100 Original scientific paper  MIDEM Society Processing of steatite ceramic with a low dielectric constant and low dielectric losses K. Makovšek1, I. Ramšak2, B. Malič1, V. Bobnar1, D. Kuščer1 1Jožef Stefan Institute, Ljubljana, Slovenija 2ETI Elektroelement d.d., Izlake, Slovenia Abstract: Steatite ceramic was processed from natural raw materials talc, clay and dolomite. They were stabilised in water electrostatically and electrosterically with polyacrylic acid at pH 9. The suspensions were spray dried and the resulting powders contained granules with a mean size of 10 µm. The powders were dry-pressed and sintered at 1275 °C and 1300 °C for 2 hours. The effect of type of stabilisation and sintering temperature on the phase composition, microstructure and dielectric properties of the ceramic was studied. The X-ray powder diffraction analysis revealed that the orthorhombic protoenstatite and tetragonal cristobalite were present in all the ceramic samples. The microstructures of the samples investigated by scanning electron microscopy were homogenous with grains surrounded with a glass phase and some pores. By energy dispersive X-ray spectroscopy we identified Mg, Si, Al and O in the grains and Mg, Si, O and minor amount of Al, Ca and Fe in the glassy phase. The dielectric constant of the ceramic measured at room temperature and 1 MHz decreased from about 8 to about 5 with increasing sintering temperature from 1275 °C to 1300 °C, while the dielectric losses were between 0.001 and 0.003. The dielectric properties of the steatite ceramic were related to the chemical composition of the glassy phase. The results show that the chemical composition of the phases and the dielectric properties of the ceramic depend on the processing temperature while the type of stabilization of raw materials in water has only a minor influence on these parameters. Keywords: steatite ceramic; microstructure; dielectric properties Priprava steatitne keramike z nizko dielektrično konstanto in nizkimi dielektričnimi izgubami Izvleček: Steatitno keramiko smo pripravili iz naravnih surovin talka, gline in dolomita, ki smo jih stabilizirali v vodi pri pH 9 elektrostatsko in elektrosterično s poliakrilno kislino. Z razprševanjem suspenzije v laboratorijskem razpršilnem sušilniku smo dobili granulat s povprečno velikostjo granul okoli 10 mm. Iz granulata smo z enoosnim stiskanjem pripravili surovce, ki smo jih sintrali pri 1275 °C in 1300 °C 2 uri. Študirali smo vpliv tipa stabilizacije in temperature sintranja na fazno sestavo, mikrostrukturo in dielektrične lastnosti keramike. Z rentgensko praškovno analizo smo ugotovili, da vsi keramični vzorci vsebujejo ortorombsko fazo protoenstatit in tetragonalno fazo kristoblit. Mikrostruktura keramike, ki smo jo preiskali z vrstičnim elektronskim mikroskopom, je bila homogena. Sestavljena je bila iz zrn, ki jih je obdajala steklasta faza, in por. Z energijsko disperzijsko spektroskopijo rentgenskih žarkov smo v zrnih dokazali prisotnost Mg, Si, Al in O, v steklasti fazi pa smo identificirali poleg Mg, Si in O tudi sledove Al, Ca in Fe. Dielektrično konstanto (e) in dielektrične izgube (tan δ) keramike smo izmerili pri sobni temperature in 1 MHz. Po sintranju pri 1275 °C je imela keramika e 8, medtem ko je bila vrednost e po sintranju na 1300 °C nižja, t.j., 5. Vrednoti tan δ so bile med 0.001 in 0.003. Ugotovili smo, da so dielektrične lastnosti steatitne keramike odvisne od kemijske sestave steklaste faze. Rezultati so pokazali, da so dielektrične lastnosti steatitne keramike in kemijska sestava faz keramike odvisne od pogojev priprave keramike, medtem ko ima tip stabilizacije osnovnih surovin v vodi na te parametre le manjši vpliv. Ključne besede: steatitna keramika; mikrostruktura; dielektrične lastnosti * Corresponding Author’s e-mail: danjela.kuscer@ijs.si Journal of Microelectronics, Electronic Components and Materials Vol. 46, No. 2(2016), 100 – 105 1 Introduction Steatite ceramic is alumosilicate that contains approxi- mately 60 % of SiO2, 30 % of MgO, 5 % of Al2O3 and low amounts of oxides such as K2O, Na2O, CaO, Fe2O3 and TiO2 originated from impure raw materials. It is char- acterized by a flexural strength between 110 and 165 MPa, an electrical resistivity of about 1011 Ωm at room temperature, a dielectric constant between 5.5 and 7.5 and dielectric