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Dielectric properties of alumina doped with TiO2 from 13 to 73 GHz
Journal of the European Ceramic Society 37 (2017) 641–646
Contents lists available at www.sciencedirect.com
Journal of the European Ceramic Society journal homepage: www.elsevier.com/locate/jeurceramsoc
Dielectric properties of alumina doped with TiO2 from 13 to 73 GHz D. Di Marco a , K. Drissi b , P.-M. Geffroy a,∗ , N. Delhote b , O. Tantot b , S. Verdeyme b , T. Chartier a a b
SPCTS UMR7315, CNRS, ENSCI, Université de Limoges, CEC, 12 Rue Atlantis 87068 Limoges, France XLIM UMR 7252, CNRS, Université de Limoges, 123 Avenue Albert Thomas, 87060 Limoges, France
a r t i c l e
i n f o
Article history: Received 22 July 2016 Received in revised form 16 September 2016 Accepted 17 September 2016 Available online 21 September 2016
a b s t r a c t Recent studies show that alumina doped with TiO2 exhibits promising dielectric properties, corresponding to low loss tangents and low temperature coefficients (or close to 0 ppmC−1 ). This paper aims to confirm these trends and study the dielectric properties of alumina doped with TiO2 from 0.5% to 12% wt. at high frequency, 13–73 GHz. This work demonstrates that alumina doped with TiO2 corresponds to potential materials for frequency converter devices working at a high frequency (up to 50 GHz). © 2016 Elsevier Ltd. All rights reserved.
Keywords: Alumina Dielectric properties Titanium oxide doping High frequency Loss tangent
1. Introduction The development of very high speed internet broadcasting by satellite towards rural areas needs to switch to a new band gap with a higher density of information, such as the Q band (approximately 40 GHz) and V band (approximately 50 GHz). The use of the Q and V bands introduces new challenges for satellite equipment and, especially, for the frequency converter. Alumina is a dielectric ceramic that is well adapted for millimetre-wave applications because of its low r and high Q; also, its temperature coefficient of resonant frequency (f ) is strongly negative, implying a heavy, complex system to offset thermal variations. Previous studies have shown that the alumina material doped with TiO2 powders presents with interesting dielectric properties, including a low loss tangent and temperature coefficient close to zero [1–4]. However, the role of TiO2 doping on the dielectric loss of alumina materials is not clearly explained in the literature because it is problematic to dissociate the impact of the microstructure or chemical impurities on the dielectric loss. In this study, a pure alumina powder has been doped with titanium salt to obtain homogeneous doping in the material. The impact of TiO2 doping from 0.5% to 12% wt. on the microstructure
∗ Corresponding author. E-mail address: [email protected] (P.-M. Geffroy). http://dx.doi.org/10.1016/j.jeurceramsoc.2016.09.016 0955-2219/© 2016 Elsevier Ltd. All rights reserved.
Table 1 Main characteristics of starting alumina powder in this study. Average grain size (m) Main impurities (ppm)
Total impurities (ppm)
Fe Na Si K
0.1 8 8 10 3 29
and on dielectric properties between 18 and 73 GHz is investigated and compared with previously published data [1–4]. 2. Experiments The pure alumina powder is doped with titanium isopropoxide to obtain very homogenous doping of materials. The characteristics of the pure alumina powders are reported in Table 1. Titanium isopropoxide was 99.999% pure with less than 3.5 ppm total metallic impurities (Sigma-Aldrich, France). Pure alumina was first dispersed, with absolute ethanol with less than 3 ppm metallic impurities (AnalaR NORMAPUR, VWR), by attrition using yttrium-stabilized-zirconia (YTZ) beads (Ø = 0.8 mm) as the grinding media. Then, titanium isopropoxide was added to the mixture with milling to ensure the best homogeneity of doping in alumina powder. After sieving and drying, powder was calcined in dense alumina crucibles up to 800 ◦ C for 20 min at 2 ◦ C.min−1 to oxidize and fix titanium onto alumina grain.
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D.D. Marco et al. / Journal of the European Ceramic Society 37 (2017) 641–646
Fig. 1. MET image of nanometric TiO2 grains dispersed onto pure alumina.
The resulting powders have very small TiO2 grains (< 5 nm) that are homogeneously dispersed onto alumina grains, as reported in Fig. 1. The alumina powder doped with TiO2 is then dispersed in osmosed water with Darvan C (R.T. Vanderbilt Company, Inc.) by attrition using yttrium-stabilized-zirconia (YTZ) beads (Ø = 0.8 mm) as the grinding media. Then, 5%wt. polyethylene glycol with an average mol. wt. 20 M (Sigma-Aldrich) is added to the alumina suspensions. Then, the suspensions are atomized using a Büchi B-290 spray dryer. Atomized powders are pressed at 100 MPa to achieve a relative green density of 54%. The pellets are shaped using uniaxial pressing, and the plates are shaped using isostatic pressing. The pellets have a thickness of 4 mm and a diameter of 8 mm, corresponding to a characterization frequency of 8 GHz. The plates are machining to produce squares with a width of 20 mm and a thickness of 0.3 mm for 55 GHz and 73 GHz characterizations. To minimize measurement errors during dielectric characterization, all samples are machined after sintering to achieve less than 10 m geometric tolerance and homogeneous surface states. To determine the dielectric properties of our alumina, two resonant methods are used. The first method we used is based on a dielectric resonator (DR) placed on a cylindrical cavity to characterize Alumina pellets at 13 GHz [5]. The characterization method consists of centring a dielectric resonator (DR) on a Teflon support, and both elements are positioned in the centre of a cylindrical metallic cavity. The cavity is excited using coaxial probes that are terminated by magnetic loops to cover all resonance frequencies of the dielectric resonator. The resonance frequency and quality factor of the entire system are measured on the fundamental TE01␦ mode of the dielectric resonator. The latter feature is specifically used with this characterization method for extracting the DR properties (complex permittivity). The metallic cavity consists of a cylindrical cavity and its top and bottom metal ground planes. The cylindrical cavity and upper ground plane are fixed. A micrometric system varies the position of the lower ground plane on which the support and dielectric resonator are placed, allowing it to be positioned very precisely in the centre of the cavity. This device allows for reduction of the impact of DR decentring. Considering an Alumina DR with a 12-mm diameter, a decentring of 300 m leads to a typical error of 0.15% for determining the permittivity and a couple percentages on the loss tangent extraction. The procedure for extracting the permittivity and loss tangent of the dielectric resonator and support, as well as the conductivity of the cavity metallic walls, was developed in a previous paper [5]. A second resonant cavity method based on a split cylinder resonator [8–10] is used to measure the permittivity and loss tangent of the complex permittivity r in the V-band (55 GHz and 73 GHz). This method allows for accurate measurements of the dielectric
Fig. 2. Shrinkage behavior of pure alumina, 0.995 Al2 O3 -0.005 TiO2 and 0.88 Al2 O3 0.12 TiO2 for a heating rate of 2 ◦ C.min−1 .
