Experimental study on thermal and mechanical properties of tailings-based cemented paste backfill with CaCl_(2)·6H_(2)O/expanded vermiculite shape stabilized phase change materials

CaCl_(2)·6H_(2)O/expanded vermiculite shape stabilized phase change materials(CEV)was prepared by atmospheric impregnation method.Using gold mine tailings as aggregate of cemented paste backfill(CPB)material,the CPB with CEV added was prepared,and the specific heat capacity,thermal conductivity,and uniaxial compressive strength(UCS)of CPB with different cement-tailing ratios and CEV addition ratios were tested,the influence of the above variables on the thermal and mechanical properties of CPB was analyzed.The results show that the maximum encapsulation capacity of expanded vermiculite for CaCl_(2)·6H_(2)O is about 60%,and the melting and solidification enthalpies of CEV can reach 98.87 J/g and 97.56 J/g,respectively.For the CPB without CEV,the specific heat capacity,thermal conductivity,and UCS decrease with the decrease of cement-tailing ratio.For the CPB with CEV added,with the increase of CEV addition ratio,the specific heat capacity increases significantly,and the sensible heat storage capacity and latent heat storage capacity can be increased by at least 10.74%and 218.97%respectively after adding 12%CEV.However,the addition of CEV leads to the increase of pores,and the thermal conductivity and UCS both decrease with the increase of CEV addition.When cement-tailing ratio is 1:8 and 6%,9%,and 12%of CEV are added,the 28-days UCS of CPB is less than 1 MPa.Considering the heat storage capacity and cost price of backfill,the recommended proportion scheme of CPB material presents cement-tailing ratio of 1:6 and 12%CEV,and the most recommended heat storage/release temperature cycle range of CPB with added CEV is from 20 to 40℃.This work can provide theoretical basis for the utilization of heat storage backfill in green mines.

Thermal Property Enhancement of a Novel Shape-Stabilized Sodium Acetate Trihydrate-Acetamide/Expanded Graphite-Based Composite Phase Change Material

Phase change materials(PCMs)are a kind of highly efficient thermal storage materials which have a bright application prospect in many fields such as energy conservation in buildings,waste heat recovery,battery thermal management and so on.Especially inorganic hydrated salt PCMs have received increasing attention from researchers due to their advantages of being inexpensive and non-flammable.However,inorganic hydrated salt PCMs are still limited by the aspects of inappropriate phase change temperature,liquid phase leakage,large supercooling and severe phase separation in the application process.In this work,sodium acetate trihydrate was selected as the basic inorganic PCM,and a novel shape-stabilized composite phase change material(CPCM)with good thermal properties was prepared by adding various functional additives.At first,the sodium acetate trihydrate-acetamide binary mixture was prepared and the melting point was adjusted using acetamide.Then the binary mixture was incorporated into expanded graphite to synthesize a novel shape-stabilized CPCM.The thermophysical properties of the resultant shape-stabilized CPCM were systematically investigated.The microscopic morphology and chemical structure of the obtained shape-stabilized CPCM were characterized and analyzed.The experiment results pointed out that acetamide could effectively lower the melting point of sodium acetate trihydrate.The obtained shape-stabilized CPCM modified with additional 18%(mass fraction)acetamide and 12%(mass fraction)expanded graphite exhibited good shape stability and thermophysical characteristics:a low supercooling degree of 1.75℃and an appropriate melting temperature of 40.77℃were obtained;the latent heat of 151.64 kJ/kg and thermal conductivity of 1.411 W/(m·K)were also satisfactory.Moreover,after 50accelerated melting-freezing cycles,the obtained shape-stabilized CPCM represented good thermal reliability.

Stabilization of coordinated motion for underwater vehicles

This paper presents a coordinating and stabilizing control law for a group of underwater vehicles with unstable dynamics. The coordinating law is derived from a potential that only depends on the relative configuration of the underwater vehicles. Being coordinated,the group behaves like one mechanical system with symmetry,and we focus on stabilizing a family of coordinated motions,called relative equilibria. The stabilizing law is derived using energy shaping to stabilize the relative equilibria which involve each vehicle translating along its longest（unstable） axis without spinning,while maintaining a relative configuration within the group. The proposed control law is physically motivated and avoids the linearization or cancellation of nonlinearities.

Preparation and Characterization of High-Temperature Non-Flowing Diurea/Paraffin/Oil Composites as Form-Stable Phase Change Materials

A series of form-stable phase change materials （FSPCMs） comprising paraffin as the latent heat storage material, diurea as the supporting material and base oil as the performance improvement agent were prepared. The diurea was synthesized in the system of paraffin/oil directly. A series of characterization was carried out for a deep understand- ing of shape stability and material properties of diurea-FSPCMs. The results showed that paraffin and base oil were packaged in the three-dimensional supra-molecular structures network which was formed by diurea. The dropping point of the prepared FSPCMs could reach 256 ℃ and the oil separation rate was as low as 1.19% at 100 ℃ for 30 h. The results of thermal properties tests showed that the prepared FSPCMs exhibited excellent thermal stability and the FSPCMs remained solid-like state in the temperature range from 25 to 200 ℃. This study proposes a novel method to prepare high-temperature non-flowing FSPCMs composites and methods to detect the thermal stability and shape stability of FSPCMs, which is helpful in understanding the shape stability mechanism and broadening the potential application of FSPCMs.