High Breakdown Field Cacu3ti4o12 Ceramics: Roles of the Secondary Phase and of Sr Doping

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Article High Breakdown Field CaCu3Ti4O12 Ceramics: Roles of the Secondary Phase and of Sr Doping

Zhuang Tang, Kangning Wu, Yuwei Huang and Jianying Li * State Key Laboratory of Electrical Insulation and Power Equipment, Xi’an Jiaotong University, Xi’an 710049, China; [email protected] (Z.T.); [email protected] (K.W.); [email protected] (Y.H.) * Correspondence: [email protected]; Tel.: +86-29-8266-5703

Academic Editor: Issouf Fofana Received: 30 March 2017; Accepted: 16 May 2017; Published: 19 July 2017

Abstract: In this work, two methods of CaCu3Ti4O12-CuAl2O4 composite and SrCu3Ti4O12-CaCu3Ti4O12 composite were prepared to improve the breakdown field in CaCu3Ti4O12 ceramics. CaCu3Ti4O12-0.5CuAl2O4 and 0.4SrCu3Ti4O12-0.6CaCu3Ti4O12 samples with proper sintering conditions were found to have greatly enhanced breakdown fields of more than 20 kV·cm−1 −1 compared to the ordinary value of 1–2 kV·cm in CaCu3Ti4O12 ceramics. In addition, reduced dielectric loss tangent of these samples remained about 0.1 at a low frequency of 0.1 Hz, indicating superior dielectric properties. No abnormal grain growth was found in either composite with a high breakdown field, which was attributed to the pining effect and consumption of Cu-rich phase at grain boundaries. Under analysis of the relaxation process by electric modulus, compared to ··· conventional CaCu3Ti4O12 ceramics, interstitial Ali and increasing interfaces were responsible for variation in activation energy in CaCu3Ti4O12-0.5CuAl2O4 composites, while the integrated action of a strong solid solution effect and weak Sr-stretching effect contributed to the elevated potential barrier height and enhanced breakdown field in 0.4SrCu3Ti4O12-0.6CaCu3Ti4O12 composites.

Keywords: CCTO-based ceramics; breakdown ﬁeld; pining effect; oxygen vacancy

1. Introduction

CaCu3Ti4O12 (CCTO) ceramics has been extensively studied due to its colossal dielectric constant (CDC) up to 105, which is frequency independent in 102–106 Hz and exhibits excellent temperature stability over 100–600 K [1]. In addition to CDC, non-ohmic characteristic has also been found in CCTO ceramics [2], which is promising in applications of energy storage capacitors and over-voltage protection devices. Both the CDC property and non-ohmic characteristics are associated with defect-related double Schottky barrier between the semiconducting grains and insulating grain boundaries [3–5]. The Schottky barrier blocks charge transport at grain boundaries, resulting in high nonlinear coefﬁcient [2] and active dipole movement for CDC property in an internal barrier layer capacitor (IBLC) model [6]. It has been accepted that electrical property of CCTO ceramics is sensitive to the preparing method and the material composition [7,8]. Particularly, changes in the compositions to fabricate CCTO-based composite ceramics can notably affect the grain boundary structure and thus the dielectric performance [9,10]. In situ synthesis is an effective way to introduce homogeneous distribution of secondary phase. The phase equilibrium relations in the Cu2O/CuO/Al2O3 [11] system showed that, ◦ above 900 C, the spinel phase of CuAl2O4 can be easily synthesized. Since the sintering temperature ◦ of CCTO ceramics is generally higher than 1000 C, it can be expected that the spinel phase of CuAl2O4, which is unlike the perovskite-like phase in previously reported literature, can be easily synthesized in situ via the addition of Al2O3. The spinel phase is generally of high resistance, and the consumption

Energies 2017, 10, 1031; doi:10.3390/en10071031 www.mdpi.com/journal/energies Energies 2017, 10, 1031 2 of 10

of Cu-rich liquid phase by CuAl2O4 can help to control the grain growth. Meanwhile, composite ceramics with CCTO and other materials are also studied to improve the non-ohmic properties. A composite system of CCTO/CaTiO3 (CCTO/CTO) has been reported by Kobayashi and Terasaki to have achieved a greatly reduced tan δ as well as an enhanced nonlinear coefficient and a breakdown electric field [10,12]. CCTO/Y2/3Cu3Ti4O12 composite ceramics have been developed as increased nonlinear coefficients with low dielectric loss, obtained [13] in our previous work. In most research, the majority of breakdown fields for CCTO ceramics are less than −1 0 2 2 kV·cm [14,15]. According to the formula of stored energy U = (ε Eb )/2, breakdown fields strongly affect the maximum stored energy. In our previous work, the breakdown voltage of CCTO could be enhanced to 13 kV·cm−1 with a dielectric constant of more than 1000 by direct introduction of secondary CuAl2O4 phase [16]. The improvement of the breakdown field is focused in this work by two different methods: CaCu3Ti4O12-CuAl2O4 composite ceramics by in situ synthesis and SrCu3Ti4O12-CaCu3Ti4O12 composite ceramics, respectively. Phase composition, microstructure, nonlinearity, and breakdown fields are investigated in this work. In addition, based on the analysis of the dielectric relaxation process, the reasons for the enhanced breakdown fields are discussed related to the potential barrier at the grain boundary.