plates without specific preparation of the samples. Fig. 3 shows the cavity used for the characterizations. The sample is inserted between the two halves of the cavity that is excited on the TE013 mode for measurements at 55 GHz and TE015 mode for measurements at 73 GHz. For a given sample, the resonant frequencies depend on the cavity dimensions as well as the thickness of the sample and its permittivity. The inner diameter of the cavity is approximately 7.8 mm. For this reason, the substrate lateral dimensions must be greater than 7.8 mm. The maximum substrate thickness should remain less than 0.4 mm to consider the radiation near the slot that separates the two cavity components negligible. Two opposite circular irises allow for coupling of the cavity to standard WR15 rectangular waveguides that are connected to two millimetre heads. The procedures for extracting the permittivity and loss tangent of the dielectric sample were developed in previous papers [5–8]. Relative errors on the permittivity and loss tangent are mainly related to the relative thickness uncertainties of the sample and dimensional dispersions of the cavity. As a consequence, a target manufacturing error of +/–10 m is necessary to maintain the permittivity extraction as low as possible, requiring that the substrate be machined with a high accuracy. The first method based on the DR, even if appreciable for its accuracy, is limited to frequencies below a few tens of GHz (typically 30 GHz). Beyond this frequency, the DR dimensions are too small to be easily handled, to maintain an accurate centring in the metallic cavity and limit dimensions errors. The second method (SCR) is therefore preferable for higher frequencies (> ∼30 GHz). 3. Results and discussion 3.1. Microstructure evolution as a function of TiO2 content The thermal shrinkage behaviour of the pure alumina powder and the alumina powders doped with 0.5%wt. and 12%wt. of TiO2 have been studied using a dilatometer (Setsys Evolution TMA, Setaram). The shrinkage behaviour of the samples as a function of the temperature for a heating rate of 2 ◦ C min−1 has been reported on Fig. 2. It should be noted here that the sintering behaviours of pure alumina powder and alumina powder doped with TiO2 are slightly different. For low TiO2 contents (i.e., <1%wt.), the sintering curve shifts towards a higher sintering temperature below 1350 ◦ C, while the sintering kinetics increase slightly up to 1350 ◦ C (in particular, between 1350 ◦ C and 1400 ◦ C). For high TiO2 contents (close to 12%wt.), the sintering curve shifts towards lower sintering temperatures when the TiO2 content increases. Additionally, the alumina was doped with 12%wt. The TiO2 sample presents with unusual behaviour, a dilatation between 1400 ◦ C
D.D. Marco et al. / Journal of the European Ceramic Society 37 (2017) 641–646
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Fig. 3. Microstructure of sintered Al2 O3 (a), 0.995 Al2 O3 -0.005 TiO2 (b), 0.95 Al2 O3 -0.05 TiO2 (c) and 0.88 Al2 O3 -0.12 TiO2 (d) for a heating rate of 2 ◦ C.min−1 . Table 2 Sintering temperature as a function of TiO2 content for a heating rate of 2 ◦ C.min−1 . TiO2 (%wt.)
Sintering temperature (◦ C)
Density (g.cm−3 )
Grain size (m)
0 1 0,5 0,9 5,0 10,0 12,0
1570 1540 1470 1484 1441 1403 1400
3.991 3.918 3.932 3.916 3.886 3.812 3.852
2.0 4.8 3.9 3.1 1.7 1.4 1.4
and 1450 ◦ C. This dilatation corresponds to the formation of the Al2 TiO5 phase [9], which leads to sample volume expansion of 0.38%. To avoid this phenomenon during sample preparation, the sintering temperature has been fixed at 1400 ◦ C for alumina doped with 12%wt. TiO2 . Sintering temperatures as a function of the TiO2 content in the alumina powder are reported in Table 2, as have been reported in the literature [10–12]. Sintering conditions have been investigated to obtain the maximum shrinkage for each sample (i.e., the maximum density). An increase in the TiO2 content leads to a decrease in the sintering temperature to prevent an important grain growth (which is not favourable to a high densification rate of the samples), see Fig. 2. Only the alumina doped with 0.5%wt. TiO2 content presents with a sintering temperature higher than one doped with the 0.1%wt. TiO2 sample. The sintering temperature seems to converge close to 1400 ◦ C for doping contents above 10%wt. The impact of TiO2 doping on the density and grain size is reported in Table 2. Accurate sample densities have been measured with a helium pycnometer (Micromeritics AccuPyc II 1340). The sample density decreases when the TiO2 content increases up to a limit of 10%wt. Up to 10%wt. of TiO2 , the density of the sintered sample seems to increase with a density measured at 3.852 g cm−3 for the alumina doped with 12%wt. TiO2 . The alumina doped with 0.5%wt. TiO2 has a density higher than one of the alumina doped with 0.1%wt. TiO2 . This has good agreement with the previous results because the alumina doped with 0.5%wt. TiO2 is sintered at a higher temperature than the alumina doped with 0.1%wt. TiO2 . A similar trend was previously described
by C. Huang [2]. Fig. 7 shows that the sample porosity increases with the TiO2 content. Indeed, the measured density values are very low compared with the data reported in the literature [2] for alumina doped nanometric TiO2 powders. We assume here that the content and incorporation method of TiO2 doping strongly modifies the kinetics of densification and grain growth at high temperature, as reported in Fig. 3. However, the impact of the TiO2 content on the grain growth at high temperature is different from the data reported in the literature [2]. First, it demonstrates a strong increase of the grain size for a doping content of 0.1%wt. of TiO2 compared to pure alumina. The grain size increases from 2 to 4.8 m. Then, the grain size slowly decreases up to 1.36 m for alumina doped with 12%wt. TiO2 sample, which corresponds to a lower limit. This trend is not in good agreement with data reported previously by C. Hang et al. [2], who reported that the grain size increases with the TiO2 content. The incorporation method of TiO2 doping in the alumina powder could have a large impact on the microstructure of sintered alumina, as reported in the literature [10,11]. The incorporation of TiO2 with a titanium isopropoxide precursor likely increases the reactivity and incorporation of TiO2 in alumina powder at a lower temperature. Then, for small TiO2 doping levels, TiO2 nanoparticles (less than 5 nm on alumina grains) react with alumina grains at low temperature and modify the kinetics of densification and grain growth. For instance, this leads to an increase in the kinetics of grain growth, as reported in Fig. 3a and b. However, for large TiO2 doping contents, it must considered here that the precipitation and coalescence of large TiO2 particles at the grain boundaries [11], limiting the grain growth during the sintering; see grey particles on Fig. 3c and d. 3.2. Result of annealing treatment on dielectric properties A previous study [1] demonstrated that sintered alumina doped with titanium dioxide powders exhibits better performance after annealing treatment, especially with a temperature coefficient of resonant frequency that is closer to 0 ppm ◦ C−1 . The aim of annealing treatment is to split up the Al2 TiO5 phase, which has poor dielectric properties, into Al2 O3 and TiO2 . X-Ray analysis has been
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Fig. 4. XRD patterns of 0.9 Al2 O3 -0.1 TiO2 ceramic sintered at 1403 ◦ C and postannealed at 800 ◦ C, 900 ◦ C, 1000 ◦ C and 1100 ◦ C during 10 h.
