Smooth Surface Research Articles

Fabrication and characterization of methylammonium lead iodide-based perovskite solar cells under ambient conditions

<p>This study investigated the fabrication and characterization of CH<sub>3</sub>NH<sub>3</sub>PbI<sub>3</sub> based perovskite solar cells (PSCs) using the one-step spin coating technique under ambient conditions, eliminating the need for expensive glovebox and thermal evaporation equipment. The perovskite layer was annealed at 65 °C for 30 seconds and 100 °C for 30 seconds, 1 and 2 minutes. The scanning electron microscope (SEM) images show a smooth and uniform surface coverage for the ETL and CH<sub>3</sub>NH<sub>3</sub>PbI<sub>3</sub> layers. SEM results also show an average grain size of 397 nm for CH<sub>3</sub>NH<sub>3</sub>PbI<sub>3</sub> and an average particle size of ~17 nm for TiO<sub>2</sub> was confirmed by transmission electron microscopy (TEM). X-ray diffraction (XRD) results confirmed the formation of tetragonal perovskite (CH<sub>3</sub>NH<sub>3</sub>PbI<sub>3</sub>) phase with high crystallinity with a crystallite size of 19.99 nm for the samples annealed for 30 seconds at 65 °C and 1 min at 100 °C. FTIR results also confirmed the presence of anatase TiO2 at wavenumber 438 cm<sup>-1</sup> and the formation of the adduct of Pb<sub>2</sub> with dimethyl sulfoxide (DMSO) and MAI is confirmed at 1,015 cm<sup>-1</sup> . From the Tauc plot the bandgap energy of TiO2 and Perovskite layers was determined to be 3.52 eV and 2.06 eV respectively. An open-circuit voltage was 0.9057 V and short circuit current density was 12.2185 mA/cm<sup>2</sup> with a fill factor of 48.05 and power conversion efficiency (PCE) of 5.199%.</p>

Polyvinyl alcohol-based multifunctional hydrogel film: a novel strategy for food preservation packaging

Food spoilage leads to the annual huge wastage of food. There is a pressing need for safe, multifunctional food preservation packaging to curb this wastage and prevent contamination. In this study, we present a novel, cross-linked P-GL hydrogel film with a smooth surface microstructure, derived from polyvinyl alcohol (PVA), gallic acid (GA), and lysozyme (L). This hydrogel film serves as an exceptional packaging material for food preservation, displaying an ability to completely block UV light and efficiently impede the penetration of water vapor and O2. Remarkably, the P-GL hydrogel film exhibits a fracture strength of approximately 37.9 MPa, demonstrating superior water solubility, thermal stability, and mechanical properties. Also, this P-GL hydrogel film can effectively eliminate E. coli and S. aureus. The P-GL hydrogel film has proven to be successful in mitigating the spoilage of blueberries through coating and covering treatments, outperforming commercial plastic wrap. As a result, the shelf life of blueberries can be extended to 20 days. Our work provides innovative insights that could potentially drive the use of safe and multifunctional hydrogel films in the food preservation packaging industry.

Lignin-enhanced wet strength and UV-Blocking properties of nanocellulose-based composite films obtained by casting/hot-pressing methods

We composited nanocellulose, lignin and polyvinyl alcohol (PVA) by using borax as a cross-linking agent, and prepared nanocellulose-based composite films by two simple and convenient methods of the casting and hot-pressing. Results showed that the molding method affected the film properties. The hot-pressing composite film exhibited smoother surface and tighter structures, while the casting one exhibited better stretchability. The composite films showed a good tensile strength (casting: 149.56 MPa; hot-pressing: 179.68 MPa). Due to the addition of lignin, the composite films were endowed with good UV-blocking capabilities, wet strength, oxidation resistance and bacteriostatic properties. The composite film has good application prospects in the field of food packaging and UV-shielding.

Experimental observations of the onset of unsteadiness for buoyant airflow along smooth and rough vertical isothermal walls

ABSTRACT The buoyant airflow along a vertical isothermal wall, under conditions close to transitional, is studied. The schlieren technique and miniature thermocouples are employed for qualitative (flow unsteadiness) and quantitative (heat transfer coefficient and local air temperature) observations. For the smooth surface, the phenomena are found to be sensitive to the enclosure configuration surrounding the plate. Roughening the heated surface with staggered rib segments of proper dimensions leads to increased heat transfer, ascribed to the onset of unsteadiness close to the segments’ edges, together with a better redistribution of the flow thanks to a favorable hydrodynamic interaction between fluid and protrusions.