2. Materials and Methods

The 0.4SrCu3Ti4O12-0.6CaCu3Ti4O12 ceramics were prepared through a conventional solid-state reaction method with the raw material of high-purity (>99%) SrCO3, CaCO3, CuO, and TiO2. Initially, powders of SrCu3Ti4O12 and CaCu3Ti4O12 were separately prepared. Stoichiometric amounts of these raw materials were mixed by ball milling in ethanol for 8 h. The dried mixture powders were calcined ◦ in air at 950 C for 15 h. Then, the calcined powders including both SrCu3Ti4O12 and CaCu3Ti4O12 were mixed together according to an Sr/Ca ratio of 4:6. Mixed powders were ground in ethanol for 8 h and sieved, and 3 wt % polyvinyl alcohol was added as a binder. Next, the presintered powders were pressed to green pellets with a 12 mm diameter and a 1.2 mm thickness under 100 MPa. Finally, the pellets were sintered in air at 1000 ◦C for 5 h and 15 h, respectively, and furnace cooled to room temperature. For in situ CuAl2O4 phase in CCTO ceramics, CCTO powder was prepared through a solid-state reaction method. After CCTO powder was suspended in an Al(NO3)3 aqueous solution, the suspension was quantitatively titrated with NH4OH to obtain Al(OH)3 precipitation on the surface of CCTO ◦ particles. The obtained powder was dried and calcined at 950 C for 4 h to dehydrate Al(OH)3 into Al2O3. Then, powder was pressed into pellets with a diameter of 12 mm and a thickness of 1.6 mm. Finally, presintered powder was sintered in air at 1100 ◦C for 4 h. The corresponding chemical reactions are as follows:

2Al(OH)3 = Al2O3 + 3H2O ↑ (1)

CuO + Al2O3 = CuAl2O4 (2) 1 Cu O + 2Al O + O = 2CuAl O . (3) 2 2 3 2 2 2 4 Phase compositions of sintered ceramics were characterized by X-ray diffraction (XRD, Regaku D/MAX IIIB, Regaku, Tokyo, Japan) in a 2θ scanning from 15 to 80◦. Microstructures and element distributions of the samples were examined by ﬁeld emission scanning electron microscopy (FESEM, JEOL JSM-7800F, JEOL, Tokyo, Japan). For measurement of electrical properties, both sides of sintered samples were polished and sputtered with gold (KYKY SBC-128, KYKY Technology, Beijing, China). I-V behavior was measured by HP 34401A multimeter with a WJ10001D precision linear high-voltage direct current (dc) power. Measurement of dielectric performance was carried out by broadband dielectric spectrometer (Concept 80, Novocontrol Technologies, Montabaur, Germany) in frequency range of 10−1–107 Hz. Energies 2017, 10, 1031 3 of 10

Energies 2017, 10, 1031 3 of 10

3. Results3. Results and and Discussion Discussion

XRDXRD patterns patterns of of undoped undoped CCTOCCTO ceramics,ceramics, CaCuCaCu3TiTi44OO1212-0.5CuAl-0.5CuAl2O42 Ocomposite4 composite and and 0.4CaCu0.4CaCu3Ti43OTi124O-0.6CaCu12-0.6 CaCu3Ti3Ti4O4O1212 compositecomposite is is shown shown in in Figure Figure 1.1 .Obviously, Obviously, all allof ofthe the samples samples exhibitexhibit one one main main perovskite perovskite phase, phase, which which isis cubiccubic perovskite-related-structure (space (space group group Im3) Im3) CCTO,CCTO, by comparisonby comparison with with the standard the standard Powder Powder Diffraction Diffraction File Database File Database [JCPDS-05-0566 [JCPDS-05-0566 #75-2188]. #75-2188]. Besides, the in-situ formed phase of spinel CuAl2O4 can be detected in Besides, the in-situ formed phase of spinel CuAl2O4 can be detected in CaCu3Ti4O12-0.5CuAl2O4 samples.CaCu However,3Ti4O12-0.5CuAl in 0.4SCTO-0.6CCTO2O4 samples. However, composites, in 0.4SCTO-0.6CCTO no second phasecomposites, is detected. no second Since phase the radiusis detected. Since the radius of Sr2+ is larger than that of Ca2+, the main peak of XRD patterns moves of Sr2+ is larger than that of Ca2+, the main peak of XRD patterns moves towards lower angles in towards lower angles in 0.4SCTO-0.6CCTO composites, which could also be verified in the inset of 0.4SCTO-0.6CCTO composites, which could also be veriﬁed in the inset of Figure1. Figure 1.

# # CCTO

* CuAl O

(a.u.) 2 4 I

34.2 34.4 34.6 # #

0.4SCTO-0.6CCTO # # # # #

CCTO-0.5CuAl O 2 4 * Intensity (a.u.) Intensity * * * CCTO

20 30 40 50 60 70 80 θ o 2 ( )

Figure 1. XRD patterns of undoped CCTO ceramics, the CaCu3Ti4O12-0.5CuAl2O4 composite, and the Figure 1. XRD patterns of undoped CCTO ceramics, the CaCu3Ti4O12-0.5CuAl2O4 composite, and 0.4 SrCu3Ti4O12-0.6 CaCu3Ti4O12 composite. Inset is the magnification of main peak for CCTO and 0.4 the 0.4SrCu3Ti4O12-0.6CaCu3Ti4O12 composite. Inset is the magniﬁcation of main peak for CCTO and SrCu3Ti4O12-0.6 CaCu3Ti4O12. 0.4SrCu3Ti4O12-0.6CaCu3Ti4O12.