Fig. 6. Measured sample loss tangent at 13 GHz as a function of TiO2 content and annealing treatment.
Table 3 Evolution of the sample density as a function of the TiO2 content and annealing treatment. TiO2 (%wt.)
Sintered density (g cm−3 )
Annealed density (g cm−3 )
0.1 0.5 0.9 5.0 10.0 12.0
3.918 3.932 3.916 3.886 3.812 3.852
3.928 3.956 3.941 3.981 3.811 3.849
Fig. 7. Measured sample temperature coefficient of resonant frequency at 13 GHz as a function of TiO2 content and annealing treatment.
Fig. 5. Measured sample permittivity at 13 GHz as a function of TiO2 content and annealing treatment.
performed at different temperatures to determine the optimal annealing temperature for our materials. Diffraction patterns are shown in Fig. 4. The dissociation of the Al2 TiO5 phase is complete for an annealing temperature of 1100 ◦ C during 10 h. This temperature is the same as ones reported in the literature [1]. The impact of the annealing treatment at 1100 ◦ C on the density as a function of the TiO2 content is reported in Table 3. A significant density increase is observed for doped alumina with 0.5%wt. and 0.9%wt TiO2 . Otherwise, no density modifications are observed, and the annealing treatment has little impact on the density. SEM analysis on the samples containing 0.5%wt. TiO2 as sintered and after annealing shows that the average grain size is similar before and after annealing treatment such that the annealing treatment has no impact on the average grain size of doped alumina. Dielectric property measurement of doped alumina before and after annealing treatment was performed on samples with 0.1%wt., 0.5%wt., 0.9%wt., 5%wt., 10%wt. and 12%wt. TiO2 at 13 GHz. Fig. 5 shows that doped alumina exhibits better permittivity than pure alumina. The annealing treatment has no effect on the permittivity for samples with less than 12%wt. TiO2 . For the sample with 12%wt.
TiO2 , the annealing slightly increases the permittivity. Therefore, the annealing treatment is favourable towards permittivity. These values are consistent with a previous study by C. Huang [2]. The permittivity of doped alumina increases with TiO2 content, and the microstructure of the sintered sample has no impact on the permittivity. The loss tangent of doped alumina in relation to TiO2 doping content is reported in Fig. 6. As expected, low titanium dioxide content improves the loss tangent compared to pure alumina. The loss tangents of doped alumina with 0.1%wt. and 0.5%wt. TiO2 exhibits better performance than those reported in the literature [1,2,4] with an optimum tan ␦ at 1.53.10−5 . For the doped alumina up to 0.5%wt. TiO2 doping, the values of tan ␦ are between those reported by McN. Alford and C. Huang [2,4]. The grain size seems to have no impact on the loss tangent because the sample with the best loss tangent is the one with the biggest grain size. The correlation between the density and loss tangent is consistent with previous studies [2,5], i.e., the loss tangent increases when the density of doped alumina decreases. The impact of the annealing treatment is also very low, and there is variation in the confidence interval apart from the point at 5%wt. TiO2 . After further analysis, it appears that this difference is linked to both contamination and shaping defects of the 13 GHz sample. In conclusion, the loss tangent of alumina doped with TiO2 strongly depends on the density, and the grain size has no impact on this dielectric factor. The temperature coefficient of the resonant frequency in relation to the TiO2 content of doped alumina is reported in Fig. 7. The temperature coefficient measured in this work for less than 1%wt. TiO2 is lower than the reference in pure alumina (−60 ppm ◦ C−1 ). The temperature coefficient of doped alumina with 0.5%wt. TiO2 , previously reported by [2] with similar material, is −20 ppm ◦ C−1 .
D.D. Marco et al. / Journal of the European Ceramic Society 37 (2017) 641–646
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Fig. 8. Permittivity of alumina sample sintered after annealing treatment at 13 GHz, 55 GHz and 73 GHz as a function of TiO2 content.
Fig. 9. Measured sample loss tangent at 13 GHz, 55 GHz and 73 GHz as a function of TiO2 content of annealed ceramics.
This means that TiO2 doping impacts the temperature coefficient from very low doping content, when TiO2 is added as a powder. We assume here that the incorporation method of TiO2 doping and, thus probably, the associated microstructure affectS the temperature coefficient of the resonant frequency. Therefore, the TiO2 content in the doped alumina is not the only parameter impacting the temperature coefficient. Nevertheless, alumina doped up to 1%wt. TiO2 exhibits significant variation in the temperature coefficient with values close to one reported in the literature for 10%wt. TiO2 content [1,2] that is still under 0 ppm ◦ C−1 . TiO2 content of 12%wt. allows for a temperature coefficient close to 0 ppm ◦ C−1 . The impact of the annealing treatment at 1100 ◦ C after sintering is consistent with data reported in the literature [1]. Only doped alumina with high TiO2 levels is affected by the annealing treatment, which allows for an increase in the temperature coefficient (or brings it closer to 0 ppm ◦ C−1 ). The dissociation of the Al2 TiO5 , which is complete at 1100 ◦ C, has a low impact on the sample microstructure. No modifications on the grain size have been observed and there is only a small increase in the density for doped alumina with low TiO2 levels. Doped alumina with 0.5%wt. TiO2 corresponds to lower loss tangent (tan ␦ = 1.5 10−5 ), but it has an important negative temperature coefficient (−60 ppm ◦ C−1 ). The density increase measured during annealing treatment has no effect on the dielectric properties. The temperature coefficient of resonant frequency stabilization at 0 ppm ◦ C−1 is reached for doped alumina with 12%wt. TiO2 after annealing, which is in contrast with 10%wt. TiO2 that is usually reported in the literature. This discrepancy should be linked to the sample microstructure, which corresponds to very large grain sizes in this study.
