钛酸铜钙型陶瓷的高介电物性和两类钙钛矿型无铅陶瓷的介电频谱的研究
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摘要
介电材料是一类利用材料的介电性质来制造电容性器件的电子材料,被广泛地应用在电容器、谐振器、滤波器、存储器等重要电子器件方面。随着近年电子器件向高性能化和尺寸微型化方向的发展,高介电材料受到越来越多的关注。在此背景下,本论文主要开展了钛酸铜钙(CaCu3Ti4O12,简称CCTO)及钛酸铜钙型(CCTO-type)高介电陶瓷材料的制备、物性及相关机理的研究,并对钛酸钡(BaTiO3)陶瓷材料和KNN基陶瓷材料的介电频谱进行了研究。
CCTO是一种具有钙钛矿型衍生结构的氧化物,不论是单晶形态还是陶瓷形态都呈现异常高的介电常数。室温下,其低频介电常数很大,基本上不依存于频率,而且在很宽的温度区域内几乎不随温度变化。为了解释CCTO陶瓷中所观察到的不同寻常的高介电行为,研究者们已经从内在机制和外在机制两个方面提出了多种可能的解释,但目前还存在着很大的争议。现在大家比较普遍接受的观点是CCTO陶瓷的高介电性质起源于内部阻挡层电容效应。从这一观点出发,氧缺陷、CuO第二相的析出以及Ti4+与Cu2+离子的变价对于形成内阻挡层电容效应起了至关重要的作用。基于内阻挡层电容效应的机制解释,高介电性质应该在化学组分及晶体结构与CCTO很类似的氧化物陶瓷中普遍存在,然而不可思议的是,文献报道只有少数的几种氧化物陶瓷具有同CCTO类似的高介电行为。因此,探索CCTO及CCTO型陶瓷材料的物理性质和相关机制,弄清上述问题是一个重要的课题。
钛酸钡铁电体的高介电性质发现于1943年。自此以后,BaTiO3基陶瓷材料被广泛地应用于制造各种电子元件。最初,BaTiO3陶瓷室温的介电常数约为1200～1500；1954年H. Kniekamp和W. Heywang首次报道晶粒尺寸为1μm左右的BaTiO3陶瓷的介电常数约为3000,揭开了研究BaTiO3陶瓷介电晶粒尺寸效应的序幕；R.J. Brandmayr等通过热压烧结制备出晶粒尺寸约为1μm的BaTiO3陶瓷,其室温下的介电常数高达6000左右,但是晶粒尺寸更小的BaTiO3陶瓷的介电常数会随着晶粒尺寸的减小剧烈下降。研究者们对BaTiO3陶瓷的介电晶粒尺寸效应做了很多的研究,并且认为由晶粒尺寸改变引起的90°电畴密度的变化对BaTiO3陶瓷的介电性质有极其重要的影响,但是由结构相变引起的电畴的变化对BaTiO3陶瓷介电频谱的影响的报道并不多见。
(K,Na) NbO3(简写为KNN)基陶瓷是近几年来受到广泛关注的一类无铅压电陶瓷材料。2004年,Y. Saito等研究者报道在多种KNN基陶瓷中发现了良好的压电性质,其中置构法制备的(K044Na0.52Li0.04)(Nbo.86Ta0.10Sb0.04)03陶瓷样品的d33值高达416pC/N。最近,本研究小组利用传统的固相反应方法成功制得了具有高压电活性的(K0.45Na0.55)098Li0.02(Nbo.77Ta0.18Sb0.05)03陶瓷样品,其d33值为413pC/N。通常,提高KNN基陶瓷的室温压电活性d33值是通过使用Li1+取代K’+或者Na1+、以及使用Ta5+或者Sb5+取代Nb5+等从而将正交-四方相变温度移至室温附近的方法实现的。其中,Sb5+取代对陶瓷压电活性的提高最为有效。KNN基无铅压电陶瓷的压电活性应该与其电畴结构有着很密切的关系。但是,目前对KNN基陶瓷的电畴结构的研究还很少,而对其与电畴振动相关的介电频谱(10MHz～10GHz)的研究则几乎没有。所以我们选取了(K0.5Na0..5)0.98Li0.02(N0.082-Ta018)03作为基本组分,用Sb5+离子部分取代Nb5+离子的方法加以修饰,来研究Sb5+取代Nb5+和相变对KNN基陶瓷的介电弛豫的影响。
本论文研究了传统固相反应法制备的CCTO、CCTO型、BaTiO3和KNN基陶瓷样品。考察了几种CCTO型陶瓷的介电性质以及烧结条件等对其微观结构、晶体结构、介电性质的影响,探讨了CCTO和CCTO型陶瓷的高介电性质的起因机制；考察了晶粒尺寸和相变对BaTiO3陶瓷的介电频谱的影响,以及Sb5+取代Nb5+对一些KNN基陶瓷的介电弛豫的影响,探讨了相关的物性微观机理。
