26.2, QuXj= GHz, Tf= -57 ppmf'c) and CeO.3SmO (er = 19.7, QuXj= 79450

Transcription

1 ( Chapter8 ) This chapter features summary of the research work described in this thesis. The significant highlights of the results presented in each chapter are outlined in chronological order. The prospects for extending the work in future directions such as in the area of dielectric resonators, low temperature cojired ceramics and polymer composites are also discussed.

2 The thesis presents microwave dielectric properties of ceria based ceramics useful as dielectric resonators (OR) and low temperature cofired ceramics (LTCC) in the microwave integrated circuits. Ceria based polymer composites useful for electronic packaging applications and Ce02-Lao.sSro.SCo03_& composites for electromechanical applications are also discussed in the thesis. The microwave dielectric properties of the ceramics are tailored by several methods such as doping, glass addition and solid solution formation in the Ce-site of Ce02 with suitable substituents. The first chapter of the thesis explains the fundamental concepts related to dielectric resonators, low temperature cofired ceramics and polymer/ceramic composites. The second chapter describes the synthesis and characterization techniques of the dielectric ceramics and polymer composites in the radio and microwave frequency region. Chapter 3 outlines the general introduction about fluorite structure and a brief history and literature survey related to the dielectric study of cerium oxide ceramics. This chapter details the synthesis, characterization and microwave dielectric properties of CeOrO.5AO-0.5Ti02 (A = Mg, Zn, Ca, Mn, Co, Ni, W) ceramics. These ceramic systems are a composite consisting of fluorite Ce02 and A Ti03 with good densification and interesting dielectric properties depending on the coexisting A Ti03 being formed. The ceramics have relative permittivities (er) in the range 17.0 to 65.5 and quality factor Q,,x!up to GHz. The 1'franges from 399 ppmfc to -62 ppmfc. The quality factor increased considerably on cooling the Ce02-0.5AO-0.5Ti02 (A = Mg, Zn, Ca, Mn, Co, Ni, W) ceramics in a manner, similar to pure Ce02. Highest Q"x!is found for the CeOr 0.5MgO-0.5Ti02 system and at 20 K it reaches as high as GHz. The effect of dopants on the microwave dielectric properties of Ce02-0.5AO-0.5Ti02 (A = Mg, Zn, Ca, Mn, Co, Ni, W) ceramics are investigated. Addition of Cr203 to Ce02-0.5CaO- 0.5Ti02 system improved the 1'f to zero ppmfc from the originally 399 ppmf'c respectively. A quality factor of90000 and GHz is found by the addition of2 wt% W03 and 2 wt% W03+O.5 wt% Ti02 respectively to the Ce02-0.5MgO-0.5Ti02 composite with er - 21 and 1'f- -50 ppmo/c. By combining 0.5 wt% W03 and 20 wt% 282

3 Ti02 to Ce02-0.5CoO-0.5Ti02 ceramic, a Q,;xJ of GHz, er = 21.2 and Tf = +5 ppml C is obtained. A Ti03 (A = Co, Mn, Ni) dielectric ceramics synthesized by the conventional solid state ceramic route have hexagonal symmetry with R'3 (148) space group. The microwave dielectric properties of ATi03 (A = Co, Mn, Ni) ceramics are reported for the first time with er in the range 19 to 25, Q,,xJfrom to GHz and Tf of about -50 ppmfc. The fourth chapter gives the preparation, characterization and microwave dielectric properties of BaO-2Ce02-nTi02 (n = ) and BaO-pCe02-4Ti02 (P = ) ceramics. BaO-2Ce02-nTi02 (n = ) ceramics are multiphase compounds containing Ce02, Ba2Ti902o and Ti02 while BaO-pCe02-4Ti02 (P = ) ceramics consists of Ce02 and BaT409 phases. The er increases from 41.5 to 69.9, Q,;xJdecreases from to GHz and Tf increases from 40 to 272 ppmfc with the value of n in BaO-2Ce02-nTi02 (n = ) ceramics due to the increasing amount of rutile. The er decreases from 32.6 to 26.1, Q,;xJincreases from to GHz and Tfincreases to the negative side from -11 to -41 ppmfc with the value of pin BaO-pCe02-4Ti02 (p = ) ceramics due to the increasing amount of ceria. The effect of different dopants on the microwave dielectric properties of BaO-3Ce02-4Ti02 ceramics is studied and BaO- 3Ce02-4Ti02 doped with 1.5 wt% CuO sintered at 1050 C/2h has er = 30, Q,,xJ= GHz and Tf= -12 ppmfc. This can be used as an ideal candidate for LTCC applications with copper electrode. The 0.5CeOz-0.5BaTi409 sample sintered at 1260 C/2h exhibits excellent microwave dielectric properties: er = 34.5, Q,;xJ = GHz and Tf = 2 ppmfc. The 12 wt% B20 3 and 1 wt% CuO added 0.5Ce02-0.5BaTi409 ceramic sintered at 950 C for 4 hours shows er = 20.6, Q,;xJ = GHz and Tf = 48 ppmfc. Silver remains unreacted with the 0.5Ce02-0.5BaT wt% B wt% CuO composite, which is one of the requirements of LT CC. Chapter 5 reports for the first time the microwave dielectric properties of two new ceramics based on cerium - Ce2(W04)3 and Ba2CeV30Il for microwave substrate applications. The Ce2(W04h and Ba2CeV3011 ceramics sintered at 1000 and 1025 C/4h have a relative permittivity of 10.8 and 14.9 with quality factors of and

