DESIGN AND PREPARATION OF SILICON NITRIDE COMPOSITE WITH HIGH FRACTURE TOUGHNESS AND NACRE STRUCTURE

A new way of designing and preparing silicon nitride ceramic composite with high fracture toughness and nacre structure has been proposed. To mimic the laminated structure of nacre, Si_3N_4 matrix ceramic layer can be obtained through compacting rolling method. To mimic the secondary toughening of nacre structure, SiC whisker is added into Si_3N_4 and acts as the secondary toughening phase. Boron nitride (BN)is selected to mimic the organic layer in nacre so as to form the weak interfaces between Si_3N_4 layers. Alumina is added into BN to adjust the bonding strength of the interface.The Si_3N_4 sheets are stacked into the die after coating with BN. After the removal of the organic matter in them, the green body is hot pressed at 1820℃for 1.5 hours under N_2 atmosphere. The fracture toughness of the so-made Si_3N_4 composite at room temperature is 20.36MPa m￣(1/2), the three-point bending strength at room temperature is 651.47MPa. The crack spreads and deflects along the interface between BN and Si_3N_4 layer and extends through the BN layer into Si_3N_4 layer. The improvement of the fracture toughness may be due to the staircase-shape-like crack which provides the long crack path, the fracture and deformation of Si_3N_4 layer, and the pullout of SiC whiskers from the Si_3N_4 layer.

Crack deflection occurs by constrained microcracking in nacre

For decades, nacre has inspired researchers because of its sophisticated hierarchical structure and remarkable mechanical properties, especially its extreme fracture toughness compared with that of its predominant constituent,CaCO3, in the form of aragonite. Crack deflection has been extensively reported and regarded as the principal toughening mechanism for nacre. In this paper, our attention is focused on crack evolution in nacre under a quasi-static state. We use the notched three-point bending test of dehydrated nacre in situ in a scanning electron microscope(SEM) to monitor the evolution of damage mechanisms ahead of the crack tip. The observations show that the crack deflection actually occurs by constrained microcracking. On the basis of our findings, a crack propagation model is proposed, which will contribute to uncovering the underlying mechanisms of nacre’s fracture toughness and its damage evolution. These investigations would be of great value to the design and synthesis of novel biomimetic materials.

Microstructure observation and mechanical behavior modeling for limnetic nacre

In the present research, microstructure of akind of limnetic shell （Hyriopsis cumingii） is observed and measured by using the scanning electron microscopy, and mechanical behavior experiments of the shell nacre are carried out by using bending and tensile tests. The dependence of mechanical properties of the shell nacre on its microstructure is analyzed by using a modified shear-lag model, and the overall stress-strain relation is obtained. The experimental results reveal that the mechanical properties of shell nacre strongly depend on the water contents of the limnetic shell. Dry nacre shows a brittle behavior, whereas wetting nacre displays a strong ductility. Compared to the tensile test, the bending test overestimates the strength and underestimates the Young＇s modulus. The modified shear-lag model can characterize the deformation features of nacre effectively.

MINERAL BRIDGES OF NACRE AND ITS EFFECTS

Nacre, or mother-of-pearl, is a kind of composites of aragonite platelets sandwiched between organic materials. Its excellent mechanical properties are thought to stem from the micro architecture that is traditionally described as a 'brick and mortar' arrangement. In this paper, a new microstructure, referred to as mineral bridge in the biomineralization, is directly observed in the organic matrix layers (mortar) of nacre. This is an indication that the organic matrix layer of nacre should be treated as a three-dimensional interface and the micro architecture of nacre ought to be considered as a 'brick-bridge-mortar' structure rather than the traditional one. Experiments and analyses show that the mineral bridges not only improve the mechanical properties of the organic matrix layers but also play an important role in the pattern of the crack extension in nacre.

Nacre extract prevents scopolamine-induced memory deficits in rodents

Objective: To investigate whether the extract from the nacreous layer of pearl oysters(nacre extract) improves impairments in memory caused by scopolamine administration in rodents.Methods: Nacre extract was prepared from the inner shell layer of pearl oyster. Effects of nacre extract on scopolamine-induced memory impairment were estimated using novel object recognition test, Y-maze test, and Barnes maze test Effect of nacre extract on mRNA expressions which are genes associated with memory in the hippocampus was investigated using semi-quantitative reverse transcription polymerase chain reaction(RT-PCR) analysis.Results: Administration of nacre extract led to the protection against scopolamine-induced impairments in object recognition, short-term memory, and spatial memory. Treatment with nacre extract reversed the mRNA expression of brain-derived neurotrophic factor(BDNF) and Homer protein homolog 1(Homer-1 a) in the hippocampus, which decreased with the treatment of scopolamine. Conclusions: These results suggest that nacre extract has attenuating effects on memory impairments induced by scopolamine through the increase in mRNA expression of BDNF and Homer-1 a.

