Introduction The rapid development and practical deployment of hyperspectral detection technology render conventional camouflage materials increasingly inadequate for covert protection in complex forest environments,primarily due to their insufficient spectral matching and limited functionality.A critical challenge in forest camouflage lies in the accurate replication of four characteristic spectral signatures of natural vegetation,i.e.,the green peak,the red edge,the near-infrared plateau,and the near-infrared water absorption peak.These features form the unique spectral"fingerprint"of vegetation,which serves as a fundamental basis for evading multi-band,high-precision detection.Among them,the green peak governs a visual concealment within the visible region,while the near-infrared water absorption peak acts as a decisive marker for distinguishing vegetation from artificial materials,which are essential for effective camouflage.However,the existing forest camouflage paints and simulation materials are constrained by inadequate structural design and limited functional integration.Most commercially available products can only partially mimic certain spectral traits of vegetation and commonly exhibit deficiencies in core performance.Note that it remains particularly difficult to simultaneously and accurately match both the green peak of healthy foliage and the near-infrared water absorption peak.This shortfall makes such materials susceptible to rapid identification under hyperspectral imaging,thereby severely limiting the practical effectiveness of the existing forest camouflage technologies.Consequently,there is an urgent need for the research and development of novel camouflage materials capable of comprehensive spectral simulation. Methods Six types of sulfate intercalated Mg/Cr layered double hydroxides(LDHs)were synthesized by a coprecipitation method.The crystalline structure of the as-prepared materials was examined by X-ray diffraction(XRD).The molecular structures and characteristic absorption features were determined by Fourier-transform infrared spectroscopy(FT-IR).The thermal decomposition behavior and mass loss profiles were analyzed by thermogravimetry-differential thermal analysis(TG-DTA).The surface morphology and particle size distribution were characterized by scanning electron microscopy(SEM).These characterization techniques provided a comprehensive structural and physicochemical profile of the synthesized LDHs.Subsequently,to meet the requirement of reproducing the green spectral peak in jungle camouflage,a functional camouflage coating was fabricated via blending the optimized LDHs material with chromium-based green pigment.This formulation enhanced the green-peak characteristics in the visible region and preserved the pronounced water-absorption peak in the near-infrared range.The spectral performance of the coating was evaluated by visible-near-infrared spectroscopy(Vis-NIR),and its thermal camouflage capability was assessed by infrared thermographic imaging. Results and discussion The results systematically indicate that a series of chromium-containing hydrotalcite materials,prepared by a controlled synthesis process,exhibit typical crystalline structural characteristics of layered double hydroxides(LDHs).The XRD patterns reveal sharp and symmetric diffraction peaks,with the characteristic peaks corresponding to the layered structure displaying a high intensity and a regular shape,indicating a good crystallinity and a well-ordered interlayer arrangement.Among the series of chromium-containing hydrotalcites,sulfate-intercalated magnesium chromium hydrotalcite(MgCr-SO42--LDHs)demonstrates unique structural advantages,i.e.,its interlayer spacing is significantly larger than that of other intercalated types of magnesium chromium hydrotalcites.Simultaneously,The TG-DTA thermal analysis indicates that this material exhibits the maximum mass loss of 26.38%at 25-400℃.This is directly related to the content of water molecules stored within the interlayers.The spatial configuration and charge distribution characteristics of the sulfate intercalation ions provide a sufficient space for the stable adsorption and storage of water molecules.As a result,MgCr-SO42--LDHs possesses the most abundant interlayer water content among the prepared samples.This rich interlayer water constitutes the core structural basis for simulating the near-infrared water absorption peak of vegetation.The visible to near-infrared spectra(400-2500 nm)further reveal the optical properties of the Mg/Cr hydrotalcite.In the visible