loss tangent is deeply impacted by the working frequency and TiO2 content. When the frequency increases from 18 to 73 GHz, the variation of the loss tangent with low and high TiO2 doping content decreases. In other words, the very low loss tangent observed on alumina doped with low TiO2 content at 13 GHz increases significantly at higher frequency, from 1.5 10−5 to 2 10−4 for 0.5%wt. TiO2 content. Moreover, Fig. 9 shows that the loss tangent degradation is not linear with the frequency. The large loss tangent for alumina doped with low TiO2 content also shows that the impact of the sample density on the loss tangent decreases when the frequency increases. At high frequencies (55 and 73 GHz), the microstructure of doped alumina is unlikely to be a determining parameter. Another parameter, not studied here, is the chemical contamination, which should impact the loss tangent. The dielectric properties of alumina doped with TiO2 from 0.1%wt. to 12%wt. at a high frequency (55 and 73 GHz) show unexpected trends. The permittivity of doped alumina is unchanged when the frequency increases. The dielectric property mainly seems to be impacted by the chemical and phase composition without being affected by the microstructure. This work also demonstrates that the loss tangent is strongly linked to the density of materials at low frequencies above 50 GHz. The microstructure has no significant impact at this range of frequencies. In conclusion, the use of alumina doped with TiO2 over 10%wt. is very interesting at high frequency, when the loss tangent of pure alumina approaches that of doped alumina with high TiO2 content. For instance, at 73 GHz, our reference in pure alumina reached a loss tangent of 1.78.10−4 , while alumina doped with 12%wt. TiO2 reached 2.90.10−4 . 4. Conclusion
3.3. High frequency behaviour of annealed alumina doped with TiO2 Annealed samples have been characterized at 55 and 73 GHz to evaluate the performances of such low loss tangent or temperature coefficient stabilized alumina doped TiO2 . The evolution of permittivity as a function of the TiO2 content and characterization frequency is reported in Fig. 8. The TiO2 content has a large impact on the permittivity of doped alumina, while there is no significant variation in the permittivity with a frequency from 13 to 73 GHz. Therefore, there is no correlation between the permittivity of doped alumina and the microstructure. The permittivity mainly depends on the TiO2 content in doped alumina. Fig. 8 also shows that the permittivity increases nearly linearly with the TiO2 content. The loss tangent of doped alumina as a function of the TiO2 content and characterization frequency are reported in Fig. 9. The
This work highlights the impact of the incorporation method of doping, metal salt in our case, on the microstructure and dielectric performances. After impregnation, titanium isopropoxide turns alumina and proper calcination into very small nanometric grains. After sintering, it produces larger grain sizes compared with those observed in the literature for similar materials [2]. Nevertheless, dielectric performances of doped alumina obtained in this work are similar to those reported in the literature for similar materials [1,2]. The TiO2 phase present in doped alumina is probably the main determining factor for dielectric properties, especially the temperature coefficient. Finally, doped alumina with TiO2 shows a similar loss tangent as one of the pure alumina at high frequency (up to 50 GHz) as well as a temperature coefficient close to 0 ppm C−1 for 12%wt. TiO2 content. These dielectric properties demonstrate that doped alumina with TiO2 is a potential material for frequency conversion in satellite
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equipment. However, the doping mechanism of TiO2 in alumina is not yet clearly understood during the sintering operation. Further studies on this approach could further improve the dielectric properties of alumina doped with TiO2 . Acknowledgement This work was supported by the French National Agency for Research (ANR) in the form of the ATOMIQ (Advanced Technologies for integrated Millimetric filtering solutions in Q and V bands) project. References [1] Y. Miyauchi, Y. Ohishi, S. Miyake, H. Ohsato, Improvement of the dielectric properties of rutile-doped Al2 O3 ceramics by annealing treatment, J. Eur. Ceram. Soc. 26 (10) (2006) 2093–2096. [2] C.-L. Huang, J.-J. Wang, C.-Y. Huang, Microwave dielectric properties of sintered alumina using nano-Scaled powders of ␣ alumina and TiO2, J. Am. Ceram. Soc. 90 (5) (2007) 1487–1493. [3] J.D. Breeze, X. Aupi, N.M. Alford, Ultralow loss polycrystalline alumina, Appl. Phys. Lett. 81 (26) (2002) 5021–5023. [4] N.M. Alford, S.J. Penn, Sintered alumina with low dielectric loss, J. Appl. Phys. 80 (10) (1996) 5895–5898.
[5] D. Di Marco, K. Drissi, N. Delhote, O. Tantot, P.-M. Geffroy, S. Verdeyme, T. Chartier, Dielectric properties of pure alumina from 8 GHz to 73 GHz, J. Eur. Ceram. Soc. 36 (14) (2016) 3355–3361. [6] P. Guillon, Y. Garault, Complex permittivity of MIC substrate, AEU Band 35 (1981) 102–104, Heft 3. [7] M.D. Janezic, J. Baker-Jarvis, Full-wave analysis of a split-cylinder resonator for nondestructive permittivity measurements, IEEE Trans. Microwav. Theory Tech. 47 (10) (1999) 2014–2020. [8] J. Rammal, O. Tantot, D. Passerieux, N. Delhote, S. Verdeyme, Monitoring of electromagnetic characteristics of split cylinder resonator and dielectric material for temperature characterization, in: European Microwave Conference, Rome, Italy, October, 2014. [9] H. Manshor, S.M. Aris, A. Zahiram, A. Azhar, E.C. Abdullah, Z.A. Ahamad, Effects of TiO2 addition on the phase, mechanical properties, and microstructure of zirconia-toughned alumina ceramic composite, Ceram. Int. 41 (2015) 3961–3967. [10] R. Bagley, I.B. Culter, D.L. Johnson, Effect of TiO2 on initial sintering of Al2O3, J. Am. Ceram. Soc 53 (1970) 136–141. [11] C. Legros, F. Herbst, C. Carry, P. Bowen, Effect of Ti-doping on the sintering behavior of transition alumina, Key Eng. Mater. 206–213 (2002) 357–360. [12] S. Lartigue-Korinek, C. Legros, C. Carry, F. Herbst, Titanium effect on phase transformation and sintering behavior of transition alumina, J. Eur. Ceram. Soc. 26 (12) (2006) 2219–2230.
