Vacuum Heat Research Articles

Microstructural Characteristics and Material Failure Mechanism of SLM Ti-6Al-4V-Zn Alloy

This study focuses on the additive manufacturing technique of selective laser melting (SLM) to produce Ti-6Al-4V-Zn titanium alloy. The addition of zinc at 0.3 wt.% was investigated to improve the strength and ductility of SLM Ti-6Al-4V alloys. The microstructure and mechanical properties were analyzed using different vacuum heat treatment processes, with the 800-4-FC specimen exhibiting the most favorable overall mechanical properties. Additionally, zinc serves as a stabilizing element for the β phase, enhancing the resistance to particle erosion and corrosion impedance of Ti-6Al-4V-Zn alloy. Furthermore, the incorporation of trace amounts of Zn imparts improved impact toughness and stabilized high-temperature tensile mechanical properties to SLM Ti-6Al-4V-Zn alloy. The data obtained serve as valuable references for the application of SLM-64Ti.

Chromium–Aluminum Coatings for Oxidation Protection of Titanium–Aluminum Intermetallic Alloys

This article explores the utilization of cathodic-arc deposition Cr-Al overlay coatings as oxidation protection for Ti-Al-Nb intermetallic alloys. The primary objective is to investigate PVD Al-Cr coatings deposited via cathodic-arc deposition without subsequent vacuum annealing. The microstructure, phase, and chemical composition of the coatings were characterized using scanning electron microscopy, energy-dispersive X-ray spectroscopy, and X-ray diffraction analysis. Isothermal exposure of samples in a laboratory air furnace was conducted, revealing the efficacy of Cr-Al coatings in protecting the Ti49-11Al-40Nb-1.5Zr-0.75V-0.75Mo-0.2Si (mass%) intermetallic alloy VTI-4 against oxidation. The findings highlight that the as-deposited coatings possess a layered structure and contain Al-Cr intermetallics. Post-exposure to the furnace without prior vacuum annealing results in coatings exhibiting a porous microstructure, raising concerns regarding oxidation protection. This investigation of Cr-Al coatings on a VTI-4 alloy substrate yields valuable insights into their nanolaminate structure and challenges associated with aluminum droplet fractions. The proposed additional vacuum heat treatment at 650 °C for 500 h effectively homogenizes the coating, leading to predominant Cr2Al and Ti-Al phases. Additionally, the formation of diffusion layers at the “coating–substrate” interface and the presence of oxide barriers contribute to the coatings’ heat resistance. Our research introduces possibilities for tailoring coating properties for specific high-temperature applications in aerospace, energy, or industrial contexts. Further refinement of the heat treatment process offers the potential for developing advanced coatings with enhanced performance characteristics.

Understanding the Effects of Post-Deposition Sequential Annealing on the Physical and Chemical Properties of Cu2ZnSnSe4 Thin Films

Cu2ZnSnSe4 thin films have been synthesized by employing two magnetron-sputtering depositions, interlaced with two sequential post-deposition heat treatments in low vacuum, Sn+Se and Se–rich atmospheres at 550 °C. By employing successive structural analysis methods, namely Grazing Incidence X–Ray Diffraction (GIXRD) and Raman Spectroscopy, secondary phases such as ZnSe coexisting with the main kesterite phase have been identified. SEM peered into the surface morphology of the samples, detecting structural defects and grain profiles, while EDS experiments showed off–stoichiometric elemental composition. The optical bandgaps in our samples were calculated by a widely used extrapolation method from recorded transmission spectra, holding values from 1.42 to 2.01 eV. Understanding the processes behind the appearance of secondary phases and occurring structural defects accompanied by finding ways to mitigate their impact on the solar cells’ properties is the prime goal of the research beforehand.