region,the material shows a distinct characteristic absorption peak at 490 nm(blue region),attributed to its intrinsic electronic transition behavior.In the near-infrared region,intense and sharp water absorption peaks clearly occur at approximately 1400 nm and 1900 nm,showing a high similarity to the near-infrared water absorption features of natural vegetation leaves.Sulfate-intercalated magnesium chromium hydrotalcite(MgCr-SO42--LDHs)is modified with chromium(III)oxide(Cr₂O₃)by an in-situ coating process via leveraging these characteristics.The near-infrared water absorption properties imparted by the abundant interlayer water in MgCr-SO42--LDHs were combined with the green optical characteristics of Cr2O3.This complementary interaction successfully offset the influence of the intrinsic blue absorption peak at 490 nm from the magnesium chromium hydrotalcite.Ultimately,a jungle camouflage coating simulation material is prepared,having both green visual characteristics and distinct near-infrared water absorption peaks.This provides a key material foundation for subsequent high-similarity spectral matching with natural vegetation. Conclusions The camouflage coating fabricated with an optimal MgCr-SO42--LDHs@Cr2O3:MgCr-SO42--LDHs ratio of 3:7 could enable a precise spectral matching with natural vegetation leaves.It accurately replicated both the green peak and the near-infrared water absorption peak,achieving a spectral similarity of up to 95.80%.Furthermore,the results of thermal imaging tests revealed no significant difference between the coating and natural vegetation.This study on dual-functional materials that simulated color and the near-infrared water absorption peak could thus offer a promising direction for the development of advanced vegetation camouflage technologies.
Introduction The sulfate source in calcium sulfoaluminate(CSA)cement is an important factor that determines its hydration characteristics,and the properties of CSA cement can be controlled via adjusting the molar ratio of calcium sulfate to ye'elimite(generally referred to as the"M-value").Sulphosilicate cement is a novel cementitious material developed via incorporating ternesite(C5S2$)and free-calcium sulfate(f-C$)into the mineral composition of CSA cement.The calculation of the M-value must comprehensively account for their contents in the clinker due to the presence of SO3 in C5S2$ and f-C$.This study optimized the conventional calculation formula for the M-value and systematically investigated its influence on the compressive strength and expansion behavior of sulphosilicate cement.In addition,the mechanism underlying the effect of anhydrite dosage on cement hydration was also elucidated through the evolution of hydration products. Methods In this experiment,sulphosilicate clinker was synthesized with limestone,fly ash,phosphogypsum,and bauxite as raw materials.The raw materials were calcined at 1250℃for 1 h,followed by rapid air quenching to obtain the clinker.Five groups of sulphosilicate cement with M-values of 0,0.9,1.8,2.7,and 3.6 were designed.Mortar specimens were prepared to evaluate compressive strength and expansion rate,while paste specimens were prepared for analyzing hydration product characteristics at specific ages.The hydration heat release was measured by an isothermal calorimeter.The phase compositions and microstructure of the hydration products were analyzed by X-ray diffractometer,thermal behavior analyzer and field-emission environmental scanning electron microscope.The pore structure was performed by a mercury intrusion porosimeter. Results and discussion Among the various sulphoaluminate cement groups,the M1.8 specimen demonstrates the maximum compressive strength of 43.9 MPa after 3-d hydration.As hydration progresses,the compressive strength of the M2.7 and M3.6 groups shows a significant growth,with the M3.6 group reaching 56.7 MPa at 14 d.However,the M3.6 specimen exhibits a notable strength retrogression,which,is decreased by 4.3 MPa,after 28-d hydration.In contrast,the compressive strength of the M0.9 group increases markedly from 14 d to 28 d,with an improvement of 12.3 MPa. For specimens with the M-value below 1,the early hydration products at 3 d are dominated by AFm phases with insufficient AFt content,resulting in an initial shrinkage.As hydration progresses,the dissolution of SO3 from anhydrite and C5S2$ promotes AFt formation,leading to a measurable expansion after 7 d.At low M-values,AFt contributes to pore refinement,while the reaction of C5S2$ and C2S in later stages generates C-(A)-S-H