Improved microwave dielectric properties of MgAl_(2)O_(4)spinel ceramics through(Li_(1/3)Ti_(2/3))^(3+)doping

A series of nominal compositions MgAl_(2-x)(Li_(1/3)Ti_(2/3))_(x)O_(4)(x=0,0.04,0.08,0.12,0.16,and 0.20)ceramics were successfully prepared via the conventional solid-state reaction route.The phase compositions,microstructures,and microwave dielectric properties were investigated.The results of x-ray diffraction(XRD)and scanning electron microscopy(SEM)showed that a single phase of MgAl_(2-x)(Li_(1/3)Ti_(2/3))_(x)O_(4)ceramics with a spinel structure was obtained at x≤0.12,whereas the second phase of MgTi_(2)O_(5)appeared when x>0.12.The cell parameters were obtained by XRD refinement.As the x values increased,the unit cell volume kept expanding.This phenomenon could be attributed to the partial substitution of(Li_(1/3)Ti_(2/3))^(3+)for Al^(3+).Results showed that(Li_(1/3)Ti_(2/3))^(3+)doping into MgAl_(2)O_(4)spinel ceramics effectively reduced the sintering temperature and improved the quality factor(Q_f)values.Good microwave dielectric properties were achieved for a sample at x=0.20 sintering at 1500℃in air for 4 h:dielectric constantε_(r)=8.78,temperature coefficient of resonant frequencyτ_(f)=-85 ppm/℃,and Q_(f)=62300 GHz.The Q_(f)value of the x=0.20 sample was about 2 times higher than that of pure MgAl_(2)O_(4)ceramics(31600 GHz).Thus,MgAl_(2-x)(Li_(1/3)Ti_(2/3))_(x)O_(4)ceramics with excellent microwave dielectric properties can be applied to 5G communications.

High sintering and dielectric performance:The improved(Mg,Zn)_(3)B_(2)O_(6) ceramics with the help of the DFT calculation

With the help of the first principle calculation,the solid-state reaction experiment was conducted to investigate the alteration in the sintering and the microwave dielectric properties of Mg_(3)B_(2)O_(6)ceramic with many Zn^(2+)substitutions.These properties were characterized using the scanning electron microscopy,network analyzer,X-ray diffraction,Raman spectroscopy,energy-dispersive spectroscopy,and thermomechanical and differential-thermal analyses.The coexistence of Mg_(3)B_(2)O_(6),Mg_(2)B_(2)O_(5)and ZnO ceramics could be observed with increasing Zn^(2+)addition,and the lattice distortion occurred in the Mg_(2)B_(2)O_(5)and Mg_(3)B_(2)O_(6)ceramics due to the substitution of Mg^(2+)with Zn^(2+).The electron density and the bond property of the MgO_(6)octahedron changed,and a quantitative method was used to discuss the variation in sintering,substitution and phase formation properties.The densification window was decreased to 1100℃,and the dielectric properties improved with the formation of a three-phase borate solid solution(dielectric constant=6.73,quality factor=112,000 GHz at 16 GHz(Q=7000),temperature coefficient of resonant frequency=-61.2 ppm℃^(-1),and relative density=97.0%).

Constitutive models and salt migration mechanisms of saline frozen soil and the-state-of-the-practice countermeasures in cold regions

A series of saline soil-related problems,including salt expansion and collapse,frost heave and thaw settlement,threaten the safety of the road traffic and the built infrastructure in cold regions.This article presents a comprehensive review of the physical and mechanical properties,salt migration mechanisms of saline soil in cold environment,and the countermeasures in practice.It is organized as follows:(1)The basic physical characteristics;(2)The strength criteria and constitutive models;(3)Water and salt migration characteristics and mechanisms;and(4)Countermeasures of frost heave and salt expansion.The review provides a holistic perspective for recent progress in the strength characteristics,mechanisms of frost heave and salt expansion,engineering countermeasures of saline soil in cold regions.Future research is proposed on issues such as the effects of salt erosion on concrete and salt corrosion of metal under the joint action of evaporation and freeze-thaw cycles.

