MICROSTRUCTURE STUDY AND CRYSTALLINE PHASE FORMATION ON Nb 2 O 5 -Ba 9 Ti 2 O 20 MICROWAVE RESONATORS

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1 MICROSTRUCTURE STUDY AND CRYSTALLINE PHASE FORMATION ON Nb 2 O 5 -Ba 9 Ti 2 O 20 MICROWAVE RESONATORS Maria d Carm de A. Nn 1, and Pedr Jsé de Castr 2 2 Labratóri Assciad de Sensres e Materiais - LAS Institut Nacinal de Pesquisas Espaciais - INPE C.P Sã Jsé ds Camps, SP - maria@las.inpe.br 1 Labratóri Assciad de Plasma - LAP Institut Nacinal de Pesquisas Espaciais - INPE C.P Sã Jsé ds Camps, SP - castr@plasma.inpe.br Keywrds: dielectric resnatrs, crystalline phases, micrstructures, micrwave measurements ABSTRACT We reprt n the develpment f barium nantitanate (Ba 2 Ti 9 O 20 ) dielectric resnatrs (DRs) fr micrwave applicatins and their characterizatin regarding ceramic and high-frequency dielectric prperties. Thse DRs shuld meet the requirements f the dielectric cnstant as well as the quality factr high values and frequency high stability, fr applicatin as lcal scillatr f an INPE Cmmunicatin Satellite. The ceramic were prepared using suitable pwder mitures cmpsed f BaCO TiO 2 withut and with additin f sme cntents f Nb 2 O 5 (0.1, 0.2 and 0.4 % ml), cmpacted by isstatic pressing with 300 MPa and sintering at 1360 C in apprpriate atmsphere. The initial pwders and the sintered ceramics were characterized by X-rays diffractin and scanning electrnic micrscpy. The values f dielectric cnstant, resnance frequency, laded quality factr due t the dielectric lsses, and cefficient f frequency variatin with the temperature was btained using a special metallic b fr these measurements. The cmparative analyses f these results shwed that the Nb 2 O 5 cntent increases the amunt f the frmed phase Ba 2 Ti 9 O 20 and the ceramic densificatin degree. 1. INTRODUCTION Several kinds f dielectric ceramic have been develped fr applicatins in micrwave telecmmunicatin and satellite bradcasting systems [1-7]. These ceramics pssess a high dielectric cnstant, lw dielectric lsses, small temperature cefficient f the resnant frequency and small specific weight. In a near future, dielectric ceramic are epected t have greater use in wireless cables, high definitin and interactive TVs, glbal psitining systems and in many ther kinds f persnal cmmunicatin systems [2,4]. Several ceramic cmpsitins based n barium and titanium cmpunds [1-3] have presented gd dielectric prperties, such as BaTi 4 O 9, BaTi 5 O 11, and Ba 2 Ti 9 O 20. This research wrk represents ne f the first attempts in rder t prduce DRs with Brazilian raw materials. Such DRs can be used fr applicatin as lcal scillatr f a Cmmunicatin Satellite perating in C-band whse cnstructin will invlve INPE. Accrding t phase equilibrium diagram f the BaO - TiO 2 system (Fig. 1(a)), the stability regin f the Ba 2 Ti 9 O 20 phase ccurs in a narrw range f cmpsitin. Fr temperature lwer than 1300 C and 81.8 ml % TiO 2 and 18.8 ml % BaO is pssible t btain the Ba 2 Ti 9 O 20 phase [1].

2 The behavir the dielectric characteristics as functin f the density f the Ba 2 Ti 9 O 20 ceramic is shwn in the Fig. 1(b) curves. The dielectric cnstant (ε) and quality factr high (Q) values were increased prprtinally t the density. The frequency high stability (τ f ) curve shws an inverse behavir in relatin t the thers. Hwever, accrding t thse results, dielectric parameter values are affected by the ceramic prsity cntent. Accrding t the research referred in [4], additins such as Nb, Ca r Ta in the Ba 2 Ti 9 O 20 ceramics imprve their dielectric characteristics. But in the literature there is nt reference abut the influence f these chemical elements n the prsity f these ceramics. Q 5000 ε r TiO BaO 4-6 GHz D teórica = 4.8 g.cm ε r Q (ppm. C) 20 τ f τ f Prsity 0 3,4 3,6 3,8 4,0 4,2 4,4 4,6 4,8 Density ( g.cm -3 ) (a) (b) Figure 1. (a) Phase equilibrium diagram fr Ba 2 Ti 9 O 20 regin frm 1100 t 1400 C f the system BaO-TiO 2 [1] and (b) effect f density n the dielectric characteristics f Ba 2 Ti 9 O 20 ceramic [1,5] The present research wrk intends t study Ba 2 Ti 9 O 20 DRs manufactured frm Brazilian raw materials, as well as the influence f Nb cntents (0.2 and 0.4 ml %) n phase amunts, micrstructure and micrwave dielectric parameters. 2. EXPERIMENTAL PROCEDURE Fr the preparatin f this type f Ba 2 Ti 9 O 20 ceramic was used BaO and TiO 2 ml cmpsitin fractin, frm BaCO 3 and TiO 2 suitable mitures. The mitures received an additin f different amunts f Nb (as Nb 2 O 5 ). The Nb ml percentages added t the initial pwder miture were: 0.2 and 0.4 ml % (r weight %). The pwder miture was cmpacted by a uniaial (40 MPa) and an isstatic (300 MPa) pressing, prducing cylindrical test bdies with relatinship pre-established H/D (where H is height and D the diameter f the test bdy), in rder t prduce sintered ceramics with suitable dimensins fr the desired resnant frequency range. The samples were sintered at 1360 C fr 3 hurs. Afterwards, the ceramic crystallgraphic phases were characterized by X-ray diffractin. The values f the dielectric parameters were measured at micrwave frequencies: resnant frequency (f), dielectric cnstant (ε), laded quality factr (Q L ), and temperature cefficient (τ f ).

