect of stepped microwave irradiation to develop carbon nanotubes on carbon fiber for enhanced performance

In the present research, carbon nanotubes were grown on carbon fiber (CF) by continuous and stepped microwave irradiation techniques. Ferrocene was used as a catalyst, and toluene was used as a microwave absorber to grow the CNTs on CF. Continuous microwave irradiation (CMI) was carried out at 720 W microwave power for 25 s. The stepped microwave irradiation (SMI) was performed at 720 W for 25 s, followed no irradiation for 15 s and further irradiation of 5 s. It was observed that SMI resulted in a dense forest of CNT without any agglomeration and damage to the CF surface compared to CMI. The ID/IG ratio of the CNTs grown with CMI and SMI was observed to be 1.03 and 0.99, respectively. This confirms the presence of denser CNT forest after SMI. The SMI showed improved wettability of the CF with respect to epoxy resin compared to CMI due to an 18% decline in contact angle. The ultimate tensile strength (UTS) of the resulting CFRP composite increased by 28% with SMI grown CNTs compared to CMI grown CNTs. The enhanced UTS is due to improved interfacial bonding between the CF layers due to denser CNTs obtained after SMI.

Biochar nanosphere-functionalized carbon fibers for in-tube solid-phase microextraction of polycyclic aromatic hydrocarbons in environmental water followed by liquid chromatography and diode array detection.

In order to improve the extraction ability of carbon fibers (CFs) for microextraction ofpolycyclic aromatic hydrocarbons (PAHs), biochar nanospheres derived from glucose were in-situ grown onto the surface of CFs via hydrothermal synthesis. The surface morphology and elemental composition of biochar nanospheres-CFs were investigated by scanning electron microscopy and X-ray photoelectron spectroscopy. Thereafter, the biochar nanosphere-CFs were pulled into the polyetheretherketone tube for solid-phase microextraction, and the tube was combined with high-performance liquid chromatography-diode array detector to online detect PAHs. With the help of π-stacking, hydrophobic, and hydrophilic effect of biochar nanospheres, the extraction efficiency of CFs was greatly enhanced (enrichment factor increased by 293% compared with the original). The conditions affecting the analytical performance (sampling volume, sampling rate, methanol content, and desorption time) were investigated. Under the optimal conditions, an online analytical method for microextraction and determination ofseveral PAHs was developed, and satisfactory results were achieved. The limits of detection were 0.003-0.010 ng mL-1 owing to high enrichment effect (2973-3600), linearity ranged from 0.010-15.0 ng mL-1, andrelative standard deviations were 0.4%-1.6% (intra-day) and 2.4%-4.4% (inter-day), respectively. The method was applied toanalyze environmental water samples (rain water, snow water, and river water), and spiked recoveries within 80.0%-119% were obtained.

Enhanced Interfacial Properties of Carbon Fiber/Maleic Anhydride-Grafted Polypropylene Composites via Two-Step Surface Treatment: Electrochemical Oxidation and Silane Treatment.

The interfacial adhesion between carbon fibers (CFs) and a thermoplastic matrix is an important aspect that should be improved in manufacturing CF-reinforced thermoplastics with high strength and rigidity. In this study, the effects of a two-step surface treatment comprising electrochemical oxidation and silane treatment of the CF surface on the mechanical properties of CF/maleic anhydride-grafted polypropylene (MAPP) composites were confirmed. The surface characteristics of the treated CFs were analyzed via scanning electron microscopy, atomic force microscopy, Fourier transform infrared spectroscopy, and X-ray photoelectron spectroscopy. The tensile testing of a single CF and interfacial adhesion of the samples before and after the surface treatment were analyzed using a single-fiber testing machine and a universal testing machine. After the silane treatment, the roughness of the CF surface increased due to the formation of a siloxane network. In addition, the interfacial shear strength increased by ∼450% compared to that of the untreated CFs due to the covalent bond between the -NH2 end group of siloxane and MAPP. This two-step surface treatment, which can be performed continuously, is considered an effective method for improving the mechanical interface strength between the CF and polymer matrix.

Influence of carbon fiber nickel electroplating on the electromagnetic interference shielding and mechanical properties of carbon fiber reinforced polyamide 6 composites

AbstractThis article investigates the use of nickel electroplated carbon fiber (Ni‐CF) fabric to enhance the electromagnetic interference (EMI) shielding capability of carbon fiber reinforced thermoplastics (CFRTs), particularly for automotive and aerospace applications. The study examines the influence of nickel electroplating parameters of carbon fiber on the morphologies of carbon fiber fabrics. Ni‐CF reinforced polyamide 6 (Ni‐CF/PA 6) was prepared by the film‐stacking method. The flexural properties analysis demonstrates a 21.1% increase in flexural strength for Ni‐CF/PA 6 composites compared to as‐received carbon fiber reinforced PA 6 (CF/PA 6) composites. Additionally, the introduction of nickel layer significantly improves the EMI shielding efficiency of CF/PA 6 composites, with the average shielding efficiency increasing from 23.5 to 31.6 dB within the X‐band and Ku‐band frequency ranges. Scanning electron microscope (SEM) analysis confirms that the nickel layer enhances the interfacial bonding between the PA 6 matrix and the Ni‐CF, which is attributed to the micro cone‐shaped structures of the nickel layer. Overall, the electroplating method effectively modifies the carbon fiber surface, resulting in composite laminates with enhanced mechanical properties and superior EMI shielding efficiency. These findings provide valuable insights for the development of advanced carbon fiber reinforced composites with improved functionalities in various applications.Highlights The electroplating method is used to process the nickel electroplated carbon fiber fabrics. The introduction of micro cone‐shaped structure nickel on the fiber surface by electroplating provides the mechanical interlocking between carbon fiber and matrix. The nickel electroplated carbon fiber can significantly increase electromagnetic interference shielding capability of carbon fiber reinforced thermoplastic composites.