一、利用传统的固相反应工艺制备了SrCu3Ti4O12、La2/3Cu3Ti4O12、Bi2/3Cu3Ti4O12和NaCu3Ti3SbO12陶瓷样品,并对它们的晶格结构、微观结构和介电性质进行了测量,以获得对在化学组成和晶格结构上与CaCu3Ti4O12非常类似的氧化物陶瓷的介电性质以及相关机制的系统理解。实验发现,恰当条件下制备的这些氧化物陶瓷样品中都观测到了异常大的介电常数,并且它们具有同之前文献报道的CCTO陶瓷高介电性质相类似的性质。室温下测量范围为40Hz～10MHz的介电频谱中,低频介电常数ε’高达104以上,1MHz附近呈现一个介电弛豫；高温下的介电频谱中,100kHz以下频率区间出现另外一个介点弛豫。特别是,利用XRD方法在这些陶瓷样品中都直接观测到了CuO第二相的析出。这些结果与之前对于CCTO陶瓷提出的其高介电性质起源于内阻挡层电容效应的机制解释是相符合的,并且证明高介电性质应该在化学组分及晶体结构与CCTO很类似的氧化物陶瓷中具有一定的普遍性。
二、通过对比被覆了不同种类的金属电极的CaCu3Ti4012、SrCu3Ti4012和NaCu3Ti3SbO12陶瓷样品的介电频谱,研究了电极对CCTO型陶瓷的高介电性质的影响。电极对比实验显示,室温下1MHz附近的介电弛豫不受金属电极的类型的影响,而100kHz以下的低频区间内的介电弛豫与金属电极的类型有着很大的关系。基于这个实验结果,我们推断CCTO及CCTO型陶瓷的这两个介电弛豫分别起源于由绝缘性的晶界和半导性的晶粒导致的内阻挡层电容效应以及由陶瓷样品和电极之间形成的肖特基势垒导致的耗尽层效应。
三、大气中利用热压烧结方法制备了高密度的CaCu3Ti4012陶瓷样品,系统地考察了它的微观结构、晶格结构和介电性质,并与常规烧结方法制备的陶瓷样品对比。常规烧结方法制备的陶瓷样品的相对理论密度普遍低于94.5%；而短时间热压烧结制备的陶瓷样品的相对理论密度就达到了98.3%,微观结构呈现晶粒尺寸双峰分布的特点。XRD结果显示热压烧结方法制备的陶瓷样品中含有少量的Cu20第二相,而经过退火处理的热压陶瓷样品则只含有CuO第二相。热压烧结方法制备的陶瓷样品和经过退火处理的热压陶瓷样品的室温介电频谱上(40Hz～100MHz)有两个介电弛豫,而常规烧结方法制备的陶瓷样品只有一个。并且热压烧结制备的CCTO陶瓷样品的低频介电常数高达2×105,经过退火处理的热压CCTO陶瓷样品的低频介电常数更是达到1×106。我们对微观结构和介电性质之间的关系进行了讨论。
四、研究了BaTiO3陶瓷样品和(K0.5Na05)0.98Lio.02(Nb0.82-yTa018Sby)03陶瓷样品的介电频谱及其随温度的变化。(1)着重考察了正交-四方相变对传统固相反应工艺制备的BaTiO3陶瓷样品的GHz附近的介电弛豫特征频率的影响。在我们的测试温度范围内,BaTiO3陶瓷样品的这个介电弛豫特征频率会随着测试温度的升高,先减小后增大,在正交-四方相变点达到最小值,并且降温过程中测得的这个介电弛豫特征频率要比升温过程中测得的数值高得多(高约250MHz)。(2)以传统固相反应工艺制备的(K0.5Na0.5)0.98Li002(Nb0.82-yTa0.18Sby)03(y=0、0.025和0.05)陶瓷样品为研究对象,考察了Sb5+取代Nb5+和相变对KNN基陶瓷GHz附近的介电弛豫特征频率的影响。实验结果显示,Sb5+取代Nb5+使陶瓷样品的介电弛豫特征频率明显地向低频方向移动,从几GHz降到几十MHz；y=0.025和0.05的陶瓷样品的介电弛豫特征频率会随着测试温度升高而增大,并且表现为热激活变化行为,正交相的激活能略大于四方相的激活能。与BaTi03陶瓷样品相比较,(K05Na0.5)0.98Li0.02(Nb0.82-yTa0.18Sby)03陶瓷样品在正交-四方相变点附近呈现出明显不同的介电弛豫特征频率随温度的变化行为。
Dielectric materials are widely used to fabricate capacitive devices such as capacitance, resonators and filters. High dielectric-permittivity materials have attracted considerable interest in recent years because they may offer an opportunity to enhance the performance and reduce the sizes of microelectronic devices. Under the circumstances, this thesis work concentrates mainly on the studies of material preparations, physical properties and the related mechanisms for CaCusTiO2and CaCu3Ti4O12-type ceramics, and the investigation on dielectric dispersion spectra of BaTiO3and a sort of KNN-based ceramics.
