Effects of Al and Co doping on the structural stability and high temperature cycling performance of LiNi_(0.5)Mn_(1.5)O_(4) spinel cathode materials

The poor structural stability and capacity retention of the high-voltage spinel-type LiNi_(0.5)Mn_(1.5)O_(4)(LNMO)limits their further application.Herein,Al and Co were doped in LNMO materials for a more stable structure and capacity.The LNMO,LiNi_(0.45)Al_(0.05)Mn_(1.5)O_(4)(LNAMO)and LiNi_(0.45)Co_(0.05)Mn_(1.5)O_(4)(LNCMO)were synthesized by calcination at 900℃ for 8 h,which was called as solid-phase method and applied universally in industry.XRD,FT-IR and CV test results showed the synthesized samples have cation disordering Fd-3m space group structures.Moreover,the incorporation of Al and Co increased the cation disordering of LNMO,thereby increasing the transfer rate of Li+.The SEM results showed that the doped samples performed more regular and ortho-octahedral.The EDS elemental analysis confirmed the uniform distribution of each metal element in the samples.Moreover,the doped samples showed better electrochemical properties than undoped LNMO.The LNAMO and LNCMO samples were discharged with specific capacities of 116.3 mA·h·g^(-1)and 122.8 mA·h·g^(-1)at 1 C charge/discharge rate with good capacity retention of 95.8% and 94.8% after 200 cycles at room temperature,respectively.The capacity fading phenomenon of the doped samples at 50℃ and 1 C rate was significantly improved.Further,cations doping also enhanced the rate performance,especially for the LNCMO,the discharge specific capacity of 117.9 mA·h·g^(-1)can be obtained at a rate of 5 C.

High temperature stability of Eu^(2+)-activated nitride red phosphors

The novel nitride-based luminescent materials have received much attention since the end of the last century. In this paper, the commercial Eu2＋-activated nitride red phosphors, Sr1.95Si5N8：Eu0.05, Sr1.85Si5N8：Eu0.15 and Ca0.99AlSiN3：Eu0.01 phosphors were an-nealed at different temperatures （beyond 300 oC） to investigate the dependence of their luminescence performance and structure vari-ability on the temperature. By photoluminescence spectra, X-ray diffraction （XRD） and thermogravimetry-differential scanning calo-rimetry （TG-DSC） analysis, the high temperature stability of the hosts and activator of the three samples were disclosed. With the an-nealing temperature increasing, the activator Eu2＋ions were firstly oxidized and then host in Sr1.95Si5N8：Eu0.05 and Sr1.85Si5N8：Eu0.15, but for Ca0.99AlSiN3：Eu0.01, only the oxidation of the host could be observed, which would lead to the luminescence degradation and even failure of these phosphors. The activator Eu2＋ions were much more stable in CaAlSiN3：Eu than Sr2Si5N8：Eu due to their crystal surroundings, and its concentration also influenced the temperature stability of Sr2Si5N8：Eu.

Research on preparation optimization of nano CeB_6 powder and its high temperature stability

High temperature self-propagating synthesis （SHS） process is very rapid, the reaction process becomes un-controlled after the SHS reaction is ignited. So the initial reaction conditions will have great effects on phase compositions and microstructures of reaction products. In this paper, the effects of the proportioning amount of Mg on the yield ratio and particle sizes of CeB6 were studied. The SHS reaction products and leached products were characterized by X-ray diffraction （XRD） and scanning electron microscopy （SEM）. The results indicated that the SHS products consisted of MgO, CeB6 and little Mg3B206. The single CeB6 phase was contained after the SHS reaction products were leached. The purity of CeB6 was higher than 99.0 mass%, and the minimum particle sizes of CeB6 were within 30-70 nm. When the propor- tioning amount of Mg was 25% more than the theoretic amount, the yield of CeB6 was 68.68%. The antioxidant ability of CeB6 was rather stronger, which was oxidized step by step, and the initial oxidation temperature was 750 ℃ which indicated that it had good high temperature stability. The apparent activation energies of oxidation reactions of CeB6 were 200.09 and 312.10 kJ/mol, respectively, and reaction orders were 0.69 and 0.40, respectively.

Effect of yttrium and manganese addition on catalytic soot combustion activity and anti-high-temperature stability of CeO_(2) catalyst

In order to analyze the influence of the addition of yttrium and manganese on the soot combustion performance and high temperature stability of CeO_(2) catalyst,a series of Y/Mn-modified CeO_(2) catalysts were prepared.The effects of structural properties,textural properties,oxygen vacancies,Ce^(3+),surface adsorbed oxygen species,reduction properties and desorption properties of oxygen species on the activity were analyzed by various characterization methods.The results of the activity test show that the addition of manganese is beneficial to enhancement of the activity,while the addition of yttrium increases the amount of reactive oxygen species,but decreases the activity.After aging at 700℃,the activity of the CeMn catalyst decreases most sharply,while the catalytic activity of the CeY catalyst can be maintained to a certain extent.Interestingly,the addition of yttrium and manganese at the same time can stabilize the activity.The fundamental reason is that yttrium and manganese move to the surface of the solid solution after aging,which increases the reduction performance of the catalyst,thus contributing to the increase of activity.Although the activity of CeYMn catalyst decreases after aging at 800℃,it is still higher than that of other catalysts aged at 700℃.

