| Cementitious composite is the most widely used building material,which is easy to form micro defects,leading to concrete cracking and safety accidents of building structures.Due to the rapid growth of radio communication and electronic equipment applications,electromagnetic(EM)wave absorption has become increasingly important in numerous infrastructures,including reducing radar cross-section(RCS),achieving EM interference(EMI)shielding and protecting human body from health risks caused by radiation.However,there are some challenges for the traditional methods of adding absorbing agencies,such as limited absorbing performance,sacrificed mechanical properties of composites,and complex mixing proportion and preparation process.Carbon nanotubes(CNTs)have excellent mechanical and electrical properties,which are ideal reinforcement materials and good absorbing agencies in composites.In addition,metamaterials(MMs)have inspiring EM properties that cannot be realized by natural materials,and metamaterials perfecet absorbers(MPAs)have been reported to absorb nearly 100% EM wave.In order to overcome the limitations of traditional methods,this study first conducts a CNT ultra high toughness cementitious composite(UHTCC),then exploits MM theory and method to develop a cement-based EM wave absorbing metastructure.The metastructure exhibits stable mechanical properties and broadband perfect absorption performance.This study has carried out relevant research under the support of National Natural Science Foundation of China(No.51878601 & No.52225803).The main researches and conclusions are listed as follows:1.Firstly,the cement composites with high dispersion and high content of CNTs were developed.In order to solve the problems of(i)limited CNT content in cement composites due to dispersion difficulties and(ii)reaggregation of CNTs in cement composites,the preparation method of aqueous solution with high CNT content was studied,and a redispersion method of CNTs in cement composites was proposed.Through the experiments of preparing CNT cement composite,the feeding sequence,mixing speed and feeding method were explored and optimized.It was found that the method of adding polyvinylpyrrolidone(PVP)to treat CNT aqueous solution,and using high-pressure spray in the feeding process is an effective redispersion method,which increases the flexural strength of cement composites by 46.6%.2.The toughening mechanism of CNTs in cement composites was investigated.The relationship between CNT content,CNT failure mode and mechanical properties of the cement matrix was studied through theoretical analysis and experimental observation.It is generally believed that the mechanical properties of cement-based materials begin to decline when the content of CNTs reaches the threshold.In this study,the CNT content was continued to increase beyond the threshold,and it was found that the flexural strength of the cement matrix first decreased then increased again,even exceeding the strength at the content threshold.It was found that there exists a “critical” CNT content after the content threshold,at which the CNT failure mode transits from breaking to pulling-out,the flexural strength of the matrix decreases slightly,and the load-displacement curve presents a double hump shape.When the CNT content exceeds the critical content,pulling-out failure begins to dominate.Therefore,the flexural strength exhibits a second increase with the increase of the CNT content.Furthermore,polyvinyl alcohol(PVA)fibers were added in the cement composite to fabricate the ultra high toughness cementitious composite.The effects of the aspect ratio and content of CNTs on the mechanical properties of UHTCC were investigated.It is found that CNT with large aspect ratio can better improve the mechanical properties.The method of using scanning electron microscope(SEM)electron beam bombards fiber surface locally was proposed,and the mechanism of multi-scale synergistic reinforcement and toughening of CNT-PVA hybrid fiber was explored.3.A systematic study was carried out on the EM wave absorbing properties of CNT fiber cementitious composite,including the effects of matrix type,wave attack front,CNT content,PVA fiber,specimen thickness.Compared with ordinary cement mortar,the UHTCC matrix with large amount of fly ash has more stable wave absorption performance.The back side of the cementitious composite to attck wave shows better absorbing performance.With the content of CNT increases,the wave absorption performance of the matrix decreases then increases,especially in high frequency ranges.PVA fiber can improve the wave absorption performance of the matrix by improving the impedance matching characteristics,extending the EM wave propagation path,and generating more intense interference peaks.The mechanism of CNT-PVA fiber-matrix synergistic EM wave absorption was analyzed by mercury intrusion porosimetry(MIP),S parameter,and SEM tests.The influence of the specimen thickness on the wave absorption performance was also investigated,and the optimal size of the carbon nanotube ductile cement-based microwave absorbing plate was proposed.Finally,a kind of CNT fiber cementitious composite with good mechanical properties and EM wave absorbing properties was proposed.The composite shows peak reflection loss of-41.8 d B,and the reflection loss is stable below-10 d B at 11.5-18 GHz.4.Based on the theory and technology of the metamaterial,an absorbing metastructure utilizing CNT periodic surface was proposed.The structure is composed of a ceramic fiber board(CFB)coated with periodical CNT,a cementitious dielectric layer(i.e.,UHTCC),a CFB dielectric layer and a reflective plate.Compared with cementitious absorber,the absorption performance of the metastructure is significantly improved.Through systematic experiments and numerical simulation,the absorbing mechanism of each component of the metastructure was analyzed.Based on the proposed CNT periodic surface,the effects of one dimentianal(1D),two one dimentianal(2D)and three one dimentianal(3D)periodic parameters on the wave absorbing performances were investigated.An innovative design of surface was proposed based on 2D inductive and capacitive surfaces.The metastructure not only realizes the low reflection loss of the capacitive surface,but also has the multi peak absorption characteristics of the inductive surface.The average reflection loss reaches-16.5 d B,and the relative bandwidth for-20 d B reaches 32.7%.In addition,an all dielectric 3D cement-based metastructure was proposed,significantly improving the absorption performance compared to cement plate.Furthermore,CNT periodic grids were coated on the 3D surface at the concave and the convex,respectively,and better wave absorption performance was achieved.5.The CNT fiber cementitious composite was utilized as the dielectric layer of the absorbing metastructure.A composite metastructure that CNTs play a dual role(the surface resonance layer and the absorbing agency of the dielectric layer)was proposed,and the ultra wideband perfect wave absorbing performance was achieved.The peak reflection loss yeilds-52.7 d B,the-20 d B bandwidth yeilds 11 GHz,and the relative bandwidth for-20 d B is more than 90%.By replacing the CNT resonance layer,the absorption peak positions exhibit regular blue shift,maintaining a stable absorption peak intensity.It is proved that the composite metastructure proposed in this study has the characteristics of stable function and flexible working frequency band.Researchers or users can adjust and expand the working frequency band of the composite metastructure by changing the surface CNT pattern,so as to create a perfect absorbing metastructure suitable for EM waves in different frequencies. |
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