The microstructure and dielectric properties of titanium oxide doped with nano CuO

JOURNAL OF ADVANCED DIELECTRICS Vol. 4, No. 3 (2014) 1450025 (7 pages) The Authors DOI: 10.1142/S2010135X14500258 The microstructure and dielectric properties of titanium oxide doped with nano CuO Juan Li*, Renli Fu*,,YueXu*, Zhenxiao Fu and Hongjuan Su *College of Material Science and Technology Nanjing University of Aeronautics and Astronautics Yudao Street, No. 29, Nanjing 210016, P. R. China Fenghua Advanced Technology Holding CO., LTD Zhaoqing 526020, P. R. China renlifu@nuaa.edu.cn Received 8 June 2014; Revised 19 August 2014; Accepted 26 August 2014; Published 12 September 2014 The TiO 2 ceramics doped with nano CuO were fabricated by the conventional solid-state reaction method. The crystal structure, microstructure, microwave dielectric properties and lattice vibrations of TiO 2 ceramics doped with nano CuO have been investigated. Nano CuO with higher sintering capability worked as flux former and effectively improved the sintering process of TiO 2 ceramics. The microwave dielectric properties of TiO 2 doped with nano CuO were much better than that doped with micron CuO at the same doping content. Raman spectra showed that the full width at half maximum of E g mode of TiO 2 doped with micron CuO was larger than nano CuO doped ones, which indicated a poor crystalline and a short phonon lifetime. TiO 2 doped with 1 wt.% nano CuO, sintered at 950 C for 2 h had compact and homogeneous microstructure and possessed the following dielectric properties: " r ¼ 106, Q f ¼ 24808, f ¼ 371 ppm/ C. Keywords: Microstructure; dielectric properties; TiO 2 ceramics; nano CuO. 1. Introduction Nowadays, with the rapid development in microwave wireless communication, there are urgent demands of good properties of dielectric resonators. For the materials used for wireless communication, they should have high dielectric constant (" r ), low dielectric loss (tan ) and a near-zero temperature coefficient of resonant frequency ( f ). However, it is difficult to achieve the three requirements in dielectric materials simultaneously. TiO 2 ceramics are found in a wide range of dielectric resonator materials. 1 3 They have a high dielectric constant " r ( 100) and a low tan value (6 10 5 at a frequency of 3 GHz and temperature of 300 K). 4 Normally, the TiO 2 ceramics with rutile phase are required sintering up to 1500 C to attain dense samples for powders with particle sizes of 0.2 3 m. 5 However, while TiO 2 are sintered at such a high temperature at air atmosphere or low oxygen partial pressure, a slight reduction of TiO 2 occurs and the oxygen vacancies and Ti 4þ /Ti 3þ interstitials would form in rutile crystal lattice. The presence of Ti 3þ and oxygen vacancies would lead to a considerable dielectric loss of the TiO 2 ceramics, Q f < 6000 GHz. 4 Moreover, strong temperature dependence of the dielectric constant results in a high temperature coefficient of resonant frequency ( f 465 ppm/ C), 4 which precludes the further application of TiO 2. Recently, Templeton et al. 4 and Pullar and Penn 6 found that doping with stable divalent and trivalent acceptor cations with ionic radii between 0.5 and 0.95 Å greatly decreased the dielectric loss of TiO 2, whereas other cations had little or no effect. The distinct effects of cation dopants in TiO 2 were attributed to different charge compensation mechanisms. 6,7 On the other hand, liquid phase sintering by addition of glass or low melting point materials such as Bi 2 O 3,B 2 O 3 and CuO is known to be an effective way to lower the sintering temperature for ceramics. 8 Several studies also demonstrated that the addition of CuO significantly lowered the sintering temperature of the rutile ceramics, and greatly enhanced the dielectric properties of TiO 2 ceramics. 9 11 Recently, oxide nanoparticles have attracted much attention, as they are effectively acting as a link between bulk materials and atomic structures. 12 Ran et al. reported that a significant enhancement in sintering activity of 3Y-TZP nanopowders was achieved by addition of CuO nanopowder, resulting in an extremely fast densification at decreased temperature. 13 Since the microwave dielectric properties of the ceramics are very sensitive to their density and the microstructures, the understanding on the control of the dielectric ceramic properties with respect to the processing parameters, such as types of additives, the sintering temperature is necessary. 13 Hence, in this study, we report the effect This is an Open Access article published by World Scientific Publishing Company. It is distributed under the terms of the Creative Commons Attribution 3.0 (CC-BY) License. Further distribution of this work is permitted, provided the original work is properly cited. 1450025-1