Effect of gas pre-decomposition device on the growth of GaN epitaxial layer

In previous studies, the influence of gas phase and surface reactions on the growth of GaN was mainly calculated through simulations. In this study, a novel gas pre-decomposition device (GPDD) was designed to experimentally investigate the effects of gas phase and surface reactions on GaN growth by changing the length and height of the isolation plates (IPs). By varying the structure of the GPDD, the effects on the growth rate and thickness uniformity of the GaN films were studied. The growth rate of the GaN sample slowed with the extension of the IPs because the longer partition plates led to insufficient gas mixing and premature consumption of the precursor trimethylgallium (TMG). The use of GPDD simultaneously achieves high crystal quality and smooth surface morphology of the GaN film. Owing to the use of GPDD, the decomposition of TMG in the pyrolysis pathway was promoted, which suppressed Ga vacancies and C impurities, resulting in weak yellow luminescence bands in the photoluminescence. This study provides a comprehensive understanding of the chemical reaction mechanism of GaN and plays an important role in promoting the development of metal-organic chemical vapor deposition equipment.

Different earthquake nucleation conditions revealed by stress drop and b-value mapping in the northern Chilean subduction zone

Stress drop is an earthquake property indicative for the characteristic relation of slip to fault dimension. It is furthermore affected by fault strength, fault topography, the presence of fluids, rupture size, slip, and velocity. In this article, the stress drop image of an entire subduction zone, namely for the seismically highly active northernmost part of Chile, is combined with mapped b-values and their corresponding magnitude distribution in order to better constrain the conditions under which earthquakes of different provenances may nucleate. The underlying recent earthquake catalog contains over 180,000 events, covering 15 years of seismicity, from which more than 50,000 stress drop estimates were computed. Their spatial average segments the subduction zone into different parts, i.e., average stress drop between seismotectonic areas is different, although this difference is small compared to the natural scatter of stress drop values. By considering stress drop variations, b-value map, magnitude distribution, and thermal models, candidate earthquake nucleation mechanisms are identified which can explain the observed distributions. This is done for two exemplary regions: (1) The plate interface, where principally lower stress drop events are found, while at the same time a high spatial heterogeneity of stress drop values is observed. This indicates relatively smooth or lubricated rupture surfaces, and locally it suggests the existence of alternating regions controlled by strong asperities, weaker material, or creep. (2) The highly active intermediate depth (ID) seismicity region, where the variation of stress drop and b-value point to a gradual change of nucleation mechanism from dehydration embrittlement at the top of the ID cloud, over dehydration driven stress transfer in its central part, to thermal runaway shear mechanisms at its bottom. In both cases, the combination of stress drop and b-value distribution helps to better understand the origin and the differences of the observed seismicity.

Toe pad morphology and adhesion in the miniaturized gecko, Chatogekko amazonicus (Gekkota: Sphaerodactylidae).

Chatogekko amazonicus is a miniaturized gecko from northern South America and is among the smallest of toe pad bearing lizards. The toe pads of C. amazonicus are miniscule, between 18% and 27% of the plantar surface area. We aimed to investigate the relationship between adhesive toe pad morphology, body size, and adhesive capabilities. Using scanning electron microscopy, we determine that the adhesive pads of C. amazonicus exhibit branched setae similar to those of other geckos, but that are generally much smaller. When compared with other gecko taxa, we show that C. amazonicus setae occupy a similar range of seta length: snout-vent length ratio and aspect ratio as other gekkonoid species (i.e. Gekkonidae, Phyllodactylidae, and Sphaerodactylidae). We demonstrate that C. amazonicus-even with its relatively small toe pads-is capable of climbing a smooth glass surface at a nearly vertical angle. We suggest that sphaerodactylids like C. amazonicus offer an excellent system for studying toe pad morphology and function in relation to miniaturization.

In Vivo Comparison of the Efficacy and Duration of Local Antibiotics on Smooth, Textured and Polyurethane Implant Surfaces.

