2D Mechanical Metamaterials with Widely Tunable Unusual Modes of Thermal Expansion

Most natural materials expand uniformly in all directions upon heating. Artificial, engineered systems offer opportunities to tune thermal expansion properties in interesting ways. Previous reports exploit diverse design principles and fabrication techniques to achieve a negative or ultralow coefficient of thermal expansion, but very few demonstrate tunability over different behaviors. This work presents a collection of 2D material structures that exploit bimaterial serpentine lattices with micrometer feature sizes as the basis of a mechanical metamaterials system capable of supporting positive/negative, isotropic/anisotropic, and homogeneous/heterogeneous thermal expansion properties, with additional features in unusual shearing, bending, and gradient modes of thermal expansion. Control over the thermal expansion tensor achieved in this way provides a continuum‐mechanics platform for advanced strain‐field engineering, including examples of 2D metamaterials that transform into 3D surfaces upon heating. Integrated electrical and optical sources of thermal actuation provide capabilities for reversible shape reconfiguration with response times of less than 1 s, as the basis of dynamically responsive metamaterials.     

影响材料热膨胀系数的主要因素

材料膨胀系数的选用对精密机械热误差修正的结果好坏有着决定性的影响。在工程实际中,各种手册或工具书中提供的材料热膨胀系数均为在确定条件下的实验测得值,并且不同参考资料上同种材料的热膨胀系数有较大的出入。文章系统地分析了影响材料热膨胀系数的参考值的主要因素,指出热膨胀系数定义的差异,测量方法的差异。试样化学成分、加工方法和形体尺寸是影响参考值的主要因素。文章为在工程实际中选用恰当的材料热膨胀系数提供了依据。     

纤维复合材料的热膨胀系数

提出了一种利用压电光声技术测量材料热膨胀系数的实验方法,并测试了单向复合材料C/C、C/Al的横向、纵向的热膨胀系数。根据已有的理论计算方法与实验结果对该方法的测试结果进行验证,证明了该检测方法的可靠性,进而又测量了C/C、C/Al材料在任一方向上的热膨胀系数。这种方法克服了理论计算过程复杂以及常规手段无法测量任一方向上热膨胀系数的缺陷。     

Thermal expansivities in fibrous composites incorporating hybrid interphase regions

The aim of this paper is the prediction of coefficients of thermal expansion in unidirectional fiber-reinforced composites. The representative volume element is a three phase composite structure subjected to a uniform temperature change. The advanced hybrid interphase concept is introduced, in which the interphase thickness depends on the property under consideration. This model involves also imperfect adhesion by immediate softening of material properties. Equations for the prediction of coefficients of thermal expansion are presented. Results are illustrated and discussed in terms of fiber volume fraction and adhesion coefficient. To validate the accuracy of these results finite element analysis has also been utilized. Predictions of coefficients of thermal expansion are in good agreement with experimental, finite element analysis and previous published results. The coefficients of thermal expansion of the considered polymer matrix composites are affected significantly by the parameters characterizing interphase.     

Thermal expansion of glasses in the solid and liquid phases

In this study, the thermal expansion of a number of mixed oxide glasses was measured in the solid and liquid regions by gamma densitometry. Conventional density measurement techniques are limited to either the solid or the liquid phase; however, with this noncontacting technique, the density and thermal expansion of both the solid and the liquid phases may be investigated as the temperature is varied and the glass viscosity varies over a wide range. This technique allows the continuous measurement of density as the liquid cools to a supercooled glass or to a crystalline solid. Lead borate glasses were examined in the range 27 to 42 mol % PbO. The liquid-phase volumetric thermal expansion coefficient was observed to decrease with increasing PbO content, in contrast to the solid-phase linear thermal expansion coefficients. In soda-baria phosphate glasses (50 mol% P₂O₅), liquid-phase volumetric expansion coefficients were found to vary as the solid-phase linear expansion coefficients. Measurements on a lithia-aluminoscilicate glass ceramic were obtained by this method to approximately 400°C above the limit of conventional dilatometry. The gamma densitometry technique can yield density, thermal expansion, glass transition and crystallization information, and quenching-rate dependence data on glasses to temperatures in excess of 1500°C, and thus provides a means for high-temperature characterization of glasses which complements conventional dilatometry, calorimetry, and thermal analysis.Paper presented at the Ninth Symposium on Thermophysical Properties, June 24–27, 1985, Boulder, Colorado, U.S.A.     

