Lightweight Materials Research Articles

Performance Evaluation of Self-Compacting Concrete Prepared Using Waste Foundry Sand on Engineering Properties and Life Cycle Assessment

The primary objective of this research is to utilize an industrial waste byproduct such as waste foundry sand (WFS) as an alternative for fine aggregate in self-compacting concrete (SCC). This research focuses on the use of WFS in SCC to enhance durability and mechanical properties, to find an alternative for fine aggregate in SCC, to reduce the disposal challenges of WFS, and to make SCC lightweight and environmentally friendly. Initially, WFS was treated with chemical (H2SO4), segregating, and sieving to remove the foreign matter and clay content. For this study, WFS is considered in varying percentages such as 0, 10, 20, 30, 40, and 50. For this investigation, M60 grade SCC is considered as per Indian standards and EFNARC guidelines. After that, this research focuses on tests on various fresh properties of SCC in each batch to find the flowability and passing ability of various mixes prepared using WFS. Similarly, the mechanical properties of SCC such as compressive, flexural, and split tensile strength tests were performed at 7, 28, and 90 days curing periods, respectively. Likewise, durability properties of SCC were found in all the mixes prepared using WFS such as water absorption, sorptivity, resistance to chemical attack, and chloride ion penetration; tests of these properties were performed at 28 and 90 days curing periods, respectively. Based on the experimental investigation of SCC, it was found that WFS can be used in M60 grade SCC as an alternative for fine aggregate up to 30% without compromising much on its properties. Finally, this establishes that using treated WFS in SCC helps in reducing the generation of waste and prevails as a meaningful utilization method. This research will also establish that the use of treated WFS will reduce the density and make SCC a lightweight, green, and sustainable material.

Robust-flexible and highly-thermostable polyimide-silica aerogels with adjustable fiber skeleton structure for efficient aerospace thermal protection

The rapid growth of the aerospace industry presents significant challenges in developing lightweight and flexible thermal protective materials for spacecraft and related equipment. One longstanding challenge has been the preparation of high-thermostable and highly flexible aerogels, which limits their practical application in flexible thermal protection. In this work, through a rational 'chain-sphere' dual-combination structure design, polyimide-silica aerogels with integrated properties of the adjustable aerogel-fiber skeleton structure, robust flexibility and high thermostable have been prepared. By incorporating 2,2-dimethylbenzidine (DMBZ), we achieve a more solid aerogel network. The aerogel membrane retains its original shape without creases or cracks even after undergoing 500 folding and unfolding tests. Additionally, the silicon nanoparticles surrounding the aerogel exhibit superior performance at high temperatures. The unique structure and components of the composite enable it to possess lightweight (0.075–0.11 g/cm−3), low thermal shrinkage (300℃∼6 %), excellent thermostability (95 % weight retention at 474℃∼557℃), and promising thermal insulating performance below 600℃. The integrated performances of the aerogel make it suitable for high-temperature thermal protection applications that require both efficient thermal insulation and robust flexibility.

Optimization and analysis of chassis for heavy electric truck under different condition

The design of heavy electric truck chassis is a critical aspect of developing sustainable and efficient transportation solutions. Heavy electric trucks, which are powered by electric motors and rely on large battery packs for energy storage. the structural integrity and strength of the chassis are of utmost importance. the integration of the battery pack into the chassis is a critical consideration. The chassis needs to accommodate the size, weight, and distribution of the battery pack while ensuring proper thermal management and protection. the unique challenges posed by these vehicles. By considering factors such as structural integrity, battery integration, lightweight materials, and other relevant design considerations, it is possible to develop efficient and sustainable heavy electric trucks that meet the demands of modern transportation.

SiCnws/CNTs/Cf-C/SiOC composites with multi-scale lossy phases for simultaneous electromagnetic wave absorption and thermal insulation

Carbon fiber reinforced carbon-based composites are considered to be an ideal lightweight material with exceptional high-temperature mechanical performance. Nevertheless, their high conductivity result in a strong reflection rather than absorption of electromagnetic wave (EMW) for the stealth application. To address this challenge, a novel carbon-based composite made of multi-scale lossy phases (Carbon nanotubes (CNTs), SiC nanowires (SiCnws), and Carbon fiber (Cf)) and impedance matching phase (SiOC ceramic) was fabricated by the precursor-derived method. The prepared SiCnws/CNTs/Cf-C/SiOC (SCC-CS) composites exhibit an effective absorption (EAB) of 2.4 GHz at a thickness of 1.9 mm and a minimum reflection loss (RLmin) of −58.44 dB (99% absorption) in the X band. The EMW absorption of the composite is attributed to the multiple loss mechanisms and favorable impedance matching with free space, caused by the multi-conductive phase and SiOC in the composite. In addition, the fabricated composites also have thermal insulation properties and can effectively achieve radar cross-sectional (RCS) reduction, which are promising aerospace composites with the integration of structure and function.