4 GHz and temperature coefficient of resonant frequency, -39 and -14 ppmf'c respectively. The Ba3V4013 which formed as a secondary phase along with Ba2CeV30 11 ceramic has been prepared separately and has relative permittivity 12, quality factor GHz and rf = -67 ppmf'c at a sintering temperature of 700 C/4h. Glasses such as B20 3, ZnO B20 3, BaO-B20 r Si02, ZnO-B20 3-Si02 and PbO-B20 3-Si02 are added to Ce2(W04)3 and Ba2CeV3011 ceramic to lower the sintering temperature for LTCC applications. The microwave dielectric properties of 0.2 wt% ZBS added Ce2(W04)3 ceramic sintered at 900 C and 1 wt% ZBS glass added Ba2CeV3011 ceramic sintered at 825 C are G r = 11.3 and 15.1, QuXj= and GHz and rf= -39 and -21 ppmfc respectively. The 0.2 wt% ZBS added Ce2(W04)3 and I wt% ZBS glass added Ba2CeV3011 ceramic are possible candidates for L TCC applications. The influence of rare earth substitution in Ce site of Ce02 ceramics are outlined in Chapter 6. Ceramic compositions in CeJ-xREx02_o (RE = La, Nd, Sm, Eu, Gd, Dy, Er, Tm, Yb and V), 0 ~ x ~ 1 have been prepared and the solubility of different rare earth and yttrium ions in ceria is found out. The microwave dielectric properties of Cel_ xrex02_o (RE = La, Nd, Sm, Eu, Gd, Dy, Er, Tm, Yb and Y) for different compositions from for OS xs 1 are measured and found that these properties depend on the solid solution formation at different levels of substitution ion. The best properties are obtained for Ceo.9Ell{1.I01.95 (er = 25.4, Q,,xj= GHz, fj = -64 ppmf'c); Ceo.gDYo (er = 26.2, QuXj= GHz, Tf= -57 ppmf'c) and CeO.3SmO (er = 19.7, QuXj= GHz, Tf= -51 ppmfc). Ceria based composites for microwave substrate and electronic packaging applications are described in Chapter 7. The first section of this chapter discusses the glass-ceria composites for L TCC applications. Different weight percentage of glasses such as B20 3, B20 3-Si02, Ah03-Si02, ZnO-B20 3, BaO-B20 3-Si02, MgO-B20 3-Si0 2, PbO-B20 3-Si02, ZnO-B20 3-Si02, 2MgO-AI20 3-5Si02, Li20-B20 3-Si02, Bh0 3-ZnO- B20 3-Si02 and Li20-MgO-ZnO-B203-Si02 are added to Ce02 powder. The 0.5 wt% of glass fluxed Ce02 gives the maximum density and best microwave dielectric properties. 284

5 Ce02 added with 20 wt% B20 3 (sintered at 900 C) shows e,.= 13.2, Q..x/= GHz, TI = -46 ppmfc and that of Ce02 added with 10 wt% Bh03-ZnO-B203-Si02 (sintered at 950 C) has er = 22.4, Q..x/= GHz, TI= ppmfc. No reaction with silver is observed for 20 wt% B20 3 and 10 wt% Bi203-ZnO-B203-Si02 added Ce02 ceramic. The second section of Chapter 7 deals with the polymer/ceria composites for electronic packaging applications. PTFE based composites are prepared by powder processing technique and HOPE composites by sigma-blend method. In both PTFE and HOPE based composites, with the increase of filler content, the packing of particles grew denser and indicated the excellent compatibility between polymer and Ce02 particles. The relative permittivity (cejj), dielectric loss (tan 0) and microhardness increased with increase in Ce02 content. The thermal conductivity (k c ) increased and coefficient of thermal expansion (a c ) decreased with increase in Ce02 content for both PTFE and HDPE composites. For 0.6 volume fraction loading of the ceramic, the PTFE composite has cejj = 5 and tan 0 = (at 7 GHz), kc = 1.85 W/moC, ac = 22.9 ppmfc and Vickers' microhardness of 17 kglmm 2. The 0.5 volume fraction of HDPE/Ce02 composite has cejj = 6.9 and tan 0 = (at 7 GHz), kc = 3.22 W/moC, ac = 78.5 pprnl C and Vickers' microhardness of 28 kglmm 2. Different theoretical approaches have been employed to predict the effective permittivity, thermal conductivity and coefficient of thermal expansion of composite systems and the results are compared with that of experimental data. Due to the reasonably good dielectric, thermal and mechanical properties of HDPE composite (0.5 VI) compared to PTFE composites, 0.5 VI HDPE Ce02 can be considered as a possible candidate for microwave substrate applications. The last section of Chapter 7 explains the giant permittivity of Ce02- Lao.5SrO.5Co03_0 (LSCO) composites. The variation of relative permittivity with sintering temperature shows that the permittivity is maximum at I 150 C for all volume fractions. The permittivity decreases with further increase in sintering temperature. The relative permittivity is maximum (er > 10 5 ) at 0.4 fj of LSCO at a sintering temperature of I 150 C. For all volume fractions of LSCO, the relative permittivity of Ce02-LSCO 285

