Experimental Study on Thermal Stress of Concrete with Different Expansive Minerals Using a Temperature Stress Testing Machine

In order to compare the compensation effect of expansive materials with different mineral sources on the temperature stress of concrete,we investigated the temperature stress of concrete when adding calcium sulfoaluminate type expansive materials(CSA)or CaO and calcium sulfoaluminate mixed type expansive materials(HCSA)at different temperatures by temperature-stress testing machine(TSTM)considering the influence of temperature history on the expansion.The experimental results show that the expansion characteristics of the two kinds of expansive materials with different mineral sources significantly vary.When adding expansive materials,the growth rate of compressive stress during the heating stage increases obviously,the maximum compressive stress is higher,while the decline rate of tensile stress in the late cooling stage becomes slow,and finally cracking temperature decreases.It is proved that concrete with HCSA has lower cracking temperatures and better temperature shrinkage compensation effect.Therefore,it is rational to choose HCSA when preparing concrete with high expansion energy to reduce thermal cracking.

Effect of Curing Temperature on the Morphology and Hydration of MgO Expansive Material in Paste

The morphologies and hydration properties of MgO expansive material at different curing temperatures were investigated.When the curing temperature varies from 25℃to 50℃,the conductivity of MgO rises from 40 to 80μs/cm,the hydration product of MgO expansive material curing in water for 28 days is from 1.5—2 mm to 2—2.5μm in length through SEM observation,and the expansion of pastes with 5%of the magnesium oxide is from 0.36%up to 1.01%,it is increased by 2.78 times.When the content of cement paste increases to 95%and cured in water for 90 days,the brucite forms as hexaflaggy?shaped crystal,which is less than 0.1μm and tends to aggregates.The effect of curing temperature on the hydration of MgO expansive material is obvious.

The phase transition, hygroscopicity, and thermal expansion properties of Yb_(2-x)Al_xMo_3O_(12)

Materials with the formula Yb2-xAlxMo3O12 （x = 0.1, 0.2, 0.3, 0.4, 0.5, 0.7, 0.9, 1.0, 1.1, 1.3, 1.5, and 1.8） were synthesized and their structures, phase transitions, and hygroscopicity investigated using X-ray powder diffraction, Raman spectroscopy, and thermal analysis. It is shown that Yb2-xAlxMo3012 solid solutions crystallize in a single monoclinic phase for 1.7 〈 x 〈 2.0 and in a single orthorhombic phase for 0.0 〈 x 〈 0,4, and exhibit the characteristics of both monoclinic and orthorhombic structures outside these compositional ranges. The monoclinic to orthorhonlbic phase transition temperature of A12Mo3012 can be reduced by partial substitution of A13＋ by Yb3＋, and the Yb2-zAlxMo3012 （0.0 〈 x 〈 2.0） materials are hydrated at room temperature and contain two kinds of water species. One of these interacts strongly with and hinders the motions of the polyhedra, while the other does not. The partial substitution of A13＋ for Yb3＋ in Yb2Mo3012 decreases its hygroscopicity, and the linear thermal expansion coefficients after complete removal of water species are measured to be -9.1 x 10-6/K, -5.5 x 10-6/K, 5.74 x 10-6/K, and 9.5 x 10 6/K for Ybl.sAlo.2（MoO4）3, Yb1.6Alo.4（MoO4）3, Ybo.4All.6（Mo04）3, and Ybo.2Al1.8（MoO4）3, respectively.

Near-zero thermal expansion inβ-CuZnV_(2)O_(7)in a large temperature range

We report a new type of near-zero thermal expansion materialβ-CuZnV_(2)O_(7)in a large temperature range from 173 K to 673 K.It belongs to a monoclinic structure(C2/c space group)in the whole temperature range.No structural phase transition is observed at atmospheric pressure based on the x-ray diffraction and Raman experiment.The high-pressure Raman experiment demonstrates that two structural phase transitions exist at 0.94 GPa and 6.53 GPa,respectively.The mechanism of negative thermal expansion inβ-CuZnV_(2)O_(7)is interpreted by the variations of the angles between atoms intuitively and the phonon anharmonicity intrinsically resorting to the negative Grüneisen parameter.