3 Micrwave characteristics were determined using a test cavity [6,7]. The micrwave measurements are perfrmed accrding t the test setup shwn in Fig. 2. In these tests we d nt use any temperature cmpensatin technique. The resnant frequencies were written dwn right away after every five degree temperature in the eperiments. The frequency cunter shuld have an accuracy f at least 10 khz in the GHz range. Netwrk Analyzer Analisadr de Rede Sweep Oscillatr Geradr de Varredura Test Cavity Caia de teste metálica Medidr de Frequência Frequency Cunter Detectr RF DR RD RF Directinal Acpladr Direcinal Cupler Electric Prbe Snda Eletrica Figure 2. Eperimental setup fr measuring the micrwave parameters It is knwn that the final value fr τ f depends n the thermal epansin cefficient f the dielectric material, the thermal epansin cefficient f the surrunding material (aluminum) and the dielectric cnstant variatin with temperature. All micrwave measurements were perfrmed fr a pure barium nantitanate sample, anther with 0.2 and the ther with 0.4 ml % nibium ide cntent. Their micrwave perfrmance was cmpared with measurements f a cmmercial imprted sample frm Trans-Tech with the same cmpsitin (barium nantitanate). 3. RESULTS AND DISCUSSIONS The Trans-Tech DR (Fig. 3a) and the ceramics withut nibium additin (sintered at 1360 C in air) (Fig. 3b) analyzed by X-ray diffractin have presented the barium nantitanate as the majr phase. The analysis results f the samples with different amunts f Nb cntent, shw the Ba 2 Ti 9 O 20 phase and anther nn identified cmpund (Fig. 3c and 3d). This ther phase is frmed by a cmpund f BaO - TiO 2 - Nb 2 O 5 system that ccurred during the ceramic synthesis and the sintering [5]. Accrding t the literature this system can present a great number f ternary phases, hwever, the accurate cmpsitin and the structure f these phases were nt still identified, due t the present small amunt in the ceramic. The analyses f X-ray diffractin fr the ceramic with nibium additin shwed that this cntributes t the stabilizatin f the Ba 2 Ti 9 O 20 phase (Fig. 3). The ceramic with Nb additin f 0.4 ml % presented value f Q c higher than the thers, prbably due t the fact f the ceramic t present significant amunt f a cmpsitin frmed between the nibium and the ther cmpnents f the system, that was nt pssible t be identified s far. Hwever the ceramic with 0.2 ml % Nb presented the least abslute value f τ f. Ba 2 Ti 9 O 20 ceramics with Nb additin develped in this wrk presented better results f τ f than that f Trans-Tech, measured in the same eperimental cnditin. This fact might be due t the presence f small quantity f Nb and Sr in the Brazilian raw materials.

$X-ray diffractin patterns fr the investigated ceramics: (a) Trans-Tech DR, (b) withut Nb additin, (c) with 0.2 ml % Nb, and (d) with 0.$

4 Ba 2 Ti 9 O 20 Cmpund with Nb (d) Intensity (a. u.) (c) (b) (a) Figure 3. X-ray diffractin patterns fr the investigated ceramics: (a) Trans-Tech DR, (b) withut Nb additin, (c) with 0.2 ml % Nb, and (d) with 0.4 ml % Nb The micrstruture f the ceramic manufactured by Trans-Tech is shwn in Fig. 4. presenting small grains and pres. The scanning electrnic micrscpy (SEM) analyses f the ceramic fracture surfaces shwed a high densificatin degree f the micrstruture and the presence f few pres (Fig. 5). The grain sizes f the Ba 2 Ti 9 O 20 ceramic withut Nb 2 O 5 additin btained in this wrk were larger than thse which are present in the DR manufactured by Trans-Tech, prbably due t presence f small amunts f Nb cntent in the raw materials. These defects are due t the packing pwder flaws ccurred during the cmpacting stage. The deffects can be minimized with the ptimizatin f the grain size distributin f these pwder mitures, that can be btained by a suitable prcedure in the grinding stage. The densificatin degree and grain size increase with the amunt f Nb 2 O 5 added in the Ba 2 Ti 9 O 20 ceramic. This indicates that this addictive caused an increase in the system atmic mbility due t the thermal treatment used fr the sintering and Ba 2 Ti 9 O 20 phase synthesis f these ceramics. 2Θ Figure.4. SEM micrgraphs f Ba 2 Ti 9 O 20 ceramic fracture surfaces frm Trans-Tech

$Figure.5 SEM micrgraphs f Ba 2 Ti 9 O 20 ceramic fracture surfaces: a) withut Nb 2 O 5 additin, b) with 0.1 ml% Nb 2 O 5, c) with 0.2 ml% Nb 2 O 5 and d) with 0.$ 6 shws the evlutin f the resnant frequency variatin with temperature during the thermal prcess in the range -20 C t +50 C, fr the samples under tests.