Plasma-Etching of the Organic Layer in Nacre

Nacre’s brick and mortar structure has been motivating innovations in biomimetic materials for decades. However, there is still room to improve understanding of the structure of the organic layer in order to engineer better biomimetic composites. A plasma-etching technique that allows for the selective removal of some organic components from individual layers is developed. We conclude that this technique enables a closer examination of the organic layer such that the locations and mechanical properties of individual components can be determined. A methodology for examining nacre samples that have not been demineralized provides a more accurate substrate for understanding the structure-property relationships of the organic layer in native nacre.

Anisotropic Mechanical Response of Nacre to Heat Treatment Under Indentation:Effect of Structural Orientation

It is generally considered that heat treatments have a negative impact on the mechanical properties of nacre due to thermal decomposition of the organic matrix.However,the present work investigated the microindentation behavior on fresh and heat-treated nacres from two orthogonal directions,and the results demonstrate that both hardness value and damage tolerance can remain almost unchanged on the cross-section with the organic matrix degeneration,despite a significant deterioration on the platelet surface.Theoretical analyses suggest that the anisotropic response of indentation behavior to heat treatment in nacre is primarily caused by its structural orientation.Specifically,compared with a single layer of irregular interplatelet interfaces in cross-sectional specimens,the multiple layers of parallel interlamellar interfaces in in-plane specimens exhibit a much greater ability to impede indenter-triggered destruction,and heat treatments would reduce the in-plane hardness but nearly have no effect on the cross-sectional hardness.Moreover,the deeper embedding of platelets in cross-sectional specimens enhances their resistance to interface cracking caused by organic matrix degradation at high temperatures,leading to a reduced sensitivity to damage.Therefore,the indentation behavior of nacre shows different tendencies in response to variations in the organic matrix state along normal and parallel directions.

Iron oxide/CNT-based artificial nacre for electromagnetic interference shielding

Biological structural materials,despite consisting of limited kinds of compounds,display multifunctionalities due to their complex hierarchical architectures.While some biomimetic strategies have been applied in artificial materials to enhance their mechanical stability,the simultaneous optimization of other functions along with the mechanical properties via biomimetic designs has not been thoroughly investigated.Herein,iron oxide/carbon nanotube(CNT)-based artificial nacre with both improved mechanical and electromagnetic interference(EMI)shielding performance is fabricated via the mineralization of Fe_(3)O_(4)onto a CNTincorporated matrix.The micro-and nano-structures of the artificial nacre are similar to those of natural nacre,which in turn improves its mechanical properties.The alternating electromagnetic wave-reflective CNT layers and the wave-absorptive iron oxide layers can improve the multiple reflections of the waves on the surfaces of the reflection layers,which then allows sufficient interactions between the waves and the absorption layers.Consequently,compared with the reflection-dependent EMI-shielding of the non-structured material,the artificial nacre exhibits strong absorption-dependent shielding behavior even with a very low content of wave-absorptive phase.Owing to the high mechanical stability,the shielding effectiveness of the artificial nacre that deeply cut by a blade is still maintained at approximately 70%−96%depending on the incident wave frequency.The present work provides a new way for designing structural materials with concurrently enhanced mechanical and functional properties,and a path to combine structural design and intrinsic properties of specific materials via a biomimetic strategy.

“Wood-nacre”:Development of a Bio-inspired Wood-Based Composite for Beam and 3D-Surface Elements with Improved Failure Mechanisms