Contents lists available at www.sciencedirect.com
Journal of the European Ceramic Society journal homepage: www.elsevier.com/locate/jeurceramsoc
Dielectric properties of alumina doped with TiO2 from 13 to 73 GHz D. Di Marco a , K. Drissi b , P.-M. Geffroy a,∗ , N. Delhote b , O. Tantot b , S. Verdeyme b , T. Chartier a a b
SPCTS UMR7315, CNRS, ENSCI, Université de Limoges, CEC, 12 Rue Atlantis 87068 Limoges, France XLIM UMR 7252, CNRS, Université de Limoges, 123 Avenue Albert Thomas, 87060 Limoges, France
a r t i c l e
i n f o
Article history: Received 22 July 2016 Received in revised form 16 September 2016 Accepted 17 September 2016 Available online 21 September 2016
a b s t r a c t Recent studies show that alumina doped with TiO2 exhibits promising dielectric properties, corresponding to low loss tangents and low temperature coefficients (or close to 0 ppmC−1 ). This paper aims to confirm these trends and study the dielectric properties of alumina doped with TiO2 from 0.5% to 12% wt. at high frequency, 13–73 GHz. This work demonstrates that alumina doped with TiO2 corresponds to potential materials for frequency converter devices working at a high frequency (up to 50 GHz). © 2016 Elsevier Ltd. All rights reserved.
Keywords: Alumina Dielectric properties Titanium oxide doping High frequency Loss tangent
1. Introduction The development of very high speed internet broadcasting by satellite towards rural areas needs to switch to a new band gap with a higher density of information, such as the Q band (approximately 40 GHz) and V band (approximately 50 GHz). The use of the Q and V bands introduces new challenges for satellite equipment and, especially, for the frequency converter. Alumina is a dielectric ceramic that is well adapted for millimetre-wave applications because of its low r and high Q; also, its temperature coefficient of resonant frequency (f ) is strongly negative, implying a heavy, complex system to offset thermal variations. Previous studies have shown that the alumina material doped with TiO2 powders presents with interesting dielectric properties, including a low loss tangent and temperature coefficient close to zero [1–4]. However, the role of TiO2 doping on the dielectric loss of alumina materials is not clearly explained in the literature because it is problematic to dissociate the impact of the microstructure or chemical impurities on the dielectric loss. In this study, a pure alumina powder has been doped with titanium salt to obtain homogeneous doping in the material. The impact of TiO2 doping from 0.5% to 12% wt. on the microstructure
∗ Corresponding author. E-mail address: [email protected] (P.-M. Geffroy). http://dx.doi.org/10.1016/j.jeurceramsoc.2016.09.016 0955-2219/© 2016 Elsevier Ltd. All rights reserved.
Table 1 Main characteristics of starting alumina powder in this study. Average grain size (m) Main impurities (ppm)
Total impurities (ppm)
Fe Na Si K
0.1 8 8 10 3 29
and on dielectric properties between 18 and 73 GHz is investigated and compared with previously published data [1–4]. 2. Experiments The pure alumina powder is doped with titanium isopropoxide to obtain very homogenous doping of materials. The characteristics of the pure alumina powders are reported in Table 1. Titanium isopropoxide was 99.999% pure with less than 3.5 ppm total metallic impurities (Sigma-Aldrich, France). Pure alumina was first dispersed, with absolute ethanol with less than 3 ppm metallic impurities (AnalaR NORMAPUR, VWR), by attrition using yttrium-stabilized-zirconia (YTZ) beads (Ø = 0.8 mm) as the grinding media. Then, titanium isopropoxide was added to the mixture with milling to ensure the best homogeneity of doping in alumina powder. After sieving and drying, powder was calcined in dense alumina crucibles up to 800 ◦ C for 20 min at 2 ◦ C.min−1 to oxidize and fix titanium onto alumina grain.
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Fig. 1. MET image of nanometric TiO2 grains dispersed onto pure alumina.
The resulting powders have very small TiO2 grains (< 5 nm) that are homogeneously dispersed onto alumina grains, as reported in Fig. 1. The alumina powder doped with TiO2 is then dispersed in osmosed water with Darvan C (R.T. Vanderbilt Company, Inc.) by attrition using yttrium-stabilized-zirconia (YTZ) beads (Ø = 0.8 mm) as the grinding media. Then, 5%wt. polyethylene glycol with an average mol. wt. 20 M (Sigma-Aldrich) is added to the alumina suspensions. Then, the suspensions are atomized using a Büchi B-290 spray dryer. Atomized powders are pressed at 100 MPa to achieve a relative green density of 54%. The pellets are shaped using uniaxial pressing, and the plates are shaped using isostatic pressing. The pellets have a thickness of 4 mm and a diameter of 8 mm, corresponding to a characterization frequency of 8 GHz. The plates are machining to produce squares with a width of 20 mm and a thickness of 0.3 mm for 55 GHz and 73 GHz characterizations. To minimize measurement errors during dielectric characterization, all samples are machined after sintering to achieve less than 10 m geometric tolerance and homogeneous surface states. To determine the dielectric properties of our alumina, two resonant methods are used. The first method we used is based on a dielectric resonator (DR) placed on a cylindrical cavity to characterize Alumina pellets at 13 GHz [5]. The characterization method consists of centring a dielectric resonator (DR) on a Teflon support, and both elements are positioned in the centre of a cylindrical metallic cavity. The cavity is excited using coaxial probes that are terminated by magnetic loops to cover all resonance frequencies of the dielectric resonator. The resonance frequency and quality factor of the entire system are measured on the fundamental TE01␦ mode of the dielectric resonator. The latter feature is specifically used with this characterization method for extracting the DR properties (complex permittivity). The metallic cavity consists of a cylindrical cavity and its top and bottom metal ground planes. The cylindrical cavity and upper ground plane are fixed. A micrometric system varies the position of the lower ground plane on which the support and dielectric resonator are placed, allowing it to be positioned very precisely in the centre of the cavity. This device allows for reduction of the impact of DR decentring. Considering an Alumina DR with a 12-mm diameter, a decentring of 300 m leads to a typical error of 0.15% for determining the permittivity and a couple percentages on the loss tangent extraction. The procedure for extracting the permittivity and loss tangent of the dielectric resonator and support, as well as the conductivity of the cavity metallic walls, was developed in a previous paper [5]. A second resonant cavity method based on a split cylinder resonator [8–10] is used to measure the permittivity and loss tangent of the complex permittivity r in the V-band (55 GHz and 73 GHz). This method allows for accurate measurements of the dielectric
Fig. 2. Shrinkage behavior of pure alumina, 0.995 Al2 O3 -0.005 TiO2 and 0.88 Al2 O3 0.12 TiO2 for a heating rate of 2 ◦ C.min−1 .