Capsular contracture is one of the most common complications after breast surgery involving silicone implants. The most likely cause of this condition is biofilm formation. In this study, the efficacy of local antibiotherapy against biofilm formation on implant surfaces was investigated. Thirty-six rats were divided into six groups. Three pockets were created on the dorsum of each rat, and 1 × 2cm implant surface samples from smooth, polyurethane and textured implants were randomly placed into pockets. All samples were inoculated with staphylococcus epidermidis. In groups 1-2-3, inoculated samples were placed into the pockets and removed after 1, 6 and 24h, respectively. In groups 4-5-6, inoculated samples immersed with rifamycin were placed and removed after 1, 6 and 24h, respectively. Bacterial load was measured with plate count method. Bacterial load was lower in groups 4-5-6 than in groups 1-2-3 (p < 0.05). In groups 4-5-6, bacterial load was lower for polyurethane than for textured surfaces at all time points (1, 6 and 24h; p < 0.05). Again, in groups 4-5-6, bacterial load was lower for smooth than for textured surfaces at 24h (p < 0.05). In groups 4-5-6, bacterial load was lower for polyurethane than for smooth surfaces at all time points, but difference was not statistically significant (1, 6 and 24h; p < 0.05). The results suggest that local antibiotic therapy was effective in reducing the bacterial load on all surfaces. The effectiveness of local rifamycin on the polyurethane surface was higher, and the duration of activity was longer than other surfaces. This journal requires that authors assign a level of evidence to each article. For a full description of these evidence-based medicine ratings, please refer to the Table of contents or the online Instructions to Authors www.springer.com/00266 .

Photothermal Welding Strategy for Mechanical Stability and High Efficiency of ETL‐Free f‐PSCs

AbstractFlexible perovskite solar cells (f‐PSCs) have drawn widespread interest owing to their distinguished advantages in excellent flexibility and relatively low cost. However, the brittle grain boundaries (GBs) and defects in flexible perovskite film tremendously influence the power conversion efficiency (PCE) and mechanical flexibility of f‐PSCs. Herein, photothermal welding, a novel method, is used to improve the perovskite films quality and the PCE of f‐PSCs with a near‐infrared (NIR) dye (indocyanine green, ICG) and polycaprolactone (PCL) as additives. Due to the strong photothermal effect, ICG molecule not only can significantly enhance NIR light harvesting, but it also can weld GBs upon exposure to an NIR laser, which is conducive to the GBs connections and device flexibility. Meanwhile, the S═O bond of ICG and C═O bond of PCL can simultaneously coordinate with Pb2+ defects in perovskite. Furthermore, they can control crystal growth to form a smooth surface of perovskite film. Consequently, the unencapsulated PSCs based on ICG/PCL displays a high champion PCE of 20.62%, with 88.4% of the original PCE after being placed in dark conditions for 600 h. The f‐PSCs delivers a champion PCE of 19.55% and exhibits excellent mechanical stability, thus providing a meaningful scientific direction to fabricate high‐flexible f‐PSCs.

Hardness Perceived When Sliding Over Roughened Surfaces.

The objective of this study was to investigate the influence of roughened surface features on the perceived hardness of various materials. Thirteen participants used a visual analog scale to evaluate the hardness of ten 3D-printed specimens by sliding a fingertip on them. The specimens had two types of surface features: flat and smooth, or with microscopic rectangular gratings. They were fabricated from two types of plastic with different Young's moduli-2.46 and 9.35 MPa. We found that both surface pattern and mechanical hardness significantly contributed to the perceived hardness of a material individually and without interaction. The roughened surfaces with rectangular gratings were judged to be harder than the flat and smooth surfaces of the same material. Among the parameters of the rectangular gratings, the groove width or periodic surface wavelength significantly contributed to the perceived hardness. Although the root cause of this phenomenon is unknown, friction caused by surface roughness is considered a potential mediator that influences the perceived hardness. The findings of this study can facilitate the manipulation of softness perception through surface design.

Low-Cost Portable Road Smoothness Testing Method Based on Pseudo-Vibration Velocity Range

Road smoothness not only directly affects the safety and comfort of vehicle travel but also relates to the efficiency and cost-effectiveness of road maintenance. Traditional road smoothness detection methods usually require professional equipment and personnel, leading to high costs and cumbersome operations. Therefore, finding a low-cost, simple, and accurate method for detecting road smoothness is of great significance. This study uses vehicle-mounted acceleration sensors to detect road smoothness, establishing a correlation between driving vibration acceleration data and the international roughness index (IRI). For this research, a driving vibration acceleration data acquisition device was developed, and the driving acceleration data from the test sections were denoised and their feature values extracted. The pseudo-vibration velocity range was used as the characteristic index representing the road surface smoothness IRI value. Testing with different vehicle types showed that the method is applicable to both sedans and SUV models, yielding a relative error of 8.9% for the sedan smoothness test model and 6.7% for the SUV smoothness test model. This study contributes to conducting large-scale road smoothness detection at a low cost, improving the efficiency of road maintenance and operations.

