Homogenization of Thermal Properties in Metaplates

Three-dimensional metamaterials endowed with two-dimensional in-plane periodicity exhibit peculiar thermoelastic behaviour when heated or cooled. By proper design of the unit cell, the equivalent thermal expansion coefficient can be programmed and can also reach negative values. The heterogeneity in the third direction of such metamaterials also causes, in general, a thermal-induced deflection. The prediction of the equivalent thermal properties is important to design the metamaterial suitable for a specific application. Under the hypothesis of small thickness with respect to the global in-plane dimensions, we make use of asymptotic homogenization to describe the thermoelastic behaviour of these metamaterials as that of an equivalent homogenous plate. The method provides explicit expressions for the effective thermal properties, which allow for a cost-effective prediction of the thermoelastic response of these metaplates.

Mechanism of Negative Thermal Expansion in Monoclinic Cu2P2O7 from First Principles

Negative thermal expansion (NTE) materials generally have high-symmetry space groups, large average atomic volumes, and corner-sharing octahedral and tetrahedral coordination structures. By contrast, monoclinic α-Cu2P2O7, which has a small average atomic volume and edge-sharing structure, has been reported to exhibit NTE, the detailed mechanism of which is unclear. In this study, we investigate the A2B2O7 polymorphs and analyze the NTE behavior of α-Cu2P2O7 using first-principles lattice-dynamics calculations. From the polymorphism investigation in 20 A2B2O7 compounds using 6 representative crystal structures, small A and B cationic radii are found to stabilize the α-Cu2P2O7-type structure. We then analyze the NTE behavior of α-Cu2P2O7 using quasi-harmonic approximation. Our calculated thermal expansion coefficients and anisotropic atomic displacement parameters were in good agreement with those of the experimental reports at low temperatures. From the mode-Grüneisen parameter distribution plotted over the entire first-Brillouin zone, we found that the phonon contributing most significantly to NTE emerges not into the special points but between them. In this phonon mode, the O connecting two PO4 tetrahedra rotates, and the Cu and O vibrate perpendicular to the bottom of the CuO5 pyramidal unit, which folds the ac lattice plane. This vibration behavior can explain the experimentally reported anisotropic NTE behavior of α-Cu2P2O7. Our results demonstrate that the most negative mode-Grüneisen parameter contributing to NTE behavior is not always located on high-symmetry special points, indicating the importance of lattice vibration analyses for the entire first-Brillouin zone.

The formation energy, phase transition, and negative thermal expansion of Fe2-xScxW3O12

Tungstates with a molecular formula A2W3O12 exhibits a wider negative thermal expansion (NTE) temperature range than molybdates but are challenging to synthesize, especially when A = Fe or Cr with metastable structures. To enhance the structural stability of Fe2W3O12, Sc with lower electronegativity is adopted to substitute Fe according to Fe2-xScxW3O12, considering the thermodynamic stability of Sc2W3O12. It is shown that the solid solutions can be easily synthesized and the phase transition temperature (PTT) can be tuned to well below room temperature (RT). Theoretical calculations and experimental results show that the formation energy decreases and the W-O bond in Fe-O-W gradually strengthens as the substitution of Sc in Fe2-xScxW3O12 increases, indicating an increase in structural stability. NTE is enhanced after phase transition with an increase in the Sc content. The reduction in PTT and the enhancement in NTE properties of Fe2W3O12 could result in a decrease in the effective electronegativity of the Fe-site elements, resulting in a low formation energy and strengthened W-O bond in Fe-O-W, which corresponds to a more stable structure.

A Proposal for a Composite with Temperature-Independent Thermophysical Properties: HfV2-HfV2O7

The HfV2-HfV2O7 composite is proposed as a material with potentially temperature-independent thermophysical properties due to the combination of anomalously increasing thermoelastic constants of HfV2 with the negative thermal expansion of HfV2O7. Based on literature data, the coexistence of both a near-zero temperature coefficient of elasticity and a coefficient of thermal expansion is suggested for a composite with a phase fraction of approximately 30 vol.% HfV2 and 70 vol.% HfV2O7. To produce HfV2-HfV2O7 composites, two synthesis pathways were investigated: (1) annealing of sputtered HfV2 films in air to form HfV2O7 oxide on the surface and (2) sputtering of HfV2O7/HfV2 bilayers. The high oxygen mobility in HfV2 is suggested to inhibit the formation of crystalline HfV2-HfV2O7 composites by annealing HfV2 in air due to oxygen-incorporation-induced amorphization of HfV2. Reducing the formation temperature of crystalline HfV2O7 from 550 °C, as obtained upon annealing, to 300 °C using reactive sputtering enables the synthesis of crystalline bilayered HfV2-HfV2O7.