Construction of a multifunctional dual-network chitosan composite aerogel with enhanced tunability

Typically, the tailorable versatility of biomass aerogels is attributed to the tunable internal molecular structure, providing broad application prospects. Herein, a simple and novel preparation strategy for developing multifunctional dual-network chitosan/itaconic acid (CSI) aerogel with tunability by using freeze-drying and vacuum heat treatment techniques. By regulating the temperature and duration of amidation reaction, electrostatic interactions between chitosan (CS) and itaconic acid (IA) was abstemiously converted into amide bond in frozen aerogel, with IA acting as an efficient in-situ cross-linking agent, which yielded CSI aerogels with different electrostatic/covalent cross-linking ratios. Heat treatment and tuning of the covalent cross-linking degree of CSI aerogel changed their microstructure and density, which led to enhanced performance. For example, the specific modulus of CSI1.5–160 °C-5 h (71.69 ± 2.55 MPa·cm3·g−1) increased by 119 % compared to that of CSI1.5 (32.73 ± 0.718 MPa·cm3·g−1), converting the material from superhydrophilic to hydrophobic (124° ± 3.6°), exhibiting favorable stability and heat transfer performance. In addition, part of -NH3+ of CS was retained in the electrostatic cross-linked network, endowing the aerogel with antibacterial properties. The findings of this study provide insights and a reliable strategy for fabricating biomass aerogel with good comprehensive performance via ingenious structural design and simple regulation methods.

Effect of heat treatment on deformation inhomogeneity of carbon steel / stainless steel bimetal

The work is devoted to the study of the effect of annealing on mechanical properties and inhomogeneity of plastic deformation of a bimetallic plate made of stainless / carbon steel with the dimensions of the working part 50×7×2 mm. To develop laser technology for producing bimetals of various compositions, the contact zone of two dissimilar steels is of greatest interest. Since the performance characteristics of the entire product as a whole depend on the structure and properties of this zone, interaction of the components of the bimetal in the process of its manufacture leads to appearance of heterogeneity of various types near the interface and in the volumes adjacent to it. The research material was obtained by laser cladding of wire AISI 304 stainless steel on a plate of low-carbon steel St3. Bimetallic samples were subjected to vacuum heating at a temperature of 700 °C at various times from 2 to 8 h. The use of data on the distributions of local strains by the speckle photography method made it possible to consider the process of plastic flow in the initial section of tension diagram and to establish the effect of annealing temperature on plastic strain localization during mechanical tests. For a quantitative assessment of deformation inhomogeneity in the main and cladding layers, we used spatiotemporal distributions of local elongations and the corresponding values of the variation coefficient. It was established that the level of deformation inhomogeneity of microvolumes at the interface during tension is higher than that of the bimetal main layers. With increase in the annealing time, increase in the variation coefficient in the joint zone is noted, which is more significant on the stainless steel side, and this increases the probability of microcracks initiation. The increased level of deformation inhomogeneity of microvolumes of the cladding layer carburized zone is contingent on the increased localization of deformation in nearby microvolumes due to structural heterogeneity.

Structural and Phase Transformation Defects Within Polycrystalline Cerium Dioxide on Heating in Vacuum and in Air

Structural and Phase Transformation Defects Within Polycrystalline Cerium Dioxide on Heating in Vacuum and in Air

Multi-matrix continuously-reinforced composites: A novel route to sustainable repair of composite structures

The worldwide increase of composites usage is accompanied by an equal increase of composite waste. While there are many reasons for discarding composite components, a primary reason is damage, with interlaminar failure being particularly common. A novel Multi-Matrix Continuously-Reinforced Composite (MMCRC) repair methodology is proposed, which incorporates repairable vitrimer resin. The procedure enables interlaminar failure repair without utilizing typical patch repair, that introduces additional damage. To achieve this a vitrimer matrix is incorporated into the damaged-prone region at the manufacturing stage. Interlaminar failure is then established and repaired using vacuum pressure and heating using an oven. With an eye to real-world application (where removing a part to be placed in an oven may not be possible), repair was also conducted utilizing localized induction heating. Both methods successfully repaired the interlaminar failure, recovering the initial strength and generating an average performance of 167% compared to the intact sample. The reasons for the increased performance are described in detail, which lead to the conclusion that the feasibility of incorporating reparable regions at the manufacturing stage has been demonstrated successfully. It is shown an interlaminar failure repair can be achieved using a less detrimental and time-consuming processing than traditional repair methodologies.

Flavor characteristics of semi-dried yellow croaker (Pseudosciaena crocea) with KCl and ultrasound under sodium-reduced conditions before and after low temperature vacuum heating.

