Betsch R. G.

Haki B. W.

Heidinger R.

Molla J.

Neil McN. Alford

Reece M.

Stuart J. Penn

Woode R. A.

Journal of Applied Physics

Crossref

Sintered alumina with low dielectric loss

Alford, NM

Penn, SJ

Spiral - Imperial College Digital Repository

Sintered alumina with low dielectric lossNeil McN. Alforda) and Stuart J. PennSchool of Electrical, Electronic and Information Engineering, South Bank University, 103 Borough Road,London SE1 0AA, United Kingdom~Received 23 April 1996; accepted for publication 7 August 1996!Low dielectric loss materials are required for applications in radio-frequency and microwavecommunications. Aluminium is the second most abundant element in the Earth’s crust andaluminium oxide~alumina! is one of the commonest ceramics. Single crystals of aluminium oxide,i.e., sapphire, possess one of the lowest dielectric losses of any material. Polycrystalline alumina hasa higher loss due to extrinsic factors. The dielectric loss of sintered alumina is studied in an attemptto determine the causes of extrinsic loss. Impurities are shown to play an important role, but themicrostructure also is a key factor. High-purity aluminas, sintered to near theoretical density, arefound to display very low loss, tand52.731025 at 10 GHz. Doping alumina with titanium dioxidewas found to reduce the tand5231025. © 1996 American Institute of Physics.@S0021-8979~96!00922-X#n-artsnengsehanteredssForsllybyat-r,deeeithasedss.n-I. INTRODUCTIONAluminium oxide ~alumina! Al2O3 is used as a ceramicpackaging material because of its high thermal conductivisuitable dielectric constant, and low dielectric loss. Mostthe properties of so-called ‘‘high-purity’’ alumina, e.g., thermal expansion, thermal conductivity, elastic modulus, meing point, Poisson’s ratio, and dielectric constant, fall withifairly narrow limits for different samples from a wide rangof sources. The dielectric loss, however, is highly variabfrom sample to sample varying over several orders of manitude. It is this issue that we address and in particular wshow that the dielectric loss of sintered materials can aproach that measured in single crystals.Dielectric loss is conventionally described by the imagnary component of the permittivity, or by tand, the ratio ofthe imaginary to the real part of the permittivity. The lossalso sometimes expressed in terms of the quality factorQwhich in the absence of other sources of loss51/tand. TheQvalues quoted in this article are corrected for other sourcesloss, e.g., the copper walls of the resonator, so thatQ51/tand. This work is concerned with the cause of dielectrloss and how it can be reduced. Losses fall into two categries: intrinsic and extrinsic. Intrinsic losses are dependentthe crystal structure and can be described by the interactof the phonon system with the ac electric field. The ac eletric field alters the equilibrium of the phonon system and thsubsequent relaxation is associated with enerdissipation.1,2 A fundamental review of the theory of intrin-sic losses is presented by Gurevich.2 These intrinsic lossesset the lower limit of losses found in pure ‘‘defect-free’single crystals. Extrinsic losses are associated with imperftions in the crystal structure, e.g., impurities, microstructurdefects, grain boundaries, porosity, microcracks, randocrystallite orientation. It is clear from our experiments, anthose of others, that the loss in sintered ceramics is limitby these extrinsic factors. The huge differences in micrstructure and perfection between single-crystal sapphire aa!Electronic mail: alfordn@vax.sbu.ac.ukJ. Appl. Phys. 80 (10), 15 November 1996 0021-8979/96/80(10)Downloaded¬23¬Dec¬2005¬to¬155.198.215.35.¬Redistribution¬subjecty,of-lt-neleg-ep-i-isofico-onionc-egy’ec-almdedo-ndpolycrystalline alumina are clear indicators why the convetional wisdom has assumed the impossibility of achievingdielectric loss approaching that of single-crystal counterpain the sintered materials. It is of general interest to examithe potential of polycrystalline ceramics and alumina is aexcellent model.A single crystal is made from a melt and the meltinpoint of alumina is 2072 °C. Sintering a ceramic involvetaking a fine powder of the material, pressing it into thdesired shape, and then heating it to temperatures less ttheir melting point~usually about 75% of the melting point!.In an effort to reduce surface energy, the powders sintogether, reducing surface area until the porosity is reducsubstantially or entirely. The sintering process involves leexpensive capital equipment and is less energy intensive.example, a single crystal of alumina in cylindrical form iaround 10 000 times more expensive than an identicashaped sintered alumina. Single-crystal sapphire suppliedESCETE measured in