Synthesis and properties of transparent PMMA/cellulose nanocomposites prepared by in situ polymerization in green solvent

AbstractIn order to contribute to the demanded sustainability transparent poly(methyl methacrylate) (PMMA) composites with cellulose nanocrystals (CNC) up to high shares of 50 wt% were prepared by in situ radical polymerization in green solvent Cyrene. The weight average molecular weights of PMMA were in the range from 144,000 to 160,000 g mol−1 and the dispersity was in the range of 1.49 to 1.89 with high conversion of monomer to polymer. The DSC thermal analysis showed that the glass transition temperature of pure PMMA was at 99°C while in composites increased from 103°C up to 129°C with the increase of CNC from 1 wt% to 50 wt%. TGA shows that the addition of CNC does not have a significant effect on the thermal stability of PMMA. In composites Young's modulus increases with the increasing CNC ratio and composites with 50 wt% of CNC showed more than twice the modulus of the PMMA matrix. The SEM images indicate uniform distribution of CNC in polymer matrix. The transparent and semitransparent PMMA/cellulose composites of improved sustainability with higher modulus of elasticity and glass transition temperature can be used as lightweight materials for a wide range of technical and everyday applications as well as optical lenses or acoustic lenses for ultrasound transducers and other devices.Highlights Cellulose nanocrystals/PMMA composites fabricated by a green in situ process. Synthesized PMMA matrices have high molecular weights. Improved elastic modulus and glass transition temperature were achieved. Prioritizing eco‐friendly chemicals for a greener production approach.

A short review on toughened epoxy based nanocomposites for EMI shields

Modern era devices and appliances require shielding from electromagnetic interference to ensure stable and efficient functioning. This is all the more pertinent in view of the safety and protection of electronic gadgets used in defence security equipment. It has been observed that the most prominent radiations of EMI fall in the range of microwave (GHz) and radio waves (MHz). Those materials which protect equipment from electromagnetic interference are called EMI shields, since they shield the equipment by eitherreflecting, absorbing or transmitting the electromagnetic radiation. However, not all materials can be used as EMI shields. The basic material requirement to be used as EMI shields include good magnetic permeability and dielectric properties as well as strong mechanical properties. Currently EMI shields made up of metals or silicones are used in the form of solid enclosures, cables, gaskets, O-rings, films, coatings, fabrics, tapes etc. With time and research new advanced lightweight materials have been developed which have shown potential for use as EMI shields. These include carbonaceous and ferrite nanofillers based polymer nanocomposites. These materials are unique because they provide shielding for a wide range of microwave frequency 2-18 GHz by a judicious choice of conductive, dielectric and magnetic nanofillers. The present review article provides an overview of the research work carried out on toughened epoxy (epoxy modified with other polymers) filled with multifunctional nanoparticles and their characterization for EMI shielding efficiency. Various factors and features have been critically analysed in this article. The entire focus of the article is aimed atToughened epoxy as a base matrix which cansupport EMI shielding in the complete frequency range of 2-18 GHz and overcome inherent disadvantages of non-toughened epoxy.

A computational framework for probabilistic modeling of galvanic corrosion for automotive applications

Abstract To address the need for reduced vehicle weight and improved environmental sustainability, the automotive industry has increasingly turned to mixing lightweight materials and alloys with metal alloys. However, this integration of dissimilar materials has heightened the risk of galvanic corrosion. This study addresses the gap in modeling of galvanic corrosion under dynamic thin film electrolyte by incorporating data derived from real-world weather conditions and finite element simulations. The presented model successfully captures the trend of galvanic corrosion rate for a given atmospheric environmental condition. The model predictions are compared with experimental data in the literature. Good agreements are observed. The model is further used for prediction of galvanic corrosion of two identical vehicles located in two different geographic locations (i.e., Miami Beach in Florida and Wendover in Nevada) in the year 2021 leveraging weather station data. Additionally, a Bayesian estimation method is used to account for uncertainties in the model parameters and estimation of the probability of failure.