This study investigated the flavor characteristics of semi-dried yellow croaker made by KCl instead of partial NaCl combined with ultrasound treatment before and after low temperature vacuum heating. The electronic tongue, electronic nose, free amino acids, 5'-nucleotides, and gas chromatography-ion mobility spectrometry were employed. Electronic nose and electronic tongue results showed that different treatment groups had different sensitive signals to smell and taste. The odor and taste of each group were mainly affected by Na+ and K+. The difference between the groups becomes larger after thermal treatment. Ultrasound and thermal treatment both changed the content of taste components. In addition, each group contained 54 volatile flavor compounds. Among them, the combined treatment method gave semi-dried large yellow croaker pleasant flavor characteristics. Besides, it also improved the content of flavor substances. In conclusion, the semi-dried yellow croaker under sodium-reduced conditions showed better performance in flavor characteristics.

Doped Carbon Dots Enable Highly Efficient Multiple‐Color Room Temperature Phosphorescence

AbstractColloidal carbon dots (C‐dots) are considered as promising heavy‐metal‐free materials to achieve room‐temperature phosphorescence (RTP) properties for promising applications, such as photoelectronic devices, information encryption, and bio‐imaging. However, most of the current obtained RTP C‐dots have a short lifetime with a relatively low quantum yield (QY). In this work, the C‐dots large‐scale synthesized via a vacuum heating approach have multiple RTP emissions (blue, green, and yellow), a long RTP lifetime of as high as 1.92 s, and a high QY of 34.4% by selecting different types of precursors, which is superior to most of reported RTP C‐dots. The multiple atoms doping and strong bonding between neighbored C‐dots promoted by vacuum heating contributes to the excellent RTP properties. As a proof‐of‐concept, the as‐obtained RTP C‐dots are used as optical ink for flexible security codes, exhibiting a bright shape with a lifetime of 1.37 s. This work offers an efficient approach for producing large‐scale high‐quality RTP C‐dots, which can be applied to anti‐counterfeiting and information encryption systems.

Study on temperature uniformity of workpieces in multi-layer trays inside vacuum sintering furnace for cemented carbide

Numerical simulation has been widely used recently to study temperature field characteristics in sintering furnaces, compensating for the limitations of traditional experimental methods in obtaining temperature field data in high-temperature sintering furnaces. We established a numerical model of the transient temperature field in a vacuum sintering furnace of cemented carbide. Numerical simulation of the transient temperature field of the vacuum heating sintering process is carried out for cemented carbide, and the transient temperature fields at different times in the furnace are obtained. By comparing with the measured data, the maximum error is within 3 %. On this basis, the uniformity of temperature within the different tray layers of the cemented carbide workpieces during the sintering process and at the end moment of sintering is further discussed. The results of the study show that the insulation stage in the sintering process can effectively improve the uniformity of the temperature of the workpieces. The temperature uniformity of the workpieces located in the middle tray layers is significantly worse than that of the workpieces in the upper and lower tray layers. Assuming the middle tray layer as the symmetrical plane, the average, maximum, minimum and coefficient of variation of the temperature of the workpieces among the trays are basically symmetrically distributed. This study provides a reference for improving the local low-temperature and high-temperature zones of the workpieces in the vacuum sintering furnace for cemented carbide.

Microstructure, mechanical properties and thermal stability of Cr/WC nano-multilayered coating synthesized by ion beam sputtering

Cr/WC nano-multilayered coatings with fixed period ratio (Cr:WC=1:2) were synthesized by using ion beam sputtering method. The microstructure, mechanical properties and the thermal stability of coatings were investigated by X-ray diffraction, Scanning Electron Microscopy, Transmission Electron Microscopy and nanoindentation. Well-defined laminar amorphous microstructure and uniform monolayers, which consists of amorphous WC1-x and Cr, can be found in the as-deposited nano-multilayered coatings with series modulation period. Although the hardness of Cr/WC nano-multilayered coating increases with the increasing of modulation period, it is still lower than the value predicted by Hill model. Almost all the toughness of Cr/WC nano-multilayered coatings are lower than that of monolayer Cr coating, however, they are higher than that of monolayer WC coating, indicating that the addition of Cr and nano-multilayered architecture can improve the toughness of monolayer WC coating. Both microstructure and mechanical properties of Cr/WC nano-multilayered coatings were stable when the vacuum heat treatment temperature was lower than 400 °C. As the heat treatment temperature was 600 °C, some nanocrystalline W2C, W and WC1-x were enclosed in the amorphous WC sublayer which is beneficial to its hardness enhancement while it is harmful to its toughness. As the heat treatment temperature was 800 °C, the nano-multilayered structure was destroyed completely and its mechanical properties were seriously degraded.