Following the natural structure of the nacre,the material studied consists of a multitude of hexagonal tiles that are glued together in an offset manner with a ductile adhesive.This so-called“wood nacre”consists of macroscopic tiles of birch wood veneer with a thickness of 0.8 mm and a size of 20 or 10 mm in diameter in order to mimic the aragonite tiles and the ductile PUR-adhesive corresponds to the layers of collagen in between.E-modulus(MOE),bending strength(MOR)and impact bending strength of the samples were determined and compared with reference samples of birch laminated wood.The hierarchical layered structure of the tiles does not cause any relevant loss in stiffness.Like nacre,“wood nacre”also shows tough fracture behaviour and a high homogenization effect.However,strain hardening and high fracture toughness of the natural model could not be fully achieved.The reason for this is the insufficient ratio between the strength and stiffness of the veneer layers and the adhesive.By adjusting the size of the tiles,increasing the strength and surface roughness of the veneers,e.g.by densification,and using more ductile adhesives that can be applied in smaller layer thicknesses,it should be possible to better reproduce the natural ratios of nacre and thus achieve a significant improvement in the material properties of“wood nacre”.In addition to the mechanical properties,the high potential of the new material lies in the possibility of producing 3D shell-shaped elements for lightweight wood hybrid construction.

Thickness-dependent mechanical properties of nacre in Cristaria plicata shell:Critical role of interfaces

The thickness dependence of mechanical properties of nacre in Cristaria plicata shell was studied under three-point bending tests.The results show that the mechanical behavior of nacre exhibits a strong thickness dependence.The bending strength firstly increases with the increase of specimen thickness and then becomes roughly constant as the thickness reaches a certain value of∼2.5mm.However,the mean value of work per unit volume increases constantly with increasing specimen thickness;meanwhile,the cracking mode changes from penetration into the platelets to deflection along the interfaces.The theoretical analyses indicate that the thickness-dependent mechanical properties of nacre are mainly caused by the variation in the number of inter-lamellar interfaces.The more the number of inter-lamellar interfaces is,the higher the strength and work of fracture of nacre under bending tests will be.However,as the number of inter-lamellar interfaces reaches a certain value(e.g.,in the present specimen with 2.5mm thickness),the strength tends to remain constant,while the work of fracture still increases.Therefore,the present research findings are expected to provide a valuable guidance for the interfacial design of nacre-like materials with high strength and toughness.

A high-strength and high-toughness nacreous structure in a deep-sea Nautilus shell:Critical role of platelet geometry and organic matrix

To explore the differences in mechanical behavior of nacre between shells that live in different water depths,the microstructures,phase composition and related mechanical properties of nacre under indentation,three-point bending and shear tests in deep-sea Nautilus and freshwater Cristaria plicata shells were systematically investigated.It is found that the nacreous structure in Nautilus shell exhibits an outstanding combination of high strength and high toughness compared with that in C.plicata shell,attributing to its larger aspect ratio of platelet and interfacial shear resistance.Specifically,the interfacial resistance is mainly generated from the adhesion of organic matrix and friction caused by nano-asperities on platelet surfaces.According to the interfacial resistance model,the stiction force originated from organic matrix adhesion is sensitive to its content,and the friction force produced by nano-asperities presents a positive correlation with their distribution density and dimension.Hence,the higher content of organic matrix of nacre with denser and larger nano-asperities on platelet surfaces in Nautilus shell contributes to a higher interfacial resistance.Therefore,it is the coupled effects of platelet geometries(i.e.aspect ratio and nano-asperity)and organic matrix that result in the high-strength and high-toughness nacreous structure in Nautilus shell,which is thus more conductive to inhabit in the deep sea with extremely high pressure.The present research findings are expected to provide beneficial references for the design of strong and tough nacre-inspired materials with appropriate platelet geometry and content of soft phase.

Cloning of a microphthalmia-associated transcription factor gene and its functional analysis in nacre formation and melanin synthesis in Hyriopsis cumingii

Microphthalmia-associated transcription factor(MITF)is an essential transactivator in melanin synthesis.To characterize the role of MITF in the pearl mussel Hyriopsis cumingii,the MITF homolog of H.cumingii was isolated.The full-length HcMitf cDNA consisted of a 1332-bp with an open reading frame that encode for a 443 amino acid protein that contain a conserved basic helix-loop-helix zipper domain.The HcMitf was found to widespread tissue distribution but expression was higher in purple mussels than in white mussels,mostly in mantle,liver,kidney,gill,and foot with the exception of the adductor mussel.HcMitf and its downstream gene tyrosinase(HcTyr)were highly expressed at the nacre deposition stage after implantation of mantle tissue to produce pearls.Using RNA interference,the expression of HcMitf was reduced by 78%(P<0.01)and expression of HcTyr was also significantly suppressed and consequently total melanin content was decreased(P<0.05).The results suggest that HcMitf plays an important role in melanin synthesis,nacre formation and shell pigmentation in the H.cumingii.

