單層電磁波吸波體之磁性填充粉末製程及電磁特性研究

Abstract

本篇論文中將針對電磁波吸收體複合材料中之磁性粉末分別採用不同之合成方法進行，包括高溫自行傳播合成法（SHS）、水熱法、檸檬酸法及凝膠/燃燒法等方式，並研究以鐵氧磁體磁性粉末所研製之電磁波吸收體，其導電與導磁材質特性，對於電磁波吸收或衰減之影響。同時，進行錳鋅、鎳鋅鐵氧磁體及以銀粒子與鎳鋅鐵氧磁體合成之核殼式磁性粉末製程研究，及深入探討這些電磁波吸收體之相關電磁特性分析。上述這些研究成果及討論分別敘述於第三章~第九章。 第三章中，我們利用凝膠法與燃燒合成法成功合成釔鋇銅氧高溫超導體，在本實驗中採用釔、鋇、銅硝酸鹽與檸檬酸及氨水作為起始原料，形成乾凝膠之先驅物，此乾凝膠於空氣中施加火源即自然進行燃燒反應過程並產出大量之灰燼，此灰燼粒徑大小介於次微米級，經煅燒（900℃/6 h）及燒結（930℃/6 h）過程後即可獲得釔鋇銅氧高溫超導體，此高溫超導體於液態氮之溫度範圍內顯現出反磁性之特性。 第四章中，我們分別使用鐵粉（Fe）、三氧化二鐵（Fe2o3）及五種不同之含鎳之原料作為反應物，利用燃燒法合成鎳鐵氧磁體（NiFe2O4）。反應物經混合及壓片過程後於充滿氧氣之環境下以電點火即開始進行燃燒過程，本實驗中含鎳原料、反應物之成份、氧氣氛之壓力及產率等方向均進行相關之研究探討。此外，X光繞射結果顯示氧氣氛能提高成品之轉化率，另在不同含鎳原料當中，三氧化二鎳（Ni2O3）對成品轉化率之幫助優於其他之原料，而鐵粉含量亦影響轉化率之高低，鐵粉含量越多對轉化率之提昇越有幫助，然而當含量多過某一特定範圍後則對轉化率產生負面之影響。使用掃描式電子顯微鏡觀察粉體粒徑大小約於0.1-0.5 μm之間，在磁滯曲線結果顯示以燃燒法合成之鎳鐵氧磁體其軟磁特性更優於以固態燒結合成之材料。 第五章則是討論氧分壓（OB）對燃燒合成之影響，分別使用鎳、鋅、鐵硝酸鹽與硝酸銨及銨基乙酸作為起始原料進行放熱之燃燒合成反應，藉由調整銨基乙酸對硝酸鹽之比例，可改變燃燒過程中之氧氣平衡值，同時獲致合成物之不同特性，最大燃燒溫度與燃燒過程所放出之氣體對氧氣平衡值之行為模式可視為自燃環境下之燃燒反應，以此法所合成之鎳鋅鐵氧磁體粉末具有奈米級之粒徑及超順磁性之特性。 第六章中則是以各金屬硝酸鹽及尿素為原料進行燃燒合成法之反應，實驗中另分別惨入鎂、鈷、銅金屬離子於鎳鋅鐵氧磁體中，以進行相關電磁特性之討論，將上述獲得之粉末與熱可塑性樹脂（TPU）混合壓片後即可製成電磁波吸收體，在2-12 GHz範圍內進行導電及導磁係數量測，量測結果經由計算可得出反射損失、匹配頻率以及匹配厚度之關係，鐵氧磁體填充物粒徑大小及掺雜之金屬離子後之鐵氧磁體粉末對電磁吸波特性所造成之影響亦在本章中詳細討論，當粒徑介於40-50 nm之間其反射損失結果相較於次微米或低於30 nm之效果為佳，同時掺雜鎂、鈷、銅金屬離子於鎳鋅鐵氧磁體粉末所製得之電磁波吸收體，於2-12 GHz範圍內反射損失可達20dB以上，此結果相較未掺雜者為佳。 第七章中我們利用檸檬酸法合成錳鋅鐵氧磁體粉末，所得之粉末與熱可塑性樹脂混合之重量比範圍控制於66.7%-88.9%之間，另外於實驗中針對錳鋅鐵氧磁體粉末亦掺雜鎂、鈷、銅等不同之金屬離子。使用布拉格曼理論分別計算其有效之導磁與導電係數，實驗結果顯示錳鋅鐵氧磁體粉末與熱可塑性樹脂混合之重量百分比及掺雜鎂、鈷、銅等不同之金屬離子對電磁波吸波效應之影響極大，掺雜鎂、鈷、銅等不同之金屬離子於錳鋅鐵氧磁體粉末之電磁波吸收體之吸波效果較佳，藉由控制掺雜鎂、鈷、銅等不同之金屬離子與混合之重量比兩大參數，可於3GHz以下獲致反射損失達20dB以上之電磁波吸收體。 第八章中我們則採用水熱法合成鎳鋅鐵氧磁體，同前製作電磁波吸收體之方式，將合成之奈米級鎳鋅鐵氧磁體粉末與熱可塑性樹脂混合壓片以製得電磁波吸收體。於本實驗中針對2~15GHz頻率範圍進行相關之電磁特性討論，同時奈米與微米粒徑大小對吸波效應之影響亦有進行相關探討，實驗所得之導磁、導電係數、頻率與厚度數據均利用布拉格曼理論計算即可獲得反射損失，對鎳鋅鐵氧磁體粉末掺雜鎂、鈷、銅等不同之金屬離子對吸波效應之影響亦進行深入研討。 第九章中我們將核殼式（core-shell）之設計理念導入於電磁波吸收體之填充物，利用水熱法合成以銀離子為核鎳鋅鐵氧磁體為殼之奈米級磁性粉末，實驗中鎳鋅鐵氧磁體對銀離子之重量比分別為6:1、4:1、2:1、1:1及1:6，每一核殼式粉體形狀接近圓形，實驗結果顯示增加銀離子含量其包覆於外層鎳鋅鐵氧磁體厚度將由200nm 遞減為15nm。本實驗中針對2~15GHz頻率範圍進行導磁、導電係數等相關量測，同時針對鎳鋅鐵氧磁體對銀離子之不同重量比所得之反射損失與匹配頻率進行計算分析，結果顯示當鎳鋅鐵氧磁體對銀離子之重量比遞減時匹配頻率將移往高頻方向。

In this thesis, the filled powders were synthesized and studied by several methods, including self-propagating high-temperature synthesis（SHS）method, hydrothermal method, citrate gel route method and sol-gel/combustion method. The complex permeability and permittivity of ferrite powders filled composite materials used for the electromagnetic-wave absorbers will be measured to investigate their role in the reflection or attenuation properties of the incident microwave. Furthermore, both synthesis and electromagnetic behaviors of MnZn ferrite, Ni-Zn ferrites and core-shell structural ferrite/silver filled composites were also studied and discussed. All the results and discussion will be systematically described in Chapter 3- Chapter 9. In Chapter 3, we combine the sol-gel and combustion synthesis techniques to fabricate Y-Ba-Cu-O high-temperature superconductor. Y (NO3)3·5H2O, Ba(NO3)2, Cu(NO3)2·3H2O, citric acid and ammonia were used as starting materials to form a precursor xerogel. And then, the dried gel undergoes a