Effect of heat treatment on resistive switching memory characteristics of NiO nanodots of tens of nanometers shattered by AFM tips

The characteristics of resistive random access memory (RRAM) at the nanometer scale are increasingly important for RRAM cells with further miniaturization. We report the RRAM characteristics of NiO nanodots in the order of tens of nanometers. NiO nanodots of approximately 70 nm were created on a single-crystal Nb-doped SrTiO3 (Nb:STO) substrate by dip-pen nanolithography. For comparison, a NiO thin film was also deposited on the Nb:STO substrate by spin coating. To reduce the size of NiO nanodots, they were shattered by an atomic force microscope tip, obtaining nanodots of approximately 10 nm. Using a conductive atomic force microscope, we characterized the current according to the applied electric field of the NiO nanodots on the Nb:STO substrate with a capacitor configured of Pt/NiO/Nb:STO substrate. When compared with the NiO thin film, the forming voltage of the NiO nanodots decreased. In addition, the shattered NiO nanodots exhibited unipolar switching characteristics, and the forming and set/reset voltages decreased with the reduction in nanodot size. Furthermore, the NiO nanodots exhibited a substantially reduced forming and set/reset voltages after heat treatment in vacuum. We suggest that the oxygen-depleted layer formed on the surface of NiO nanodots by heat treatment plays a crucial role in the reduction of the forming and operation voltages.

Recycling of Cathode active materials from Spent Lithium-ion Batteries (LIBs): Effective Methodology for Environmental Remediation

In this work, three different solvents such as N, N-Dimethylformamide (DMF), N-Methyl-2-pyrrolidone (NMP) and Dimethyl sulfoxide (DMSO) were used to recycle cathode active materials (CAM) from spent lithium ion batteries (LIBs). Both calcination and solvent dissolution processes were used. The cathode active materials recycled from DMF, NMP and DMSO solvent dissolution process were named as CAM 1, CAM 2 and CAM 3, respectively. Initially, cathode electrode cut into small pieces and then baked at 600 °C for 7 h in tube furnace under argon environment. In the send step baked electrode transferred to solvent and then continue reflux reaction 24 h. In the final step the Al foils were separated from the solution and the cathode active material were collected by vacuum filtration and heat treatment. The structure and morphology of the cathode active materials were characterized by using XRD, Raman, SEM and TEM instruments. The SEM morphology shows that PVDF polymer and acetylene black removed from the cathode active materials by calcination and solvent dissolution process. EDAX and mapping shows that CAM 3 is more pure and uniform morphology than CAM 1 and CAM 2 cathode active materials. The elemental analysis shows that CAM 1, CAM 2 and CAM 3 are NMC 532 cathode powder with a chemical formula Li (Ni5Mn3Co2) O2. XRD results show that R3m layered structure with strong crystalline peaks for all three recycled cathode active materials. Raman analysis confirms the presence of metal oxide.

Ultrathin Superhydrophobic Coatings for Air‐Stable Inorganic Solid Electrolytes: Toward Dry Room Application for All‐Solid‐State Batteries

AbstractInorganic solid electrolytes (SEs), such as sulfides and halides, are crucial for developing practical all‐solid‐state batteries (ASSBs) owing to their high ionic conductivities and mechanical sinterabilities. However, their sensitivity to humid air necessitates stringent dry‐room conditions during processing, which increases production costs. This study demonstrates that ultrathin (≈5 nm) superhydrophobic polydimethylsiloxane (PDMS) or fluorinated PDMS (F‐PDMS) protective layers can enhance the stability of air‐sensitive sulfide (Li6PS5Cl (LPSCl)) and halide (Li2.5Zr0.5In0.5Cl6) SEs in ASSBs. The (F)‐PDMS coatings are applied using a scalable, straightforward vapor‐phase deposition process, achieving high Li+ conductivity retention (92%, from 2.5 to 2.3 mS cm−1 at 30 °C). The protective layers effectively inhibit LPSCl degradation under practically relevant dry room conditions (dew point of −50 °C or −10 °C): e.g., from 2.3 to 0.97 mS cm−1 for PDMS‐coated LPSCl versus from 2.5 to 0.57 mS cm−1 for bare LPSCl. Surprisingly, the superhydrophobic coatings facilitate the recovery of Li+ conductivity via vacuum heat treatment. This new phenomenon, known as regeneration, is achieved by the facile elimination of adsorbed water. Furthermore, the regenerated (F)‐PDMS‐coated LPSCl demonstrates significant performance in NCM||Li‐In ASSB cells. These findings suggest that superhydrophobic (F)‐PDMS coatings are a promising solution for practical all‐solid‐state technologies.

