CN1655296A - 由Mn－Zn铁氧体制成的电磁波吸收体 - Google Patents

由Mn－Zn铁氧体制成的电磁波吸收体 Download PDF

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CN1655296A
CN1655296A CN200410004920.7A CN200410004920A CN1655296A CN 1655296 A CN1655296 A CN 1655296A CN 200410004920 A CN200410004920 A CN 200410004920A CN 1655296 A CN1655296 A CN 1655296A
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ferrite
electromagnetic wave
wave absorb
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mole
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小林修
伊藤清
法月正志
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Minebea Co Ltd
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Abstract

一种由Mn－Zn铁氧体制成的电磁波吸收体，该Mn－Zn铁氧体包括：含40.0－49.9摩尔％的Fe2O3，4.0－26.5摩尔％的ZnO，余下的由MnO组成的尖晶石主相；以及包含CaO作基本组分的第二相，在该铁氧体中，尖晶石主相的质量构成了尖晶石主相和第二相总质量的50.0－99.0％。

Description

由Mn-Zn铁氧体制成的电磁波吸收体
技术领域
本发明涉及一种由Mn-Zn铁氧体制成的电磁波吸收体，其在高频段下具有优异的吸收性能。
背景技术
吸收电磁波的电磁波吸收体可以影响电阻元件的欧姆损耗，派生元件的介电损耗或者磁材料的磁损耗。影响磁损耗的电磁波吸收体，其吸收性能可用由下面的(1)式计算得到的反射系数来评价：
Z in = Z o μ r ϵ r tanh [ j 2 π c fd μ r ϵ r ]
这里，μ是磁导率，ε是介电常数，c是光速，f是电磁波频率，d是电磁波吸收体的厚度，Z是阻抗特性。一般来讲，在一个给定的频段下具有20dB或者更大的反射系数的电磁波吸收体被认为在这一频段下具有充分的吸收性能。
影响磁损耗的电磁波吸收体可以由铁氧体材料制成，比如Mn-Zn铁氧体或者Ni-Zn铁氧体。Mn-Zn铁氧体主要用于要在相对较低的频段30-500MHz范围内吸收电磁波的电磁波吸收体上，而Ni-Zn铁氧体用于要在相对较高的频段500MHz往上的范围内吸收电磁波的电磁波吸收体上。由于Ni-Zn铁氧体很昂贵，希望在高频段应用中也可以使用较低廉的Mn-Zn铁氧体。
在由Mn-Zn铁氧体制成的电磁波吸收体中，随着频率的提高，涡电流相应地增加，所以会引起损耗的增大。因此，为了在高频段内可以充分的吸收电磁波，必须提高Mn-Zn铁氧体的电阻率。当其中的Fe2O3含量超过50.0摩尔％的化学计量的Mn-Zn铁氧体烧结时，Fe3+被还原成Fe2+，Fe3+与Fe2+之间容易发生电子转移，由此电阻率降到1Ωm以下。结果，可用Mn-Zn铁氧体作电磁波吸收体的频段只能达到几百kHz，从此往上，Mn-Zn铁氧体的磁导率就显著降低，软磁性消失，这样就失去了用作电磁波吸收体的功能。
为了提高电阻率，Mn-Zn铁氧体中可以包含CaO，SiO2或者其它类似的物质作添加剂以提高其在晶粒边界的电阻，并同时可以在大约1200℃的低温下烧结以将其晶粒尺寸由大约20μm减小到大约5μm，由此提高晶粒边界的比率。然而，在这样的Mn-Zn铁氧体中，由于晶粒边界本身的电阻低，很难得到大于1Ωm的电阻率。而且，如果加入的CaO达0.20质量％或更多，在烧结时会产生反常的晶粒生长，其性能会受到显著破坏。
例如，在公开号为H09-180925的日本专利申请中公开了一种具有提高了的电阻率的Mn-Zn铁氧体，其中包含的基本组分为：20.0-30.0摩尔％的MnO，18.0-25.0摩尔％的ZnO，余下的由Fe2O3组成，其具有的DC电阻率为0.3Ωm或者更大，在1kHz下的介电常数ε为100000或者更低。这种通过在其中添加CaO，SiO2，SnO2和/或TiO2而制得的Mn-Zn铁氧体具有提高了的电阻，但因此能得到的电阻率仅能达到2.0Ωm，这对于在高频段吸收电磁波来讲仍然不是足够的好。