our laboratory has a tand of 131025.The lowest claimed dielectric loss in sintered aluminaroom temperature~'25 °C! and microwave frequencies wasby Woodeet al.3 who measured a tand of 4.331025 ~Q523 356! at approximately 9 GHz. This value was a significant improvement over earlier studies, e.g., Mollaet al.,4who measured tand5331024 ~Q53333!. However, in com-prehensive studies on very pure alumina, Molla, Heidingeand Ibarra5,6 and Kobayashi and Katoh7 recorded low lossesin the region tand5231025. Molla and co-workers5 sug-gested that these losses were intrinsic in origin. Wooet al.3 noted that purity alone was a poor indicator of thdielectric loss tangent. In order to verify this statement whave measured several so-called high-purity aluminas wvarying degrees of impurities present. In contrast, it wfound that purity does appear to influence theQ; however,although an impure alumina always gives a poorQ, a verypure alumina is not a guarantee of a highQ. Both high purityand the correct processing are required for a highQ. It mustbe pointed out that the pure alumina may be purposely dopwith certain elements in order to assist the sintering proceIt is therefore vital that these additions do not adversely i5895/5895/4/$10.00 © 1996 American Institute of Physicst¬to¬AIP¬license¬or¬copyright,¬see¬http://jap.aip.org/jap/copyright.jspstestichrsoaandin-asoftyeofn-asedfluence theQ. For example, aluminas with high levels oimpurities such as alkali salts~particularly sodium! are seento possess a higher dielectric loss. Also, metallic impuritimight be expected to reduceQ and this is observed in thestudies of the impure alumina powders discussed below.II. EXPERIMENTTo investigate the difference between high-purity powders, ceramic pucks were produced from a range of commcially available high-purity powders. The powders werpressed in a 13-mm-diam stainless-steel die press at a psure of 100 MPa. The pressed samples were sintered in a1550 °C for 300 min. The sample density was measurednoting the sample mass and dimensions. The dielectric cstant and dielectric loss were measured at 9 GHz usingparallel-plate resonator~modified Hakki–Coleman8! tech-nique. To reduce the influence of the copper plates on theQmeasurement the dielectric sample is placed in the gaptween the copper plates on a 4-mm-high, 10-mm-diamedisk of low-loss quartz. The sintered alumina samples wealso approximately 10-mm-diam, 4-mm-thick disks. ThTE01d mode was examined using a Hewlett–PackaHP8719C vector network analyzer with 1 Hz resolution. Adielectric measurements were carried out at room tempeture in air at a relative humidity of approximately 30%. Nspecial precautions were taken to prevent the adsorptionwater to the sample surface which might be expected to haan adverse effect on the dielectric loss. The disks were msured in the ‘‘as-fired’’ condition, i.e., without surface preparation such as polishing. The loss measurements are psented in terms of theQ factor, i.e.,~tand!21. The effect ofthe copper plates on theQ was corrected for using the Itoh–Rudokas model.9 Great care was taken to ensure that thsampleQ was measured correctly. In order to test the mesurement equipment a range of ceramic dielectric resonamaterials from different manufacturers~Morgan Matroc,Tekelec, Murata! were tested. In all cases the sampleQwhich was measured agreed closely with the manufacturemeasurements. These samples generally possessed a hdielectric constant so that in practice the experiment wmore straightforward due to the fact that more of the eletromagnetic field is confined in the sample. In alumina, wia dielectric constant of approximately 10, care must be takto avoid radiation losses and this was achieved by carefuadjusting the distance between the gaps between the copplates and the sample. A major factor was to ensure paralism of the copper plates with respect to the sample as onlfew microns misalignment can have an adverse effect onQ. Results have been confirmed using a cavity resonatechnique. Table I shows the impurity analyses from thpowder manufacturers of powders A–E and also gives tsintered density and measuredQ at 9 GHz. The major im-purities consist of Na, Mg, Si, Fe, Ca, and Cu. All the powders used were commercial alpha alumina powders wsimilar surface areas. Other alumina powders were atested which had surface areas of approximately 1 m2 g21.The sintered density of these powders was too low for meaingful comparisons to be made.5896 J. Appl. Phys., Vol. 80, No. 10, 15 November 1996Downloaded¬23¬Dec¬2005¬to¬155.198.215.35.¬Redistribution¬subjecfes-er-eres-ir atbyon-TABLE I. Chemical analyses~ppm! and surface areas of aluminum oxidepowders used. Sintered density andQ at 9 GHz is also shown.PowderImpurity Na2O 800 300 600 20 6~ppm! MgO 400 1000 1800 16 5SiO2 400 400 400 85 18Fe2O3 300 100 200 29 11CaO 200 40 300 ••• •••SO3 ••• ••• ••• ••• •••CuO ••• ••• ••• 13 4Surface area~m2 g21! 