热键是什么键巨介电常数材料CCTO的可变程跳跃电导研究
林鹏;黄海涛;叶茂;曾燮榕;柯善明
婆婆丁泡水喝的功效与作用
【摘 要】The ac conductivity and dielectric properties of CaCu3Ti4O12 (CCTO) ceramics were investigated in a temperature range of-120℃to 300℃and a frequency range of 1 Hz to 10 MHz. Two different conduction process, which can be well described by Mott’s variable-range-hopping (VRH) mechanism, were obrved in different temperature regions. The high temperature VRH conduction is related to the cond ionization of oxygen vacancy. The low temperature dielectric properties of CCTO could be described by the so-called universal dielectric respon (UDR) when a polaron relaxation is considered.%文章研究了巨介电常数材料CaCu3Ti4O12(CCTO)在宽温区(－120℃～300℃)及宽频域(1 Hz～10 MHz)的交流电导及介电性能。在低温区和高温区，CCTO表现出两种不同的导电过程，均可以由Mott提出的可变程跳跃电导机制(Variable-Range-Hopping，VRH)来描述。研究发现高温VRH过程与氧空位的二次离子化相关，而低温过程符合普适介电响应方程，其介电弛豫行为起源于极化子的弛豫。
【期刊名称】《集成技术》
【年(卷),期】2014(000)006
【总页数】6页(P8-13)
【关键词】CCTO;巨介电常数;可变程跳跃电导;极化子弛豫
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【作 者】林鹏;黄海涛;叶茂;曾燮榕;柯善明
【作者单位】深圳大学材料学院 深圳 518060; 深圳特种功能材料重点实验室 深圳 518060;香港理工大学应用物理系 香港 999077;深圳大学材料学院 深圳 518060; 深圳特种功能材料重点实验室 深圳 518060;深圳大学材料学院 深圳 518060; 深圳特种功能材料重点实验室 深圳 518060;深圳大学材料学院 深圳 518060; 深圳特种功能材料重点实验室 深圳 518060
【正文语种】中 文
【中图分类】TB34
CaCu3Ti4O12(CCTO) has been reported to have a perovskite structure and a colossal dielectric constant (CDC) in the order of 105, which is almost independent of temperature from 400 K to 100 K but drops dramatically to less than 102 below 100 K[1]. Since then a huge amount of work[1-5]has been accomplished in an attempt to understand the origin of the remarkable dielectric properties. Similar dielectric behavior has been obrved in chargedensity-wave (CDW) systems[6]. CDW materials are generally metals in low dimensions with a critical temperature, below which an insulating state could be obrved. CCTO is unlikely a CDW material, becau it is cubic and does not display any metallicity[2]. An internal barrier layer capacitance (IBLC) mechanism has been widely ud to explain the colossal dielectric constants[5]. In the IBLC picture, the insulating grain boundary layers between miconducting grains act as barrier layers which block the current flow.
However, Ramirez et al. argued that the Maxwell-Wagner (MW) type mechanism could not be solely responsible for the anomalous relaxation near 100 K in CCTO[2]. The CDC behavior has also been reported on a number of materials, such as A2FeBO6(A＝Ba, Sr,
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and Ca; B＝Nb, and Ta, etc.)[7], La1-xSrxMnO3[8], Pr0.7Ca0.3MnO3[9], TbMnO3[10], and Li/Ti doped NiO[11]. It indicates that this phenomenon may be governed by a unified mechanism of relaxational excitations. The anomalous low temperature relaxation in manganites has been attributed to localized hopping of polarons between lattice sites within a characteristic timescale[8,9]. Zhang and Tang[12]also found that the state of mixed valences of Ti ions in CCTO induces a bulk polaron conduction by variable range hopping (VRH) at low temperatures.
身份证挂失补办The complex frequency-dependent ac conductivity characterizes in depth of the charge transport behavior by hopping of localized charge carriers (such as polarons)[13,14]. In the prent letter, we report measurements of the complex ac conductivity of CCTO over a temperature range from —130℃to 300℃. Besides the low temperature polaronic conduction[12], a high temperature polaronic conduction behavior was also detected with a higher hopping energy. The low temperature dielectric properties of CCTO could be described by the socalled universal dielectric respon when a polaron relaxation is considered.
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Single pha CCTO ceramics were prepared through a conventional mixed oxide route and the detailed processing parameters can be found elwhere[4]. The single pha was confirmed by X-ray diffraction (XRD). Silver paint was coated on both surfaces of the sintered disks and fired at 650℃ for 20 minutes. The sample pellets are 12 mm in diameter and about 1 mm in thickness. The dielectric properties and ac conductivity were measured by using a frequencyrespon analyzer (Novocontrol Alpha-analyzer) over a broad frequency range (1 Hz—10 MHz) at different temperatures from —130℃ to 300℃.
Fig. 1 shows the frequency dependence of the conductivityat various temperatures. For theconductivityshown in Fig. 1(a), similar to an earlier report[12], there is a rapid increa at low frequencies and a slow increa at high frequencies. Thein the high frequency range can be described by the “universal dielectric respon” (UDR)[15]
whereis the dc bulk conductivity, f is the frequency and 0＜s＜1. Equation (1) is typical of thermally assisted tunneling between localized states. In CCTO, it has been attributed to the localized charge carriers[12]. It should be mentioned thatud here is not the measured dc conductivity of CCTO, but the extrapolated value at low frequency.
The steplike increa in Fig. 1(a) shifts to higher frequencies with increasing temperature. The localized charge carriers contribute to the conductivity by a hopping process. The frequency dependence of the conductivity in the hopping regime for only one hopping center has been given by Pollak[16], Where N is the number of charge carriers, E is the magnitude of the applied electric field, τ is the relaxation time related to the critical frequency. Equation (2) clearly predicts a steplike increa ofin Fig. 1(a). Such a steplike increa inis accompanied by a loss peak in the imaginary part of the permittivitythrough the Kramors-Kronig relationship and is also related to the steplike increa in the real part of the dielectric permittivity火腿炒蛋
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