CCTO is the oxide that has a cubic perovskite-related crystal structure and exhibits an enormously large dielectric permittivity at low frequencies in both forms of single crystals and ceramics. The dielectric permittivity is almost a constant in the low frequency range below100kHz at room temperature and is nearly independent of temperature over a wide temperature region. In order to interpret the observed giant dielectric-permittivity (giant-ε') phenomenon for CCTO, several possible mechanisms have been proposed so far from both intrinsic and extrinsic viewpoints and still remain controversial. On the other hand, that giant-ε' phenomenon in CCTO ceramics originating from an internal barrier layer capacitance (IBLC) effect seems to be the most widely accepted explanation at present. In connection with it, oxygen vacancies, CuO segregation as well as aliovalences of Ti and Cu ions were suggested to contribute greatly to the IBLC formation. By this line of reasoning, giant-ε' phenomena are then anticipated to occur popularly in the large family of compositionally and structurally CCTO-like oxides. Nevertheless, only few members were reported to show similarly the giant-ε' behaviors besides CCTO, which is incomprehensible. Therefore, it is essential to quest the physical properties and interrelated mechanisms of CCTO and CCTO-like ceramics, and understand fully the various above issues.
The high dielectric properties of BaTiO3were discovered in1943. Since then, BaTiO3-based ceramics have been widely used in manufacturing various electronic components. Initially, the value reported for the dielectric permittivity of BaTiO3ceramics was only about1200～1500. In1954, H. Kniekamp and W. Heywang first found that the dielectric permittivity in BaTiO3ceramics with an average grain size of about1μm is over3000, marking the start of reserch on dielectric grain-size effect in BaTiO3ceramics. In the next decades, many scholars observed high dielectric permittivity of3500～4000in fine-grained BaTiO3ceramics (with the average grain size of～1μm); more surprisingly, R. J. Brandmayr obtained a hot-pressed BaTiO ceramic that has an average grain size is about1μm and the dielectric permittivity of about6000. However, the dielectric permittivity sharply decreases with further decreasing the grain size. A lot of work have been done on the dielectric grain size effect in BaTiO3ceramics, and it is considered that the density of90°-domain has the great influence on dielectric properties. However, no studies have been carried out about the influence of domain configuration change due to the tetragonal-orthohombic phase transition on their dielectric dispersion spectra.
As typical lead-free piezoelectric materials,(K,Na)NbO3-based (abbreviated as KNN-based) ceramics have attracted much attention in past few years. In2004, Saito et al. reported that excellent piezoelectric properties are found in some KNN-based ceramics, and the ceramic with chemical composition of (K0.44Na0.52Li0.04)(Nb0.86Ta0.10Sb0.04)O3prepared by a reactive template grain growth technique has a d33value of416pC/N. Recently, we have succeeded in obtaining high piezoelectric properties (K0.45Na0.55)0.98Li0.02(Nb0.77Ta0.18Sb0.05)O3ceramic with d33value of413pC/N through the conventional solid-state reaction method. It is consdiered that the enhancement of piezoelectric activity in KNN-based ceramics with the substitution of Li1+for K1+or Na1+and the substitution of Ta5+or Sb5+for Nb5+should be mainly ascribed to the shifting the tetragonal-orthohombic phase transition temperature to room temperature. Further, it has been found that the substitution of Sb5+for Nb5+is most effective in the enhancing the piezoelectric activity for KNN-based ceramics. On the other hand, domain structure should have also played some important roles in the observed high piezoelectric activity in KNN-based ceramics, studies on domain structure are still few. In particular, systematical studies on domain-related dielectric relaxation have scarcely been reported. In this work, dielectric spectra of (K0.5Na0.5)0.98Li0.02(Nb0.82-yTa0.18Sby)O3ceramics were thus systematically investigated.