Microstructures of CCTO-0.5CuAl2O4 and 0.4SCTO-0.6CCTO ceramics and pure CCTO in

differentMicrostructures sintering times of CCTO-0.5CuAl are given in Figure2O4 and2. For 0.4SCTO-0.6CCTO in-situ CuAl2O4 phase ceramics samples, and grain pure size CCTOwith in uniform distribution ranges from 1 to 4 μm and the second phase of CuAl2O4 locates intergranularly different sintering times are given in Figure2. For in-situ CuAl 2O4 phase samples, grain size with in samples sintered for 4 h. However, abnormal growth of CCTO grains occurs in samples sintered uniform distribution ranges from 1 to 4 µm and the second phase of CuAl2O4 locates intergranularly for 20 h, which is a common phenomenon in CCTO ceramics, leading to a grain size of more than 50 in samples sintered for 4 h. However, abnormal growth of CCTO grains occurs in samples sintered μm. Meanwhile, the second phase of CuAl2O4 is partially coated by giant CCTO grains. For for 20 h, which is a common phenomenon in CCTO ceramics, leading to a grain size of more 0.4SCTO-0.6CCTO ceramics, no abnormal grain growth is observed in either sintering condition. µ thanTwo 50 categoriesm. Meanwhile, of grains theare found second in phaseSEM images, of CuAl including2O4 is cuboid partially grains, coated with by a size giant of more CCTO than grains. For 0.4SCTO-0.6CCTO2 μm and spherical grains ceramics, with noa size abnormal less than grain 2 μm. growth In the grain is observed growth process, in either the sintering second phase condition. Twohas categories a significant of grains inhibitory are found effect inon SEM the migration images, includingof the grain cuboid boundary, grains, which with is a called size of pinning more than 2 µmeffect. and sphericalThe effect is grains represented with a through size less the than Zener 2 µ modelm. In [17]. the grain growth process, the second phase has a signiﬁcant inhibitory effect on the migration of the grain boundary, which is called pinning effect. G 4 -1 The effect is represented through the Zener model= [17f]. (4) r 3 G 4 where G is the limit size of the matrix grain in the= presencef −1 of the second phase pinning, r and f are (4) respectively the particle diameter and the volumer 3fraction of the second phase. It is considered that whereas Gtheis thegrain limit size size of the of thesecond matrix phase grain decreases in the presence and the ofvolume the second fraction phase of the pinning, second rphaseand f are increases, the matrix grain size apparently declines. respectively the particle diameter and the volume fraction of the second phase. It is considered that as In this paper, for CCTO-CuAl2O4 composites sintered for 4 h, the second phase of CuAlO4 is the grain size of the second phase decreases and the volume fraction of the second phase increases, uniformly located among the CCTO grains, which can effectively consume the Cu-rich phase and the matrix grain size apparently declines. obstruct the contact of CCTO grains. The pinning effect of CuAlO4 affects the mass transfer process andIn this thus paper, impedes for the CCTO-CuAl abnormal 2grainO4 composites growth of sinteredCCTO grains. for 4 For h, the CCTO-CuAl second phase2O4 composites of CuAlO 4 is uniformlysintered located for 20 h among, Figure the2b shows CCTO that grains, partial which CuAlO can4 is effectivelyswallowed by consume the CCTO the grains, Cu-rich leading phase to and obstructreduced the contactCuAlO of4 located CCTO grains.intergranularly The pinning and effecta weakened of CuAlO pining4 affects effect. the massIn 0.4SCTO-0.6CCTO transfer process and thus impedes the abnormal grain growth of CCTO grains. For CCTO-CuAl2O4 composites sintered for 20 h, Figure2b shows that partial CuAlO 4 is swallowed by the CCTO grains, leading to reduced Energies 2017, 10, 1031 4 of 10

CuAlO located intergranularly and a weakened pining effect. In 0.4SCTO-0.6CCTO composites, it is Energies4 2017, 10, 1031 4 of 10 obvious that the grain size of 0.4SCTO-0.6CCTO composites are sharply decreased compared to pure CCTOcomposites, samples init is Figure obvious2e,f. that The the pining grain size effect of in0.4SCTO-0.6CCTO 0.4SCTO-0.6CCTO composites is slightly are differentsharply decreased from that in CCTO-CuAlcompared2O to4 purecomposites. CCTO samples No nominal in Figure second 2e,f. phaseThe pining exists effect in the in composites0.4SCTO-0.6CCTO since onlyis slightly one main phasedifferent of CCTO from is detectedthat in inCCTO-CuAl XRD. However,2O4 composites. the interfaces No innominal 0.4SCTO-0.6CCTO second phase composites exists in the consist of interfacescomposites between since only SCTO/SCTO, one main phase SCTO/CCTO, of CCTO is and detected CCTO/CCTO, in XRD. However, which can the effectively interfaces hinderin 0.4SCTO-0.6CCTO composites consist of interfaces between SCTO/SCTO, SCTO/CCTO, and grain growth. Moreover, under the transport action of SrTiO3 [18,19], partial Cu-rich phase transfers fromCCTO/CCTO, grain boundary which into can grain, effectively thus decreasing hinder grain the growth. formation Moreover, of Cu-rich under phase the transport at the grain action boundary of SrTiO3 [18,19], partial Cu-rich phase transfers from grain boundary into grain, thus decreasing the and impeding abnormal grain growth. formation of Cu-rich phase at the grain boundary and impeding abnormal grain growth.