Thermomechanical analysis of multilayered plates in terms of Reddy-type higher-order theory

A Reddy-type higher-order plate theory considering transverse normal thermal strain (HPTC) has been presented for thermomechanical analysis of multilayered plates. Displacement variables in model HPTC have not increased although transverse normal deformation is considered. Based on the proposed model, analytical solutions of simply-supported plates under thermal and mechanical combined loads are presented. Analyzing thermomechanical behaviors of composite plates, effects of number of layers, transverse normal strain and thermal expansion coefficients on displacements, and stresses of composite plates have been studied. Numerical results showed that transverse normal strain and thermal expansion coefficients have a significant impact on behaviors of composite plates.     

Thermal Expansion of Hybrid Chiral Mechanical Metamaterial with Patterned Bi-Strips

Hybrid chiral mechanical metamaterials with center squares connecting by bi-layer strips (bi-strips) with patterned interfaces are designed and fabricated via multimaterial 3D printing. Due to the thermal mismatch between the bi-strips and the chirality-induced rotation, the designs will undergo either thermal expansion or shrinkage under constant temperature increase, resulting in widely tuned overall thermal expansion coefficients (CTEs) for the chiral mechanical metamaterials. Analytical models of both the bi-strips with arbitrary dissimilar interface morphology and the chiral designs under temperature change are developed to predict the curvature of the bi-strips and the overall CTEs of the chiral designs. Two design regions with opposite trends are observed and explored. The models are verified via systematic finite element (FE) simulations and experiments on 3D-printed specimens. This investigation enlarges the design space of chiral mechanical metamaterials for achieving desired CTEs in a wide range.     

三维编织复合材料热物理性能实验

针对不同编织工艺参数的三维四向编织复合材料,进行了热环境下的热物理性能实验研究,获得了热环境下三维四向编织复合材料的热物理性能变化规律及其分布特征,分析了环境温度和编织角对材料的热膨胀系数(CTE)、热传导系数(CTC)、比热(SH)以及热扩散率(TD)的影响,得到了一些重要结论。这些结果为三维编织复合材料的热物理性能数值分析以及进一步研究材料的热力耦合行为奠定了实验基础。     

Development of a Laser Interferometric Dilatometer for Measurements of Thermal Expansion of Solids in the Temperature Range 300 to 1300 K

A laser interferometric dilatometer has been developed for measuring linear thermal expansion coefficients of reference materials for thermal expansion in the temperature range 300 to 1300 K. The dilatometer is based on an optical heterodyne interferometer capable of measuring length change with an uncertainty of 0.6 nm. Linear thermal expansion coefficients of silicon were measured in the temperature range 700 to 1100 K. The performance of the present dilatometer was tested by a comparison between the present data and the data measured with the previous version of the present dilatometer and the data recommended by the Committee on Data for Science and Technology (CODATA). The present data agree well with the recommended values over all the temperature range measured. On the other hand, the present values at lower temperatures are in poor agreement with the previous experimental data. The combined standard uncertainty in the present value at 900 K is estimated to be 1.1×10^–8 K^–1.     

预测复合材料导热系数的热阻网络法

借助计算机模拟复合材料的空间结构,直接迭代求解热阻网络,得到复合材料的导热系数。分析了在随机分布条件下取样尺度对导热系数的影响,以及二维和三维条件下导热的差别。与文献中实验数据的比较表明,所述方法能够较好地预示颗粒弥散型复合材料的导热系数。     

Determination of the Thermophysical Properties of Halogenated Hydrocarbons in a Heat-Conducting Calorimeter

The values of the coefficients of thermal expansion and compressibility, isobaric heat capacity and thermal diffusivity of three brominated saturated hydrocarbons of butyl bromide, hexyl bromide and heptyl bromide are measured in a heat-conducting calorimeter at a temperature of 298–363 K in the pressure range of 0.098–147 MPa. The experimental data on the isobaric heat capacity are compared to the calculation results. Generalized dependences are suggested to determine the heat capacity and thermal diffusivity.     

