メタマテリアル

Structural metamaterials are a type of mechanical metamaterial that provide properties such as crushability and lightweight characteristics. Using projection micro-stereolithography, microlattices can be created using forms much like trusses and girders. Materials four orders of magnitude stiffer than conventional aerogel, but with the same density have been created. Such materials can withstand a load of at least 160,000 times their own weight by over-constraining the materials.^[62][63]

A ceramic nanotruss metamaterial can be flattened and revert to its original state.^[64]

While metamaterials derive their extraordinary properties from engineered micro- or nano-scale architectures that manipulate wave behaviour, metastructures operate at the macro-scale, leveraging geometric design and modular assembly to achieve multifunctional mechanical performance across larger systems. Fully bio-based composite and modular metastructure cells based on trussed geometry encompassing bamboo rods and plant-based polymer joints demonstrate scalable mechanical performance, supporting up to 700 kg in compression with a mass of only 30 g.^[65]

Typically, materials found in nature, when homogeneous, are thermally isotropic, meaning heat diffuses at roughly the same rate in all directions. Thermal metamaterials, as a subclass of mechanical metamaterials, achieve anisotropic and tailored thermal responses through architected internal structures. The term arose around 2008, when Fan, Gao, and Huang^[66] demonstrated shaped graded materials with apparent negative thermal conductivity, and introduced the concept of a thermal cloak through transformation thermotics. By carefully designing their geometry at nano, micro, meso, or macro scales, these materials exhibit effective thermal conductivities not accessible in natural materials. Their classification as mechanical metamaterials stems from the fact that their unusual thermal behavior arises from engineered structure rather than chemical composition. Examples include composites with highly aligned fibers, particle arrays, or carbon nanotubes, where directional organization enables controlled heat flow.

Metamaterials may be fabricated that include some form of nonlinear media, whose properties change with the power of the incident wave. Nonlinear media are essential for nonlinear optics. Most optical materials have a relatively weak response, meaning that their properties change by only a small amount for large changes in the intensity of the electromagnetic field. The local electromagnetic fields of the inclusions in nonlinear metamaterials can be much larger than the average value of the field. Besides, remarkable nonlinear effects have been predicted and observed if the metamaterial effective dielectric permittivity is very small (epsilon-near-zero media).^{[67][68][69][70][71][72][73]} In addition, exotic properties such as a negative refractive index, create opportunities to tailor the phase matching conditions that must be satisfied in any nonlinear optical structure and can strongly modify the known nonlinear effects and enable new ones.^[74][75][76]

メタ流体は、粘度、圧縮性、光学特性などのプログラム可能な特性を提供します。あるアプローチでは、直径 50 ～ 500 ミクロンの空気充填エラストマー球をシリコンオイルに懸濁させました。球は圧力下で圧縮され、圧力が解放されると元の形状に戻ります。これらの 2 つの状態では特性が異なります。無加圧状態では、光を散乱して不透明になります。加圧下では、半月形に崩壊して光を集束し、透明になります。圧力応答により、センサーまたは動的な油圧流体として機能することができます。コーンスターチのように、ニュートン流体または非ニュートン流体として機能します。圧力下では非ニュートン流体になります。つまり、せん断力に応じて粘度が変化することです。^{[ 77 ]}

2009年、Marc BrianeとGraeme Milton ^{[ 78 ]}は、正または負のホール係数材料のみで構成された3Dの3材料ベースの複合材料の符号を原理的に反転できることを数学的に証明しました。その後、2015年にMuamer Kadicら^{[ 79 ]は、等方性材料に単純な穴を開けることでホール係数の符号が変化することを示しました。この理論的主張は最終的にChristian Kernら[ 80 ]}によって実験的に実証されました。

2015年には、Christian Kernらによって、単一材料の異方性穿孔がさらに珍しい効果、すなわち平行ホール効果をもたらすことが実証されました。^{[ 81 ]}これは、導電性媒体内部の誘導電場が電流と磁場に対して直交するのではなく、実際には磁場と平行になることを意味します。