This thesis treats the CCTO, CCTO-type, BaTiO3and some KNN-based ceramics prepared by the conventional solid-state reaction as the study objects. For CCTO-type ceramics, dielectric properties as well as the effects of sintering condition on their microstructure, crystalline structure, dielectric and electric properties are investigated, and the related high-dielectric mechanisms in CCTO and CCTO-type ceramics are discussed. For the BaTiO3ceramics, attention was paid to the influences of the grain size and the tetragonal-orthohombic phase transition on the dielectric spectra. For the (K0.5Na0.5)0.98Li0.02(Nb0.82-yTa0.18Sby)O3ceramics, attention was focused on the influences of the Sb-substitution and and the tetragonal-orthohombic phase transition on the dielectric spectra.
1. SrCu3Ti4O12, La2/3Cu3Ti4O12, Bi2/3Cu3Ti4O12and NaCu3Ti3SbO12ceramics were investigated to obtain a systematical understanding about the dielectric behaviors of the compositionally and structurally CaCu3Ti4O12-like oxides and the underlying related mechanism. In opposite to the literature results, giant dielectric-permittivity phenomena with low-frequency ε' larger than104have been observed in all of these ceramics. Within the measuring frequency range of40Hz～100MHz, a single dielectric relaxation with the characteristic frequency around1MHz is seen at room temperature an additional one in low frequency region is also observed at high temperatures. Furthermore, the existence of CuO secondary phase has been confirmed in them. In general, the results can be explained by the similar mechanism of internal barrier layer capacitance effect formerly proposed for CaCu3Ti4O12ceramics and indicate that the giant-ε' phenomena should be quite common in the large family of compositionally and structurally CaCu3Ti4O12-like oxide ceramics.
2. By comparing the dielectric dispersion spectra of CaCu3TiO4, SrCu3Ti4Oi2, and NaCu3Ti3SbOi2ceramics coated with different metal electrodes, the electrode influence on the giant dielectric-permittivity properties in CCTO-type ceramics was studied. Within the measuring frequency range of40Hz-100MHz, a single dielectric relaxation with the characteristic frequency around1MHz is seen at room temperature or below whereas an additional one in low frequency region is also observed at high temperatures. On the basis of the experimental results, the two dielectric relaxations are ascribed to an internal barrier layer capacitance effect and an electrode polarization effect, respectively.
3. An attempt to preparing high-density CaCu3Ti4O12ceramics was made by the hot-press sintering technique under ambient atmosphere. Microstructure, crystalline structure and dielectric property of the obtained CaCu3Ti4O12ceramics were investigated and compared with those of the conventionally sintered CaCu3Ti4O12ceramics. While the conventionally sintered ceramics show generally their theoretical relative density values lower than94.5%, the hot-pressed one with a short sintering duration reaches the theoretical relative density value of98.3%and exhibits the microstructure of bimodal grain distribution. X-ray diffraction analysis showed that CU2O minor phase exists in the hot-pressed ceramic and only CuO secondary phase is present after a further thermal treatment in ambience. Both the hot-pressed ceramics and its further thermally treated one exhibit two dielectric relaxations at room temperature, whereas the conventionally sintered ceramics have only one dielectric relaxation. Moreover, the hot-pressed CCTO ceramics has an extremely large low-frequency permittivity of2×105, which further increases to nearly1×106through the ambient thermal treatment. The relation between microstructure and dielectric property is discussed.
4. The dielectric dispersion spectra and their evolution with temperature in BaTiO3and (K0.5Na0.5)0.98Li0.02(Nb0.82-yTa0.18Sby)O3ceramics were studied.(1) for BaTiO3ceramics, the influence of tetragonal-orthohombic phase transition on dielectric dispersion spectra was examined On the cooling process from a high temeprarure, the characteristic relaxational frequency of the dielectric relaxation related to the motion of domain walls decreases gradually at first with decreasing the testing temperature, drops drastically to a minimum in the vicinity of To-T and increases conversely with further decreasing the testing temperature. Besides, difference in the characteristic relaxational frequency is observed between the heating process and the cooling process. In particular, the values measured during the cooling process are signicantly higher than those measured during the heating process in the tetragonal phase, about250MHz.(2) For (K0.5Na0.5)0.98Li0.02(Nb0.82-yTa0.18Sby)O3ceramics, the influences of Sb5+-substitution for Nb5+ions and tetragonal-orthohombic phase transition on dielectric dispersion spectra were investigated. The result shows that the Sb5+-substitution decreases largely the characteristic relaxational frequency of the dielectric relaxation from several GHz to several tens of MHz at room temperature. Furthermore, the characteristic frequency increases monotonically with temperature and shows the thermally-activated behavior with slightly larger activation energy values in orthohombic phase than those in tetragonal phase. Thus, distinctly different behaviors in the changes of dielectric characteristic relaxational frequency with temperature have been observed between (K0.5Na0.5)0.98Li0.02(Nb0.82-yTa0.18Sby)O3ceramics and BaTiO3ceramics around TO-T.
