| Metamaterials have extraordinary material properties and open up new routes for the study of regulating physical fields,which is a multifaceted interdisciplinary discipline involving many fields such as materials science,mechanics,thermal,and electromagnetism.Physical field regulating devices such as invisible cloaks are important tools for regulating physical fields,and the devices based on neutral inclusion have no restrictions on material properties and are easy to fabricate.In this paper,based on the idea of neutral inclusion in the micromechanics of composite materials,we investigate architected metamaterial construction methods and various physical field control methods in response to the demanding shape requirements of neutral inclusion and the demand for controlling various physical fields in the development of science and technology.The physical fields involved include diffusion fields(thermal and direct current electric fields)and quasi-static wave fields(electromagnetic,acoustic,and elastic waves).The main research works are as follows:Firstly,a method to study architected metamaterials is proposed based on neutral inclusion.The architected metamaterials can be constructed by approximating the circular neutral inclusions to square cells and arranging the cells periodically under the premise that the volume fraction of each component material remains unchanged.The method is not only applicable to both diffusion and quasi-static wave fields but can also achieve more accurate effective material parameters on a macroscopic scale through the construction of architected metamaterials.At the same time,continuous macroscopic effective material parameters can be obtained by this method.Based on this method,an architected thermal metamaterial for regulating transient heat flow is constructed by using the differences in effective thermal conductivity,density,and specific heat capacity calculation methods.The material can regulate transient heat flow velocity while ensuring constant steady-state heat flow transport capacity;Secondly,we design a double-layer invisible cloak based on neutral inclusion to obtain an accurate solution for the invisible condition of thermal-electric dual-field cloak,extend the range of applicable materials of thermal-electric dual-field invisible cloak,and further extend the range based on architected metamaterials.The invisible cloak has an accurate invisible effect under thermal and direct current electric fields;Based on the architected metamaterials,we propose a method to construct invisible cloak that is applicable to both diffusion and quasistatic wave fields,combining the idea of "hiding drops in water".After determining the material parameters of the core and the cloak and the volume fraction of the core(analogous to "water droplet"),the same material as the core and the cloak is selected,and the same volume fraction is guaranteed to construct the background(analogous to "water").Without considering the multi-field coupling,the invisible cloak designed based on this method can meet the invisible requirements of both diffusion fields and quasi-static wave fields,expanding the number of physical fields applicable to the invisible cloak to more than three and providing a new idea for the research of integrated multifunctional materials.Meanwhile,the invisible effect of the invisible cloak depends only on the volume fraction of each component material,independent of the material parameters;Then,based on the idea of neutral inclusion and architected metamaterial,combined with the idea of cyclotomic method,the circular neutral inclusion is divided into polygons,and when the number of sides of the polygon tends to infinity,the polygon is approximated as a circle so as to construct the approximate polygonal invisible cloak.The invisible effect of a polygonal invisible cloak increases with the increase of the number of sides,and it has a high invisible efficiency under the physical fields of different transmission directions;meanwhile,combining with the mathematical differentiation,an arbitrarily shaped bare core with a continuous smooth boundary is selected as the research object,and its boundary curve is divided into infinitely small micro-arcs,and the circular neutral inclusion theory is approximated to each micro-arc to construct an arbitrarily shaped invisible cloak with a continuous smooth boundary.The experimental and finite element simulation results show that the arbitrarily shaped invisible cloak can effectively eliminate or weaken the perturbation of the bare core to the external physical fields;Finally,based on the invisible cloak,two methods are proposed to combine the physical and geometric symmetries to achieve the flow stabilization function and the field source conversion function,respectively.Since the circular invisible cloak has high geometric and physical symmetry and can eliminate the perturbation of the bare core to the external physical fields,a generalized stabilizer can be constructed by splitting the cloak in two parts and placing it at a narrow cross section of the background material.The stabilizer can maintain uniform and stable physical field transport in the presence of changing cross sections of the transport medium,and the flow stabilization function is effective for both diffusive and quasi-static wave fields.In addition,the stabilizer has a switching function in the wave fields,which can form wave paths and breaks.On the basis of the stabilizer,a field source conversion device can be constructed by further combining physical and geometric symmetry.The field source conversion device can convert the signals of diffuse and quasi-static wave fields of smaller size to the signals of physical fields of larger size with uniform transmission,and its conversion function is effective for both diffuse and quasi-static wave fields. |