Interface modification of carbon fiber composites by interlocked brushite nanopins and brushite nanosheets

The interface region of carbon fiber/epoxy composites (CE) plays an essential role in mechanical and biotribological properties for bone replacement application. However, the debonding interface of fiber/matrix in CE presents a significant obstacle to their practical deployment. In this study, two component brushites with interlocked structure reinforcement have been designed and synthesized into CE for synchronously upgrading the tensile strength and biotribological property. Especially, brushite nanopins (BSp) are radially grafted on the surface of carbon fibers in order to optimize the interface between carbon fiber and epoxy matrix via forming intertwined filamentous architectures. In addition, brushite nanosheets (BSs) are vertical growth on the surface of BSp in order to promote the penetration of the epoxy and improve the cohesion of epoxy matrix. Thus, the mechanical and biotribological properties of BSp and BSs (BSps) modified CE (BSps-CE) have been markedly enhanced. In contrast with CE, the tensile strength of BSps-CE (97.22 MPa) is increased by 27%, and the elastic modulus is increased by 54%. The wear rate of BSps-CE (0.283×10−15 m3(N·m)−1) is greatly reduced by 169% compared with CE (0.763×10−15 m3(N·m)−1). The boost of mechanical strength and the reduce of wear rate means that the service life of the BSps-CE as an artificial implant in human body is prolonged, and the wear debris is reduced. This work demonstrates that it is feasible for BSps to improve the mechanical and biotribological properties of CE by interfacial modification. Superior mechanical and biotribological performances of BSps-CE show its potential for orthopedic implant and fracture internal fixing applications.

Shape evolution and characteristics of carbon fiber reinforced polymer surface in laser ablation

Carbon fiber reinforced polymer (CFRP) has been far-ranging used for structural components in structural engineering. Laser paint removal is promising in the repair and maintenance of CFRP structural components. However, there are still many unknowns relating to laser paint removal, and the existing theory and practice are insufficient to guide the industrial application of laser paint removal from CFRP laminate. This paper uses nanosecond laser cleaning experiments on the paint of CFRP laminate surface to study the cleaning mechanism and cleaning effect of different process parameters for removing the paint. The surface morphology and three-dimension morphology of laser-cleaned samples are observed to reveal the cleaning quality. In addition, after laser paint removal, the nanostructures are generated on the carbon fibers (CFs) surface. The thermodynamic behavior of CFRP laminate during laser paint removal is discussed by the combination of experimental results and theoretical analysis. The contact angle measurements and single lap joints experiments demonstrate laser-cleaned surfaces present better wettability and higher tensile shear strength than other cleaning methods. This study provides a theoretical and experimental basis for the application of laser paint removal from CFRP laminate.

Detect and visualize non-uniform yarn orientations on preformed CFRP parts using automatic scanning and image processing

This article presents a methodology to measure and visualize the 3D yarn orientation distribution on the surface of preformed carbon fiber reinforced polymer (CFRP) parts. An image acquisition system composing of a camera and a 6-axis robot was established to scan and capture images of a series sampling points on surface of the CFRP part. The acquired images were analyzed by an image processing algorithm based on Canny edge detector, Sobel operator and histogram analysis, to quickly determine yarn orientations at each sampling point. This method was proved to achieve <4.43° and 12.80° of average error in yarn angle and yarn orientation measurement, respectively. The measured yarn orientations and angles were visualized as color-coded maps to display the 3D yarn orientation and angle distribution on the preformed part. 2D interpolation was added to smoothen the color-coded map and accurately approximate yarn orientations and angles outside the sampling points. Compared to the existing yarn orientation/angle measurement methods based on image processing, the presented methods had following advantages: (1) successful detection on both preforms and cured parts with stable accuracy; (2) high measurement speed; (3) accurate images acquisition on severely curved surfaces; (4) economical measurement cost without requirement for customized sensors.

Enhanced Thermal Shock Resistance of High-Temperature Organic Adhesive by CF-SiCNWs Binary Phase Structure.

The development of high-temperature organic adhesive for bonding ultra-high-temperature ceramics with excellent thermal shock resistance has important significance to thermal protection systems for high-temperature environment application. In this study, high-temperature organic adhesive (HTOA) with carbon-fiber-SiC nanowires (CF-SiCNWs) binary phase enhancement structure was prepared. The method is that the SiCNWs grow on the chopped carbon-fiber surface and in the matrix of modified HTOA during high-temperature heat treatment with the help of a catalyst by a tip-growth way and with a vapor-liquid-solid (V-L-S) growth pattern. The results showed that the CF-SiCNWs binary phase enhancement structure plays a significant role in improving thermal shock resistance of high-temperature organic adhesive. The retention rate of the joint bond strength for the bonding samples after 20 cycles of thermal shock testing reaches 39.19%, which is higher than for the ones without CF, whose retain rate is only 6.78%. The shear strength of the samples with the CF-SiCNWs binary phase enhancement structure was about 10% higher than for those without the enhancement structure after 20 cycles of thermal shock.