Chemical-mechanical stability of the hierarchical structure of shell nacre

The hierarchical structure and mechanical property of shell nacre are experimentally investigated from the new aspects of chemical stability and chemistry-mechanics coupling. Through chemical deproteinization or demineralization methods together with characterization techniques at micro/nano scales,it is found that the nacre of abalone,haliotis discus hannai,contains a hierarchical structure stacked with irregular aragonite platelets and interplatelet organic matrix thin layers. Yet the aragonite platelet itself is a nanocomposite consisting of nanoparticles and intraplatelet organic matrix framework. The mean diameter of the nanoparticles and the distribution of framework are quite different for different platelets. Though the interplatelet and in-traplatelet organic matrix can be both decomposed by sodium hydroxide solution,the chemical stability of individual aragonite platelets is much higher than that of the microstructure stacked with them. Further,macroscopic bending test or nanoindentation experiment is performed on the micro/nanostructure of nacre after sodium hydroxide treatment. It is found that the Young's modulus of both the stacked microstructure and nanocomposite platelet reduced. The reduction of the microstructure is more remark than that of the platelet. Therefore the chemical-mechanical stability of the nanocomposite platelet itself is much higher than that of the stacked microstructure of nacre.

Extraction and Purification of Matrix Protein from the Nacre of Pearl Oyster Pinctada fucata

A soluble matrix protein P14 with an apparent molecular mass of 14.5 kDa was isolated from fragmented nacre of pearl oysters （Pinctada fucata） treated with 10% NaOH solution to investigate the nacre matrix proteins and their effect on the CaCO3 crystal. The protein was characterized by gel exclusion chromatography and reversed-phase high performance liquid chromatography after demineralization by 10% acetic acid. The X-ray diffraction pattern of P14 crystals indicates that P14 plays an important role in nacre biomineralization. P14 can induce aragonite formation, stimulate CaCO3 crystal formation, and accelerate aragonite precipitation. Heating of the acid insoluble nacre residue, which was named conchiolin, in 10% sodium dodecyl sulfate solution supplemented with 10% β-mercaptoethanol solution for 10-20 min at about 100℃ gave two other soluble proteins having molecular masses of 19.4 kDa and 25.0 kDa. The present study suggests that these two proteins are linked to the insoluble organic matrix by disulfide bridges because the extraction yield increases when β-mercaptoethanol is added to the medium.

Silk Fibroin Nacre

Producing lightweight,mechanically strong,ductile,and biocompatible materials remains a significant challenge in material engineering due to the conflict between structural and mechanical features.Inspired by the“brick-and-mortar”structure of nacre,a construction with a naturally optimized structure-performance-function relationship,this study developed silk fibroin(SF)nacre as a silk protein-based nacre by integrating ice-templating and thermoplastic molding techniques.SF nacres are similar to natural nacre in microstructure,and their strength and toughness are even superior to natural nacres.These mechani-cal properties permit machining by extreme processing techniques,such as ion beam lithography.Furthermore,SF nacre can be used to modulate the polarization of laser beams and generate bright structural colors.Biocompatibility,mechanical robustness,good processability,and tunable coloration allow SF nacres to be used as a plastic replacement for structural engineering and biomedical use,showing promising advancement of such implantable devices towards clinical translation.

基于攻击效能的网络攻击法分类与形式化描述

网络攻击方法的分类和形式化描述是对网络攻击进行深入研究的基础。分析了现有攻击分类的不足,提出了基于攻击效能的分类方法(NACR) ,将攻击方法、攻击工具和漏洞紧密联系在一起;然后,在 NACR 的基础上提出了一种新的形式化描述方法,给出了与攻击效能密切相关的参量定义,阐述了攻击方法之间的相互关系;最后,通过一个攻击实例具体阐述了网络攻击方案的形式化描述问题。

Multiscale Mechanics and Optimization of Gastropod Shells

A vast majority of mollusks grow a hard shell for protection. The structure of these shells comprises several levels of hierarchy that increase their strength and their resistance to natural threats. This article focuses on nacreous shells, which are composed of two distinct layers. The outer layer is made of calcite, which is a hard but brittle material, and the inner layer is made of nacre, a tough and ductile material. The inner and outer layers are therefore made of materials with distinct structures and properties. In this article, we demonstrate that this system is optimum to defeat attacks from predators. A two-scale mod- eling and optimization approach was used. At the macroscale, a two-layer finite element model of a seashell was developed to capture shell geometry. At the microscale, a representative volume element of the microstructure of nacre was used to model the elastic modulus of nacre as well as a multiaxial failure criterion, both expressed as function of microstructural parameters. Experiments were also performed on actual shells of red abalone to validate the results obtained from simulations and gain insight into the way the shell fails under sharp perforation. Both optimization and experimental results revealed that the shell displays optimum performance when two modes of failure coincide within the structure. Finally, guidelines for designing two-layer shells were proposed to improve the performance of engineered protective systems undergoing similar structural and loading conditions.