of nano CuO, as additives, on the sintering ability, densification, phase compositions, microstructure and microwave properties of TiO 2 ceramics. 2. Experimental Procedures The TiO 2 ceramics doped with different content of nano CuO (up to 8 wt.%) were prepared by using solid-state ceramic route. High purity powders including TiO 2 (purity: 99.99%; Aladdin, Shanghai, China), nano CuO powders (AR, particle size: 60 nm, Nanjing, China) and micron CuO (AR, Nanjing, China) as raw materials were weighed according to the desired molar ratio. The powders were ground in a nylon jar for 20 h with zirconia balls and distilled water as media. The milled powders were dried, granulated, and pressed into cylinders of 15 mm in diameter and 6 8 mm in height by uniaxial pressing under a pressure of 100 MPa. These pellets were sintered at temperatures of 925 C, 950 C and 975 C for 2 h in air with a heating rate of 5 C/min. The bulk densities of the sintered ceramics were measured by Archimedes method. The crystalline phases of sintered samples were identified by using X-ray diffraction (XRD) with CuK radiation (Bruker D8 advance, Germany). Polished and thermally etched surfaces of the specimens were observed using field emission scanning electron microscopy (HITACHI-SU8010, Japan), fitted with an energy dispersive spectrometer for element analysis. Raman measurements were carried out by a Laser Raman spectrometer (Renishaw invia, UK) at room temperature. The 100 mw output of the 532-nm line of a Nd YAG laser was used as the excitation source. The obtained Raman spectra were recorded with a resolution of approximately 1 cm 1. Microwave dielectric constants (" r ) and the quality factor values (Q f ) at microwave frequencies were measured by Hakki Coleman dielectric resonator method using a Network Analyzer (Advantest R3767C, Tokyo, Japan). The temperature coefficient of resonant frequency ( f ) was calculated from the data collected at the temperature range of 25 80 C using the following formula f ¼ f 80 f 25 f 25 ð80 25Þ ; where f 80 and f 25 represent the resonant frequency at 85 C and 25 C, respectively. Fig. 1. XRD patterns of TiO 2 doped with different CuO content sintered at 950 C for 2 h: (a) 1 wt.% CuO, (b) 3 wt.% CuO and (c) 8 wt.% CuO. on the JCPDS file number 75-1757 for the TiO 2 with rutile structure and on the JCPDS file number 48-1548 for CuO. No additional peaks were observed in all the samples. Thus, a reasonable conclusion can be drawn that there is no reaction forming a binary phase between TiO 2 and CuO. This is in good agreement with the literature that CuO-doped TiO 2 samples were mixtures of rutile phase TiO 2 and CuO, with no observable formation of the secondary phase as a result of interactions between TiO 2 and CuO. 10 3.2. The densification behaviors and microstructure of TiO 2 doped with nano CuO The bulk density of TiO 2 doped with nano CuO sintered at different temperatures is shown in Fig. 2, the data were taken through the average of three test samples. The samples 3. Results and Discussion 3.1. The Phase composition of TiO 2 doped with nano CuO Figure 1 shows X-ray powder diffraction (XRD) patterns of TiO 2 /nano CuO ceramics sintered at 950 C for 2 h. All the peaks of the samples in the XRD patterns were indexed based Fig. 2. Bulk density of TiO 2 doped with different CuO content sintered at 925 C, 950 C, and 975 C for 2 h, respectively. 1450025-2

J. Li et al. J. Adv. Dielect. 4, 1450025 (2014) Fig. 3. The microstructures of TiO2 doped with different content of nano CuO, sintered at 950 C, 2 h: (a) x ¼ 1 wt.%, (b) x ¼ 4 wt.%, (c) x ¼ 7 wt.% and (d) x ¼ 8 wt.%. sintered at 950 C had higher bulk densities than the samples sintered at 925 C and 975 C. For all sintering temperatures, there was a steady increase in the bulk densities when the content of nano CuO was increased from 1 wt.% to 7 wt.% and it was because CuO (6.505 g/cm3) with higher density than TiO2 (4.249 g/cm3) appeared more and more in the twophase system. The degradation of bulk density with 8 wt.% nano addition, however, was due to trapped porosity caused by the evaporation of liquid phase and also due to abnormal grain growth. Based on the CuO TiO2 phase diagram, a eutectic point is at T ¼ 919 C. Cu-based eutectic melts were formed in the interspace among grains during sintering process, which were beneficial to the diffusion of ions, so the densification process was improved.14,15 However, the effect of liquid phase sintering became saturated as excess CuO was added. The SEM micrographs of TiO2/nano CuO ceramics sintered at 950 C for 2 h are illustrated in Fig. 3. It could be found that there are two areas in contrast as marked in Fig. 3(b). Based on the EDS results (see it in Table 1), the dark area (marked as A) is identified as TiO2, and bright area (marked as B) is primarily CuO. The addition of nano CuO led to the formation of liquid phase and effectively enhanced the sintering ability of TiO2 ceramics during the sintering process at high temperature.15 For the 1 wt.