gel and stratlingite(C2ASH8),refining the microstructure of hydration products and enhancing long-term strength.In contrast,specimens with higher M-values(i.e.,1.8,2.7,and 3.6)exhibit expansion as early as 3 d.Increased anhydrite content and prolonged hydration amplify the crystallographic stress exerted by AFt crystals on the surrounding matrix,causing coarsening of pores,and elevated total porosity,and resulting in strength retrogression during 14-28 d of curing. Conclusions This study was to propose a formula for calculating the gypsum coefficient M-value in sulphosilicate cement,i.e.,CG=0.13×M×(Y-A×2.24-T×1.27)/Sg,establishing the relationship between gypsum dosage and cement performance,while elucidating the influence of M-values on the mechanical properties and hydration mechanisms.The experimental results demonstrated that within the optimal M-range of 0.9-1.8,cement could achieve continuous strength development for all curing ages.Higher M-values(2.7-3.6)enhanced mid-term strength but induced strength regression at 28 d.Increasing M-values effectively suppressed early-age shrinkage and induced progressive expansion,and expansion rates were positively correlated to M-values at all hydration stages.Hydration products primarily consisted of AFt,AH3,and C-(A)-S-H gel.At M0 and M0.9,AFm phases formed during early hydration,followed by C2ASH8 in later stages.Conversely,M-values of≥1.8 accelerated C4A3$ hydration but inhibited both C5S2$ dissolution and C2ASH8 formation.Microstructurally,M0.9 specimens exhibited pore refinement and reduced porosity through gel-phase densification,enhancing strength development,whereas M3.6 samples suffered from coarse pore proliferation due to excessive anhydrite content promoting oversized AFt crystals,ultimately degrading 28-d strength.At the M-value of 0.9-1.8,the system achieved balanced high strength and minimal volumetric deformation,making it suitable for general engineering applications.Elevating the M-value to 1.8-2.7 induced a controlled expansion behavior,enabling targeted use in shrinkage compensation scenarios.However,at the M value of 3.6,delayed strength retrogression and microstructural deterioration could occur,thus establishing 3.6 as a critical upper limit for M-values.
Introduction Magnesium silicate hydrate(M-S-H)cementitious material is regarded as a potential alternative to conventional Portland cement due to its excellent sulfate erosion resistance,heavy metal ion solidification performance,and low pH value during hydration.However,the industrialization process of M-S-H cementitious materials is currently restricted due to the high cost and unstable supply of traditional silicon sources that rely on industrial by-products(such as silica fume,fly ash,and slag).Natural layered silicates such as montmorillonite and kaolin have a similar tetrahedral-octahedral stacking unit structure to M-S-H,and can exhibit a good pozzolanic activity after mechanical or thermal activation.Exploring the use of calcined montmorillonite and kaolin as alternative silicon sources to partially or completely replace silica fume broadens the source of raw materials for M-S-H cementitious materials and reduces production costs.This work was to investigate the effects of partial or complete replacement of silica fume by calcined montmorillonite(MT)and calcined kaolin(KL)on the structure and strength of M-S-H cementitious materials,clarifying the action mechanism of aluminum in the two clay minerals on the M-S-H hydration process and layered structure. Methods Light-burned magnesia(LM)was used as a magnesium source and silica fume(SF)as a reference silicon source.Montmorillonite and kaolin were calcined in a muffle furnace at 750℃for 1 h at a heating rate of 10℃/min to obtain calcined montmorillonite and calcined kaolin.The reference sample was prepared via mixing MgO and silica fume at a mass ratio of 4:6.On this basis,different mass fractions(i.e.,10%,30%,50%,70%,and 100%)of calcined montmorillonite and kaolin were used to replace silica fume,and the samples were named as MT-10～MT-100 and KL-10～KL-100,respectively according to the replacement ratio.The mortar strength specimens were prepared with the samples above,superplasticizer,tap water,and standard sand,and the compressive strength was tested according to the standard GB/T 17671-1999"Methods of testing cement-Determination of strength".The paste was prepared at a water-to-solid ratio of 0.5 and 0.7%superplasticizer based on the total mass of solids,cured in sealed glass tubes with liquid paraffin,and terminated hydration with