Figure 2. Microstructure of CaCu3Ti4O12-0.5CuAl2O4 sintered for 4 h (a) and 20 h (b); Figure 2. a b 0.4SrCu3TiMicrostructure4O12-0.6 CaCu3Ti4O of12 sintered CaCu3Ti for4O 512 h-0.5CuAl (c) and 152 Oh 4(dsintered); and pure for CCTO 4 hsintered ( ) and for 4 20 h (e h) ( ); 0.4SrCuand 310Ti 4hO (f12). -0.6CaCu3Ti4O12 sintered for 5 h (c) and 15 h (d); and pure CCTO sintered for 4 h (e) and 10 h (f). The current density (J)-breakdown field (E) characteristics of CCTO, CCTO-0.5CuAl2O4, and 0.4SCTO-0.6CCTO ceramics is given in Figure 3a. The current range is from 0.5 to 500 μA. The The current density (J)-breakdown field (E) characteristics of CCTO, CCTO-0.5CuAl2O4, and breakdown field is defined as the electric field when the current density is around 0.6 mA·cm−2. 0.4SCTO-0.6CCTO ceramics is given in Figure3a. The current range is from 0.5 to 500 µA. The Results of the breakdown field and the nonlinear coefficient are shown in Figure 3b. It can be found breakdown field is defined as the electric field when the current density is around 0.6 mA·cm−2. Results that the CCTO-0.5CuAl2O4 and 0.4SCTO-0.6CCTO samples show an extremely high breakdown of the breakdown field and the nonlinear coefficient are shown in Figure3b. It can be found that the electric field Eb of about 20 kV·cm−1, which is almost 10 times of former works in both pure CCTO CCTO-0.5CuAlceramics [3,20]2O 4andand CCTO-based 0.4SCTO-0.6CCTO composite samples ceramics show [9]. The an breakdown extremely highfield breakdownof 0.4SCTO-0.6CCTO electric field E of about 20 kV·cm−1, which is almost 10 times of former works in both pure CCTO ceramics [3,20] b Energies 2017, 10, 1031 5 of 10

Energies 2017, 10, 1031 5 of 10 and CCTO-based composite ceramics [9]. The breakdown field of 0.4SCTO-0.6CCTO samples sintering ◦ −1 at 1000 samplesC for 5 sintering h even at reaches 1000 °C 23.8 for 5 kV h even·cm reachesand 23.8 that kV·cm of CCTO-0.5CuAl−1 and that of CCTO-0.5CuAl2O4 samples2O also4 samples achieves 21 kVEnergies·cmalso−1 achieves2017, which, 10, 1031 21 are kV·cm the highest−1, which values are the everhighest reported. values ever As thereported. SEM As images the SEM show, images the show, pining5 of 10 the effect of the secondpining phaseeffect of can the effectively second phase reduce can theeffectivel grainy size,reduce which the grain can lead size, to which the enhancement can lead to the of the −1 samplesenhancement sintering of at the1000 breakdown °C for 5 h even field. reaches In addi 23.8tion, kV·cm the consumptionand that of CCTO-0.5CuAl of Cu-rich by2O 4CuAlO samples2 in breakdown field. In addition,−1 the consumption of Cu-rich by CuAlO2 in CCTO-0.5CuAl2O4 and the alsoCCTO-0.5CuAl achieves 21 kV·cm2O4 and, whichthe transport are the actionhighest of values SrTiO 3ever in 0.4SCTO-0.6CCTO reported. As the SEM result images in even show, higher the E b. transport action of SrTiO in 0.4SCTO-0.6CCTO result in even higher E . The nonlinear coefficients are piningThe nonlineareffect of coefficientsthe 3second arephase calculated can effectivel in the ycurrent reduce range the grainof 50–500 size,b μwhichA. It iscan clearly lead shownto the in calculated in the current range of 50–500 µA. It is clearly shown in Figure3b that nonlinear coefficients enhancementFigure 3b that of nonlinearthe breakdown coefficients field. do In not addi decrtion,ease the with consumption enhanced breakdown of Cu-rich fields, by CuAlO maintaining2 in do notCCTO-0.5CuAl decreaseexcellent nonlinear with2O enhanced4 and properties. the transport breakdown action fields, of SrTiO maintaining3 in 0.4SCTO-0.6CCTO excellentnonlinear result in even properties. higher Eb. The nonlinear coefficients are calculated in the current range of 50–500 μA. It is clearly shown in Figure 3b that25 (a)nonlinear coefficients do not decrease with enhanced breakdown Breakdown fields, field maintaining excellent nonlinear properties. (b) Nonlinear coefficient 20 20 -1 25 Breakdown field 15(a) (b) Nonlinear coefficient 0.6CCTO-0.4SCTO 1000oC/5h

20 /kV·cm o 20

-1 0.6CCTO-0.4SCTO 1000 C/15h E 10 10 o CCTO-0.5CuAl2O4 1100 C/4h 15 CCTO1100oC/4h 5 0.6CCTO-0.4SCTO 1000oC/5h