Synergistic effects of multi-strengthening mechanisms in exceptionally high-strength W-Ni-Fe composites

Tungsten heavy alloys (WHAs) are typical tungsten-based composites and are considered as important materials in kinetic energy penetrators that currently use depleted uranium munitions. However, the incongruity between strength and plasticity is the “Achilles’ heel”, which restricts further application in national defense and military industries. Herein, a novel fine-grained La2O3 strengthened W-Mo-Ni-Fe composites were fabricated successfully via using nanoscale solid-solution composite powder and the optimized two-stage sintering as well as vacuum heat-treatment techniques. In the model W-xMo-4.9Ni-2.1Fe-1La2O3 composites (x = 2.5, 5, 10 and 20 wt%), the heat-treated alloys had a highly homogeneous microstructure, consisting of equiaxed W grains, matrix phase and dispersed La2O3 reinforced particles. Increasing Mo content promoted microstructure refinement and caused the formation of MoNi3 intermetallic compound (IMC). As a result, benefiting from the multiple strengthening mechanisms containing fine-grained, solid-solution and oxide dispersion strengthening effects, the 88 W-5Mo-4.9Ni-2.1Fe-1La2O3 composite exhibited a superb synergy of tensile strength and ductility (1116 MPa and ∼ 15%). Moreover, the analysis of the lattice strain fluctuations and dislocation behaviors of WHAs were conducted by means of the geometric phase analysis (GPA) approach for the first time. The plastic deformation mechanism was studied to elucidate the fracture characteristics and slip systems with Schmid factor (SF) and Kernel Average Misorientation (KAM) analysis. Our findings not only shed light on the understanding of the deformation mechanisms of WHAs, but also provide a new avenue for the design of WHAs with outstanding mechanical properties.

Oxidation-Induced Changes in the Lattice Structure of YSZ Deposited by EB-PVD in High-Vacuum Conditions

Yttria-stabilized zirconia (YSZ), a thermal barrier coating material characterized by low thermal conductivity, is typically deposited via electron beam-physical vapor deposition. Notably, oxygen depletion occurs during this process, causing color changes in YSZ. Therefore, YSZ is speculated to undergo phase transformation during this process, which demands careful consideration owing to its effect on the life of thermal coatings. To study this phenomenon, bulk samples were prepared, subjected to vacuum heat treatment to induce oxygen depletion, and followed by oxidative heat treatment. Experimental results showed that the color of the samples changed to black after the vacuum heat treatment and to a lighter color after the oxidative heat treatment. In addition, X-ray diffraction and Raman analyses were performed. The monoclinic phase formation was confirmed during the vacuum heat treatment; however, it disappeared after the oxidation heat treatment. The coating obtained in a high vacuum atmosphere exhibited a black color and cubic phase, which changed to a bright color and tetragonal phase after the oxidation heat treatment.

The improved size distribution and NIR luminescence of ZGGO:Cr3+ nanoparticles induced by Y3+ doping

Zn2Ga2.98-xYxCr0.02Ge0.75O8 (ZGGO:Cr0.02,Yx) persistent luminescent nanoparticles (PLNPs) with different concentrations were synthesized by a hydrothermal method combined with a vacuum heat treatment. With increasing Y3+ concentration from 0 to 0.05, the average size reduces from 53 to 28 nm and the particle shape changes from irregular to nearly spherical due to Y3+ doping leading to the suppressed growth rate of nanocrystals via surface charge modification. On the basis of spectral analyses and the first-principles calculation, the near infrared (NIR) emission peaks at 703 and 743 nm originate from the 4T2→4A2 transition of Cr3+ occupying octahedral Ga3+ sites and the 4A2→4T1 transition of Cr3+ occupying tetrahedral Ga3+ sites, respectively. Meanwhile, it is found that Y3+ ions are more likely to occupy tetrahedral Ga3+ sites and it leads to not only the creation of more anti-site defect pairs and the enhanced afterglow intensity but also the increased rigidity and the improved dispersibility of PLNPs in different simulated body fluids induced by complete surface modification (i.e. SiO2 coating). The above results indicate that ZGGO: Cr0.02,Y0.03@SiO2 PLNPs have potential applications in bioimaging.