CCTO是一种具有钙钛矿型衍生结构的氧化物,不论是单晶形态还是陶瓷形态都呈现异常高的介电常数。室温下,其低频介电常数很大,基本上不依存于频率,而且在很宽的温度区域内几乎不随温度变化。为了解释CCTO陶瓷中所观察到的不同寻常的高介电行为,研究者们已经从内在机制和外在机制两个方面提出了多种可能的解释,但目前还存在着很大的争议。现在大家比较普遍接受的观点是CCTO陶瓷的高介电性质起源于内部阻挡层电容效应。从这一观点出发,氧缺陷、CuO第二相的析出以及Ti4+与Cu2+离子的变价对于形成内阻挡层电容效应起了至关重要的作用。基于内阻挡层电容效应的机制解释,高介电性质应该在化学组分及晶体结构与CCTO很类似的氧化物陶瓷中普遍存在,然而不可思议的是,文献报道只有少数的几种氧化物陶瓷具有同CCTO类似的高介电行为。因此,探索CCTO及CCTO型陶瓷材料的物理性质和相关机制,弄清上述问题是一个重要的课题。
钛酸钡铁电体的高介电性质发现于1943年。自此以后,BaTiO3基陶瓷材料被广泛地应用于制造各种电子元件。最初,BaTiO3陶瓷室温的介电常数约为1200～1500；1954年H. Kniekamp和W. Heywang首次报道晶粒尺寸为1μm左右的BaTiO3陶瓷的介电常数约为3000,揭开了研究BaTiO3陶瓷介电晶粒尺寸效应的序幕；R.J. Brandmayr等通过热压烧结制备出晶粒尺寸约为1μm的BaTiO3陶瓷,其室温下的介电常数高达6000左右,但是晶粒尺寸更小的BaTiO3陶瓷的介电常数会随着晶粒尺寸的减小剧烈下降。研究者们对BaTiO3陶瓷的介电晶粒尺寸效应做了很多的研究,并且认为由晶粒尺寸改变引起的90°电畴密度的变化对BaTiO3陶瓷的介电性质有极其重要的影响,但是由结构相变引起的电畴的变化对BaTiO3陶瓷介电频谱的影响的报道并不多见。
(K,Na) NbO3(简写为KNN)基陶瓷是近几年来受到广泛关注的一类无铅压电陶瓷材料。2004年,Y. Saito等研究者报道在多种KNN基陶瓷中发现了良好的压电性质,其中置构法制备的(K044Na0.52Li0.04)(Nbo.86Ta0.10Sb0.04)03陶瓷样品的d33值高达416pC/N。最近,本研究小组利用传统的固相反应方法成功制得了具有高压电活性的(K0.45Na0.55)098Li0.02(Nbo.77Ta0.18Sb0.05)03陶瓷样品,其d33值为413pC/N。通常,提高KNN基陶瓷的室温压电活性d33值是通过使用Li1+取代K’+或者Na1+、以及使用Ta5+或者Sb5+取代Nb5+等从而将正交-四方相变温度移至室温附近的方法实现的。其中,Sb5+取代对陶瓷压电活性的提高最为有效。KNN基无铅压电陶瓷的压电活性应该与其电畴结构有着很密切的关系。但是,目前对KNN基陶瓷的电畴结构的研究还很少,而对其与电畴振动相关的介电频谱(10MHz～10GHz)的研究则几乎没有。所以我们选取了(K0.5Na0..5)0.98Li0.02(N0.082-Ta018)03作为基本组分,用Sb5+离子部分取代Nb5+离子的方法加以修饰,来研究Sb5+取代Nb5+和相变对KNN基陶瓷的介电弛豫的影响。
本论文研究了传统固相反应法制备的CCTO、CCTO型、BaTiO3和KNN基陶瓷样品。考察了几种CCTO型陶瓷的介电性质以及烧结条件等对其微观结构、晶体结构、介电性质的影响,探讨了CCTO和CCTO型陶瓷的高介电性质的起因机制；考察了晶粒尺寸和相变对BaTiO3陶瓷的介电频谱的影响,以及Sb5+取代Nb5+对一些KNN基陶瓷的介电弛豫的影响,探讨了相关的物性微观机理。
一、利用传统的固相反应工艺制备了SrCu3Ti4O12、La2/3Cu3Ti4O12、Bi2/3Cu3Ti4O12和NaCu3Ti3SbO12陶瓷样品,并对它们的晶格结构、微观结构和介电性质进行了测量,以获得对在化学组成和晶格结构上与CaCu3Ti4O12非常类似的氧化物陶瓷的介电性质以及相关机制的系统理解。实验发现,恰当条件下制备的这些氧化物陶瓷样品中都观测到了异常大的介电常数,并且它们具有同之前文献报道的CCTO陶瓷高介电性质相类似的性质。室温下测量范围为40Hz～10MHz的介电频谱中,低频介电常数ε’高达104以上,1MHz附近呈现一个介电弛豫；高温下的介电频谱中,100kHz以下频率区间出现另外一个介点弛豫。特别是,利用XRD方法在这些陶瓷样品中都直接观测到了CuO第二相的析出。这些结果与之前对于CCTO陶瓷提出的其高介电性质起源于内阻挡层电容效应的机制解释是相符合的,并且证明高介电性质应该在化学组分及晶体结构与CCTO很类似的氧化物陶瓷中具有一定的普遍性。