/kV·cm 0.6CCTO-0.4SCTO 1000oC/15h E 10 10 0 o 0 CCTO-0.5CuAl2O4 1100 C/4h 0.6CCTO- 0.6CCTO- CCTO- CCTO 0.00.20.40.60.4SCTO 0.4SCTO 0.5CuAl O o CCTO1100oC/4h-2 2 4 1100 C/4h 5 o o o J /mA·cm 1000 C/5h 1000 C/15h 1100 C/4h 0 Figure0 3. J-E plots (a) and breakdown field, nonlinear0.6CCTO- coefficient0.6CCTO- (b) of CCTO-CCTO, CCTO-0.5CuAlCCTO 2O4, 0.00.20.40.6 o Figure 3. J-E plots (a) and breakdown ﬁeld, nonlinear coefﬁcient0.4SCTO (b0.4SCTO) of CCTO,0.5CuAl CCTO-0.5CuAl2O4 1100 C/4h 2O4, and and 0.4SCTO-0.6CCTO ceramics.-2 o o 0.4SCTO-0.6CCTO ceramics.J /mA·cm 1000 C/5h 1000 C/15h 1100oC/4h

FigurePermittivity 3. J-E plots and (a) anddielectric breakdown loss field,at room nonlinear temperature coefficient (ofb) ofCCTO, CCTO, CCTO-0.5CuAl2O2O4, 4, and and 0.4SCTO-0.6CCTO ceramics. Permittivity0.4SCTO-0.6CCTO and dielectric ceramics are loss presented at room in temperatureFigure 4. It can of be CCTO, seen that CCTO-0.5CuAl permittivity of2 Oboth4, and 0.4SCTO-0.6CCTOCCTO-0.5CuAl ceramics2O4 and 0.4SCTO-0.6CCTO are presented in ceramics Figure4 .are It much can besmaller seen than that that permittivity of pure CCTO of both ceramicsPermittivity but still and reach dielectric more than loss 1000. at Nevertheless,room temperature compared of CCTO,to high dielectricCCTO-0.5CuAl loss of2 Omore4, and than CCTO-0.5CuAl2O4 and 0.4SCTO-0.6CCTO ceramics are much smaller than that of pure CCTO ceramics 0.4SCTO-0.6CCTO2 in CCTO ceramics ceramics at 0.1 are Hz, presented the stable in Figurepermittivity 4. It canat lowbe seenfrequency that permittivity (less than 10of Hz)both in but still reach more than 1000. Nevertheless, compared to high dielectric loss of more than 2 in CCTO CCTO-0.5CuAlCCTO-0.5CuAl2O24O and4 and 0.4SCTO-0.6CCTO 0.4SCTO-0.6CCTO ceramics suppress are the much dielectric smaller loss thaneffectively, that of resulting pure CCTO in the ceramicsceramicslow at 0.1dielectric but Hz, still the lossreach stable of more about permittivity than 0.1 1000. even Nevertheless, at at 0.1 low Hz. frequency A compared similar (less phenomenon to high than dielectric 10 Hz)is also inloss CCTO-0.5CuAlobserved of more thanin the 2O4 and 0.4SCTO-0.6CCTO2 CCTO/MgTiOin CCTO ceramics3 [9] suppressand at 0.1CCTO/CaTiO Hz, the the dielectric stable3 [21] permittivitycomposite loss effectively, ceramics, at low resulting frequencyand it has in (less thebeen lowthan attributed dielectric10 Hz) to in the loss of aboutCCTO-0.5CuAl 0.1missing even charge at 0.12O 4 carriers.Hz.and 0.4SCTO-0.6CCTO A similarIn addition, phenomenon according suppress to is thethe also dielectricformula observed ofloss stored ineffectively, the energy CCTO/MgTiO resultingU = (ε′E bin2)/2, 3the [the9 ] and low dielectric loss of about 0.1 even at 0.1 Hz. A similar phenomenon is also observed in the CCTO/CaTiOmaximum3 [ 21U ]calculated composite for ceramics,CCTO-0.5 CuAl and2O it4 hasand been0.4SCTO-0.6CCTO attributed to samples the missing are correspondingly charge carriers. CCTO/MgTiO3 [9] and−3 CCTO/CaTiO3 [21] composite ceramics, and it has been attributed to the In addition,44.1 and according 50.9 kJ·m to in the this formula work, which of stored is a great energy improvementU = (ε0E over2)/2, that the of maximumCCTO ceramicsU calculated of 2–7 b 2 missingkJ·m−3 incharge previous carriers. works In [1,3,20,21].addition, according to the formula of stored energy U = (ε′Eb )/2, the −3 for CCTO-0.5 CuAl2O4 and 0.4SCTO-0.6CCTO samples are correspondingly 44.1 and 50.9 kJ·m maximum U calculated for CCTO-0.5 CuAl2O4 and 0.4SCTO-0.6CCTO samples are correspondingly −3 in this44.1 work, and which50.9 kJ·m is−3 a in great this work, improvement which is a overgreat thatimprovement of CCTO over ceramics that of ofCCTO100 2–7 ceramics kJ·m inof 2–7 previous workskJ·m [1,3−,320 in, 21previous]. works [1,3,20,21]. 104 10010