plates without specific preparation of the samples. Fig. 3 shows the cavity used for the characterizations. The sample is inserted between the two halves of the cavity that is excited on the TE013 mode for measurements at 55 GHz and TE015 mode for measurements at 73 GHz. For a given sample, the resonant frequencies depend on the cavity dimensions as well as the thickness of the sample and its permittivity. The inner diameter of the cavity is approximately 7.8 mm. For this reason, the substrate lateral dimensions must be greater than 7.8 mm. The maximum substrate thickness should remain less than 0.4 mm to consider the radiation near the slot that separates the two cavity components negligible. Two opposite circular irises allow for coupling of the cavity to standard WR15 rectangular waveguides that are connected to two millimetre heads. The procedures for extracting the permittivity and loss tangent of the dielectric sample were developed in previous papers [5–8]. Relative errors on the permittivity and loss tangent are mainly related to the relative thickness uncertainties of the sample and dimensional dispersions of the cavity. As a consequence, a target manufacturing error of +/–10 m is necessary to maintain the permittivity extraction as low as possible, requiring that the substrate be machined with a high accuracy. The first method based on the DR, even if appreciable for its accuracy, is limited to frequencies below a few tens of GHz (typically 30 GHz). Beyond this frequency, the DR dimensions are too small to be easily handled, to maintain an accurate centring in the metallic cavity and limit dimensions errors. The second method (SCR) is therefore preferable for higher frequencies (> ∼30 GHz). 3. Results and discussion 3.1. Microstructure evolution as a function of TiO2 content The thermal shrinkage behaviour of the pure alumina powder and the alumina powders doped with 0.5%wt. and 12%wt. of TiO2 have been studied using a dilatometer (Setsys Evolution TMA, Setaram). The shrinkage behaviour of the samples as a function of the temperature for a heating rate of 2 ◦ C min−1 has been reported on Fig. 2. It should be noted here that the sintering behaviours of pure alumina powder and alumina powder doped with TiO2 are slightly different. For low TiO2 contents (i.e., <1%wt.), the sintering curve shifts towards a higher sintering temperature below 1350 ◦ C, while the sintering kinetics increase slightly up to 1350 ◦ C (in particular, between 1350 ◦ C and 1400 ◦ C). For high TiO2 contents (close to 12%wt.), the sintering curve shifts towards lower sintering temperatures when the TiO2 content increases. Additionally, the alumina was doped with 12%wt. The TiO2 sample presents with unusual behaviour, a dilatation between 1400 ◦ C
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Fig. 3. Microstructure of sintered Al2 O3 (a), 0.995 Al2 O3 -0.005 TiO2 (b), 0.95 Al2 O3 -0.05 TiO2 (c) and 0.88 Al2 O3 -0.12 TiO2 (d) for a heating rate of 2 ◦ C.min−1 . Table 2 Sintering temperature as a function of TiO2 content for a heating rate of 2 ◦ C.min−1 . TiO2 (%wt.)
Sintering temperature (◦ C)
Density (g.cm−3 )
Grain size (m)
0 1 0,5 0,9 5,0 10,0 12,0
1570 1540 1470 1484 1441 1403 1400
3.991 3.918 3.932 3.916 3.886 3.812 3.852
2.0 4.8 3.9 3.1 1.7 1.4 1.4
and 1450 ◦ C. This dilatation corresponds to the formation of the Al2 TiO5 phase [9], which leads to sample volume expansion of 0.38%. To avoid this phenomenon during sample preparation, the sintering temperature has been fixed at 1400 ◦ C for alumina doped with 12%wt. TiO2 . Sintering temperatures as a function of the TiO2 content in the alumina powder are reported in Table 2, as have been reported in the literature [10–12]. Sintering conditions have been investigated to obtain the maximum shrinkage for each sample (i.e., the maximum density). An increase in the TiO2 content leads to a decrease in the sintering temperature to prevent an important grain growth (which is not favourable to a high densification rate of the samples), see Fig. 2. Only the alumina doped with 0.5%wt. TiO2 content presents with a sintering temperature higher than one doped with the 0.1%wt. TiO2 sample. The sintering temperature seems to converge close to 1400 ◦ C for doping contents above 10%wt. The impact of TiO2 doping on the density and grain size is reported in Table 2. Accurate sample densities have been measured with a helium pycnometer (Micromeritics AccuPyc II 1340). The sample density decreases when the TiO2 content increases up to a limit of 10%wt. Up to 10%wt. of TiO2 , the density of the sintered sample seems to increase with a density measured at 3.852 g cm−3 for the alumina doped with 12%wt. TiO2 . The alumina doped with 0.5%wt. TiO2 has a density higher than one of the alumina doped with 0.1%wt. TiO2 . This has good agreement with the previous results because the alumina doped with 0.5%wt. TiO2 is sintered at a higher temperature than the alumina doped with 0.1%wt. TiO2 . A similar trend was previously described
by C. Huang [2]. Fig. 7 shows that the sample porosity increases with the TiO2 content. Indeed, the measured density values are very low compared with the data reported in the literature [2] for alumina doped nanometric TiO2 powders. We assume here that the content and incorporation method of TiO2 doping strongly modifies the kinetics of densification and grain growth at high temperature, as reported in Fig. 3. However, the impact of the TiO2 content on the grain growth at high temperature is different from the data reported in the literature [2]. First, it demonstrates a strong increase of the grain size for a doping content of 0.1%wt. of TiO2 compared to pure alumina. The grain size increases from 2 to 4.8 m. Then, the grain size slowly decreases up to 1.36 m for alumina doped with 12%wt. TiO2 sample, which corresponds to a lower limit. This trend is not in good agreement with data reported previously by C. Hang et al. [2], who reported that the grain size increases with the TiO2 content. The incorporation method of TiO2 doping in the alumina powder could have a large impact on the microstructure of sintered alumina, as reported in the literature [10,11]. The incorporation of TiO2 with a titanium isopropoxide precursor likely increases the reactivity and incorporation of TiO2 in alumina powder at a lower temperature. Then, for small TiO2 doping levels, TiO2 nanoparticles (less than 5 nm on alumina grains) react with alumina grains at low temperature and modify the kinetics of densification and grain growth. For instance, this leads to an increase in the kinetics of grain growth, as reported in Fig. 3a and b. However, for large TiO2 doping contents, it must considered here that the precipitation and coalescence of large TiO2 particles at the grain boundaries [11], limiting the grain growth during the sintering; see grey particles on Fig. 3c and d. 3.2. Result of annealing treatment on dielectric properties A previous study [1] demonstrated that sintered alumina doped with titanium dioxide powders exhibits better performance after annealing treatment, especially with a temperature coefficient of resonant frequency that is closer to 0 ppm ◦ C−1 . The aim of annealing treatment is to split up the Al2 TiO5 phase, which has poor dielectric properties, into Al2 O3 and TiO2 . X-Ray analysis has been
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Fig. 4. XRD patterns of 0.9 Al2 O3 -0.1 TiO2 ceramic sintered at 1403 ◦ C and postannealed at 800 ◦ C, 900 ◦ C, 1000 ◦ C and 1100 ◦ C during 10 h.
