Preparation And Properties Of (Mg,Co,Ni,Cu,Zn)O-based High-entropy Oxides

The idea of introducing the concept of "high entropy" in materials design is to maximize the conformational entropy in order to stabilize equimolar mixtures and build more stable systems.This high entropy design concept brings four core effects to materials:(1)high entropy effect,which expands the solid solution between elements and provides stability for random solid solution formation;(2)slow diffusion effect(hysteresis diffusion effect),which inhibits the growth of second phase nanoparticles;(3)lattice distortion effect,which facilitates strengthening and hardening;and(4)cocktail effect,which produces unexpected properties after mixing multiple elements.properties.The new materials obtained through the concept of high entropy design are high entropy materials.In this thesis,we systematically explore the novel preparation methods and conditions of(Mg,Co,Ni,Cu,Zn)O system high-entropy oxides as powders and their electrochemical properties as anode materials for lithium-ion batteries;in addition,we also adopt the emerging microwave sintering method to prepare(Mg,Co,Ni,Cu,Zn)O high-entropy oxides and explore their structures and wave absorption properties.(1)Solid-phase reactions were used to prepare(Mg,Co,Ni,Cu,Zn)O high-entropy oxide ceramic powders,and the effects of calcination temperatures(800 °C,850 °C,900 °C,950 °C,and 1000 °C)on their crystal structures were systematically investigated.To explore a new method for preparing(Mg,Co,Ni,Cu,Zn)O high-entropy oxide powders,the(Mg,Co,Ni,Cu,Zn)O high-entropy oxide powders were prepared using microwave sintering,and the effects of calcination temperatures(750 ℃,800 ℃,825 ℃,850 ℃)on their crystal structures were investigated.(2)Based on the preparation of(Mg,Co,Ni,Cu,Zn)O high-entropy oxide powders,the electrochemical properties of them were explored as anode materials for lithium-ion batteries.It was further compounded with graphene to enhance the kinetic performance and energy storage performance of the high-entropy metal oxide anode material.The high-entropy oxide electrode modified with graphene surface can reach a first discharge capacity of 1225 mAh g-1 at 100 mA g-1,and the discharge capacity can be maintained at about 950 mAh g-1 after 200 cycles.a reversible cycle capacity of 460 mAh g-1 was obtained under the condition of multi-turn cycles at higher current densities.The results of the multiplicative performance at different current densities show that the composite has a cyclic discharge capacity of 1001,840,620,482 and 393 mAh g-1 at 100,200,500,1000 and 2000 mA g-1,respectively.Meanwhile,the electrochemical kinetic test results demonstrate a pseudocapacitance of the composite of about 25.8 % at 0.5 mV s-1.The results indicate that this composite forms a stable structure,which gives the composite excellent electrochemical stability.(3)microwave sintering as a special sintering method,the principle of the use of microwaves with a special waveband and the basic fine structure of the material coupled to generate heat,the dielectric loss of the material so that the overall heating of the material to the sintering temperature and densification method,and then the formation of high temperature conditions to prepare the desired material.Firstly,(Mg,Co,Ni,Cu,Zn)O high entropy oxides were prepared using microwave sintering,which indicates that(Mg,Co,Ni,Cu,Zn)O high entropy oxides perform well for wave thermal conversion and have the potential to be used as wave absorbing materials.Inspired by this,we prepared(Mg,Co,Ni,Cu,Zn)O high-entropy oxides by microwave sintering and compounded them with carbon additions of 20 wt%,30 wt%,40 wt%,and 50 wt%(denoted as C-20,C-30,C-40,and C-50,respectively),and further investigated the wave absorption properties of single high-entropy oxides and composites with different carbon additions.The wave absorption properties of the single high-entropy oxide and the composites with different carbon additions were further investigated.The values of the real part of the dielectric constant(ε′)curve for the sample C-0without carbon addition were stable at about 1.5.The values of the real part of the dielectric constant(ε′)for the sample C-20 ranged from 2.1 to 4.7;the values of the real part of the dielectric constant(ε′)for the samples C-30 and C-40 gradually increased and remained in a stable fluctuation;the values of the dielectric constant(ε′)for the sample C-50 with the most carbon addition were stable and fluctuated.The values of the imaginary part of the dielectric constant(ε′′)of the C-0 sample are stable around 0;the values of the imaginary part of the dielectric constant(ε′′)of the C-20 sample range from0.35 to 3.1;with the increase of carbon addition,the values of the imaginary part of the dielectric constant(ε′′)of the C-30,C-40 and C-50 samples gradually increase and show an overall decrease.The lowest reflection loss is-3.43 dB for C-0 sample without carbon addition,-30.68 dB for C-20 composite,-40.43 dB for C-30 composite,-40.43 dB for C-40 composite and-40.43 dB for C-40 composite.It is worth mentioning that the C-30 composite with a thickness of 5 mm has a reflection loss band width below-10 dB of 5.61GHz(6.89-12.50 GHz).Two semicircles can be observed in the Cole-Cole curves of samples C-0 and C-20,indicating that the dielectric losses of samples C-0 and C-20 are mainly polarization losses,while the Cole-Cole curves of samples C-30,C-40 and C-50 show multiple semicircles along with long straight lines,indicating that the dielectric losses of samples C-30,C-40 and C-50 mainly originate from conductivity loss and polarization loss.