isopropanol at a certain age,then dried in a vacuum oven at 40℃for 48 h.The mineral phase,thermal behavior,functional groups,microstructure,and element distribution of the samples were characterized by X-ray diffraction(XRD),thermogravimetric analysis(TGA),Fourier transform infrared spectroscopy(FTIR),scanning electron microscopy(SEM),and solid-state 29Si and 27Al magic-angle spinning nuclear magnetic resonance(MAS NMR). Results and discussion Calcined kaolin exhibits a great early strength enhancement effect,and its compressive strengths at 3 d and 7 d have a positive correlation with the dosage(i.e.,correlation coefficients are 0.996 and 0.905,respectively).At the dosage of calcined kaolin of 70%,the strength at 3 d reaches 39.37 MPa and the strength at 7 d reaches 49.58 MPa,which are 464.0%and 400.3%higher than those of the reference sample,respectively.In contrast,the strength enhancement effect of calcined montmorillonite is weak.The strengths at 3 d and 7 d increase slowly with the increase of montmorillonite dosage.At the montmorillonite dosage of 70%,the strength at 3 d is only 8.94 MPa and the strength at 7 d is 14.28 MPa,which are 36.5%and 53.7%higher than those of the reference sample,respectively.The strength increases slowly in the montmorillonite dosage range of 10%-50%. The microscopic analysis shows that calcination at 750℃causes more thorough damage to the crystal structure of kaolin than that of montmorillonite.The XRD diffraction peaks of kaolin disappear completely after calcination,and the layered structure collapses completely,resulting in a higher pozzolanic activity.After calcination,the interlayer spacing of montmorillonite decreases,and some characteristic diffraction peaks are still retained,indicating that its crystal order is not completely destroyed and the pozzolanic activity is lower than that of calcined kaolin.The FTIR and NMR analysis further confirms that the structural hydroxyl groups of calcined kaolin are completely removed,and a large number of active Al and Si sites are exposed.Although montmorillonite also undergoes dehydroxylation and structural damage after calcination,the degree of disorder is lower than that of kaolin. Aluminum in both calcined montmorillonite and kaolin can enter the M-S-H phase in the early stage of hydration,and form magnesium aluminosilicate hydrate(M-S-A-H)gel via substituting Si4+in the silicate tetrahedron or Mg2+in the magnesium-oxygen octahedron,which significantly improves the polymerization degree of the product.The formed M-S-A-H is external hydration products,mainly formed through element diffusion between particles.The analysis by the XRD,FTIR,and DTG shows that Mg-Al layered double hydroxide(Mg-Al LDH)is generated within the first 28 d of hydration in both raw material systems.The enhancement of cementitious material strength by calcined kaolin and montmorillonite is related to the formation of M-S-A-H.The strength improvement effect of calcined kaolin is higher than that of calcined montmorillonite due to the more thorough destruction of the crystal structure,higher degree of network disorder,more active Al and Si sites,and higher content of Al(V). Conclusions Calcined kaolin and montmorillonite could be used as alternative silicon sources for M-S-H cementitious materials.Calcined kaolin had a great early strength enhancement effect,and its compressive strength had a positive correlation with the dosage.At the dosage of 70%,the compressive strength at 7 d was increased by more than 400%,compared with the sample prepared with pure SF.The strength enhancement effect of calcined montmorillonite could be weak,and the strength was only increased by 53.7%at the same dosage and age.The difference in strength enhancement effect between calcined kaolin and montmorillonite could be due to the fact that the calcination at 750℃caused more thorough damage to the crystal structure of kaolin than that of montmorillonite,resulting in a higher pozzolanic activity.Aluminum in both calcined montmorillonite and kaolin could enter the M-S-H phase in the early stage to form M-S-A-H gel,significantly improving the polymerization degree of the product.The formed M-S-A-H were external hydration products,and Mg-Al LDH was generated within the first 28 d of hydration.