Direct synthesis of lightweight fiber sponges with interconnected nanopores structured fibers for absorbing low-frequency noise

Noise pollution from transportation industries poses many challenges to people’s health and the global economy. Most noise-absorbing materials suffer from the difficulty of achieving good noise absorption while maintaining light weight. In this study, we propose a feasible strategy to assemble porous fiber sponges (PPFS) based on electrospinning method. Modulating the phase separation rate of charged jet allows for the direct assembly of fiber sponges consisting of interconnected nanopores (30–70 nm) structured fibers. The resultant PPFS possess lightweight feature (volume density of 9.06 mg cm−3) and good mechanical properties (full recovery from 60% deformation). The sponges also show excellent low-frequency sound-absorbing performance (the sound absorption coefficient at 500 Hz is 0.78). Meanwhile, the noise reduction coefficient (NRC) of the fiber sponges reaches 0.57 when thickness is 30 mm, which could absorb white noise and high decibel noise efficiently. This work offers a fresh solution to develop lightweight and efficient sound-absorbing materials.

Surface properties enhancements by optimizing synthesizing parameters of AA8026 + Zr2O3/TiO2 hybrid composite

Aluminium alloy is one of the widely used lightweight materials and is utilized in the production of a wide variety of components. The AA8026 properties enhancements through optimizing the synthesizing parameters through the Taguchi route is the aim of this investigation. Thereby establishing control over the stir casting process to alter the wear and microhardness of AA8026 aluminium alloy by hybrid reinforcement. There were 16 samples of AA8026 + Zr2O3/TiO2 composite prepared as per Taguchi L16 orthogonal array. The process parameters varied four levels such as stirring time from 15 min to 30 min, quantity of reinforcement from 3 wt% to 12 wt%, molten temperature from 750 °C to 900 °C, and stirring speed from 450 rpm to 600 rpm. The fabricated samples are investigated for microhardness and wear properties. From the analysis, it is inferred that stirring speed is highly influenced by wear and microhardness properties. The results exemplify that 0.086 mm3/m was minimum wear and 180.2 HV was maximum microhardness achieved.

Substitution potential of La/Ce/Er for high-cost Sc in rare-earth Al alloys: A comprehensive first-principles study

Al–Sc alloys have high strength, plasticity, and corrosion resistance, making them ideal lightweight materials for the aerospace and automotive industries. However, the practical application of these alloys is constrained by the high cost of Sc. We investigate the substitution potential of lower-cost rare-earth elements X (X = La, Ce and Er) for the expensive Sc in Al–Sc alloys through first-principles calculations. Assessing substitution formation energy, substitutional segregation, and their impact on (001), (110), and (111) γ-Al/γ′-Al3Sc interfaces, results highlight Er as the most promising candidate among the three, thermodynamically replacing Sc in Al3Sc precipitates. Er atoms selectively segregate to γ-Al/γ′-Al3Sc interfaces, enhancing stability by reducing interface energies. The morphology of precipitates undergoes a transition from a truncated cuboctahedron to an {111}-faceted octahedron. Increased Er segregation further bolsters interfacial fracture strength, attributed to segregation-induced charge accumulation. Our work could offer reference for the design of rare-earth Al alloys with low-cost and high-performance.

Comprehensive Study on the Properties of AZ91/x-Si3N4 Composites for Their Prospective Application

Metal alloy matrix composites are generally lightweight structural materials with a high strength-to-weight ratio. They can be extensively used in various fields of modern engineering applications, such as aerospace and automotive components and biomedical engineering. This study focuses on the development and characterization of lightweight metal alloy matrix composites for industrial applications, with a particular emphasis on magnesium (Mg) alloys as a replacement for aluminum-based alloys. Mg alloys offer significant weight advantages, being 33% lighter than aluminum and 75% lighter than steel, making them highly desirable for use in various engineering fields. In the present study, Mg (AZ91) alloy reinforced with x-Si3N4 composites (x = 0, 1, 3, 5, 7, 9 wt.%) were fabricated using a liquid state process. The AZ91/x-Si3N4 composites were evaluated through physical, mechanical, wear, and microstructural characterization. The experimental results, supported by statistical analysis, demonstrated that the incorporation of Si3N4 particles amplified the mechanical properties, wear resistance, and porosity of the composites. However, the presence of the reinforced particles resulted in reduced forgeability and elongation, limiting certain deformation characteristics. The existence of the reinforced particles within the composites was confirmed through SEM analysis, providing visual evidence of their distribution and interaction within the Mg alloy matrix. Finally, it was concluded that the implication of the study could be sought for the light structural parts of aerospace, automotive, biomedical, and prosthetic applications.