High-entropy(Sm_(0.2)Eu_(0.2)Gd_(0.2)Dy_(0.2)Er_(0.2))_(2)Hf_(2)O_(7) ceramic with superb resistance to radiation-induced amorphization

Nuclear engineering materials are required to possess outstanding extreme environmental tolerance and irradiation resistance.A promising novel pyrochlore-type of(Sm_(0.2)Eu_(0.2)Gd_(0.2)Dy_(0.2)Er_(0.2))2 Hf_(2)O_(7)high-entropy ceramic(HE-RE2 Hf_(2)O_(7))for control rod was prepared by solid-state reaction method.The ion irradiation of HE-RE_(2) Hf_(2)O_(7)with 400 keV Kr+at 400℃was investigated using a 400 kV ion implanter and compared with single-component pyrochlore Gd2 Hf_(2)O_(7)to evaluate the irradiation resistance.For HE-RE2 Hf_(2)O_(7),the phase transition from pyrochlore to defective fluorite is revealed after irradiation at 60 dpa.After irradiation at 120 dpa,it maintained crystalline,which is comparable to Gd2 Hf_(2)O_(7)but superior to the titanate pyrochlores previously studied.Moreover,the lattice expansion of HE-RE2 Hf_(2)O_(7)(_(0.2)2%)is much lower than that of Gd2 Hf_(2)O_(7)(0.62%),indicating excellent irradiation damage resistance.Nanoindentation tests displayed an irradiation-induced increase in hardness and a decrease in elastic modulus by about 2.6%.Irradiation-induced segregation of elements is observed on the surface of irradiated samples.In addition,HE-RE2 Hf_(2)O_(7)demonstrates a more sluggish grain growth rate than Gd2 Hf_(2)O_(7)at 1200℃,suggesting better high-temperature stability.The linear thermal expansion coefficient of HE-RE2 Hf_(2)O_(7)is 10.7×10-6 K-1 at 298–1273 K.In general,it provides a new strategy for the design of the next advanced nuclear engineering materials.

High electrocaloric effect in barium titanate-sodium niobate ceramics with core-shell grain assembly

Electrocaloric effect(ECE)is promising in realizing solid-state cooling as an alternative to the conventional refrigeration with environmentally harmful coolant and low efficiency.High ECE in lead-free ferroelectric ceramics is highly desirable for the EC cooling.In this work,different from the researches that tune the ECE by conventional compositional design or external stress engineering,we fabricated the(1-x)BaTiO_(3)-xNaNbO_(3)(BTO-xNN)lead-free ceramics with a core-shell grain structure arising from the inhomogeneous stoichiometry of element distribution,leading to the internal compressing stress in the grains.It is interesting that the phase transition behavior,including the phase transition temperature and the diffusion property,is regulated by the core-shell grain structure induced internal stress,which can be capitalized on for the favorable ECE.Cooperated with 0.02 NN,a high ECE,e.g.adiabatic temperature change(ΔT)of 3.6 K and isothermal entropy change(ΔS)of 4.5 J kg^(-1) K^(-1),is attained in the BTO ceramic.As the internal stress further increases with more NN,the BTO-0.06NN exhibits an extremely stable ECE with a variety rate below ±4% in a wide temperature range from 300 K to 360 K.This work provides a novel approach to explore pronounced ECE in lead-free ferroelectrics for eco-friendly refrigeration.

Novel bandgap-based under-voltage-lockout methods with high reliability

Highly reliable bandgap-based under-voltage-lockout （UVLO） methods are presented in this paper. The proposed under-voltage state to signal conversion methods take full advantages of the high temperature stability characteristics and the enhancement low-voltage protection methods which protect the core circuit from error operation; moreover, a common-source stage amplifier method is introduced to expand the output voltage range. All of these methods are verified in a UVLO circuit fabricated with a 0.5 μm standard BCD process technology. The experimental result shows that the proposed bandgap method exhibits a good temperature coefficient of 20 ppm/℃, which ensures that the UVLO keeps a stable output until the under-voltage state changes. Moreover, at room temperature, the high threshold voltage VTH＋ generated by the UVLO is 12.3 V with maximum drift voltage of ±80 mV, and the low threshold voltage VTH- is 9.5 V with maximum drift voltage of±70 mV. Also, the low voltage protection method used in the circuit brings a high reliability when the supply voltage is very low.

Polycrystalline diamond compact with enhanced thermal stability

Polycrystalline diamond compacts（PDC）, which are composed of diamond and WC/Co substrate, and synthesized at high pressure and high temperature（HPHT）, are widely applied as the tooth of drilling bit. However, the thermal stability of PDC will be reduced when diamond transforms into graphite due to cobalt in PDC acting as a catalyst during the drilling work. In this study, a new three-layer structured PDC with enhanced thermal stability has been successfully synthesized at pressures of 5.5–7.0 GPa and temperatures of 1650–1750？C. In this structure, the diamond-Si C composite acts as the working layer,and the diamond-Si C-Co composite and WC/Co cements are as the intermediate layer and substrate,respectively. It is found that the initial oxidizing temperature of the three-layered PDC is enhanced up to820？C, which is significantly higher than that（～780？C） of the conventional PDC counterpart.