Introduction The hydration of binder is an exothermic reaction,which results in an obvious temperature increase in concrete in the early hydration period.The shrinkage of hardened concrete due to the temperature drop is a main cracking trigger of concrete.A kind of temperature rising inhibitor is developed to decrease the hydration heat of binder in early hydration age,which can reduce the cracking risk of concrete.Cyclodextrin is a main functional composition of the temperature rising inhibitor.C3A is an important clinker mineral influencing the early exothermal characteristics of Portland cement.In this paper,the effect of cyclodextrin on the hydration of tricalcium aluminate-gypsum(C3A-CaSO4·2H2O)was investigated.This work could favor understanding the action mechanism of the temperature rising inhibitor to reduce the cracking risk of concrete structures. Methods Pure C3A was calcined,the chemical pure gypsum and cyclodextrin was used.The hydration exothermal curves of C3A-CaSO4·2H2O pastes containing different dosages of cyclodextrin were measured.The hydration products of C3A-CaSO4·2H2O pastes containing different dosages of cyclodextrin in different ages were in-situ determined by quantitative X-ray diffraction(QXRD).The morphology of hydration products on the surface of C3A particles immersed in different solutions was characterized by scanning electron microscopy(SEM).The etching situation on the surface of C3A particles washed by different solutions was determined by three-dimensional white light interferometric surface profilometry. Results and discussion The beginning time of second hydration of C3A moves up and its exothermic rate decreases,but its reacting time prolongs with the increase of cyclodextrin dosage.The heat output of C3A during its second hydration stage varies little.The consumption of C3A and CaSO4·2H2O increases continuously and the exhausting time of gypsum reduces with the increase of cyclodextrin dosage.The forming quantity of ettringate in the paste containing cyclodextrin is greater than that in controlling paste.The transformation of ettringate to AFm is suppressed after the exhaust of gypsum.Cyclodextrin can expedite the dissolution of C3A in CaSO4 solution to form more deeper etch pits on the surface of C3A particles,which speeds up the hydration of C3A.Needle-like ettringite changes to stick-like one,and the transformation of ettringite to AFm restrains when cyclodextrin exists in pastes. Conclusions Cyclodextrin could promote the initial hydration of C3A to move up the beginning of the second hydration of C3A.The consumption of C3A and CaSO4·2H2O increased continuously in the first hydration stage and the exhausting time of gypsum reduced with the increase of cyclodextrin dosage.Cyclodextrin reduced the reaction speed and prolonged reacting time of C3A during its second hydration stage,its heat output changed little.Cyclodextrin could enhance the dissolution of C3A in gypsum solution to form more deep etch pits on the surface of C3A particles,speeding up the hydration of C3A.Cyclodextrin could change needle-like ettringate to stick-like one and suppress the transformation of ettringate to AFm.
Introduction Solid oxide fuel cells(SOFCs)are high-efficient solid-state energy conversion devices.However,all-ceramic self-supported SOFCs face several challenges such as high brittleness,difficulty in mechanical processing,poor thermal shock resistance,and limited weldability,which result in high manufacturing costs and restrict the applications in mobile power systems.In contrast,metal-supported SOFCs(MS-SOFCs)with metal materials as the external structural support,exhibit remarkable mechanical strength,low cost,and rapid start-up capability,making it highly promising for mobile applications.The anode is a critical component of MS-SOFCs,serving as the site where fuel oxidation occurs to generate electrons.Its microstructure significantly influences the density and effectiveness of the triple-phase boundaries(TPB),where the gas phase,the ionic phase,and the electronic phase intersect.The TPB density largely determines the polarization resistance,with its low-frequency component being inversely related to anode gas diffusion.A common strategy to enhance gas transport is the incorporation of pore-formers,such as graphite,into the anode raw materials.Most studies focus on the type,particle size,and content of pore-formers,which directly affect the pores number,size,and distribution.In this work,atmospheric plasma spraying(APS)was employed to fabricate three types of anodes and corresponding cells.APS reduces thermal input to the metal substrate,effectively preventing oxidation,deformation,and elemental interdiffusion between the metal support and the anode at high temperatures.This study systematically investigates the influence of graphite incorporation methods on the microstructural