Application of laser treatment technology for boiling heat transfer augmentation

Abstract Boiling heat transfer can be enhanced when the heater’s surface morphology is altered. The paper discusses the use of the laser beam to produce efficient heat exchangers. Two types of samples were investigated with distilled water and ethyl alcohol as boiling agents. The specimens differed with the height of the microfins: 0.19 mm and 0.89 mm. It was observed that both of them enhanced boiling heat transfer in comparison to the smooth reference surface. However, the sample with higher microfins performed better, especially in the region of low temperature differences, where the heat flux was about three times higher than in the case of the smaller microfins. The comparison of the experimental data with selected models of boiling heat transfer revealed significant differences with regard to the heat flux. The laser-made samples dissipated larger heat fluxes than it could be anticipated according to the models. It might be linked with high surface roughness of the area between the microfins, generated as a result of the laser beam interaction with the surface.

Comparison of SiNx Dielectric Layer Grown by Plasma‐Enhanced Chemical Vapor Deposition, Low‐Pressure Chemical Vapor Deposition, and Metal‐Organic Chemical Vapor Deposition in Diamond‐ and GaN‐Based Integrated Devices

To address the issue of heat dissipation caused by the high output power density of gallium nitride (GaN) devices, using diamond‐integrated devices is an effective solution. Recent studies have suggested that incorporating a dielectric layer, such as silicon nitride (SiNx), between diamond and GaN can improve adhesion while also reducing thermal boundary resistance (TBR). In this study, plasma‐enhanced chemical vapor deposition (CVD), low‐pressure CVD, and metal‐organic CVD (MOCVD) techniques are utilized to grow the SiNx layer. The interface behavior of diamond/SiNx/GaN is analyzed through scanning electron microscopy, transmission electron microscopy (TEM), scanning TEM, and energy‐dispersive X‐ray spectroscopy, while time‐domain thermoreflectance measurement is used to characterize thermal properties. After analyzing the impact of the growth dielectric layer on the interface thermal resistance of the three growth modes, it is concluded that the dielectric layer produced by the MOCVD technique exhibits a smoother surface and lower TBR compared to the other two methods. Therefore, the use of the MOCVD technique is recommended to achieve optimal thermal performance in diamond/SiNx/GaN systems.

Development of Nanopillar Arrays Nanopatterning Without Lift‐Off for Transferable GaN‐Based µLEDs

AbstractThe mass production of µLEDs requires an upscaling approach on 200 mm wafers, which implies the deployment of a technology that achieves zero defectivity without liftoff. In this report, Nanoimprint lithography (NIL) processing is successfully optimized for nanostructuring GaN‐based Silicon‐On‐Insulator (SOI) substrates. The etching of SiO2/GaN/AlN/Si/SiO2 layers using different plasmas is conducted and multi‐layer nanopillars 100–200 mm in diameter are fabricated. This approach generates zero‐defect arrays of pillars, which is particularly advantageous for the growth process. In addition, the SiO2 at the bottom of the pillar allows it to twist during the subsequent GaN regrowth, as this layer becomes soft at the growth temperature &gt;1000 °C. This ability to deform enables a coalescence of pillars into layers with reduced dislocation density. As a result, high‐quality GaN microplatelets and µLEDs are grown via a bottom‐up approach based on pendeoepitaxy using metal–organic vapor phase epitaxy (MOVPE). The fabricated µLEDs have a very smooth surface with a roughness of 0.6 nm which facilitated the implementation of an easy and simple transfer protocol. Adhesive tape and metalmetal bonding, are used to bond the µLEDs onto a metal‐coated silicon substrate. The reported findings offer exciting new insights into the development of high‐performance displays.