复合材料粘弹性本构关系与热应力松弛规律研究Ⅱ:数值分析

给出了预测复合材料粘弹性松弛模量、等效热应力松弛系数和等效时变热膨胀系数的均匀化方法的有限元数值实现步骤, 研究了单向纤维复合材料随温度变化的粘弹性本构关系, 以及热应力松弛规律和热膨胀系数的时变特征。单向纤维复合材料的一维热变形分析数据显示了热应变对时间的强烈依赖关系;以数值形式给出的等效热应力松弛模量对时间的依赖关系表明, 等效的热应力松弛模量对时间的依赖性较弱, 其冲击模量和渐近模量只相差0.4 %。     

Lithium aluminosilicate reinforced with carbon nanofiber and alumina for controlled-thermal-expansion materials

Abstract

Materials with a very low or tailored thermal expansion have many applications ranging from cookware to the aerospace industry. Among others, lithium aluminosilicates (LAS) are the most studied family with low and negative thermal expansion coefficients. However, LAS materials are electrical insulators and have poor mechanical properties. Nanocomposites using LAS as a matrix are promising in many applications where special properties are achieved by the addition of one or two more phases. The main scope of this work is to study the sinterability of carbon nanofiber (CNFs)/LAS and CNFs/alumina/LAS nanocomposites, and to adjust the ratio among components for obtaining a near-zero or tailored thermal expansion. Spark plasma sintering of nanocomposites, consisting of commercial CNFs and alumina powders and an ad hoc synthesized β-eucryptite phase, is proposed as a solution to improving mechanical and electrical properties compared with the LAS ceramics obtained under the same conditions. X-ray diffraction results on phase compositions and microstructure are discussed together with dilatometry data obtained in a wide temperature range (?150 to 450 °C). The use of a ceramic LAS phase makes it possible to design a nanocomposite with a very low or tailored thermal expansion coefficient and exceptional electrical and mechanical properties.     

零膨胀材料设计与模拟验证

零膨胀材料对提高航空航天结构和电子设备等的热几何稳定性有重要意义。采用拓扑优化技术设计各相材料在单胞域的分布形式, 以获得零膨胀材料的微结构形式。给出了由二相实体材料和空心构成的各向同性零膨胀材料的设计方案, 讨论了初始设计依赖性问题, 分析了该依赖性的存在原因。采用有限元技术代替实际测试, 分析了所设计材料的试件在均匀温度变化下的变形, 验证了所设计材料的零膨胀(低膨胀) 性质, 说明通过拓扑优化技术设计材料的微结构是设计零膨胀材料的有效方法。     

考虑刚度特性的零热膨胀复合材料高许用温变设计

材料的宏观零热膨胀可以通过两种不同的正热膨胀材料在单胞尺度上的复合实现。该类材料虽然能在较大温度波动环境下保持较高的几何稳定性,但两种材料之间过大的热应力很容易导致材料失效,从而限制其许用温度变化范围(简称许用温变)。为此,提出以单位温升的最大热应力作为零热膨胀材料许用温变的衡量指标,通过解析和有限元数值仿真两种方法,对三种典型弯曲变形机制的零热膨胀材料进行许用温变和刚度特性分析,揭示了单胞可设计参数对其性能的影响规律。结果表明:在满足零热膨胀条件下,通过合理的单胞结构设计和选材设计,可以实现刚度与许用温变双目标共赢。      

Lithium aluminosilicate reinforced with carbon nanofiber and alumina for controlled-thermal-expansion materials