evolution of the anode and the resulting cell performance,providing important theoretical insights into the operational mechanisms of SOFC anodes. Methods Porous 430L stainless steel substrates were used as supports.Three different NiO-GDC(Gd0.2Ce0.8O1.9)anode powders were prepared.C1 is the baseline without a pore-former.C2 contains 40%(in volume fraction)graphite,which is mixed by spray granulation process to produce composite particles.C3 is made by mechanically mixing 40%of the same graphite with C1 powder.All powders are spherical with good fluidity.Anode layers were deposited via APS.Subsequently,the C2 and C3 anodes were heat-treated in air to remove the graphite pore-former at 750℃for 2 h.A ScSZ(Sc2O3-ZrO2)electrolyte and an LSCF(La0.6Sr0.4Co0.2Fe0.8O3-δ)cathode were subsequently deposited by APS to build single cells.The microstructure and porosity of the anodes were characterized using scanning electron microscopy(SEM)and image analysis software.The surface roughness was measured by profilometer.The electrochemical performance,including the open-circuit voltage(OCV),current-voltage-power(I-V-P)and electrochemical impedance spectroscopy(EIS),were evaluated in the range of 600-750℃using humidified H2 as fuel and air as oxidant.The equivalent circuit fitting of EIS data is carried out to quantitatively analyze the contribution of charge transfer,surface adsorption/dissociation and gas diffusion in polarization impedance. Results and discussion The incorporation of graphite pore-former significantly modified the pore size distribution and total porosity within the anodes.The measured porosity of C1,C2,and C3 was 26%±2%,37%±3.1%,and 42%±2.3%,respectively.The C1 anode featured a relatively dense structure with uniformly distributed pores,which primarily consisted of submicron cracks and fine pores originating from the thermal stress inherent to the APS process.In contrast,the C2 anode showed a notable increase in both the number and size of pores,which were homogeneously dispersed without significant agglomeration.The C3 anode,however,contained a substantial amount of large pores,mostly～5 μm in diameter,attributed to the agglomeration of graphite particles during mechanical mixing,resulting in coarse and irregular pore structures after heat treatment.Furthermore,the addition of graphite modified the thermal response characteristics of the agglomerated powder during the spraying process,promoting the formation of a more uniformly melted microstructure in the anode layer.The average surface roughness(Ra)values for C1,C2,and C3 were 6.64,7.06 nm,and 7.66 μm,respectively,indicating that graphite addition increased anode surface roughness.This phenomenon is due to the thermal decomposition of graphite during the plasma spraying process,where high temperatures cause partial oxidation of graphite in the open atmosphere,thereby generating CO2 gas.The release of this gas from the incompletely solidified anode surface etches irregular pits and protrusions,ultimately leading to increased surface roughness.The cell without graphite pore-former(C1)consistently demonstrated the highest OCV and maximum power density,reaching 1.0 V and 957 mW·cm-2 at 750 °C,respectively.EIS analysis revealed that the anodes with graphite pore-former(C2 and C3)exhibited improved charge transfer capability,thereby reducing the high-frequency polarization resistance.Despite this,the overall output performance of C2 and C3 did not show effective enhancement,which is attributed to their increased ohmic resistance(Ro)and lower OCV.The elevated surface roughness and inherent porosity in the C2 and C3 anodes adversely affected the quality of the subsequently sprayed electrolyte layer,introducing microcracks and gas permeation pathways.This resulted in increased Ro and reduced OCV,ultimately weakening the benefits gained from the reduced polarization resistance. Conclusions The method of graphite pore-former addition significantly affects the anode microstructure and overall cell performance.Compared with mechanical mixing,spray granulation produces a superior and uniform pore structure.However,contrary to conventional expectation,the introduction of graphite pore-forming agent into the APS anode reduces overall cell performance due to induced electrolyte defects,which elevated Ro and lowered OCV.Future optimization should focus on strategies to reduce anode surface roughness and refine pore structure without affecting the quality of electrolyte deposition.It is anticipated that this will further enhance the output performance of MS-SOFCs.