Fig. 6. Measured sample loss tangent at 13 GHz as a function of TiO2 content and annealing treatment.
Table 3 Evolution of the sample density as a function of the TiO2 content and annealing treatment. TiO2 (%wt.)
Sintered density (g cm−3 )
Annealed density (g cm−3 )
0.1 0.5 0.9 5.0 10.0 12.0
3.918 3.932 3.916 3.886 3.812 3.852
3.928 3.956 3.941 3.981 3.811 3.849
Fig. 7. Measured sample temperature coefficient of resonant frequency at 13 GHz as a function of TiO2 content and annealing treatment.
Fig. 5. Measured sample permittivity at 13 GHz as a function of TiO2 content and annealing treatment.
performed at different temperatures to determine the optimal annealing temperature for our materials. Diffraction patterns are shown in Fig. 4. The dissociation of the Al2 TiO5 phase is complete for an annealing temperature of 1100 ◦ C during 10 h. This temperature is the same as ones reported in the literature [1]. The impact of the annealing treatment at 1100 ◦ C on the density as a function of the TiO2 content is reported in Table 3. A significant density increase is observed for doped alumina with 0.5%wt. and 0.9%wt TiO2 . Otherwise, no density modifications are observed, and the annealing treatment has little impact on the density. SEM analysis on the samples containing 0.5%wt. TiO2 as sintered and after annealing shows that the average grain size is similar before and after annealing treatment such that the annealing treatment has no impact on the average grain size of doped alumina. Dielectric property measurement of doped alumina before and after annealing treatment was performed on samples with 0.1%wt., 0.5%wt., 0.9%wt., 5%wt., 10%wt. and 12%wt. TiO2 at 13 GHz. Fig. 5 shows that doped alumina exhibits better permittivity than pure alumina. The annealing treatment has no effect on the permittivity for samples with less than 12%wt. TiO2 . For the sample with 12%wt.
TiO2 , the annealing slightly increases the permittivity. Therefore, the annealing treatment is favourable towards permittivity. These values are consistent with a previous study by C. Huang [2]. The permittivity of doped alumina increases with TiO2 content, and the microstructure of the sintered sample has no impact on the permittivity. The loss tangent of doped alumina in relation to TiO2 doping content is reported in Fig. 6. As expected, low titanium dioxide content improves the loss tangent compared to pure alumina. The loss tangents of doped alumina with 0.1%wt. and 0.5%wt. TiO2 exhibits better performance than those reported in the literature [1,2,4] with an optimum tan ␦ at 1.53.10−5 . For the doped alumina up to 0.5%wt. TiO2 doping, the values of tan ␦ are between those reported by McN. Alford and C. Huang [2,4]. The grain size seems to have no impact on the loss tangent because the sample with the best loss tangent is the one with the biggest grain size. The correlation between the density and loss tangent is consistent with previous studies [2,5], i.e., the loss tangent increases when the density of doped alumina decreases. The impact of the annealing treatment is also very low, and there is variation in the confidence interval apart from the point at 5%wt. TiO2 . After further analysis, it appears that this difference is linked to both contamination and shaping defects of the 13 GHz sample. In conclusion, the loss tangent of alumina doped with TiO2 strongly depends on the density, and the grain size has no impact on this dielectric factor. The temperature coefficient of the resonant frequency in relation to the TiO2 content of doped alumina is reported in Fig. 7. The temperature coefficient measured in this work for less than 1%wt. TiO2 is lower than the reference in pure alumina (−60 ppm ◦ C−1 ). The temperature coefficient of doped alumina with 0.5%wt. TiO2 , previously reported by [2] with similar material, is −20 ppm ◦ C−1 .
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Fig. 8. Permittivity of alumina sample sintered after annealing treatment at 13 GHz, 55 GHz and 73 GHz as a function of TiO2 content.
Fig. 9. Measured sample loss tangent at 13 GHz, 55 GHz and 73 GHz as a function of TiO2 content of annealed ceramics.
This means that TiO2 doping impacts the temperature coefficient from very low doping content, when TiO2 is added as a powder. We assume here that the incorporation method of TiO2 doping and, thus probably, the associated microstructure affectS the temperature coefficient of the resonant frequency. Therefore, the TiO2 content in the doped alumina is not the only parameter impacting the temperature coefficient. Nevertheless, alumina doped up to 1%wt. TiO2 exhibits significant variation in the temperature coefficient with values close to one reported in the literature for 10%wt. TiO2 content [1,2] that is still under 0 ppm ◦ C−1 . TiO2 content of 12%wt. allows for a temperature coefficient close to 0 ppm ◦ C−1 . The impact of the annealing treatment at 1100 ◦ C after sintering is consistent with data reported in the literature [1]. Only doped alumina with high TiO2 levels is affected by the annealing treatment, which allows for an increase in the temperature coefficient (or brings it closer to 0 ppm ◦ C−1 ). The dissociation of the Al2 TiO5 , which is complete at 1100 ◦ C, has a low impact on the sample microstructure. No modifications on the grain size have been observed and there is only a small increase in the density for doped alumina with low TiO2 levels. Doped alumina with 0.5%wt. TiO2 corresponds to lower loss tangent (tan ␦ = 1.5 10−5 ), but it has an important negative temperature coefficient (−60 ppm ◦ C−1 ). The density increase measured during annealing treatment has no effect on the dielectric properties. The temperature coefficient of resonant frequency stabilization at 0 ppm ◦ C−1 is reached for doped alumina with 12%wt. TiO2 after annealing, which is in contrast with 10%wt. TiO2 that is usually reported in the literature. This discrepancy should be linked to the sample microstructure, which corresponds to very large grain sizes in this study.