二、通过对比被覆了不同种类的金属电极的CaCu3Ti4012、SrCu3Ti4012和NaCu3Ti3SbO12陶瓷样品的介电频谱,研究了电极对CCTO型陶瓷的高介电性质的影响。电极对比实验显示,室温下1MHz附近的介电弛豫不受金属电极的类型的影响,而100kHz以下的低频区间内的介电弛豫与金属电极的类型有着很大的关系。基于这个实验结果,我们推断CCTO及CCTO型陶瓷的这两个介电弛豫分别起源于由绝缘性的晶界和半导性的晶粒导致的内阻挡层电容效应以及由陶瓷样品和电极之间形成的肖特基势垒导致的耗尽层效应。
三、大气中利用热压烧结方法制备了高密度的CaCu3Ti4012陶瓷样品,系统地考察了它的微观结构、晶格结构和介电性质,并与常规烧结方法制备的陶瓷样品对比。常规烧结方法制备的陶瓷样品的相对理论密度普遍低于94.5%；而短时间热压烧结制备的陶瓷样品的相对理论密度就达到了98.3%,微观结构呈现晶粒尺寸双峰分布的特点。XRD结果显示热压烧结方法制备的陶瓷样品中含有少量的Cu20第二相,而经过退火处理的热压陶瓷样品则只含有CuO第二相。热压烧结方法制备的陶瓷样品和经过退火处理的热压陶瓷样品的室温介电频谱上(40Hz～100MHz)有两个介电弛豫,而常规烧结方法制备的陶瓷样品只有一个。并且热压烧结制备的CCTO陶瓷样品的低频介电常数高达2×105,经过退火处理的热压CCTO陶瓷样品的低频介电常数更是达到1×106。我们对微观结构和介电性质之间的关系进行了讨论。
四、研究了BaTiO3陶瓷样品和(K0.5Na05)0.98Lio.02(Nb0.82-yTa018Sby)03陶瓷样品的介电频谱及其随温度的变化。(1)着重考察了正交-四方相变对传统固相反应工艺制备的BaTiO3陶瓷样品的GHz附近的介电弛豫特征频率的影响。在我们的测试温度范围内,BaTiO3陶瓷样品的这个介电弛豫特征频率会随着测试温度的升高,先减小后增大,在正交-四方相变点达到最小值,并且降温过程中测得的这个介电弛豫特征频率要比升温过程中测得的数值高得多(高约250MHz)。(2)以传统固相反应工艺制备的(K0.5Na0.5)0.98Li002(Nb0.82-yTa0.18Sby)03(y=0、0.025和0.05)陶瓷样品为研究对象,考察了Sb5+取代Nb5+和相变对KNN基陶瓷GHz附近的介电弛豫特征频率的影响。实验结果显示,Sb5+取代Nb5+使陶瓷样品的介电弛豫特征频率明显地向低频方向移动,从几GHz降到几十MHz；y=0.025和0.05的陶瓷样品的介电弛豫特征频率会随着测试温度升高而增大,并且表现为热激活变化行为,正交相的激活能略大于四方相的激活能。与BaTi03陶瓷样品相比较,(K05Na0.5)0.98Li0.02(Nb0.82-yTa0.18Sby)03陶瓷样品在正交-四方相变点附近呈现出明显不同的介电弛豫特征频率随温度的变化行为。
Dielectric materials are widely used to fabricate capacitive devices such as capacitance, resonators and filters. High dielectric-permittivity materials have attracted considerable interest in recent years because they may offer an opportunity to enhance the performance and reduce the sizes of microelectronic devices. Under the circumstances, this thesis work concentrates mainly on the studies of material preparations, physical properties and the related mechanisms for CaCusTiO2and CaCu3Ti4O12-type ceramics, and the investigation on dielectric dispersion spectra of BaTiO3and a sort of KNN-based ceramics.