7 5 3 5–10 9–16Density ~%! 97.8 98.2 79.2 98 99Q ~at 9 GHz! 3500 1000 4500 37 000 35 000abe-terreerdllra-oofveea--re-ea-torr’sigherasc-An increased level of impurities in the alumina powderwas observed to cause a deterioration in theQ. Indeed, insome less pure aluminas tested theQ was so poor as to beunmeasureable. The two purest aluminas gave the highesQbut the purest alumina, alumina E did not achieve the highQ and this may be due to a powder surface area effect whhas a large influence on the manner in which the powdesinter. The effect of porosity on the dielectric loss was alschecked and found to have a very significant effect. AluminD was chosen and sintered at temperatures between 10001550 °C for 5 h in order to vary the final density. Figure 1shows the effect of sample density on theQ and demon-strates the extreme importance of microstructure in determing the extrinsic loss. The theoretical density of alumina wtaken as 3.97 Mg m23 and the correlation coefficient be-tweenQ and density wasr50.87, which is significant at the99.9% confidence level. Because of the variation inQ withdensity we have compared samples with varying levelsimpurity at near-theoretical density. At the highest densithe Q was found to be 37 000. The microstructure of thsamples with varying porosity is seen in Figs. 2~a! and 2~b!.The difference in microstructure is clearly seen in termsgrain size and porosity. We also measured the dielectric costant of the aluminas and found that it behaved muchexpected decreasing as pore volume increased and followa Maxwell mixture law10 fairly closely.thenllyperlel-y athetorehe-ithlson-FIG. 1. The effect of density on theQ at 9 GHz of polycrystalline alumina.N. McN. Alford and S. J. Pennt¬to¬AIP¬license¬or¬copyright,¬see¬http://jap.aip.org/jap/copyright.jspFIG. 2. Scanning electron micrograph showing variation in microstructure with density:~a! alumina sintered at 1100 °C, 5 h, 40% porosity;~b! aluminasintered at 1550 °C, 5 h near full density.--rftnc-10ityIn the second series of experiments a high-purity almina~alumina D! was doped with titania, TiO2. The additionof TiO2 to sapphire single crystal has been noted bHeidinger11 where he examined sapphire in which 0.02 wt %TiO2 was added to the melt. The tand of the Ti:sapphire at300 K was not improved over an undoped single crystmade by the Czochralski technique and a single crystal maby the Bridgeman technique at 145 GHz. The tand at thisfrequency of all three of the single crystals was approxmately 231024; however the TiO2 doping level was lower incomparison with the levels reported below. In the followinexperiments, the TiO2 was added to the alumina by thor-oughly mixing TiO2 powder with the alumina powder. Theaddition of TiO2 influences the sintering temperature awhich the material achieves high density and for this reasthe aluminas doped with TiO2 were sintered at 1500 °C. Theresults of doping the alumina with TiO2 on theQ are shownin Fig. 3 and Table II. It is seen that theQ increases to amaximum of 50 300 at a doping level of 0.5 wt % TiO2; thisis about 50% of theQ of a high-quality single crystal.lyg.g-atFIG. 3. The effect of TiO2 doping on theQ of alumina at 9 GHz.J. Appl. Phys., Vol. 80, No. 10, 15 November 1996Downloaded¬23¬Dec¬2005¬to¬155.198.215.35.¬Redistribution¬subjecu-yaldei-gtonSamples of TiO2 sintered in air to full density display a verylow Q ~1500! compared with that of a single crystal of TiO2which had a measuredQ of 7000. Pure stoichiometric TiO2is white in color, the ceramics sintered in air show a darkening to a brown-tan color, this being due to the fact that TiO2is easily reduced.12–14The samples of alumina doped with titania display a bluish tinge and a slight darkening from their usual white coloand this may be due to a stoichiometric deficiency ooxygen.14 It might be expected that doping with TiO2 wouldreduce theQ due to the inclusion of a high-loss componenand possible reduction of the oxide. The fact that there is aenhancement in theQ is surprising and it is not clear atpresent whether the TiO2 plays an active role in reducing theloss or whether the addition of TiO2 has simply improved themicrostructure by acting as a sintering aid. The microstruture of the 0.5 wt % TiO2 doped alumina is shown in Fig. 4where it is seen that the grain size varies between 2 andmm. The grain structure is quite different in comparison withthe undoped alumina which achieved near-theoretical densat 1550 °C@Fig. 2~b!