Materials with a very low or tailored thermal expansion have many applications ranging from cookware to the aerospace industry. Among others, lithium aluminosilicates (LAS) are the most studied family with low and negative thermal expansion coefficients. However, LAS materials are electrical insulators and have poor mechanical properties. Nanocomposites using LAS as a matrix are promising in many applications where special properties are achieved by the addition of one or two more phases. The main scope of this work is to study the sinterability of carbon nanofiber (CNFs)/LAS and CNFs/alumina/LAS nanocomposites, and to adjust the ratio among components for obtaining a near-zero or tailored thermal expansion. Spark plasma sintering of nanocomposites, consisting of commercial CNFs and alumina powders and an ad hoc synthesized β-eucryptite phase, is proposed as a solution to improving mechanical and electrical properties compared with the LAS ceramics obtained under the same conditions. X-ray diffraction results on phase compositions and microstructure are discussed together with dilatometry data obtained in a wide temperature range (−150 to 450 °C). The use of a ceramic LAS phase makes it possible to design a nanocomposite with a very low or tailored thermal expansion coefficient and exceptional electrical and mechanical properties.     

A synthetic chemomechanical machine driven by ligand-receptor bonding

The ability to create synthetic chemomechanical machines with engineered functionality promises large technological rewards. However, current efforts in molecular chemistry are restrained by the formidable challenges faced in molecular structure and function prediction. An alternative approach to engineering machines with tailorable chemomechanical functionality is to design Brownian ratchet devices using molecular assemblies. We demonstrate this through the creation of autonomous molecular machines that sense, mechanically react, and extract energy from ligand-receptor binding. We present a specific instantiation, measuring approximately 100 nm in length, which actuates upon detection of a streptavidin ligand. Machines were designed through the tailoring of energy landscapes on 3D DNA origami motifs. We also analyzed the response over a logarithmic concentration ratio (device:ligand) range from 1:10(1) to 1:10(5).     

Isothermal Hybridization Kinetics of DNA Assembly of Two‐Dimensional DNA Origami

The Watson–Crick base‐pairing with specificity and predictability makes DNA molecules suitable for building versatile nanoscale structures and devices, and the DNA origami method enables researchers to incorporate more complexities into DNA‐based devices. Thermally controlled atomic force microscopy in combination with nanomechanical spectroscopy with forces controlled in the pico Newton (pN) range as a novel technique is introduced to directly investigate the kinetics of multistrand DNA hybridization events on DNA origami nanopores under defined isothermal conditions. For the synthesis of DNA nanostructures under isothermal conditions at 60 °C, a higher hybridization rate, fewer defects, and a higher stability are achieved compared to room‐temperature studies. By quantifying the assembly times for filling pores in origami structures at several constant temperatures, the fill factors show a consistent exponential increase over time. Furthermore, the local hybridization rate can be accelerated by adding a higher concentration of the staples. The new insight gained on the kinetics of staple‐scaffold hybridization on the synthesis of two dimensional DNA origami structures may open up new routes and ideas for designing DNA assembly systems with increased potential for their application.     

Semiempirical Estimation of Thermal Expansion Coefficients and Isobaric Heat Capacities of Fluorite-Type Compounds

The thermal expansion coefficients and isobaric heat capacities of fluorite-type compounds have been estimated using the Morse potential and the Debye model. The Born repulsion parameters of various compounds, which are necessary for determining the parameters of the Morse potential, have been determined empirically for elements belonging to every period of the periodic table. Using the parameters thus determined, the Debye temperature, the thermal expansion coefficient, and the Gruneisen constant of fluorite-type compounds have been calculated and then the isochoric and the isobaric heat capacities have been calculated over a wide range of temperatures. The calculated thermal expansion coefficients and isobaric heat capacities thus obtained are in good agreement with experimental values except for the anomalous temperature regions due to vacancy formation and phase transitions.     

Analysis of Interfacial Coating Effect on Thermal Stress in Composites

A preliminary model for the analysis of thermo-mechanical behaviour of interfacial coatings on thefibers in unidirectional composites have been developed on the solution of thermo-elastic mechan-ics. Thermal stress would be introduced into the composite during cooling because of the mismatchof thermo-mechanical properties among their components. The low modulus coating can effectivelyreduce the interracial stress caused by different thermal expansion coefficient between fibers and ma-trix, no matter how high or low the expansion coefficients of coatings are in CF/Al and SiC/Ticomposite systems, however, high modulus coating can decrease the interfacial compressive stress,only when the thermal expansion coefficient of coating is lower.