% nano CuOdoped TiO2 ceramics, compact microstructure with crystal grains in dense contact was obtained (see it in Fig. 3(a)). As nano CuO content was increased, the grain growth was promoted and the grain size increased. During sintering process, Cu 2þ might tend to migrate to the grain boundaries and form low-melting flux phase, and the TiO2 mass transfer from grain to grain is thus enhanced via the flux phase bridge.11,16,17 However, as shown in Fig. 3(d), when the concentration of nano CuO increased to 8%, the sample was coarse-grained and porous, and it showed that typical microstructure resulted from abnormal grain growth. Moreover, enrichment in the liquid phase concentration at the grain boundaries was clearly visible. These would deteriorate the microwave dielectric properties of specimens. To further understand the effect of nano CuO on the microstructure of TiO2 ceramics, TiO2 ceramics with 1 wt.% micron CuO were also prepared under the same process 1450025-3

Table 1. The Energy Dispersive Spectrometer Analysis (EDS) data of TiO 2 ceramics with comparison of properties of the TiO 2 ceramics with 1 wt.% nano CuO corresponding to Fig. 3(b). Atom (%) Spot Ti Cu O A 38.12 1.14 60.74 B 7.37 41.48 51.15 conditions for comparison. Figure 4 shows the SEM images of the fractured surfaces of TiO 2 doped with 1 wt.% nano and micron CuO, sintered at 950 C for 2 h. It is observed that there were less closed pores in nano CuO-doped TiO 2 ceramics. The bulk densities for 1 wt.% nano and micron CuO-doped TiO 2 ceramics are 4.233 and 4.115 g/cm 3,respectively. It is clear that densification mechanisms occur when the distance between the centers of the crystallites diminishes. The formation of pores leads to an increase in the distance between the crystallites. Therefore, it inhibits the densification in some orientations at the samples using large particles and involves higher porosity. 12 Besides, for the specimen doped with micron CuO, the effect of a little amount of liquid phase can be observed, which would damage its microwave dielectric properties. The sintering mechanism and increase in relative density with the addition of nano CuO in this study can be explained as follows. The sintering velocity is roughly in inverse proportion to particles size. The finer particles have higher surface. Moreover, fine particles have many defects including vacancy, local stress and aberration. During sintering, surface and defect atoms tend to become normal atoms with lower energy and thus, the nano CuO has higher sintering capability and promotes the sintering process. 12 3.3. Dielectric properties of TiO 2 doped with nano CuO Figure 5 shows the dielectric constant of TiO 2 ceramics sintered at 925 C, 950 C and 975 C for 2 h as a function of nano CuO content. It is found that there was no distinct correlation between " r values and sintering temperatures. However, for all sintering temperatures, the dielectric constant decreased by increasing the content of nano CuO. It has been reported that the relative permittivity is significantly affected by the density and secondary phases in composite. The " r values of TiO 2 and CuO were reported to be 105 at 5 GHz 1 3 and 18.1 at 1 MHz, 18 respectively. Therefore, the addition of nano CuO to the TiO 2 ceramic decreased the relative permittivity. The maximum dielectric relative permittivity has been achieved in the sample doped with 1 wt.% nano CuO. When excessive amount of nano CuO was used, the dielectric constant of ceramic decreased dramatically. The f value of the TiO 2 /nano CuO ceramics sintered at 950 C for 2 h as a function of nano CuO content is Fig. 4. The SEM images of the fractured surfaces of TiO 2 doped with (a) x ¼ 1 wt.% nano CuO and (b) x ¼ 1 wt.% micron CuO sintered at 950 C, 2 h. Fig. 5. Dielectric constant of TiO 2 doped with different contents of nano CuO sintered at 925 C, 950 C and 975 C, 2 h. 1450025-4

Table 2. Comparison of properties of the TiO 2 ceramics with the addition of nano and micron CuO as additives. 1 wt.% CuO Sintering temperature ( C) Bulk density (g/cm 3 ) " r Q f f Nanoparticles 950 4.233 106 24,808 371 Micron sized 950 4.115 105 13,905 377 Fig. 6. Temperature coefficient of resonant frequency ( f )oftio 2 doped with different contents of nano CuO sintered at 950 C, 2 h. demonstrated in Fig. 6. The f values of the ceramic do not considerably change with the variation in the concentration of nano CuO. It is well known that there are several factors that contribute to the temperature coefficient of resonant frequency, such as composition, additive, and the second phase of the materials. 19 As the f values of TiO 2 and CuO are 465 4 and 110 ppm/ C, 10 respectively, increasing nano CuO content would make the f value become less positive. However, it was not observed in our experiments. This may be attributed to the little doping content of nano CuO. The quality factor values (Q f ) of the TiO 2 /nano CuO ceramics at different sintering temperatures for 2 h as a function of nano CuO content are illustrated in Fig. 7. The maximum Q f value is found to be 24,808 for the samples added with 1 wt.