4 10 3 10 δ ' ε 101 tan

3 δ 10 2

' 10 ε 1 0.1

ε' CCTO tanδ CCTO tan CCTO-0.5CuAlO CCTO-0.5CuAlO 2 0.4SCTO-1000-5 0.4SCTO-1000-5 10 1 0.4SCTO-1000-15 0.4SCTO-1000-15 10 0.10.01 ε10' -1 CCTO 10 1 tanδ CCTO103 105 107 CCTO-0.5CuAlO CCTO-0.5CuAlO 0.4SCTO-1000-5 f 0.4SCTO-1000-5/Hz 101 0.4SCTO-1000-15 0.4SCTO-1000-15 0.01 Figure 4. Dielectric10 performance-1 10 1at room 10temperature3 10 5of CCTO,10 CCTO-0.5CuAl7 2O4, and 0.4SCTO-0.6CCTO ceramics. f /Hz

Figure 4. Dielectric performance at room temperature of CCTO, CCTO-0.5CuAl2O4, and Figure 4. Dielectric performance at room temperature of CCTO, CCTO-0.5CuAl O , and 0.4SCTO-0.6CCTO ceramics. 2 4 0.4SCTO-0.6CCTO ceramics.

Energies 2017, 10, 1031 6 of 10

The influence of CuAl2O4 and SrCu3Ti4O12 on the relaxation process of CCTO was also investigated in this research. Electrical modulus is considered to be more effective in analyzing the relaxation process of polycrystalline ceramics since the large density of the mobile charge carriers is not taken into account in this formalism [22]. The electric modulus is defined as the reciprocal of

Energiescomplex2017 permittivity:, 10, 1031 6 of 10 εε′′′ * 11 ++′′′ M ==*2222 =jMjM = (5) The inﬂuence of CuAlεε2O4 and′′′′′′′′′−+j εεSrCu3Ti4O ε12 on the ε relaxation + ε process of CCTO was also investigated in this research. Electrical modulus is considered to be more effective in analyzing 2 2 2 2 thewhere relaxation M′ = ε′ process/(ε′ + ε″ of) polycrystallineand M″ = ε″/(ε′ ceramics + ε″ ) are since the the real large and density imaginary of the part mobile of complex charge carrierselectric ismodulus. not taken Three into accountrelaxation in thispeaks formalism can be observed [22]. The for electric all the modulus samples is in deﬁned electric as modulus the reciprocal plots ofin complextemperature permittivity: range 153–433 K, shown in Figure 5. The peak frequencies shift to higher frequencies as temperature increases, and their corresponding activation energy levels are calculated in Figure 6, 0 00 according to Arrhenius’ law.1 Breakdown1 fields εEb and activationε energy of relaxations are given in M∗ = = = + j = M0 + jM00 (5) Table 1. ε∗ ε0 − jε00 ε02 + ε00 2 ε02 + ε00 2

0 0 02 002 00 02 002 whereTableM = ε1./( εElectrical+ ε ) andparametersM” = ε /(ofε CCTO+ ε ) areceramics the real of and undoped imaginary CCTO part ofceramics complex and electric modulus.CaCu3Ti Three4O12/CuAl relaxation2O4 composite peaks ceramics. can be observed for all the samples in electric modulus plots in temperature range 153–433 K, shown in Figure5. The peak frequencies shift to higher frequencies Sample Eb (kV·cm−1) L1 (eV) L2 (eV) L3 (eV) as temperature increases, and their corresponding activation energy levels are calculated in Figure6, undoped CCTO 2.2 0.11 0.47 0.65 according to Arrhenius’ law. Breakdown ﬁelds Eb and activation energy of relaxations are given in CaCu3Ti4O12-0.5CuAl2O4 21 0.15 0.35 0.84 Table1. 0.4SCTO-0.6CCTO 23.8 0.14 0.48 0.88 10-1 10-2 Peak 3 (b) CCTO-0.5CuAl O (a) CCTO 2 4 Peak 1 4x10-6 Peak 1 Peak 2 " -3 M 10 -6 " 2x10 -3 Peak 2 M -1 1 3 10 10 10 10 " f (Hz) 10-5 M

T increasing T increasing -4 -7 10 10 Peak 3 -1 1 3 5 7 10 10 10 10 10 10-1 101 103 105 107 f /Hz f /Hz -2 10 (c) 0.4SCTO-0.6CCTO Peak 3 Peak 1 3E-4

'' Peak 2 M 2E-4 10-3 -1 1 3

'' 10 10 10 f (Hz) M

10-4

T increasing

10-1 101 103 105 107 f /Hz

Figure 5. Frequency dependence of electric modulus in the temperature range of 153–433 K for Figure 5. Frequency dependence of electric modulus in the temperature range of 153–433 K for CCTO CCTO ceramics (a), the CaCu3Ti4O12-0.5CuAl2O4 composite (b), and the 0.4SCTO-0.6CCTO ceramics (a), the CaCu3Ti4O12-0.5CuAl2O4 composite (b), and the 0.4SCTO-0.6CCTO composite (c). composite (c).

Table 1. Electrical parameters of CCTO ceramics of undoped CCTO ceramics and CaCu3Ti4O12/CuAl2O4 composite ceramics.