6 shws the evlutin f the resnant frequency variatin with temperature during the thermal prcess in the range -20 C t +50 C, fr the samples under tests.

The ceramic 00Nb reached the frequency stability frm 20 C n, while the Trans-Tech sample (DR-TT), frm 35 C n.

psitive temperature cefficient. This fact might be due t a greater quantity f Nb in these DRs [5]. Table 1.

5 Figure.5 SEM micrgraphs f Ba 2 Ti 9 O 20 ceramic fracture surfaces: a) withut Nb 2 O 5 additin, b) with 0.1 ml% Nb 2 O 5, c) with 0.2 ml% Nb 2 O 5 and d) with 0.4 ml% Nb 2 O 5 The results f the dielectric measurements at micrwave frequencies fr the ceramics analyzed are presented in Table 1 and Fig. 6 shws the evlutin f the resnant frequency variatin with temperature during the thermal prcess in the range -20 C t +50 C, fr the samples under tests. Accrding t the Table 1 and cnfirmed by the curves in Fig. 6, the Ba 2 Ti 9 O 20 DRs 00Nb and DR-TT have a negative temperature cefficient. The ceramic 00Nb reached the frequency stability frm 20 C n, while the Trans-Tech sample (DR-TT), frm 35 C n. On the ther hand, bth 02Nb and 04Nb, that have nibium additin, presented a ppsite behavir, that is t say, their resnant frequency increases with temperature, hence yielding a psitive temperature cefficient. This fact might be due t a greater quantity f Nb in these DRs [5]. Table 1. Measured values f the micrwave parameters fr the ceramic resnatrs DR H ± 0,01 a ± 0,01 f ± Q L ε± 0,3 τ f cde [mm] [mm] [GHz] (ma.) [ppm/ C] 00Nb , Nb , Nb , DR-TT , Nb: ceramic withut Nb 2 O 5 additin; 02Nb: ceramic with 0.2 ml% Nb; 04Nb: ceramic with 0.4 ml% Nb and DR-TT: Trans-Tech DR

6 FREQÜÊNCIA RESONANT DE FREQUENCY RESSONÂNCIA [GHz] [GHz] 6,5261 6,5254 6,5247 6,5240 6,5233 6,056 6,054 6,052 6,050 5,9705 5,9700 5,9695 5,9690 5,9685 5,7930 5,7925 5,7920 5,7915 DR-TT 04Nb 02Nb 00Nb TEMPERATURE TEMPERATURA [ O C] Figure 6. Measurements f resnant frequency variatin with temperature in the range -20 C t +50 C fr DRs referred in Table 1 4. CONCLUSIONS The preliminary results btained in this wrk are very prmising fr btaining and Ba 2 Ti 9 O 20 phase stabilizing f dielectric resnatrs, including thse with Nb additin in these ceramics. We can say that these results are better than were epected. The simultaneus synthesis and sintering prcedure resulted in a suitable way t stabilize the Ba 2 Ti 9 O 20 phase. The values f the micrwave parameters btained in this wrk shwed t be influenced by the amunt f Nb 2 O 5 added t the barium nantitanate ceramic. Such results might be related t the Brazilian raw materials that have small amunt f Nb and Sr. The general micrwave prperties f manufactured ceramics are similar t thse presented by cmmercial imprted samples; even cncerning thermal frequency stability, 00Nb and 02Nb DRs btained better values. Other nibium rate as well as ther element additin (Sr and a Nb-Sr miture) will be investigated in relatin t dielectric prperties in micrwave frequencies. REFERENCES [1] D. Hennings; P. Schnabel, Philips J. Res., v.38, n. 6 (1983), p [2] J.M. Wu; H.W. Wang,, J. Amer. Cer. Sc., v. 71, n. 10 (1988), p [3] P. Tandn; J. Gyette; Y.K.Bse, J. Mater. Sci. Let., v. 14 (1995), p [4] H.M. O Bryan Jr; J. Thmsn Jr, J. Amer. Cer. Sc., v. 66, n. 12 (1983), p. 66 [5] M.C.A. Nn; P.J. Castr, 36 th Brazilian Ceramic Meeting, (1992), p [6] P.J. Castr; M.C.A. Nn, J. Micrwave and Opteletrnics, v.4, n.1 (1999), p. 27. [7] P.J. Castr. Dctr Thesis, POLI/USP, Brazil, 1989.

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