Phase transition and thermal expansion property of Cr_(2-x)Zr_(0.5x)Mg_(0.5x)Mo_3O_(12) solid solution

Compounds with the formula Cr2-xZr0.5xMg0.5xMo3O12（x = 0.0, 0.3, 0.5, 0.9, 1.3, 1.5, 1.7, 1.9） are synthesized, and the effects of Zr4＋ and Mg2＋ co-incorporation on the phase transition, thermal expansion, and Raman mode are investigated. It is found that Cr2-xZr0.5xMg0.5xMo3O12 crystallize into monoclinic structures for x 〈 1.3 and orthorhombic structures for x _〉 1.5 at room temperature. The phase transition temperature from a monoclinic to an orthorhombic structure of Cr2Mo3O12 can be reduced by the partial substitution of （ZrMg）6＋ for Cr3＋. The overall linear thermal expansion coefficient decreases with the increase of the （ZrMg）6＋ content in an orthorhombic structure sample. The co-incorporation of Zr4＋ and Mg2＋ in the lattice results in the occurrence of new Raman modes and the hardening of the symmetric vibrational modes, which are attributed to the MoO4 tetrahedra sharing comers with ZrO6/MgO6 octahedra and to the strengthening of Mo-O bonds due to less electronegativities of Zr4＋ and Mg2＋ than Cr3＋, respectively.

Negative thermal expansion: Mechanisms and materials

Negative thermal expansion(NTE)of materials is an intriguing phenomenon challenging the concept of traditional lattice dynamics and of importance for a variety of applications.Progresses in this field develop markedly and update continuously our knowledge on the NTE behavior of materials.In this article,we review the most recent understandings on the underlying mechanisms(anharmonic phonon vibration,magnetovolume effect,ferroelectrorestriction and charge transfer)of thermal shrinkage and the development of NTE materials under each mechanism from both the theoretical and experimental aspects.Besides the low frequency optical phonons which are usually accepted as the origins of NTE in framework structures,NTE driven by acoustic phonons and the interplay between anisotropic elasticity and phonons are stressed.Based on the data documented,some problems affecting applications of NTE materials are discussed and strategies for discovering and design novel framework structured NET materials are also presented.

The Coiling of Split Dandelion Scape Induced by Cell Hygroscopicity

It is of great significance to study the water absorption and expansion of cells,which are essential for the growth and life regulation of plants,not only for understanding the complex life phenomena of plants,but also for the manufacture of biomimetic materials.The main purpose of this study is to shed light on the mechanism of a novel coiling phenomenon of the split dandelion scape soaked in water and the high concentration mannitol solution.We discovered that the difference between water absorption/dehydration capacities of inner cells and lateral cells of the split dandelion scape generates asymmetric expansion/contraction between inner and lateral layers,resulting two opposite coiled patterns.Analogous to the deformable hypothesis of the thermal expansion model,the water absorption-induced cell expansion model was proposed to explain the coiling process of the split dandelion scape.Both the simulation and experimental results revealed that the coiling of dandelion scape is strongly depended on the response of cells to the stimulation of the solution environment.Showing considerable potential load bearing capacity of plants,the coiling phenomenon can be an inspiration to the designs of soft robots and bionic sensors for humidity control.

o-ZrW_(1.6)Mo_(0.4)O_8:A Novel Orthorhombic Intermediate Phase Formed During the Synthesis of the Negative Thermal Expansion Cubic ZrW_(1.6)Mo_(0.4)O_8 Material by the Precursor Dehydration Route

The mechanism of the precursor dehydration route was revealed for the synthesis of NTE c-ZrW_ 1.6 Mo_ 0.4 O_8. The hydrate precursor was dehydrated at 473 K and transformed to a NTE cubic compound above 800 K. A novel intermediate phase o-ZrW_ 1.6 Mo_ 0.4 O_8 occurs between the temperature range of 573—800 K. The XRD pattern of novel intermediate was refined with the structural model of LT-ZrMo_2O_8 by using Rietveld method. The residuals are R_ wp =7.80% and R_p=5.79%. The space group is Pmn2_1 and the lattice parameters are a=0.5917(4) nm, b=0.7273(4) nm, c=0.9148(6) nm, and Z=2.

Structure and Properties of Nanostructured Vacuum-Deposited Foils of Invar Fe–(35–38 wt%)Ni Alloys

The process of electron beam vacuum deposition of the Fe-（35-38 wt%）Ni alloys at substrate temperatures Ts from 300 to700 ℃ were used to produce vacuum-deposited foils with the FCC structure, differing by the size of characteristic microstructural elements （grains and subgrains）. It was shown that refinement of foil microstructural elements to nanoscale is accompanied by their microhardness increase up to 4-5 GPa. The change of the thermal expansion coefficient （TEC） of the nanostructured （NS） foil of the Fe-35.1Ni alloy within the temperature range from -50 to 150 ℃ has some deviation from that observed for cast Invar alloy of the same composition. It has been found that the main factors affecting the peculiarities of thermal expansion of the NS foil can be related to the presence of small fraction of BCC- phase in them, high level of crystalline lattice microstrains and inhomogeneous magnetic order in FCC- phase. It was shown that as a result of additional thermal treatment of NS foils their invar properties become similar to that observed for cast Invar alloy but mechanical properties remain on the same level.