−1 Sample Eb (kV·cm ) L1 (eV) L2 (eV) L3 (eV)

undoped CCTO 2.2 0.11 0.47 0.65 CaCu3Ti4O12-0.5CuAl2O4 21 0.15 0.35 0.84 0.4SCTO-0.6CCTO 23.8 0.14 0.48 0.88 Energies 2017, 10, 1031 7 of 10 Energies 2017, 10, 1031 7 of 10

20 18 (a) CCTO (b) CCTO-0.5CuAl O 2 4 0.15 eV 16 Peak 1 0.11 eV 12 Peak 2 12 Peak 3 (Hz) (Hz) 8

f 0.35 eV f 6 ln Peak 1 ln 0.47 eV 4 Peak 2 0.84 eV 0.65 eV Peak 3 0 0 468 -1 0246-1 1000/T (K ) 1000/T (K ) 12

0.14 eV 8 0.88 eV 0.48 eV (Hz) f 4 Peak 1 ln Peak 2 Peak 3 0 (c) 0.4SCTO-0.6CCTO 2468 1000/T (K-1)

Figure 6. Frequency dependence of the electric modulus in the temperature range of 153–433 K for Figure 6. Frequency dependence of the electric modulus in the temperature range of 153–433 K for undoped CCTO ceramics (a), CaCu3Ti4O12/CuAl2O4 composite (b) and 0.4SCTO-0.6CCTO (c). undoped CCTO ceramics (a), CaCu3Ti4O12/CuAl2O4 composite (b) and 0.4SCTO-0.6CCTO (c). Peak 1 in Figure 5 is commonly considered as the intrinsic bulk or grain relaxation in CCTO ceramicsPeak 1 in[6]. Figure Activation5 is commonlyenergy of Peak considered 1 is 0.11 eV as for the CCTO intrinsic ceramics, bulk while or grain this value relaxation is about in 0.13 CCTO ceramicseV for [6 the]. Activation CCTO-0.5CuAl energy2O4 ofcomposite Peak 1 is and 0.11 0.14 eV eV for for CCTO the 0.4SCTO-0.6CCTO ceramics, while this composite. value is In about the 0.13 CCTO-0.5CuAl2O4 composite, since the radius of Al3+ (0.535 Å) is close to that of Ti4+ (0.605 Å), Al3+ eV for the CCTO-0.5CuAl2O4 composite and 0.14 eV for the 0.4SCTO-0.6CCTO composite. In the tends to substitute Ti4+ to form the AlTi′. As the grains3+ of CCTO are n-type semiconducting,4+ the 3+ CCTO-0.5CuAl2O4 composite, since the radius of Al (0.535 Å) is close to that of Ti (0.605 Å), Al 3+ carrier concentration4+ will be reduced according0 to the substitution. Additionally, in recent work, Al tends to substitute Ti to form the AlTi . As the grains of CCTO are n-type semiconducting, the concentration is much higher than its solid solubility limit in CCTO [23]. Thus, interstitial Ali··· can 3+ carrier concentration will3+ be reduced according to the substitution. Additionally, in recent work, Al be generated from Al filling in the gaps of unit cell, which are caused by Cu-deficiency. Therefore,··· concentrationcharge carriers is much in n-type higher semico thannducting its solid grains solubility are pa limitrtially in CCTOcompensated, [23]. Thus, and defect interstitial structures Ali can 3+ be generatedcan be restrained, from Al leadingfilling to in thea slight gaps increase of unit cell,in the which intrinsic are causedenergy bylevel. Cu-deficiency. The decline of Therefore, the chargedielectric carriers response in n-type in Figure semiconducting 4 at high frequency grains are can partially also be explained compensated, by this and inhibition defect structureseffect. In the can be restrained,0.4SCTO-0.6CCTO leading to a slightcomposites, increase since in thecation-O intrinsic bo energynds do level.not need The declinemuch ofenergy the dielectric [24], oxygen response in Figurevacancies4 at highcan be frequency considered can a reasonable also be explained source for by electron this inhibition formation. effect. However, In the the 0.4SCTO-0.6CCTO solid solution composites,effect of sinceSr2+ in cation-O CCTO and bonds Ca2+ do innot SCTO need has much an inhibitory energy [ 24effect], oxygen on the vacancies intrinsic oxygen can be consideredvacancy a reasonabledefects. sourceAfter Ca for2+ electronor Sr2+ is involved formation. in solid However, solution, the the solid generation solution of effect oxygen of Sr vacancy2+ in CCTO is difficult and Ca2+ in SCTOto achieve, has an thus inhibitory increasing effect the onintrinsic the intrinsic activation oxygen energy vacancy [25]. defects. After Ca2+ or Sr2+ is involved In the single-phase CCTO ceramics, Peak 2 is derived from the intrinsic defect closely related in solid solution, the generation of oxygen vacancy is difficult to achieve, thus increasing the intrinsic to the oxygen vacancy, corresponding to the deep trap energy level in the depletion layer. activation energy [25]. Introduction of the interstitial Ali··· makes the composition of the positive charge in the depletion layerIn the more single-phase complex. CCTODielectric ceramics, loss related Peak with 2 is derived Peak 2 fromand relevant the intrinsic activation defect energy closely should related to the oxygenreduce by vacancy, interstitial corresponding Ali··· suppressing to the the deep generation trap energy of oxygen level in vacancy. the depletion However, layer. it can Introduction be seen of ··· the interstitialfrom Figure Al 4i thatmakes the dielectric the composition loss for CCTO-0.5CuAl of the positive2O charge4 composite in the at depletion intermediate layer frequency more complex. is ··· Dielectricgreatly loss strengthened related with compared Peak 2 with and the relevant CCTO activation sample, indicating energy should that the reduce relaxation by interstitialmechanismAl i suppressingcorresponding the generation to Peak 2 ofmay oxygen not transform vacancy. into However, the oxygen it can vacancy be seen dominance. from Figure The4 that depletion the dielectric of ··· lossthe for deep CCTO-0.5CuAl trap caused 2byO 4thecomposite interstitial at Al intermediatei in the depletion frequency layer may is greatlybe the reason strengthened for the decrease compared withactivation the CCTO energy sample, of Peak indicating 2. that the relaxation mechanism corresponding to Peak 2 may not transform into the oxygen vacancy dominance. The depletion of the deep trap caused by the interstitial ··· Ali in the depletion layer may be the reason for the decrease activation energy of Peak 2. Energies 2017, 10, 1031 8 of 10