self-propagating combustion process when being ignited in air and yields voluminous ashes. These ashes are sub-micron in size and require low temperatures of calcination and sintering (900℃/6 h and 930℃/6 h, respectively) to obtain the product of Y-Ba-Cu-O superconductor with orthorhombic phase. The sintered body shows diamagnetism at liquid nitrogen temperature. In Chapter 4, Ni-ferrite (NiFe2O4) powder was synthesized by a self-propagating high –temperature synthesis (SHS) process using Fe, Fe2O3 and various Ni-containing species (Ni-CSs) as reactants. The reactants were thoroughly mixed and pressed into a cylindrical compact. The compact was ignited by electrical heating under oxygen partial pressures varying between 0.02 to 1.5 Mpa. The effects of the type of Ni-containing species, the reactant composition, the oxygen pressure on the combustion temperature, and the product conversion were investigated. X-ray diffraction (XRD) analysis showed that oxygen pressure promoted product conversion. Among several Ni-containing species tested, Ni2O3 was found to give the highest conversion. The product conversion increased with increasing iron content and then decreased with further increase in iron content. Scanning electron microscope (SEM) observations showed that the product synthesized is mostly in the form of agglomerated fine particles with submicron size (0.1-0.5 μm). Hysteresis behavior analysis showed that the product as synthesized possessed better soft magnetic properties when compared with commercial Ni-ferrite prepared by the solid reaction method. In Chapter 5, Ni0.5Zn0.5Fe2O4 powder was synthesized via an exothermic reaction between nitrates [Ni(NO3)2•6H2O, Zn(NO3)2•6H2O, Fe(NO3)3•9H2O, and NH4NO3] and glycine [NH2CH2COOH]. By adjusting the glycine-to-nitrates ratio, the oxygen balance (OB) values of the reactant mixtures can be varied in which the combustion phenomena is altered and thereby the as-synthesized products with different characteristics are obtained. An interpretation based on the measurement of maximum combustion temperature (Tc) and the amounts of gas evolved during reaction for various OB values has been proposed regarding the nature of combustion and its correlation with the characteristics of as-synthesized products. After instrumental analyses, it is shown that the as-synthesized powders are nanoscale crystallites with a large specific surface area as well as possessing a superparamagnetic behavior. In Chapter 6, NiZn ferrite with the chemical formula (Ni0.5-xZn0.5-xFe2O4, M=Co, Cu, or Mg, and x=0 or 0.05) were synthesized by a combustion synthesis method by using metallic nitrates and urea ( (NH2)2CO) as reactants. The nanoparticles of these materials were mixed with a thermal-plastic polyurethane (TPU) elastomer to be a microwave-absorbing composite. The complex permittivity ( ) and the complex permeability ( ) of the absorber were measured in the frequency range of 2-12 GHz. The reflection loss (R. L.), matching frequency (fm) and matching thickness (dm) were calculated using the theory of the