在例如公开号为H07-230909的日本专利申请中公开了另外一种Mn-Zn铁氧体，其包含的基本组分为45.0-48.6摩尔％的Fe2O3，适当摩尔％的Mn2O3(与Fe2O3一起总和构成50.0摩尔％)，28.0-50.0摩尔％的MnO，余下的由ZnO组成，并且进一步包含0.01-0.50质量％的SiO2和CaO作添加剂，在其中存在1.0摩尔％或更少(不含0)的Fe2+。该Mn-Zn铁氧体被用作一种偏转轭的磁芯材料，通过将Fe2O3的含量限制在50.0摩尔％以下而获得提高了的电阻率。该Mn-Zn铁氧体以用在64-100kHz的频段为目的，不适于用在超过1MHz的高频段。
并且，在例如专利号为3108803的日本专利中还公开了另外一种Mn-Zn铁氧体，其包含的基本组分为44.0-50.0摩尔％(不含50.0％)的Fe2O3，4.0-26.5摩尔％的ZnO，0.1-8.0摩尔％的TiO2和/或SnO2，余下的由MnO组成，其具有150Ωm或更高的电阻率。该Mn-Zn铁氧体通过将Fe2O3的含量限制在50.0摩尔％以下而获得提高了的电阻率。
为了在高频段下具有很好的功能，电磁波吸收体必须在磁导率，介电常数和电阻率上呈现出适当的特性。由传统的Mn-Zn铁氧体制成的电磁波吸收体仅能在有限的频段内起作用，所以，不能单独烧结传统的Mn-Zn铁氧体来制造可在包括高频段的扩展频率范围内起作用的电磁波吸收体。
发明内容
本发明是考虑到上述问题而做出的，本发明的一个目的就是提供一种可降低成本的由Mn-Zn铁氧体制成的电磁波吸收体，其可以满足需要，吸收性能优异，并同样适合于在高频段下使用。
本发明者已进行了研究并发现其中Fe2O3含量低于50.0摩尔％的化学计量组成，同时含有非常微量的Mn2O3和FeO的烧结的Mn-Zn铁氧体，甚至在含有难以想象的高浓度的CaO时，也不会遭受性能的破坏，而且，他们已经证实，当适当安排构成铁氧体材料的尖晶石主相和包括CaO基本组分的第二相之间的比率时，该烧结的Mn-Zn铁氧体能够构成在宽范围的高频段内可以有效吸收电磁波的电磁波吸收体，本发明已经实现。
为了得到上述目的，依据本发明的电磁波吸收体由包括如下组分的Mn-Zn铁氧体制成：含40.0-49.9摩尔％的Fe2O3，4.0-26.5摩尔％的ZnO，余下的由MnO组成的尖晶石主相；以及包含CaO作基本组分的第二相。在该铁氧体中，尖晶石主相的质量构成了尖晶石主相和第二相总质量的50.0-99.0％。
在用于本发明的电磁波吸收体的Mn-Zn铁氧体中，Fe2O3的含量为40.0-49.9摩尔％，低于50.0摩尔％的化学计量组成。在制造电磁波吸收体时，为抑制可造成软磁性破坏的Mn3+的生成，Mn-Zn铁氧体要在还原气氛下烧结。然而，如果在还原气氛中烧结的Mn-Zn铁氧体中含有的Fe2O3超过50.0摩尔％，超过50.0摩尔％的Fe2O3被还原成Fe2+，由此会降低该Mn-Zn铁氧体的电阻率。这是本发明中将Fe2O3的含量设在低于50.0摩尔％的原因，这样，甚至当Mn-Zn铁氧体在还原气氛中烧结时，在本发明的电磁波吸收体中也基本没有Fe2+存在。
由于如上所述，在本发明的电磁波吸收体中基本没有已知可破坏软磁性和电阻率的Mn3+，也基本没有已知可显著降低电阻率的Fe2+，所以可以同时获得高的电阻率和优异的软磁性。此时，优选的Mn3+含量以Mn2O3表示为0.8摩尔％或更低，优选的Fe2+的含量以FeO表示为0.2摩尔％或更低。
本发明的电磁波吸收体中包含4.0-26.5摩尔％的ZnO，太小的ZnO含量会引起初始磁导率的下降，太大的ZnO含量会引起饱和磁化强度和居里温度的降低。
除了尖晶石主相，本发明的电磁波吸收体还包含主要包括CaO的第二相。当加入到Mn-Zn铁氧体中时，CaO会在晶粒边界上偏析，由此使电阻率提高。然而，在传统的Mn-Zn铁氧体中，当加入0.20质量％或更多的CaO时，会产生严重破坏性能的反常的晶粒生长。另一方面，在本发明中，尖晶石主相含有低于50.0摩尔％的Fe2O3，微量的Mn2O3和FeO，甚至当加入的CaO达1.00质量％或更多时，也不会产生反常的晶粒生长。
由于CaO在Mn-Zn铁氧体的晶粒边界层上形成一个绝缘层，所以可以通过适当安排具有大磁导率的尖晶石主相与包含CaO的非磁性的第二相之间的混合比率来控制电磁波吸收体的磁导率。而且，可以通过适当安排具有大介电常数的尖晶石主相与具有小介电常数的第二相之间的混合比率来控制电磁波吸收体的介电常数。由于太大的CaO含量破坏软磁性，太小的CaO含量导致不能控制磁导率和介电常数，尖晶石主相和第二相之间的质量比必须设在99∶1-50∶50的范围内。