Facile Synthesis and Properties of Highly Porous Quartz Fiber-Reinforced Phenolic Resin Composites with High Strength.

Lightweight and high-strength insulation materials have important application prospects in the aerospace, metallurgical, and nuclear industries. In this study, a highly porous silica fiber reinforced phenolic resin matrix composite was prepared by vacuum impregnation and atmospheric drying using quartz fiber needled felt as reinforcement and anhydrous ethanol as a pore-making agent. The effects of curing agent content on the structure, composition, density, and thermal conductivity of the composite were studied. The mechanical properties of the composite in the xy direction and z direction were analyzed. The results showed that this process can also produce porous phenolic resin (PR) with a density as low as 0.291 g/cm3, where spherical phenolic resin particles are interconnected to form a porous network structure with a particle size of about 5.43 μm. The fiber-reinforced porous PR had low density (0.372~0.397 g/cm3) and low thermal conductivity (0.085~0.095 W/m·K). The spherical phenolic resin particles inside the composite were well combined with the fiber at the interface and uniformly distributed in the fiber lap network. The composite possessed enhanced mechanical properties with compressive strength of 3.5-5.1 MPa in the xy direction and appeared as gradual compaction rather than destruction as the strain reached 30% in the z direction. This research provides a lightweight and high-strength insulation material with a simple preparation process and excellent performance.

Impact response characteristics and damage mechanism of continuous carbon-fiber-reinforced magnesium matrix composite materials

The continuous carbon-fiber-reinforced magnesium matrix composite (C_f/Mg) is a new type of composite material made of a magnesium alloy sheet and carbon-fiber-reinforced composite material with alternating layers. It has the advantages of high damage tolerance, light weight, and corrosion resistance, and has become the first choice of lightweight materials in aerospace, transportation, and other fields. Because impact-resistance serves as an important index for its structural design and performance stability, a continuous carbon-fiber-reinforced magnesium matrix composite with indirect bonding using a modified epoxy resin was designed in this study. Moreover, C_f/Mg was chosen to be the object of research through the selection of constituent materials such as magnesium alloys, carbon fibers, resins, and other properties, which matched the aim of realizing a lightweight material. With the help of hot-compression molding technology, dynamic impact mechanical behavior, and damage analysis technology, the structural design and preparation process optimization of epoxy resin carbon-fiber prepregs and magnesium alloy layer materials, as well as the dynamic impact mechanical response characteristics and damage evolution process under different impact energy conditions, were studied. Accordingly, through a combination of low-velocity impact experiments and numerical simulations, the effects of continuous multiple impacts at the same location with the same impact energy, as well as impacts with the same impact energy and different punch diameters, on the low-velocity impact damage behavior and dynamic impact response characteristics of C_f/Mg were investigated. The results of this study show that when the 5J impact is applied four times consecutively, the peak impact load gradually increases with the increasing number of impacts. Moreover, the peak impact loads of the second, third, and fourth impacts increase by 6.09%, 10.7%, and 14.5%, respectively, compared with the results of the first impact.

A Bioinspired Design of Protective Al2O3/Polyurethane Hierarchical Composite Film Through Layer-By-Layer Deposition.