Peak 3 in Figure5 is supposed to be related to M–W relaxation at grain boundary, increasing apparently from 0.65 in CCTO ceramics to 0.84 and 0.88 eV in composite ceramics. The activation energy is considered closely associated with conduction process at low frequency and controlled by the potential barrier at the grain boundary. For in-situ formed CuAl2O4 samples, on one hand, the introduction of the second phase leads to a more sophisticated interface in CCTO-CuAl2O4 composites, such as the interface between CCTO/CCTO, CCTO/CuAl2O4, and CuAl2O4/CuAl2O40 [26], ··· attributing to the rise of interface density. Combined with the interstitial Ali suppressing on the generation of oxygen vacancy, potential barrier height is elevated in CCTO-CuAl2O4 composites. On the other hand, the potential barriers of the grain boundary for 4 h and 20 h samples are calculated as 0.81 eV and 0.74 eV, respectively, which can explain the discrepancy in the pining effect. The potential barriers for samples sintered for 4 h are larger than that of samples sintered for 20 h, indicating that the pining effect is weakened in these samples. The discrepancy in the pining effect can lead to distinct nonlinear properties. For SCTO-CCTO composites, since the solid solution effect of Sr2+ in CCTO and Ca2+ in SCTO and has an inhibitory effect on the intrinsic oxygen vacancy defects, the donor density in 0.4SCTO-0.6CCTO is in a relatively low level. Meanwhile, potential barrier height at the grain boundary is inversely proportional to donor density, leading to the elevated potential barrier height of 0.88 eV in 0.4SCTO-0.6CCTO composites. Nevertheless, the space for the A site in CCTO is very rigid since the expected Ca–O distance is 2.72 Å, while the observed distance is 2.61 Å [27]. As the radius of Sr is much larger than that of Ca, Sr–O bonds are in an extremely overbonded situation. The Sr–O distance is expected to be 2.82 Å while the observed Sr–O distance is 2.64 Å [27]. When the Sr/Ca ratio is increasing on the A site, the larger Sr stretches the Ti–O bonds and inﬂuences the tilted TiO6 octahedra, thus enhancing the polarizability of the tilted TiO6 octahedra and possibly breaking the balance of Cu–O planar [28]. At this point, the generation of oxygen vacancy much more easily increases the donor density when Sr/Ca ratio increases. Only when the Sr/Ca ratio is 4:6, the integrated action of the strong solid solution effect and the weak Sr-stretching effect contributes to the elevated potential barrier height and enhanced breakdown ﬁeld.

4. Conclusions

In summary, CaCu3Ti4O12-0.5CuAl2O4 composites prepared by an in-situ method and 0.4SrCu3Ti4O12-0.6CaCu3Ti4O12 composites by a solid-state method were found to exhibit remarkably enhanced breakdown ﬁelds. In addition, permittivity still maintains more than 1000 while the dielectric loss remains about 0.1 even at a low frequency of 0.1 Hz, indicating excellent dielectric properties:

◦ −1 1. The breakdown field for CCTO-0.5CuAl2O4 sintering at 1100 C for 4 h can reach 21 kV·cm . The microstructure shows that no abnormal grain growth appears in samples with high breakdown fields, which can be attributed to the pining effect and consumption of the Cu-rich phase at the grain boundaries, contributing directly to the increase in breakdown field. Interstitial ··· Ali and increasing interfaces owing to the second CuAl2O4 phase are possible reasons for the variation in activation energy compared to the CCTO ceramics. 2. The breakdown field for 0.4SCTO-0.6CCTO sintering at 1000 ◦C for 5 h can even achieve −1 23.8 kV·cm , which are the highest values ever reported. Under the transport action of SrTiO3, some Cu-rich phase transfers from the grain boundary into the grain, decreasing the formation of Cu-rich phase at the grain boundary and impeding abnormal grain growth. Integrated action of a strong solid solution effect and a weak Sr-stretching effect contributes to the elevated potential barrier height and enhanced breakdown field.

Acknowledgments: This work is supported by the National Natural Science Foundation of China (No. 51177121) and (No. 51221005) and the Natural Science Foundation of Shaanxi Province (No. 2015JM5243). Author Contributions: Jianying Li and Zhuang Tang conceived and designed the experiments; Yuwei Huang and Zhuang Tang performed the experiments; Kangning Wu and Zhuang Tang analyzed the data; Kangning Wu Energies 2017, 10, 1031 9 of 10 contributed analysis tools; Zhuang Tang wrote the paper; Jianying Li and Kangning Wu offered suggestions about the paper. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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