第二相中除了CaO，可以包含其它通常用作Mn-Zn铁氧体添加剂的组分。这些组分是，例如SiO2，V2O5，MoO3，ZrO2，Ta2O5，HfO2，Nb2O5，CuO以及其它类似物质，其能促进烧结作用并提高电阻。由于这些组分的含量太大会引起反常的晶粒生长，它们优选的含量为1.00质量％或更低的SiO2，0.20质量％或更低的V2O5，0.20质量％或更低的MoO3，0.20质量％或更低的ZrO2，0.20质量％或更低的Ta2O5，0.20质量％或更低的HfO2，Nb2O5以及12.0质量％或更低的CuO。
本发明的电磁波吸收体按如下制造。将作为尖晶石主相组分的Fe2O3，ZnO和MnO原材料粉末按预定混合比进行称量，混合，并在依据尖晶石主相的组成而恰当确定的800-1000℃的温度下煅烧，然后用例如通用球磨进行研磨。将可能需要的CaO和其它添加剂加入到经过如上处理的粉末中并混合，得到具有目标组分的混合物粉末。然后，依据通常的制造方法将粉末造粒，其中加入粘结剂，像聚乙烯醇，聚丙稀酰胺，甲基纤维素，聚环氧乙烷或者甘油，然后在如80MPa或更高的压力下将其压成具有预定形状的生坯。将生坯在1000-1300℃的温度下，在炉中于通过充入惰性气体如氮气来控制氧分压的气氛中烧结，并在相同的气氛中冷却。可供选择地，生坯可在空气中烧结。
附图简述
图1是本发明的电磁波吸收体样品和对比样品的吸收特性图，显示了作为频率函数的反射系数。
具体实施方式
将用本发明的具体实施例来对其进行阐释，但应该明白本发明并不局限于此。
<实施例>
按表1所示的组分制造5种不同的测试样品，其中包括2种对比样品。将原材料粉末Fe2O3，ZnO和MnO进行混合，用磨碎机搅拌，在空气中于850℃下煅烧2小时，用磨碎机研磨1小时，得到了混合粉末。然后，将表1中所示数量的CaO加入到表1中处理的粉末中，然后将这样得到的粉末用磨碎机搅拌1小时，加入聚乙烯醇造粒，在80MPa的压力下压成环形芯(生坯)。将生坯在炉中于1200℃下烧结2小时，烧结气氛通过往炉中充入氮气来进行控制，然后在相同的气氛下冷却，获得了本发明的样品1-3和对比样品1-2，每个外径7.0mm，内径3.0mm，高度10.0mm。
所有样品中的Mn2O3和FeO含量由滴定测量(使用用滴定法)，并在表1中列出。并用同轴管S-参数测量技术测量在不同频率下的磁导率和介电常数，由此计算评价电磁波吸收性能的反射系数，计算结果如图1中所示。
表1
样品号          尖晶石主相[摩尔％]   第二相[质量％](3)     滴定分析[摩尔％]
    Fe2O3 (1)     MnO(2)     ZnO     CaO     Mn2O3     FeO
对比样品1     47.0     40.0     13.0     0.0     0.1     0.1
发明样品1     47.0     40.0     13.0     1.0     0.1     0.1
发明样品2     47.0     40.0     13.0     25.0     0.1     0.1
发明样品3     47.0     40.0     13.0     50.0     0.1     0.1
对比样品2     47.0     40.0     13.0     60.0     0.1     0.1
注释：(1)Fe2O3指FeO和Fe2O3
(2)MnO指Mn2O3和MnO
(3)相对于尖晶石主相和第二相的总质量的比率
从图1可以看出，发明样品1和2在30-500MHz的频段内都有20dB或更大的反射系数，在低频段下可以起到电磁波吸收体的作用。发明样品3在高于500MHz的频段下具有20dB或更大的反射系数，这就证明，可单独使用该烧结铁氧体来制造在高频段(500-1000MHz)内适用的电磁波吸收体。另一方面，对比样品1的反射系数曲线具有一个尖峰，这样其仅能在一个有限的频段内用作电磁波吸收体，对比样品2中的磁性尖晶石主相的质量比太小，所以不能在整个频段内得到优异的无线电波吸收特性。

Claims (1)

1.一种由Mn-Zn铁氧体制成的电磁波吸收体，该Mn-Zn铁氧体包括：含40.0-49.9摩尔％的Fe2O3，4.0-26.5摩尔％的ZnO，和其余的MnO的尖晶石主相；以及包含CaO作基本组分的第二相，其中，尖晶石主相的质量构成了尖晶石主相和第二相总质量的50.0-99.0％。
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