CCTO is the oxide that has a cubic perovskite-related crystal structure and exhibits an enormously large dielectric permittivity at low frequencies in both forms of single crystals and ceramics. The dielectric permittivity is almost a constant in the low frequency range below100kHz at room temperature and is nearly independent of temperature over a wide temperature region. In order to interpret the observed giant dielectric-permittivity (giant-ε') phenomenon for CCTO, several possible mechanisms have been proposed so far from both intrinsic and extrinsic viewpoints and still remain controversial. On the other hand, that giant-ε' phenomenon in CCTO ceramics originating from an internal barrier layer capacitance (IBLC) effect seems to be the most widely accepted explanation at present. In connection with it, oxygen vacancies, CuO segregation as well as aliovalences of Ti and Cu ions were suggested to contribute greatly to the IBLC formation. By this line of reasoning, giant-ε' phenomena are then anticipated to occur popularly in the large family of compositionally and structurally CCTO-like oxides. Nevertheless, only few members were reported to show similarly the giant-ε' behaviors besides CCTO, which is incomprehensible. Therefore, it is essential to quest the physical properties and interrelated mechanisms of CCTO and CCTO-like ceramics, and understand fully the various above issues.
The high dielectric properties of BaTiO3were discovered in1943. Since then, BaTiO3-based ceramics have been widely used in manufacturing various electronic components. Initially, the value reported for the dielectric permittivity of BaTiO3ceramics was only about1200～1500. In1954, H. Kniekamp and W. Heywang first found that the dielectric permittivity in BaTiO3ceramics with an average grain size of about1μm is over3000, marking the start of reserch on dielectric grain-size effect in BaTiO3ceramics. In the next decades, many scholars observed high dielectric permittivity of3500～4000in fine-grained BaTiO3ceramics (with the average grain size of～1μm); more surprisingly, R. J. Brandmayr obtained a hot-pressed BaTiO ceramic that has an average grain size is about1μm and the dielectric permittivity of about6000. However, the dielectric permittivity sharply decreases with further decreasing the grain size. A lot of work have been done on the dielectric grain size effect in BaTiO3ceramics, and it is considered that the density of90°-domain has the great influence on dielectric properties. However, no studies have been carried out about the influence of domain configuration change due to the tetragonal-orthohombic phase transition on their dielectric dispersion spectra.
As typical lead-free piezoelectric materials,(K,Na)NbO3-based (abbreviated as KNN-based) ceramics have attracted much attention in past few years. In2004, Saito et al. reported that excellent piezoelectric properties are found in some KNN-based ceramics, and the ceramic with chemical composition of (K0.44Na0.52Li0.04)(Nb0.86Ta0.10Sb0.04)O3prepared by a reactive template grain growth technique has a d33value of416pC/N. Recently, we have succeeded in obtaining high piezoelectric properties (K0.45Na0.55)0.98Li0.02(Nb0.77Ta0.18Sb0.05)O3ceramic with d33value of413pC/N through the conventional solid-state reaction method. It is consdiered that the enhancement of piezoelectric activity in KNN-based ceramics with the substitution of Li1+for K1+or Na1+and the substitution of Ta5+or Sb5+for Nb5+should be mainly ascribed to the shifting the tetragonal-orthohombic phase transition temperature to room temperature. Further, it has been found that the substitution of Sb5+for Nb5+is most effective in the enhancing the piezoelectric activity for KNN-based ceramics. On the other hand, domain structure should have also played some important roles in the observed high piezoelectric activity in KNN-based ceramics, studies on domain structure are still few. In particular, systematical studies on domain-related dielectric relaxation have scarcely been reported. In this work, dielectric spectra of (K0.5Na0.5)0.98Li0.02(Nb0.82-yTa0.18Sby)O3ceramics were thus systematically investigated.
This thesis treats the CCTO, CCTO-type, BaTiO3and some KNN-based ceramics prepared by the conventional solid-state reaction as the study objects. For CCTO-type ceramics, dielectric properties as well as the effects of sintering condition on their microstructure, crystalline structure, dielectric and electric properties are investigated, and the related high-dielectric mechanisms in CCTO and CCTO-type ceramics are discussed. For the BaTiO3ceramics, attention was paid to the influences of the grain size and the tetragonal-orthohombic phase transition on the dielectric spectra. For the (K0.5Na0.5)0.98Li0.02(Nb0.82-yTa0.18Sby)O3ceramics, attention was focused on the influences of the Sb-substitution and and the tetragonal-orthohombic phase transition on the dielectric spectra.