loss tangent is deeply impacted by the working frequency and TiO2 content. When the frequency increases from 18 to 73 GHz, the variation of the loss tangent with low and high TiO2 doping content decreases. In other words, the very low loss tangent observed on alumina doped with low TiO2 content at 13 GHz increases significantly at higher frequency, from 1.5 10−5 to 2 10−4 for 0.5%wt. TiO2 content. Moreover, Fig. 9 shows that the loss tangent degradation is not linear with the frequency. The large loss tangent for alumina doped with low TiO2 content also shows that the impact of the sample density on the loss tangent decreases when the frequency increases. At high frequencies (55 and 73 GHz), the microstructure of doped alumina is unlikely to be a determining parameter. Another parameter, not studied here, is the chemical contamination, which should impact the loss tangent. The dielectric properties of alumina doped with TiO2 from 0.1%wt. to 12%wt. at a high frequency (55 and 73 GHz) show unexpected trends. The permittivity of doped alumina is unchanged when the frequency increases. The dielectric property mainly seems to be impacted by the chemical and phase composition without being affected by the microstructure. This work also demonstrates that the loss tangent is strongly linked to the density of materials at low frequencies above 50 GHz. The microstructure has no significant impact at this range of frequencies. In conclusion, the use of alumina doped with TiO2 over 10%wt. is very interesting at high frequency, when the loss tangent of pure alumina approaches that of doped alumina with high TiO2 content. For instance, at 73 GHz, our reference in pure alumina reached a loss tangent of 1.78.10−4 , while alumina doped with 12%wt. TiO2 reached 2.90.10−4 . 4. Conclusion
3.3. High frequency behaviour of annealed alumina doped with TiO2 Annealed samples have been characterized at 55 and 73 GHz to evaluate the performances of such low loss tangent or temperature coefficient stabilized alumina doped TiO2 . The evolution of permittivity as a function of the TiO2 content and characterization frequency is reported in Fig. 8. The TiO2 content has a large impact on the permittivity of doped alumina, while there is no significant variation in the permittivity with a frequency from 13 to 73 GHz. Therefore, there is no correlation between the permittivity of doped alumina and the microstructure. The permittivity mainly depends on the TiO2 content in doped alumina. Fig. 8 also shows that the permittivity increases nearly linearly with the TiO2 content. The loss tangent of doped alumina as a function of the TiO2 content and characterization frequency are reported in Fig. 9. The
This work highlights the impact of the incorporation method of doping, metal salt in our case, on the microstructure and dielectric performances. After impregnation, titanium isopropoxide turns alumina and proper calcination into very small nanometric grains. After sintering, it produces larger grain sizes compared with those observed in the literature for similar materials [2]. Nevertheless, dielectric performances of doped alumina obtained in this work are similar to those reported in the literature for similar materials [1,2]. The TiO2 phase present in doped alumina is probably the main determining factor for dielectric properties, especially the temperature coefficient. Finally, doped alumina with TiO2 shows a similar loss tangent as one of the pure alumina at high frequency (up to 50 GHz) as well as a temperature coefficient close to 0 ppm C−1 for 12%wt. TiO2 content. These dielectric properties demonstrate that doped alumina with TiO2 is a potential material for frequency conversion in satellite
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equipment. However, the doping mechanism of TiO2 in alumina is not yet clearly understood during the sintering operation. Further studies on this approach could further improve the dielectric properties of alumina doped with TiO2 . Acknowledgement This work was supported by the French National Agency for Research (ANR) in the form of the ATOMIQ (Advanced Technologies for integrated Millimetric filtering solutions in Q and V bands) project. References [1] Y. Miyauchi, Y. Ohishi, S. Miyake, H. Ohsato, Improvement of the dielectric properties of rutile-doped Al2 O3 ceramics by annealing treatment, J. Eur. Ceram. Soc. 26 (10) (2006) 2093–2096. [2] C.-L. Huang, J.-J. Wang, C.-Y. Huang, Microwave dielectric properties of sintered alumina using nano-Scaled powders of ␣ alumina and TiO2, J. Am. Ceram. Soc. 90 (5) (2007) 1487–1493. [3] J.D. Breeze, X. Aupi, N.M. Alford, Ultralow loss polycrystalline alumina, Appl. Phys. Lett. 81 (26) (2002) 5021–5023. [4] N.M. Alford, S.J. Penn, Sintered alumina with low dielectric loss, J. Appl. Phys. 80 (10) (1996) 5895–5898.
[5] D. Di Marco, K. Drissi, N. Delhote, O. Tantot, P.-M. Geffroy, S. Verdeyme, T. Chartier, Dielectric properties of pure alumina from 8 GHz to 73 GHz, J. Eur. Ceram. Soc. 36 (14) (2016) 3355–3361. [6] P. Guillon, Y. Garault, Complex permittivity of MIC substrate, AEU Band 35 (1981) 102–104, Heft 3. [7] M.D. Janezic, J. Baker-Jarvis, Full-wave analysis of a split-cylinder resonator for nondestructive permittivity measurements, IEEE Trans. Microwav. Theory Tech. 47 (10) (1999) 2014–2020. [8] J. Rammal, O. Tantot, D. Passerieux, N. Delhote, S. Verdeyme, Monitoring of electromagnetic characteristics of split cylinder resonator and dielectric material for temperature characterization, in: European Microwave Conference, Rome, Italy, October, 2014. [9] H. Manshor, S.M. Aris, A. Zahiram, A. Azhar, E.C. Abdullah, Z.A. Ahamad, Effects of TiO2 addition on the phase, mechanical properties, and microstructure of zirconia-toughned alumina ceramic composite, Ceram. Int. 41 (2015) 3961–3967. [10] R. Bagley, I.B. Culter, D.L. Johnson, Effect of TiO2 on initial sintering of Al2O3, J. Am. Ceram. Soc 53 (1970) 136–141. [11] C. Legros, F. Herbst, C. Carry, P. Bowen, Effect of Ti-doping on the sintering behavior of transition alumina, Key Eng. Mater. 206–213 (2002) 357–360. [12] S. Lartigue-Korinek, C. Legros, C. Carry, F. Herbst, Titanium effect on phase transformation and sintering behavior of transition alumina, J. Eur. Ceram. Soc. 26 (12) (2006) 2219–2230.