Enamel surface roughness following debonding resin clean up using high speed air-turbine and electric handpieces. In vitro study

BackgroundHigh speed electric handpieces have recently been growing in popularity among dental professionals. Advantages include smoother surface preparation and increased cutting efficiency.AimThe primary objective was to compare enamel surface roughness following resin cleanup after bracket debonding using highspeed air turbine versus electric handpiece. The secondary objective was to record the time needed for resin-clean up.MethodForty deidentified freshly extracted human premolars were cleaned and sectioned at the cement-enamel junction. The crowns were embedded in acrylic blocks. Enamel surface roughness parameters (Ra, Rz, Rp and Rv) were measured using a stylus profilometer. Brackets were bonded using a light-cure orthodontic adhesive and stored in distilled water for 24 h. Following bracket debonding, the specimens were randomly divided into 2 groups: First group: resin clean-up was carried out using a 12-fluted carbide bur mounted on a high-speed air turbine; and second group: where an electric handpiece was used. Surface roughness parameters were measured following resin clean up and after polishing using pumice and a rubber cup. Time needed for resin clean-up was recorded. Differences in enamel surface roughness and time between groups were compared using repeated measures ANOVA and independent samples t-test, respectively at P ≤ 0.05.ResultsThe electric handpiece groups showed significantly higher values for Ra, Rz and Rp both following resin cleanup and polishing. Time taken for resin cleanup was significantly longer for the electric handpiece group.ConclusionConsidering both surface roughness and time, electric handpiece do not seem to add greater effectiveness or efficiency to resin cleanup following orthodontic bracket debonding.

Co-Delivery of Dragon’s Blood and Alkanna tinctoria Extracts Using Electrospun Nanofibers: In Vitro and In Vivo Wound Healing Evaluation in Diabetic Rat Model

The increasing prevalence of diabetic wounds presents a significant challenge due to the difficulty of natural healing and various obstacles. Dragon’s blood (DB) and Alkanna tinctoria (AT) are well recognized for their potent healing abilities, which include potent antibacterial and anti-inflammatory activities. In this study, electrospun nanofibers (NFs) based on polyvinyl pyrrolidone (PVP) were co-loaded with both DB and AT, aiming to magnify their efficacy as wound-dressing applications for diabetic wound healing. The evaluation of these NFs as wound dressings was conducted using a streptozotocin-induced diabetic rat model. Electrospun NFs were prepared using the electrospinning of the PVP polymer, resulting in nanofibers with consistent, smooth surfaces. The loading capacity (LC) of AT and DB into NFs was 64.1 and 70.4 µg/mg, respectively, while in the co-loaded NFs, LC was 49.6 for AT and 57.2 µg/mg for DB. In addition, X-ray diffraction (XRD) revealed that DB and AT were amorphously dispersed within the NFs. Top of Form The loaded NFs showed a dissolution time of 30 s in PBS (pH 7.4), which facilitated the release of AT and DB (25–38% after 10 min), followed by a complete release achieved after 180 min. The antibacterial evaluation demonstrated that the DB-AT mixture had potent activity against Pseudomonas aeruginosa (P. aeruginosa) and Staphylococcus aureus (S. aureus). Along with that, the DB-AT NFs showed effective growth inhibition for both P. aeruginosa and S. aureus compared to the control NFs. Moreover, wound healing was evaluated in vivo in diabetic Wistar rats over 14 days. The results revealed that the DB-AT NFs improved wound healing within 14 days significantly compared to the other groups. These results highlight the potential application of the developed DB-AT NFs in wound healing management, particularly in diabetic wounds.

Comparison of Intralesional Triamcinolone Acetonide Alone with Intralesional Triamcinolone Acetonide-5-Fluorouracil Combination Injection in Keloid: A Case Report

Keloids are abnormal cutaneous wound healing responses extending beyond the borders of the initial wound, usually appearing pink-purplish to hyperpigmented nodules or plaques with a hard consistency, irregular shape, uneven border, and smooth shiny surface. Most often occur on the chest, shoulder, upper arms, earlobes, and cheeks. This case report aims to compare a case of keloid treated with intralesional triamcinolone acetonide (TAC) alone with intralesional triamcinolone acetonide-5-fluorouracil (TAC + 5-FU) combination injection. A 21-year-old Minahasa male complains of growing pruritic scars in the back area and right and left upper arms since five years ago. Physical examination of the right and left upper arms revealed multiple hyperpigmented nodules and plaques, irregularly shaped, smooth, and shiny surfaces with defined borders and varying sizes. A clinical diagnosis of keloid was made. Treatment was initiated with weekly intralesional TAC alone on the left upper arm vs. intralesional TAC + 5-FU combination injection on the right upper arm. The evaluation was made based on the clinical and modified Vancouver scar scale. One of the most commonly used therapeutic options for keloid is TAC. However, the combination of TAC + 5-FU may be opted for due to its mechanism through the corticosteroid mechanism of action in conjunction with the antimetabolite activity of 5-FU. The combination may yield a more effective and faster outcome with fewer side effects. Intralesional combination TAC + 5-FU injection may be a therapeutic option for keloid with minimal side effects.