Amodified numerical model for moisture-salt transport in unsaturated sandy soil under evaporation

Soil salinization,caused by salt migration and accumulation underneath the soil surface,will corrode structures.To analyze the moisture-salt migration and salt precipitation in soil under evaporation conditions,a mathematical model consisting of a series of theoretical equations is briefly presented.The filling effect of precipitated salts on tortuosity factor and evaporation rate are taken into account in relevant equations.Besides,a transition equation to link the solute transport equation before and after salt precipitation is proposed.Meanwhile,a new relative humidity equation deduced from Pitzer ions model is used to modify the vapor transport flux equation.The results show that the calculated values are in good agreement with the published experimental data,especially for the simulation of volume water content and evaporation rate of Toyoura sand,which confirm the reliability and applicability of the proposed model.

Laboratory investigation on the mechanical behavior of ice-saturated frozen loess

To investigate the mechanical properties of ice-saturated frozen soil, a series of triaxial tests under various confining pressures（0.5 to 9.0 MPa） on ice-saturated frozen loess with ice content of 23.7% were carried out at a temperature-6 °C, and at 1.25 mm/min of loading rate. The triaxial tests include two loading modes, one with monotonic loading（i.e., triaxial compression）, and another with static cycle loading. The test results under triaxial compression show that the strength and deformation behaviors of ice-saturated frozen loess are affected by confining pressure. According to the test results of triaxial loading-unloading cycle test, the elastic modulus evolution with the number of cycles under different confining pressures are analyzed.

A multi-index coupling system of ice slope stability and its application

Based on the Simplified Bishop Method, the minimum safety factor of ice slope both with and without tension cracks is calculated in combination with triaxial compression tests. It is found that there exists a critical depth for each crack. Then, factors influencing ice slope stability such as slope ratio, slope height, ice cohesion, internal friction angle, unit weight and temperature were analyzed. Meanwhile, a regression equation between the aforementioned factors and safety factor is obtained, with which sensitivity analysis is carried out. The performance function is built in combination with random distribution of physical and mechanical parameters to analyze the reliability index. The Advanced First Order Second Moment Method is employed on the solution to the perfor- mance function. The one-way coupling system of ice slope stability is therefore formed based on safety factor and reliability index. Finally, an illustrated example of ice slope is provided, which shows that failure probability is relatively high, up to 6.18%, alt- hough safety factor is 2.77. Thus, it is objective and reasonable to apply the coupled system method to the slope stability rating.

Calculation theories and analysis methods of thermodynamic stability of embankment engineering in cold regions

The calculation theories and analysis methods of thermodynamic stability of embankment engineering in cold regions are systematically summarized.The engineering theories and methods taken to control frost heave and thaw settlement in seasonal frozen soil regions may not be applicable in permafrost regions.The active cooling technology of roadbed should be utilized to limit the effects caused by both climate changes and human engineering activities.The paper mainly discussed the calculation theories and analysis methods of four kinds of embankment structures,i.e.,the crushed-rock embankment,duct-ventilated embankment,thermosyphon embankment,and composite embankment.It is expected that a scientific basis could be provided for the theory,design,and application of embankment constructions in cold regions.

Amelioration of sintering and multi-frequency dielectric properties of Mg_(3)B_(2)O_(6):A mechanism study of nickel substitution using DFT calculation

With the support of density functional theory(DFT)calculation,the amelioration of sintering and dielectric properties of the Mg_(3)B_(2)O_(6)(MBO)ceramic was realized through the substitution of magnesium with nickel.The TE-mode cylindrical cavity method was used to measure the dielectric properties at different frequencies.The thermo-mechanical analysis and simultaneous thermal analysis were used to characterize the chemical and mechanical properties.The phase composition was determined through the X-ray diffraction(XRD)and Raman spectrum.The microstructure was investigated using the scanning electron microscopy(SEM).Magnesium substitution with nickel(4 mol%)could ionize the B-0 bond of BO3,modify the vibration mode,improve the order degree,densify the microstmcture,decrease the intrinsic densification temperature,and ameliorate the dielectric properties of the MBO ceramics.The maximum values were achieved for the ceramics with 4 mol%nickel and sintered at 1175℃,that is,97.2%for relative density,72,600 GHz(10 GHz),75,600 GHz(11.4 GHz),and 92,200 GHz(15 GHz)for Q×f,7.1(10 GHz),7.01(11.4 GHz),and 6.91(15 GHz)for£r,and-56.3 ppm/℃ for if.