Combinations of essential oils as effective green stabilizers for single base propellants

AbstractNitrocellulose‐based propellants undergo natural (and inevitable) autocatalytic degradation reactions. The presence of a stabilizer is essential to avoid and reduce the degradation reactions by capturing the NOx molecules. Traditional stabilizers, such as diphenylamine, and ethylcentralite, react with NOx through their amine and/or amide groups, leading to n‐nitroso groups, which are known to have carcinogenic, mutagenic and reprotoxic properties. Molecules that are not prone to form by‐products containing nitrosamines have been studied as substitutes, such as guaiacol and curcumin. However, the aromatic rings on these molecules have their acidity augmented when capturing the NOx groups and, paradoxically, increasing hydrolysis rates. In order to find a solution for the problem, we evaluated the use of essential oils containing terpenes and/or phenolic compounds to introduce a buffering capacity. Several propellant formulations with those compounds were subjected to traditional stability tests (German, Bergman–Junk and Storage tests), Pressure Vacuum Stability Test and Heat Flow Microcalorimetry. Finally, industrial batches were produced for the evaluation of real (ballistic) performance. Results indicated that the usage of essential oils increases stability properties not only through a sum of effects, but also in a synergistic way. Ballistic results were also adequate.

The crack-healing and strength recovery mechanisms for the new developed Ti(C,N)-TiSi2-WC composite ceramic tool materials with crack-healing ability

Because the ceramic tool life will be reduced due to the crack growth, it is necessary to develop the new ceramic tools with crack-healing ability for improving the ceramic tool life. Ti(C,N)-TiSi2-WC composite ceramic tools were fabricated by vacuum hot pressing sintering technology. The Vickers hardness tester was used to prefabricate cracks with different lengths. The crack-healing abilities of materials were researched by air and vacuum heat treatments. The effects of different healing conditions on crack-healing behaviors were investigated based on the strength recovery and microstructure of crack morphology. The healing conditions included healing temperature, healing time, crack length and healing atmosphere. It could be found that the surface cracks were entirely healed after air heat treatment, but locally healed after vacuum heat treatment. After air heating at 800 °C for 90 min, the restoration ratio of flexural strength of the crack-healed samples returned to 109.02% compared to that of the smooth specimen. The strength of the crack-healed specimen would be fully recovered when the crack length was less than 400 μm. After vacuum heating at 1000 °C for 60 min, the recovery of the flexural strength of the crack-healed samples returned to 49.73%. X-ray diffraction and scanning electron microscope technology were used to analyze compositions and microstructure of crack-healed specimens. In the air atmosphere, the liquid filling of SiO2 and TiO2 and crack closure caused by WSi2 and MoSi2 were confirmed as the crack-healing mechanisms. SiO2 and TiO2 were produced by high temperature oxidation reaction of TiSi2 and WSi2. WiS2 and MoSi2 were new phases formed during sintering and heat treatment, and their formation could be explained by thermodynamically feasible reactions. In the vacuum atmosphere, local filling of the crack with silicide solid solutions such as WSi2, MoSi2 and NiSi was the main crack-healing mechanism.

Inhibitory Mechanism of EGCG on Advanced Glycation End Products Formation During Low-Temperature Vacuum Heating of Russian Sturgeon

ABSTRACT Advanced glycation end products (AGEs) are generated in all stages of food thermal treatment. In order to reveal the inhibitory mechanism of AGEs, a range of concentration of (−)-epigallocatechin-3-gallate (EGCG) has been added during low-temperature vacuum heating (LTVH), and high-resolution mass spectrometry-based proteomics was applied for identifying and quantifying N-glycosylated proteins, N-glycosylated peptides, and N-glycosylated sites. The results indicated that, compared with atmospheric cooking, LTVH significantly decreased the generation of AGEs. The addition of EGCG has a positive effect on the inhibition of AGEs when the concentration is lower than 2,000 μg/mL, and 200 μg/mL was the optimal concentration.