Preparation,Characterization And Magnetic Properties Of Several Nanomagnetic Materials By Super (Near) Critical Hydrothermal Method

Thousands of years ago,the curiosity made people discover magnetics.For centuries,the research and development about magnetism promotes the progress of science and technology.Earlier,People's attention was mainly focused on macroscopic magnetic applications.For example,the compass,electromagnets,permanent magnets,etc.were used in the field of mechanical work.In the 1950s,atomic-scale magnetic phenomena were discovered and have been studied about applications,such as exchange effects,spin-orbit coupling,crystal field effects and so on.In recent decades,scientists have discovered that solid-state magnetism is,to a large extent,a nanostructure phenomenon.The research of nano-magnetic materials is of great significance in the field of magnetic materials.Because of the nanomaterials'properties,nanomagnetic materials produce abnormal magnetic properties,which are widely used in various areas.The application of nano-magnetic materials is closely related to its magnetic properties,and its magnetic properties are inseparable with the nano-structure of magnetic materials?morphology,size,nano-particle surface structure,composition,etc.?.However,the preparation method and processing technology have an important influence on the nanostructures.Due to the characteristics of nanomaterials,when the nanomaterials are small in size,their surface activity is strong,they are easy to combine with surrounding atoms,reunite,and the particle size becomes larger.However,nanomagnetic materials are easy to reunite due to their magnetic properties.Therefore,it is of great significance to find an effective method to control the morphology,size and structure of nanomagnetic materials.The super?near?critical hydrothermal method is a common method for preparing nanomaterials.And it is widely used due to its characteristics of small size,narrow size distribution,good dispersion,and no pollution to the environment.At present,remarkable achievements have been made in the synthesis of nanomagnetic materials by super?near?critical water.Supercritical water can dissolve gas and most of organic matters.In the synthesis of nano-magnetic materials by the super?near?critical water,the gas and the organic substances can be introduced as"soft templates"or reaction media,nanomagnetic materials can be more effectively synthesized with controllable morphyology.As an important half-metal ferromagnetic material,CrO₂ has a very high spin polarization rate,and is widely used in spintronic devices and magnetic storage materials.However,CrO₂ is metastable at normal temperature and pressure,so the preparation of nanostructured CrO₂ is a challenging task.Through the supercritical hydrothermal synthesis,using CrO₃ as raw material and Ti nano-pit array as template,high-purity CrO₂ nanoparticle arrays were prepared in high oxygen pressure conditions.The growth mechanism of CrO₂ nanoparticle arrays was discussed.The study found that CrO₂ nanoparticles are closely packed together in honeycomb hexagons,and the concentration of CrO₃ has important influence on the morphology and size of the samples.By adjusting the concentration of CrO₃,the CrO₂nanoparticles arrays with the nanopartiales size of 65 nm were preparated,and its the surface density of 1.5�10¹⁰ cm².CrO₂ nanoparticle arrays exhibit better magnetic properties.The research reveals that CrO₂ nanoparticle arrays can be praparated in large-scale,and have a high application prospect in magnetic storage media.CrOOH has been drawing attention as a common dye,catalyst and carrier,and the precursors used to preparate chromium oxide?CrO₂,Cr₂O₃?.The size-controlled syntheses of extremely tiny CrOOH nanorods?the diameter of 2 nm,the length of 16nm?with different polymorphs were carried out by supercritical hydrothermal method using PVA?cold water soluble starch,glycol?as soft template and CrO₃ as raw material.The effect of reaction conditions on the morphology and structure of the sample was systematically studied.We find that the optimum temperature for the synthesis of CrOOH nanorods is 400�C,and the molar ratio of the reactants to the soft template(Cr⁶⁺:[OH])in the precursor solution had an important influence on the morphology and structure of the product.Beside,the soft templates play key roles on the morphology of the sample.So CrOOH nanorods with different length-to-diameter ratio can be synthesized by adjusting the molar ratio of Cr⁶⁺and[OH]and the different solft templates.This study provided new ideas and methods for the preparation of CrO₂ nanorods.The size and morphology of the precursor have an important influence on the size and morphology of the product.In this dissertation,CrO₂ nanoparticles were prepared by using the tiny CrOOH nanorods with the structural advantages.Method one:the CrOOH nanorods were dispersed by ball milling via the gas phase nano-SiO₂as dispersant,and the mixure was used as precursor,subsequently,CrO₂ nanoparticles were obtained under high oxygen pressure at 400�C.The influence of dispersant content on the morphology of the products was studied.When the content of gas-phase nano-SiO₂ is high,CrOOH can be effectively dispersed to obtain CrO₂nanoparticles with a saturation magnetization of 38 emu/g and the size of 15-40 nm.Method two:through the St???ber's hydrolysis method,CrOOH nanorods were coated by SiO₂,and then annealled at 400�C under high oxygen pressure.CrO₂ nanorods coated SiO₂ were obtained,and were systematically studied about the magnetic properties.It was found that SiO₂ coating makes that the CrO₂ maintain substantially the nanorod structure of CrOOH.The synthesized CrO₂ nanorods coated by SiO₂ have single magnetic domain structure and superparamagnetism,and its freeze temperature is 115 K.The results of this study show that the superparamagnetic CrO₂nanoparticles can be applied in the field of biomedicine and magnetic liquids.NiO is an antiferromagnetic material,whereas NiO nanoparticles exhibit ferromagnetism or superparamagnetism.And sometimes NiO nanoparticles could have bias effect.Due to the interesting magnetic properties and the applications in magnetoelectronics,NiO nanoparticles have attracted widespread attention.NiO nanoparticles with weak room temperature ferromagnetism were synthesized in one step using Ni?NO₃?₂�6H₂O as raw material and PVA as soft template under near-critical hydrothermal conditions.Under the H₂ gas environment,the NiO nanoparticles were annealed at 300�C and 350�C,respectively,the product is still NiO nanoparticles,but has a bias effect.And the their magnetic properties were systematically studied.The NiO nanoparticles with SiO₂ coating by the St???ber hydrolysis method were subjected to annealing in H₂ gas atmosphere at 400�C and500�C.Then,the NiO and Ni nanoparticles coated by SiO₂,and Ni nanoparticles coated by SiO₂ were obtained,and their magnetic properties were also studied systematically.The results show that the weak ferromagnetism of NiO nanoparticles is caused by the spin magnetic moments,which generated by uncompensated on the surface of nanoparticles.After annealing,more uncompensated spin magnetic moments are generated on the NiO surface,and the interaction between uncompensated spin magnetic moments and the antiferromagnetic NiO nanoparticle core results in a bias effect.The bias effect of NiO and Ni nanoparticles coated by SiO₂ disappears.It is mainly due to that more NiO nanoparticles are reduced to Ni nanoparticles through annealing at 400�C,which result in decreaseing of the uncompensated spin magnetic moment generated on the surface of the NiO nanoparticles,make the interaction weaker and the bias effect disappear.Both samples annealed at 400�C and 500�C are superparamagnetic.Because of the non-toxic,inherent biocompatibility,high saturation magnetization,and superparamagnetism at room temperature under critical size,Fe₃O₄ is widely used in biomedicine applications.Using Fe?NO₃?₃�9H₂O as raw material,PVA or glucose as soft templates,Fe₃O₄ nanoparticles were synthesized in one step by near-critical hydrothermal method.Different reaction conditions have important influence on its size,morphology and magnetic properties.Fe₃O₄ coated by the organics can be considered to be modified by surface biomolecules and used to target drugs or carriers to treat diseases such as tumors.The results of this study provide new ideas for the synthesis of biopharmaceuticals.