Chitosan, Poly(vinyl alcohol) and Chitosan/Poly(vinyl alcohol) based active films loaded with white turmeric powder for food packaging applications

In recent years packaging materials made of biodegradable polymers received considerable attention. The packaging system containing antimicrobial agents was one of the new methods that have an advantage over the conventional system. This work aims to fabricate and characterize antibacterial films based on chitosan (CS) and poly(vinyl alcohol) (PVA) incorporated with white turmeric powder (WT) for food packaging applications. The active films of chitosan/cetrimide/white turmeric (CCW), poly(vinyl alcohol)/cetrimide/white turmeric (PCW) and PVA/CS/CT/WT (PCCW) with varied concentrations of WT were prepared by solvent casting method. The influence of WT on the physical and structural properties of the prepared films were studied using different analytical techniques. The results of SEM analysis indicated interactions of polymer matrix with WT contributed to homogeneous, compact and smooth surface morphology. The Fourier Transform Infrared (FTIR) Spectroscopy and X-ray diffraction (XRD) analysis revealed hydrogen bond formation with a polymer matrix that improved the physical and thermal properties. The research findings revealed that there was an improvement in the tensile strength, moisture adsorption, solubility and hydrophobicity of CCW active films with an increase in WT content. At the same time, there was an improvement in inhibiting bacterial growth with increasing concentration of WT for all the fabricated films. The results showed better mechanical and antibacterial properties for PCCW films. All the films exhibited biodegradation under environmental conditions. All these results suggested WT incorporated CS and PVA based films can be used in the food packaging industry to enhance the safety and quality of the packaging material.

Local Influence for the Thin-Plate Spline Generalized Linear Model

Thin-Plate Spline Generalized Linear Models (TPS-GLMs) are an extension of Semiparametric Generalized Linear Models (SGLMs), because they allow a smoothing spline to be extended to two or more dimensions. This class of models allows modeling a set of data in which it is desired to incorporate the non-linear joint effects of some covariates to explain the variability of a certain variable of interest. In the spatial context, these models are quite useful, since they allow the effects of locations to be included, both in trend and dispersion, using a smooth surface. In this work, we extend the local influence technique for the TPS-GLM model in order to evaluate the sensitivity of the maximum penalized likelihood estimators against small perturbations in the model and data. We fit our model through a joint iterative process based on Fisher Scoring and weighted backfitting algorithms. In addition, we obtained the normal curvature for the case-weight perturbation and response variable additive perturbation schemes, in order to detect influential observations on the model fit. Finally, two data sets from different areas (agronomy and environment) were used to illustrate the methodology proposed here.

Microstructure Evolution of Polyacrylonitrile-Based Fibers during Thermal Pre-Oxidation

In this work, pre-oxidized polyacrylonitrile fibers are treated with ultrasonic etching and solution etching to produce ultra-thin sections. The evolution of the fibers’ microstructure in the pre-oxidation process is observed, and the transformation model of the microstructure of the pre-oxidized fibers is proposed. Scanning electron microscopy and high-resolution transmission electron microscopy were used to observe the microstructure changes of the fibers. Fourier transform infrared spectroscopy and X-ray diffraction were used to observe the chemical structure transformation and crystallization degree of the fibers in the pre-oxidation process. The results revealed that pre-oxidized fibers exhibited a smooth surface, while their interior consisted of fibrils. The longitudinal microfibrils were connected by the transverse microfibrils and amorphous regions. The fracture morphology of the fibers shifted from ductile to brittle, and the cross-section gradually became smoother. The linear molecular chain of PAN transformed into a ring structure as pre-oxidation progressed, subsequently leading to the cross-linking of this ring structure into an orderly trapezoidal configuration. The connection between the fibrils was enhanced, and the fiber structure became more compact and stable.