6 CHAPTERB composites is almost independent of temperature. Ce02-LSCO composites can replace the present cermets used in electromechanical and embedded passive devices due to its high permittivity. The following table gives the important and selective ceria based microwave dielectric resonator and L TCC materials, their sintering temperatures and microwave dielectric properties developed during the course of this Ph. D. work. 286

7 CHAPTER 8 SI. ST p Br Q~f 'rf No Material (GC) (glcm 3 ) (GHz) (ppml"c) 1 Ce CeOrO.5CaO-.5Ti Ce02-0.5MgO-0.5Ti # 4 CeOrO.5ZnO-0.5Ti CeOrO.5MnO-0.5Ti Ce02-0.5CoO-0.5Ti Ce02-0.5NiO-0.5Ti Ce02-0.5W0 3 -O.5Ti Ce02-0.5CaO-.5Ti wt% Cr CeOrO.5MgO-0.5Ti wt% W03 11 Ce02-0.5MgO-0.5Ti wt% W wt% Ti02 12 Ce02-0.5CoO-0.5Ti wt% W03 13 Ce CoO-0.5Ti wt%W03+20 wt% Ti02 14 CeOrO.5CoO-0.5Ti wt% Ce02 15 CoTi MnTi NiTi BaO-2CeOr 7Ti BaO-3Ce02-4Ti Ce02-0.5BaTi Ba2CeV Ce2(W04) Ba3V CeO.3Ndo CeO.9EUo.I Ceo.SDYO CeO.3SmO Ce02-0.5BaTi wt% B wt% CuO 29 Ce2(W04) wt% ZBS Ba2CeV wt% ZnO-B20 3-Si02 31 Ce wt% B Ce wt% BZBS n 287

8 As is evident from the table, dielectric resonators with I 0 ~ er ~ 70, 4000 ~ QuXJ:!> GHz and +399 ~ Lf -65 ppmf'c have been developed during this investigation. In the present work several temperature stable polymer-ceria composites having relative permittivity in the range and dielectric loss in the range have been developed. Giant permittivity (e,. > 10 5 ) CeOrLSCO composites suitable for electrostrictive applications have also been developed in the present study. The possible error in the measurement of permittivity is of the order of 0.3%. Such an error is possible when dimensional uncertainties of the sample are in the order of 0.15%. The uncertainty in the quality factor using TEols mode cavity method with optimized enclosure is of the order of ± 2xlO'6. In the cavity perturbation method, the experimental error is found to be less than 2% in the case of permittivity and 1.3% in the case of dielectric loss. The uncertainty in loss tangent measurements is of the order of ± 2x 10,5 or ± 0.3 tan O. Within a batch and from batch to batch deviation of about 10-20% in QuXJ (± 1000 GHz), 3% in relative permittivity (± I) and 10% in Lt (± 5 ppmfc) is observed. These deviations are essentially due to the slight variations in porosity, microstructure and crystal defects. The scope for the extension of the work described in this thesis lies mainly in three areas. One of the newest and most prolific of the advances in wireless communication has been the development and implementation of tape casting as a manufacturing process for the production of thin sheets of ceramic materials for LTCC technology. The primary one is to prepare L TCC tapes using the LTCC materials developed in the present study. The sintering behaviour, shrinkage, dielectric properties, mechanical properties and chemical compatibility with electrode materials of the LTCC tapes should also be investigated. Although the sintering temperature of the materials in the present investigation is reduced by the addition of low melting oxides and glasses, it negatively affected the microwave dielectric properties. Hence chemical synthesizing techniques like hydrothermal, co-precipitation, citrate-gel, sol-gel etc. would be of great interest. 288

9 CHAPTER B It is established that the dielectric properties of single crystals are superior to their polycrystalline counterparts. Hence another future directing in dielectric resonator research is to grow single crystals of A Ti0 3 (A = Co, Mn, Ni) ceramics for microwave applications. However, growing large single crystals of these materials is challenging. The polymer composites developed in the present investigation must be utilized to fabricate substrates for antenna and printed circuit board applications. Broadband DRAs of different geometries and DRA array using temperature stable DRs should also be fabricated using the developed DR materials. 289