% of nano CuO sintered at 950 C. Generally, the quality factor is affected by extrinsic factors such as impurities, porosity, grain size and defect concentration. When 8 wt.% nano CuO was added, evaporation of liquid phase in the sintering process and abnormal grain growth resulted in more intergranular pores, therefore, the quality factor decreased dramatically. It is worth noting that the Q f values obtained in the present study are much higher compared to micron-sized CuO used. 10 As mentioned above, the TiO 2 ceramics with addition of 1 wt.% micron CuO were also prepared under the same process conditions for comparison, the sintering condition, density, dielectric properties are tabulated in Table 2. The Table 2 clearly shows that under the same conditions, the Q f values of TiO 2 ceramics doped with 1 wt.% nano CuO were almost double compared to those with micron CuO, whereas there was not much variation in " r and f.itis reported that the microwave dielectric loss can be attributed to both intrinsic and extrinsic losses. The extrinsic losses are caused by porosity, grain boundaries and secondary phases. 20 Microstructure observation revealed that nano CuO with higher sintering capability promoted the sintering process of TiO 2 ceramics and led to the well-densified and uniform body compared to the micron CuO used, and as a result, the Q f value was increased. Raman spectroscopy is a powerful technique to identify phase composition and give an insight into the short range character of the crystalline materials. Figure 8 represents the room temperature Raman spectra of TiO 2 with 1 wt.% nano and micron CuO ceramics over the range of 100 1000 cm 1. The B 2g mode at 820 830 cm 1 is too weak to be seen and the others are all labeled. The Raman modes B 1g,E g,a 1g Fig. 7. Quality factor (Q f )oftio 2 doped with different concentration of nano CuO sintered at 925 C, 950 C and 975 C, 2 h. Fig. 8. Raman spectra of TiO 2 doped with 1 wt.% nano and micron CuO sintered at 950 C, 2 h. 1450025-5

shift to higher frequencies with the addition of nano CuO. It is noticed that the intensity of Raman band in nano CuOdoped TiO 2 ceramics at around 235 cm 1 is not stronger than that of micron CuO-doped samples, which might be due to increased ordering. 21 It is known that Raman peaks are of narrow line width, which is intimately correlated with highly ordered structure and high Q characteristics of materials. 22 The full width at half maximum of the E g mode extracted by the peak fitting in micron CuO-doped ceramics is larger than that of nano CuO-doped samples, which indicates a poor crystalline and a short phonon lifetime. 23 Thus, the TiO 2 ceramics with nano CuO addition possessed more highly ordered structure compared to micron CuO-doped samples. This should be the intrinsic reason why the nano CuO-doped TiO 2 ceramics had higher Q f values. The microwave dielectric loss can be affected by the intrinsic factors such as lattice vibration and the extrinsic factors such as bulk density, secondary phase and so on. In our study, from the SEM micrographs and Raman scattering spectroscopy, we conclude that the improved sintering process and highly ordered structure in TiO 2 ceramics by the addition of nano CuO contributed to the higher Q f values compared to the micron CuO used. 4. Conclusion The TiO 2 x wt.% CuO (x ¼ 1 8) ceramics were prepared by the conventional solid-state ceramic route. The effects of nano CuO on the densification, microstructure, lattice vibrations and dielectric properties of TiO 2 ceramics were discussed. Nano CuO caused the liquid-phase sintering mechanism and effectively improved the sintering process of TiO 2 ceramics due to its higher sintering capability. Moreover, the Raman result shows that TiO 2 ceramics with nano CuO had narrower width of the Raman mode E g and lower relative intensity of Raman band at around 235 cm 1 compared to the micron CuO used under the same process conditions, which can significantly reduce the sources for microwave intrinsic losses. These two results accounted for the markedly improved Q f values of TiO 2 ceramics with the addition of nano CuO compared to the micron CuO used. TiO 2 ceramics doped with 1 wt.% nano CuO sintered at 950 C for 2 h possessed the microwave dielectric properties of " r ¼ 106, Q f ¼ 24;808, and f ¼ 371 ppm/ C. As the content of nano CuO increased, the grain size of the TiO 2 ceramics experienced a continuous increase, while the relative density, the dielectric constant (" r ), and the Q f values decreased. Acknowledgment The authors acknowledge the financial support of the Science and Technology projects of Guangdong Province (2011A091103002). References 1 A. E. 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