# requiring a higher temperature com-pared with the TiO2-doped alumina which achieved near-theoretical density at 1500 °C~Fig. 4!. The 0.5 wt %TiO2-doped alumina shown in Fig. 4 possesses a slightlarger grain size than the undoped alumina shown in Fi2~b! even though the sintering temperature was lower sugesting that the TiO2 has indeed acted as a very effectivesintering aid. Energy-dispersive x-ray analysis revealed thTABLE II. Effect of TiO2 dopant on theQ of alumina at 9 GHz.TiO2 ~wt %! Q0 37 0000.25 40 5990.5 50 3101 36 1705 12 65010 73765897N. McN. Alford and S. J. Pennt¬to¬AIP¬license¬or¬copyright,¬see¬http://jap.aip.org/jap/copyright.jspedantss;foratd,ro--p-reFIG. 4. Scanning electron micrograph of sintered alumina doped with 0.5TiO2 sintered at 1500 °C, 5 h.irentlyinessa-atn-fulAthe Ti is located in discrete grains at the triple points of thalumina grains. Transmission electron microscopy peformed on 12 of these grains further indicates that the Al:ratio is very close to of 2:1 suggesting that aluminum titaate, Al2TiO5, has formed. The samples appear highstrained as seen in Fig. 5 where the presence of dislocaloops is clearly seen. It is possible that the strain presentthe sintered samples has a direct effect on the dielectric lohowever, this is the subject of further study. The 10 wtTiO2-doped alumina grain morphology is entirely differenconsisting of larger, rounded, and equiaxed grains due topresence of larger amounts of aluminum titanate.a-rerys..EFIG. 5. Transmission electron micrograph showing aluminum-titanate-rregion at the triple point of alumina grains and the presence of dislocatloops. Marker is 500 nm.5898 J. Appl. Phys., Vol. 80, No. 10, 15 November 1996Downloaded¬23¬Dec¬2005¬to¬155.198.215.35.¬Redistribution¬subjecer-Tin-lytioninss;%ttheSubstituting Nb and Y for Ti also produces an enhancQ but Zr did not alterQ indicating that at least certain ele-ments close to Ti in the periodic table when used as a dopdo not increase loss and indeed could even reduce the lohowever, in these doping experimentsQ was found to beextremely sensitive to the sintering regime. The reasonsthe reduction in dielectric loss with the addition of TiO2 arenot fully understood; however the possibilities are: first, ththe reduction of the titanium oxide is responsible; seconthat the presence of strain due to TiO2 addition is a majorfactor; or, finally, that the addition of the TiO2 has simplyacted as a sintering aid and has improved the overall micstructure of the samples.III. CONCLUSIONSIn conclusion, we have shown that high-purity polycrystalline sintered alumina can achieve low dielectric loss. Doing the alumina with titania, TiO2, has produced a materialwith an exceptionally low dielectric loss at room temperatuand at microwave frequencies approaching that of sapphsingle crystals. Low-temperature measurements are currein progress and indicate that significant further reductionthe loss is achievable. Further experiments are in progrinvolving the use of superconducting shields at low temperture and the indications in preliminary experiments are ththere is a considerable enhancement in theQ at cryogenictemperatures.ACKNOWLEDGMENTSThanks are due to John Gallop of NPL and Tony Ceteno and Jonathan Breeze of Matra Marconi Space for usediscussions. Special thanks are due to Norbert Klein of KFJülich for carrying out measurements at cryogenic tempertures the results of which are due to be reported. We aindebted to Kevin Schrapel and Mike Reece at Queen Maand Westfield College, London for x-ray and TEM analysiThis work is supported in part by an EPSRC grant.1V. B. Braginsky and V. S. Ilchenko, Phys. Lett. A120, 300 ~1987!.2V. L. Gurevich and A. K. Tagantsev, Adv. Phys.40, 719 ~1991!.3R. A. Woode, E. N. Ivanov, M. E. Tobar, and D. G. Blair, Electron. Lett30, 417 ~1994!.4J. Molla, A. Ibarra, J. Margineda, J. M. Zamarro, and A. Hernandez, IEETrans Instrum. Meas.42, 817 ~1993!.5J. Molla, R. Heidinger, and A. J. Ibarra, Nucl. Mater.212–215, 1029~1994!.6A. Ibarra, R. Heidinger, and J. Molla, J. Nucl. Mater.191–194, 530~1992!.7Y. Kobayashi and M. Katoh, IEEE Trans. Microwave Theory Tech.33,586 ~1985!.8B. W. Haki and P. D. Coleman, IEEE Trans. Microwave Theory Tech.8,402 ~1960!.9D. Kajfez and P. Guillon,Dielectric Resonators~Artech House, Zu¨rich,1986!.10W. D. Kingery, H. K. Bowen, and D. R. Uhlmann,Introduction to Ce-ramics ~Wiley, New York, 1976!, Vol. 2.11R. Heidinger, J. Nucl. Mater.212–214, 1101~1994!.12M. Reece and R. Morrell, J. Mater. Sci.26, 5566~1991!.13R. G. Betsch, H. L. Park, and W. B. White, Mater. Res. Bull.26, 613~1991!14F. A. Grant, Rev. Mod. Phys.31, 646 ~1959!.%ichionN. McN. Alford and S. J. Pennt¬to¬AIP¬license¬or¬copyright,¬see¬http://jap.aip.org/jap/copyright.jsp

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Sintered alumina with low dielectric loss

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