Structural materials such as ceramics, metals, and carbon fiber-reinforced plastics (CFRP) are frequently threatened by large compressive and impact forces. Energy absorption layers, i.e., polyurethane and silicone foams with excellent damping properties, are applied on the surfaces of different substrates to absorb energy. However, the amount of energy dissipation and penetration resistance are limited in commercial polyurethane foams. Herein, a distinctive nacre-like architecture design strategy is proposed by integrating hard porous ceramic frameworks and flexible polyurethane buffers to improve energy absorption and impact resistance. Experimental investigations reveal the bioinspired designs exhibit optimized hardness, strength, and modulus compared to that of polyurethane. Due to the multiscale energy dissipation mechanisms, the resulting normalized absorbed energy (≈8.557MJ m-3) is ≈20 times higher than polyurethane foams under 50% quasi-static compression. The bioinspired composites provide superior protection for structural materials (CFRP, glass, and steel), surpassing polyurethane films under impact loadings. It is shown CFRP coated with the designed materials can withstand more than ten impact loadings (in energy of 10 J) without obvious damage, which otherwise delaminates after a single impact. This biomimetic design strategy holds the potential to offer valuable insights for the development of lightweight, energy-absorbent, and impact-resistant materials.

Nano-Enhanced Phase Reinforced Magnesium Matrix Composites: A Review of the Matrix, Reinforcement, Interface Design, Properties and Potential Applications.

Magnesium matrix composites are essential lightweight metal matrix composites, following aluminum matrix composites, with outstanding application prospects in automotive, aerospace lightweight and biomedical materials because of their high specific strength, low density and specific stiffness, good casting performance and rich resources. However, the inherent low plasticity and poor fatigue resistance of magnesium hamper its further application to a certain extent. Many researchers have tried many strengthening methods to improve the properties of magnesium alloys, while the relationship between wear resistance and plasticity still needs to be further improved. The nanoparticles added exhibit a good strengthening effect, especially the ceramic nanoparticles. Nanoparticle-reinforced magnesium matrix composites not only exhibit a high impact toughness, but also maintain the high strength and wear resistance of ceramic materials, effectively balancing the restriction between the strength and toughness. Therefore, this work aims to provide a review of the state of the art of research on the matrix, reinforcement, design, properties and potential applications of nano-reinforced phase-reinforced magnesium matrix composites (especially ceramic nanoparticle-reinforced ones). The conventional and potential matrices for the fabrication of magnesium matrix composites are introduced. The classification and influence of ceramic reinforcements are assessed, and the factors influencing interface bonding strength between reinforcements and matrix, regulation and design, performance and application are analyzed. Finally, the scope of future research in this field is discussed.

Comparative Study on the Effect of Synthetic Foaming Agent and Natural Foaming Agent on the Engineering Properties of Foam Concrete

The abstract discusses the imperative for the construction industry to pursue sustainable practices, particularly in materials selection. It introduces the study examining how different foaming agents impact the properties of foam concrete, a lightweight material crucial for sustainable construction. The construction industry's contribution to global sustainability efforts necessitates exploring eco-friendly alternatives for building materials. This study investigates the impact of synthetic and natural foaming agents on the engineering properties of foam concrete, with a focus on environmental considerations aligned with global sustainability goals. Foam concrete, a lightweight material without coarse aggregate, relies on the type of foaming agent used. This paper presents a comparative study of the effects of natural and synthetic foaming agents on foam concrete's engineering properties, including compressive strength, water absorption, and density. Experimental results indicate that foam concrete produced with a natural foaming agent exhibits greater compressive strength and density while absorbing less water than foam concrete with a synthetic foaming agent. These findings reduce the dead load on structures and minimize labour costs during construction.

Experimental Analysis of Mechanical Properties of Fiber Reinforced Cenosphere Lightweight Concrete with Higher Temperature Effect

The aim of this research is to explore the impact of polypropylene fibers and glass fibers on the mechanical characteristics of lightweight concrete when subjected to elevated temperatures. Different fiber ratios ranging from 0.25% & 0.50% by volume of concrete were tested to evaluate their impact on various concrete mixtures. Cenosphere is lightweight material as used with cement as bonding material. Pumice aggregate, which is known for its lightweight properties, was utilized as the coarse aggregate. The study aimed to explore various characteristics of lightweight concrete such as slump value, unit weight, and compressive, tensile, and flexural strength. The concrete samples were subjected to different temperatures ranging from 200°C to 400°C for two hours. The findings revealed that the addition of glass fibers improved the tensile strength of the concrete and enhanced the compressive and flexural strength of the lightweight concrete. However, the incorporation of polypropylene fibers resulted in more cracks and pore formation in the concrete. Additionally, the compressive and flexural strength of the concrete decreased with an increase in temperature up to 400°C.