Hydrate formation and its effects on the thermal expansion properties of HfMgW3O12

HfMgW₃O₁₂ is a representative material with negative thermal expansion in the ABM₃O₁₂ (A = Zr, Hf; B = Mg, Mn, Zn, M = W, Mo) family. Herein we report a novel feature of hydration in HfMgW₃O₁₂ and its effect on the thermal expansion and its structures which have not been determined previously. It is found that hydrate formation in HfMgW₃O₁₂ occurs under ambient or moisture conditions and restrain the low energy librational and translational and even high energy bending and stretching motions of the polyhedra. The coefficient of thermal expansion could be tailored from negative to zero and positive depending on the hydration levels. The unhydrated HfMgW₃O₁₂ adopts an orthorhombic structure with space group Pna2₁ (33) without phase transition at least from 80 K to 573 K, but pressure-induced structure transition and amorphization are found to occur at about 0.19 Gpa and above 3.93 GPa, respectively.

Negative Thermal Expansion of Ni-Doped MnCoGe at Room-Temperature Magnetic Tuning

Compounds that exhibit the unique behavior of negative thermal expansion (NTE)-the physical property of contraction of the lattice parameters on warming-can be applied widely in modern technologies. Consequently, the search for and design of an NTE material with operational and controllable qualities at room temperature are important topics in both physics and materials science. In this work, we demonstrate a new route to achieve magnetic manipulation of a giant NTE in (Mn_0.95Ni_0.05)CoGe via strong magnetostructural (MS) coupling around room temperature (∼275 to ∼345 K). The MS coupling is realized through the weak bonding between the nonmagnetic CoGe-network and the magnetic Mn-sublattice. Application of a magnetic field changes the NTE in (Mn_0.95Ni_0.05)CoGe significantly: in particular, a change of Δ L/ L along the a axis of absolute value 15290(60) × 10^-6-equivalent to a -31% reduction in NTE-is obtained at 295 K in response to a magnetic field of 8 T.

Anisotropic Near-Zero Thermal Expansion in REAg xGa4- x ( RE = La-Nd, Sm, Eu, and Yb) Induced by Structural Reorganization

In this work, we have discovered the anisotropic near-zero thermal expansion (NZTE) behavior in a family of compounds REAg _xGa_{4- x} ( RE = La-Nd, Sm, Eu, and Yb). The compounds adopt the CeAl₂Ga₂ structure type and were obtained as single crystals in high yield by metal flux growth technique using gallium as active flux. Temperature-dependent single crystal X-ray diffraction suggests that all the compounds exhibit near zero thermal expansion along c direction in the temperature range of 100-450 K. Temperature-dependent X-ray absorption near-edge spectroscopic study confirmed ZTE behavior is due to the geometrical features associated within the crystal structure. The anisotropic NZTE behavior was further established by anisotropic magnetic measurements on selected single crystals. The atomic displacement parameters, apparent bond lengths, bond angles, and structural distortion with respect to the temperature reveal that geometric features associated with the structural distortion cause the anisotropic NZTE along c-direction. The preliminary magnetic studies suggest all the compounds are paramagnetic at room temperature except LaAgGa₃. Electrical resistivity study reveals that compounds from this series are metallic in nature.

Fabrication of Zr2WP2O12/ZrV0.6P1.4O7 composite with a nearly zero-thermal-expansion property

Sintered bodies of Zr₂WP₂O₁₂ (ZWP) and ZrV_0.6P_1.4O₇ (ZVP) were fabricated, and their linear thermal expansion coefficients (TEC) were found to be -2.92 × 10^-6 and 3.27 × 10^-6 °C^-1, respectively, in the range 25-500 °C. In an attempt to fabricate composites with a zero-thermal-expansion property, sintered ZWP/ZVP composites with ZVP/ZWP volume ratios of 0.5/0.5, 0.53/0.47, 0.55/0.45, and 0.6/0.4 were fabricated. Scanning electron microscopy revealed that sintering of ZVP/ZWP composites progressed well compared with that of ZWP. A porous ZVP/ZWP composite with a relative density of ca. 83% was fabricated at a ZVP/ZWP volume ratio of 0.53/0.47. X-ray diffractometry and energy dispersive X-ray spectrometry clarified that the ZVP/ZWP composite mainly consisted of ZWP and ZVP grains. Thermomechanical analysis confirmed that the ZVP/ZWP composite exhibited very low thermal expansion with a slight hysteresis with a TEC of -0.29 × 10^-7 °C^-1 in the range 25-500 °C.

A novel negative thermal expansion material of Zr0.70V1.33Mo0.67O6.73

A novel negative thermal expansion (NTE) material of Zr_0.70V_1.33Mo_0.67O_6.73 was synthesized.

Superstructure ZrV2O7 nanofibres: thermal expansion, electronic and lithium storage properties

ZrV₂O₇ has attracted much attention as a negative thermal expansion (NTE) material due to its isotropic negative structure. However, rarely has investigation of the lithium storage behaviors been carried out except our first report on it. Meanwhile, the electrochemical behaviors and energy storage characteristics have not been studied in depth and will be explored in this article. Herein, we report on the synthesis, characterization and lithium intercalation mechanism of superstructure ZrV₂O₇ nanofibres that were prepared through a facile solution-based method with a subsequent annealing process. The thermal in situ XRD technique combined with the Rietveld refinement method is adopted to analyze the change in the temperature-dependent crystal structure. Benefiting from the nanostructured morphology and relatively high electronic conductivity, it presents acceptable cyclic stability and rate capability. According to the operando evolution of the XRD patterns obtained from electrochemical in situ measurements, the Li intercalation mechanism of the solid solution process with a subsequent conversion reaction can be concluded. Finally, the amorphous state of the electrodes after the initial fully discharged state can effectively enhance the electrochemical performances.