In vivo and in vitro biomineralization in the presence of the inner-shell film of pearl oyster

The inner shell surface is the biomineralization site in shell formation and an inner-shell film covers it. This surface is composed of two regions： an outer calcitic region and an inner aragonitic region. In this study, some amalgamated calcite crystals were found in the calcitic region and some aragonitic ＂imprints＂ were found in the central part of the aragonitic region. The ＂imprints＂ are probably the trace of mantle cells that adhered to the inner shell surface when the shell was produced. Furthermore, to build a novel in vitro biomineralization system, the inner-shell film was detached from the shell and introduced to the calcitic crystallization solution. Crystallization experiments showed that nacre proteins could induce aragonite crystals in the novel system but inhibited calcite growth in the absence of the inner-shell film. These data suggested that the inner-shell film may induce aragonite growth in vivo by combining nacre proteins.

PFMG1 promotes osteoblast differentiation and prevents osteoporotic bone loss

OBJECTIVE To investigate the effect of Pinctadafucata mantle gene 1(pfmg1)on osteoporotic bone lossand the role in osteoblast differentiation and matrix mineralization,and to explore the molecular mechanismof how PFMG1 functions through both animal and cellular experiments.METHODS For animal experiments,female BALB/c mice were subjected to sham-operation(sham)or ovariectomy(ovx)at 5weeks of age,control and pfmg1 lentiviral particles were packaged and injected through tail vein to ovx mice(2×107TU/mouse),respectively.Bone mineral density(BMD)was detected 2 months after the surgery,and the proximal tibia was scanned in three dimensions byμCT.For cellular experiments,GST-PFMG1 protein was expressed and purified,then added to MC3T3E1 cell culture medium.MTT,ALP activity and the level of matrix mineralization were detected after the treatment.RESULTSEctopic expression of pfmg1 gene enhanced the BMD level of ovx mice.μCT images revealed that PFMG1 improvedthe osteoporotic characteristics caused by ovariectomy,including the decreases in trabecular number(Tb.N),trabecular thickness(Tb.Th),and in trabecular bone volume as a percentage of total bone volume(BV/TV);and the increases in trabecular spacing(Tb.Sp)and trabecular bone pattern factor(TBPf).The alkaline phosphatase(ALP)activity and the level of matrix mineralization increased,while the MTT activity decreased after treated with PFMG1 in the osteoblast cell line MC3T3E1.CONCLUSION PFMG1 from the mental of P.fucatacould promote osteoblast differentiation and matrix mineralization in vitro,and couldprevent bone loss caused by ovariectomy in vivo.These findings showed the potential of PFMG1 from nacre as a therapeutic drug for osteoporosis.

Bio-inspired clay nanosheets/polymer matrix/mineral nanofibers ternary composite films with optimal balance of strength and toughness

Although remarkable progress has been witnessed in mimicking the nacre-like architecture in laboratory,it remains a great challenge for understanding the unique balancing mechanism of toughness and strength in biological materials. Here,taking advantage of the synergistic effect of different dimensional nanoscale building blocks,we fabricate nacre-like films that reconcile high strength and toughness.The obtained ternary lamellar composite films are constructed by one-dimensional xonotlite nanowires and two-dimensional montmorillonite nanosheets with the assistance of poly（vinyl alcohol）. The ternary composite films show high strength（（241.8±10.2）MPa） and toughness（（5.85±0.46） MJ m^-3）,both of which are higher than that of the single nanofibrillar xonotlite network films or the binary montmorillonite/poly（vinyl alcohol） composite films. The excellent mechanical properties of the nacre-like ternary composite films are aroused by the synergistic toughening mechanism of the different dimensional building blocks. This strategy provides a facile approach to integrate the nacre-like composite films with potential applications in tissue engineering scaffold,strong air barrier coatings,and fire-retardant packing materials.