Preparation And Properties Of NiFe₂O₄/FeNi Composites

Microwave absorbing materials have been widely used in civil and military spects,and gradually developed into the research focus of green environmental management and military stealth technology.Among them,ferrite and metal powder absorbing materials have low price and excellent microwave absorption characteristics,so they stand out in many absorbing materials.With the rapid development of the radar technology and stealth technology,it puts forward higher requirements on absorbing materials,traditional or single absorbing materials can't meet the high,medium and low full frequency range of electromagnetic wave shielding requirements,so we must use new materials and new technologies to help solve the problem.In this paper,Ni nanopowders,FeNi alloy nanopowders and NiFe₂O₄ nanopowders were prepared respectively.Their morphology,magnetic properties and absorption properties were characterized.NiFe₂O₄/FeNi composites were prepared by using hydrothermal coprecipitation method.Using the advantages of the electromagnetic properties of the two materials to contributioning microwave absorption.The main research results are as follows:Using NiCl₂·6H₂O,NaOH as raw materials,N₂H₄·H₂O as reducing agent,PVPK30 as surfactant,Ni nanopowders were prepared by the ultrasonic reduction method.The effects of Ni²⁺concentration,N₂H₄/Ni²⁺molar ratio,reaction temperature and reaction time on the reaction products were studied.The best solution:Ni²⁺concentration is 0.2 mol/L,N₂H₄/Ni²⁺mole ratio is 4:1,reaction temperature is 80?,reaction time is 90 min.The PVP molecular chain is stretched on the surface of Ni grains,the Ni nucleation adsorption of PVP molecules has a rapid growth rate,forming the aristate spherical Ni nanoparticles.The grain size of the sample increases with the reaction time.The saturation magnetization increases with the increase of grain size.However,the coercivity increases first and then decreases with the grain size,reaching a maximum value of 23.20 KA/m at 42nm.The single domain critical size of Ni nanoparticles is about 42nm,which is in good agreement with Lex 49.1nm.The electromagnetic shielding performance of the aristate spherical Ni nanopowders in the3.04⁴.96GHz band is higher than 10 dB,the reflectivity is as high as-19.64 dB at 2.16GHz.Using NiCl₂·6H₂O,FeCl₂·4H₂O,NaOH as raw materials,N₂H₄·H₂O as reducing agent,FeNi?1/1,1/2,1/3?nanopowders were prepared by the hydrothermal reduction method.Put FeNi?1/1?samples in Ar/N₂ atmosphere at 500?and 700?heat treatment 2 h respectively to make FeNi after annealing.The effects of chemical composition and heat treatment on the morphology,magnetic properties and microwave absorption properties of the alloys were analyzed.The results showed nano-FeNi samples were all spherical particles with pure phase.The saturation magnetization decreases with the increase of nickel content in the alloys,and increases with the increase of annealing temperature.Coercive force decreases obviously with the annealing temperature,at 700?for0.68 KA/m.The samples are converted from semi-hard magnetic to typical soft magnetic materials.The annealing treatment increases the permeability of FeNi?1/1?alloys,which is beneficial to improve the absorption properties.The effective absorption band of FeNi?1/1?nanopowders after annealing at 700?is6.12⁹.38 GHz in the thickness of 3mm.The maximum absorption peak appears at 3.84GHz,when the thickness is 5 mm,the reflection loss is-28.36 dB.Using Ni?NO₃?₃·6H₂O,FeSO₄·7H₂O as raw materials,NaOH as precipitant,NiFe₂O₄nanopowders were prepared by the hydrothermal method.NiFe2O4 samples are spinel structure with pure phase.They are block structure and agglomerate into chain distribution.When the coating thickness is 3mm,the NiFe2O4 nanoparticles have obvious microwave absorption at 10.51¹5.38 GHz band,the maximum reflection loss at 13.10 GHz is-20.49dB.Using Ni?NO₃?₃·6H₂O,FeSO₄·7H₂O,FeNi as raw materials,NaOH as precipitant,PEG-400as surfactant,NiFe2O4/FeNi composites were prepared by the hydrothermal coprecipitation method.The effects of Ni²⁺concentration,different cladding ratio,pH and heat treatment temperature were investigated.The results show that when Ni²⁺concentration is 0.1 mol/L,NiFe₂O₄/FeNi=1:1,pH value of 11,the heat treatment temperature of 500?,the cladding layer is the most complete,formed"steamed bread bag"phenomenon.The saturation magnetization increases with the increases of cladding ratio and coercive force decreases.The microwave absorption properties of NiFe₂O₄/FeNi composites are significantly better than FeNi or NiFe2O4 nanopowders.Its effective absorption band width is 2 times wider than FeNi alloys.With the increases of NiFe₂O₄ in NiFe₂O₄/FeNi composites,the absorption band shifts to the high frequency band,moving from 7.67 GHz to 12.24GHz.And the microwave absorption properties increase from-18.87 dB to-29.08 dB.