Negative thermal expansion and associated anomalous physical properties: review of the lattice dynamics theoretical foundation

Negative thermal expansion (NTE) is the phenomenon in which materials shrink rather than expand on heating. Although NTE had been previously observed in a few simple materials at low temperature, it was the realisation in 1996 that some materials have NTE over very wide ranges of temperature that kick-started current interest in this phenomenon. Now, nearly two decades later, a number of families of ceramic NTE materials have been identified. Increasingly quantitative studies focus on the mechanism of NTE, through techniques such as high-pressure diffraction, local structure probes, inelastic neutron scattering and atomistic simulation. In this paper we review our understanding of vibrational mechanisms of NTE for a range of materials. We identify a number of different cases, some of which involve a small number of phonons that can be described as involving rotations of rigid polyhedral groups of atoms, others where there are large bands of phonons involved, and some where the transverse acoustic modes provide the main contribution to NTE. In a few cases the elasticity of NTE materials has been studied under pressure, identifying an elastic softening under pressure. We propose that this property, called pressure-induced softening, is closely linked to NTE, which we can demonstrate using a simple model to describe NTE materials. There has also been recent interest in the role of intrinsic anharmonic interactions on NTE, particularly guided by calculations of the potential energy wells for relevant phonons. We review these effects, and show how anhamonicity affects the response of the properties of NTE materials to pressure.

A linear bridging angle in the pyrovanadate group within the crystal structure of Hg₂V₂Te₂O₁₁

The title compound, dimercury(II) divanadium(V) ditellurium(IV) undecaoxide, Hg2V2Te2O11, is a new representative within the family of divalent oxovanadato(V)tellurates(IV). The anionic framework is made up of disphenoidal [TeO4] polyhedra that are linked by corner-sharing to two neighbouring pyrovanadate units, resulting in chains of six-membered rings propagating parallel to [1-10]. The bridging O atom of the pyrovanadate unit is located on an inversion centre, leading to a staggered conformation and a linear V-O-V angle between the two [VO4] tetrahedra. The anionic chains are connected by interjacent six-coordinate Hg(2+) cations into a three-dimensional framework. The 5s(2) lone electron pair of the Te(IV) atom is stereochemically active and protrudes into the free space of the chain links.

In situ investigation of the surface morphology evolution of the bulk ceramic Y2Mo3O12 during crystal water release

The surface morphology evolution of the bulk ceramic Y2Mo3O12 during the release of crystal water is followed in situ for the first time using atomic force microscopy. It is found that both the shape and size of individual grains and the integration morphology of the sample exhibit dynamic changes with increasing temperature. We believe that the surface morphology evolution of the sample with increasing temperature is closely correlated with the forces induced by the contraction and expansion of the lattice during crystal water release in two different stages.

Two Decades of Negative Thermal Expansion Research: Where Do We Stand?

Negative thermal expansion (NTE) materials have become a rapidly growing area of research over the past two decades. The initial discovery of materials displaying NTE over a large temperature range, combined with elucidation of the mechanism behind this unusual property, was followed by predictions that these materials will find use in various applications through controlled thermal expansion composites. While some patents have been filed and devices built, a number of obstacles have prevented the widespread implementation of NTE materials to date. This paper reviews NTE materials that contract due to transverse atomic vibrations, their potential for use in controlled thermal expansion composites, and known problems that could interfere with such applications.

Quenchable high-density amorphous polymorphs of zirconium tungstate

Zirconium tungstate turns amorphous above 2 GPa. Amorphous-Zr(WO(4))(2) is studied in situ using synchrotron x-ray diffraction and Raman spectroscopy at high pressure. The height and the position of the first peak of the structure factor as a function of pressure exhibit discontinuous changes suggesting an amorphous to amorphous transformation around 19 GPa. The pressure dependence of the Raman mode frequencies of the tungstate tetrahedra also exhibits a change at the same pressure. The high-density amorphous form appears to have higher oxygen coordination as compared to the low-density amorphous form.

The room-temperature superstructure of ZrP2O7 is orthorhombic: there are no unusual 180 degrees P-O-P bond angles

The structure of room-temperature ZrP2O7 is shown to be orthorhombic by a combination of high-resolution synchrotron powder diffraction and single-crystal synchrotron diffraction data. Small nontwinned single crystals were obtained by synthesizing the compound using solvothermal methods at temperatures below the cubic to orthorhombic phase transition. The average P-O-P angle is 146 degrees. DFT calculations (B3LYP/AUG-cc-pVDZ) on the isolated P2O7(4-) anion yield a P-O-P angle of 153.42 degrees and indicate that the barrier to inversion is of the order 3.6 kJ mol(-1).