1. SrCu3Ti4O12, La2/3Cu3Ti4O12, Bi2/3Cu3Ti4O12and NaCu3Ti3SbO12ceramics were investigated to obtain a systematical understanding about the dielectric behaviors of the compositionally and structurally CaCu3Ti4O12-like oxides and the underlying related mechanism. In opposite to the literature results, giant dielectric-permittivity phenomena with low-frequency ε' larger than104have been observed in all of these ceramics. Within the measuring frequency range of40Hz～100MHz, a single dielectric relaxation with the characteristic frequency around1MHz is seen at room temperature an additional one in low frequency region is also observed at high temperatures. Furthermore, the existence of CuO secondary phase has been confirmed in them. In general, the results can be explained by the similar mechanism of internal barrier layer capacitance effect formerly proposed for CaCu3Ti4O12ceramics and indicate that the giant-ε' phenomena should be quite common in the large family of compositionally and structurally CaCu3Ti4O12-like oxide ceramics.
2. By comparing the dielectric dispersion spectra of CaCu3TiO4, SrCu3Ti4Oi2, and NaCu3Ti3SbOi2ceramics coated with different metal electrodes, the electrode influence on the giant dielectric-permittivity properties in CCTO-type ceramics was studied. Within the measuring frequency range of40Hz-100MHz, a single dielectric relaxation with the characteristic frequency around1MHz is seen at room temperature or below whereas an additional one in low frequency region is also observed at high temperatures. On the basis of the experimental results, the two dielectric relaxations are ascribed to an internal barrier layer capacitance effect and an electrode polarization effect, respectively.
3. An attempt to preparing high-density CaCu3Ti4O12ceramics was made by the hot-press sintering technique under ambient atmosphere. Microstructure, crystalline structure and dielectric property of the obtained CaCu3Ti4O12ceramics were investigated and compared with those of the conventionally sintered CaCu3Ti4O12ceramics. While the conventionally sintered ceramics show generally their theoretical relative density values lower than94.5%, the hot-pressed one with a short sintering duration reaches the theoretical relative density value of98.3%and exhibits the microstructure of bimodal grain distribution. X-ray diffraction analysis showed that CU2O minor phase exists in the hot-pressed ceramic and only CuO secondary phase is present after a further thermal treatment in ambience. Both the hot-pressed ceramics and its further thermally treated one exhibit two dielectric relaxations at room temperature, whereas the conventionally sintered ceramics have only one dielectric relaxation. Moreover, the hot-pressed CCTO ceramics has an extremely large low-frequency permittivity of2×105, which further increases to nearly1×106through the ambient thermal treatment. The relation between microstructure and dielectric property is discussed.
4. The dielectric dispersion spectra and their evolution with temperature in BaTiO3and (K0.5Na0.5)0.98Li0.02(Nb0.82-yTa0.18Sby)O3ceramics were studied.(1) for BaTiO3ceramics, the influence of tetragonal-orthohombic phase transition on dielectric dispersion spectra was examined On the cooling process from a high temeprarure, the characteristic relaxational frequency of the dielectric relaxation related to the motion of domain walls decreases gradually at first with decreasing the testing temperature, drops drastically to a minimum in the vicinity of To-T and increases conversely with further decreasing the testing temperature. Besides, difference in the characteristic relaxational frequency is observed between the heating process and the cooling process. In particular, the values measured during the cooling process are signicantly higher than those measured during the heating process in the tetragonal phase, about250MHz.(2) For (K0.5Na0.5)0.98Li0.02(Nb0.82-yTa0.18Sby)O3ceramics, the influences of Sb5+-substitution for Nb5+ions and tetragonal-orthohombic phase transition on dielectric dispersion spectra were investigated. The result shows that the Sb5+-substitution decreases largely the characteristic relaxational frequency of the dielectric relaxation from several GHz to several tens of MHz at room temperature. Furthermore, the characteristic frequency increases monotonically with temperature and shows the thermally-activated behavior with slightly larger activation energy values in orthohombic phase than those in tetragonal phase. Thus, distinctly different behaviors in the changes of dielectric characteristic relaxational frequency with temperature have been observed between (K0.5Na0.5)0.98Li0.02(Nb0.82-yTa0.18Sby)O3ceramics and BaTiO3ceramics around TO-T.
引文
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