absorbing wall. Effects of both the particle size of ferrite and the dopant presented in the ferrite on the electromagnetic properties and microwave-absorbing characteristics were investigated. It was found that nanoparticles around 40-50 nm exhibit higher reflection loss than those of micro-sized powders and than those with size less than 30 nm. In addition, the Co-doped, Mg-doped, and Cu-doped NiZn ferrite-TPU composites could be designed to be greater than 20 dB in the frequency range of 2-12 GHz to become promising materials for microwave absorbing application. These doped ferrite-containing composites have more effective microwave-absorbing characteristics than undoped one. In Chapter 7, Theμr and εr were investigated for ferrite-TPU composite with ferrite weight percentage varying from 66.7%-88.9%, and the ferrite doped with metal (Mg, Co, Cu) had been prepared by citrate gel route. The effective complex permeability and permittivity specta in the pure MnZn ferrite composites and various ferrite weight percentage were calculated using the Bruggeman effective medium theory. It was found that the dopant content and ferrite/TPU ratio influences the electromagnetic wave absorbing characteristics of MnZn ferrite composite. The results show that the absorbers with doped (Mg, Co, Cu) ferrite have excellent electromagnetic absorption. The reflection loss more than 20dB at the frequencies below 3 GHz can be achieved by controlling the ferrite content and doped elements in the ferrite-TPU composites. In Chapter 8, Ni0.5Zn0.5Fe2O4 ferrite powders were synthesized by a hydrothermal method. The nanoparticles of these materials were mixed with a thermal-plastic polyurethane (TPU) polymer in order to form a suitable composite for electromagnetic wave absorber in a frequency range from 2.0 to 15.0 GHz. The effect of particle size on the reflection loss was investigated by comparing nanosized and microsized powders. The reflection loss as a function of frequency ( f ), thickness of the absorber(d), the real and imaginary part of permittivity (ε’ /ε”) and the real and imaginary part of permeability (μ’/μ”) were obtained by calculation using the Bruggeman effective medium theory. The effect of Co2+, Cu2+, and Mg2+ on the reflection loss was also studied In Chapter 9, Ni0.5Zn0.5Fe2O4 spinel ferrites (shell) and silver powders (core) of core–shell nanoparticles were first synthesized by the hydrothermal method at different ratio (ferrites/silver=6/1,4/1,2/1,1/1,1/6). Each core–shell particle has a almost spherical shape. Increasing the ratio of silver content would be increasing the X-ray diffraction intensity of nanoparticles and decreasing the thickness of shell from 200nm to 15nm. The permittivity (ε΄, ε˝) and permeability(μ΄, μ˝) of absorbing materials were be measured in the frequency range of 2∼15 GHz. The reflection loss and matching frequency were calculated with different ferrite/silver ratio. It was found that the matching frequency of absorbers shift to high frequency when the ferrites/silver ratio of nanopowders was decreased.