Preparation of egg-structured ceramsites from molybdenum tailings with improved properties

Conversion of solid waste tailings into building materials is an effective way to turn waste into resources as well as resolve the problems of their accumulation and pollution. Herein, in this paper, light-weight and high strength molybdenum tailings ceramsites (LHMCs) were prepared using molybdenum tailings as the main raw material supplemented with fly ash, sludge, steel slag and other solid wastes by egg-structure (three-layer structure) design and high-temperature sintering. The effects of raw material ratio, amount of carbon powder agent, sintering temperature, and sintering time on the properties of LHMCs were investigated, and the optimal preparation process was proposed to be preheating at 400 °C for 60 min and sintering at 1150 °C for 40 min. The results of the physicochemical properties, morphology, and component analysis of LHMCs show that the LHMCs with egg-structure including porous core, reinforced middle layer, and dense outer layer can significantly increase the strength of the ceramsites under the premise of ensuring the low density of the ceramsites. Meanwhile, the doping of molybdenum tailings improves the physical properties of the ceramsites and achieves the reuse of waste tailings. Importantly, the optimized LHMCs can meet the performance requirements of 900-grade ceramsites. This paper proposes a strategy for the preparation of tailings-doped light-weight and high-strength ceramic building materials, which provides a new idea for the recycling of solid waste.

Multi-objective optimization of machining variables for wire-EDM of LM6/fly ash composite materials using grey relational analysis

Abstract With the enhancement in science and technology, necessity of complex shapes in manufacturing industries have become essential for more versatile applications. This leads to the demand for lightweight and durable materials for applications in aerospace, defense, automotive, as well as sports and thermal management. Wire electric discharge machining (WEDM) is an extensively utilized process that is used for the exact and indented shaped components of all materials that are electrically conductive. This technique is suitable in practically all industrial sectors owing to its widespread application. The present investigation explores WEDM for LM6/fly ash composites to optimize different process variables for attaining performance measures in terms of maximum material removal rate (MRR) and minimum surface roughness (SR). Taguchi’s L27 OA design of experiments, grey relational analysis, and analysis of variance (ANOVA) were employed to optimize SR and MRR. It has been noted from ANOVA that reinforcement (R) percentage and pulse on time are the most influential aspects for Grey Relational Grade (GRG) with their contributions of 28.22 and 18.18%, respectively. It is found that the best process variables for achieving the highest MRR and lowest SR simultaneously during the machining of the composite are gap voltage of 30 V, pulse on time of 10 µs, pulse off time of 2 µs, wire feed of 8 m/min, and R of 9%. The predicted GRG is 0.84, and the experimental GRG value is 0.86. The validation experiments at the optimized setting show close agreement between predicted and experimental values. The morphological study by optical microscopy revealed a homogenous distribution of reinforcement in the matrix which enhances the composite’s hardness and decreases the density.

Preparation and Analysis of New Hybrid PAEK Nano-Composites Containing MWCNT, Inorganic and Organic Nano-Particles

In modern era light weight material with complex shape and high material properties is required for advanced applications at room temperature. In this concern, the experimental study analysis to observe the effect of different nanofiller material like GO, MWCNT, NH2 and COOH modified MWCNT as organic nanofiller and WS2 and MoS2 as inorganic nanofiller reinforced in pure PAEK. This experimental study focuses on incorporation of organic and inorganic nanofillers separately and combine as hybrid nanofiller nanocomposite. On preparation of PAEK nano-composite and hybrid nanocomposite the comparative study of mechanical properties like tensile, flexural and impact is carried out. The nano-composites were prepared by melt compounding method. Scanning Electron Microscopy confirm the homogenous dispersion and alignment of nanofilers. The hybrid nano-composites show highest tensile strength as compared to the organic and in-organic nanofiller nanocomposite. As the percentage of MWCNT increases from 0.2 to 1 wt% , the properties of nano-composites decreases due agglomeration tendency of MWCNT that results in low load transfer capacity of nanocomposite, therefore agglomeration tendency of MWCNT required to be controlled for preparation of nano-composite and this is achieved by reinforcing Hybrid nanofillers to pure PAEK matrix. However, organic filler shows 75 MPa to 83 MPa tensile properties and which is lower compared to in-organic and hybrid filler due agglomeration tendency of organic nanofiller confirmed by SEM analysis.