losses of about 0.001 [1]. These charac- teristics and the ability to fabricate final products in a 101 wide variety of forms, make steatite ceramic attractive in many applications such as high-frequency insula- tors, appliance insulators, resistor cores, casings for thermostats and fuses. Steatite ceramic is usually fabricated from natural raw materials, namely talc and clay components, that are wet ground and patterned into a green body by ce- ramic technologies, examples being dry pressing of spray-dried powder and extrusion or casting of the suspension. After sintering at temperatures between 1250 and 1400 oC, the ceramic contains the grains of Mg-Al-Si-O that crystallise as protoensteatite and a glassy phase, whereas SiO2 as crystoballite can also be present in small quantities [2,3]. Protoenstatite consists of tetrahedral chains usually of corner-shearing SiO4 units that are separated by parallel chains of octahe- dral edge-sharing MO6 units containing cations such as Mg and Al [4]. The high-temperature orthorhombic protoenstatite phase is prone to transform on cooling into clinoen- statite, a monoclinic phase [5]. The resulting unit vol- ume change of 2.8 % may lead to formation of cracks and thus to deterioration of mechanical properties of the ceramic. The protoenstatite- clinoenstatite phase transformation is prevented when the protoenstatite grains are surrounded by the glassy phase and when the grains are in the micrometre range [1]. Thus, the mi- crostructure of the steatite ceramic has to be homoge- neous with micrometre-sized grains. The microstructure and the phase composition of the steatite ceramic depend on the chemical composition and the morphology of the raw materials, as well as on the processing conditions, such as homogenisation of raw materials, consolidation, temperature and time of firing. The presence of agglomerates in the powder has a detrimental effect on the ceramic since agglomerates either persist during processing or even lead to the for- mation of new heterogeneities during densification [6]. Agglomerates can be eliminated from the powder by colloidal processing [7]. By controlling the interparticle forces using polyelectrolytes such as ammonium poly- acrylate the talc was effectively dispersed in water [8]. After spray-drying and sintering the steatite ceramic was dense and homogeneous. Electrical properties of steatite ceramics are mostly re- lated to the amount and the composition of the glassy phase [1]. It was shown that the presence of alkalis in the glassy phase increases the electrical conductiv- ity and dielectric losses, while the presence of alkaline earth oxides results in low-loss steatite ceramics [1,9]. In this work we processed the steatite ceramic from talc and clay components stabilized in water electro- statically and electrosterically by polyacrylic acid, re- spectively. The suspensions were spray-dried and the resulting powder was pressed into powder compacts. They were sintered at 1275 °C and 1300 °C, respectively. The aim of this work was to relate the dielectric permit- tivity and the dielectric losses of the resulting ceramic to the phase composition, the microstructure and the chemical composition of the phases. 2 Experimental Talc, clay and dolomite were dispersed in water (Milli Q) without any additive and with polyacrylic acid (PAA, Aldrich) in the amount of 0.3 wt. % per g of the powder. The suspensions had the solid/water mass ratio 33/67. During the mixing of the suspensions with a magnetic stirrer for 2 hours, 0.2 wt. % of polyvinyl alcohol (PVA, Alfa Aesar) and 0.8 wt. % of polyethylene glycol (PEG, Aldrich) were added. Then the suspensions with pH 9 were homogenized in a planetary ball mill (Retsch) for 2 hours at 200 min-1. The suspensions were spray-dried in a laboratory spray drier (Buechi B-290) in air at 190 °C and 7.5 bar. Powder compacts were prepared from granulated powders. The powders were placed in a steel mold with a diameter of 12 mm and uniaxially pressed at 100 MPa. The samples were then sintered at 1275 °C and 1300 °C, respectively, for 2 h with heating and cooling rates of 5 °C/min. The ceramics prepared from the powder without PAA, sin- tered at 1275 °C and 1300 °C, were denoted S1275 and S1300, respectively. The ceramics prepared from the powder with PAA, sintered at 1275 °C and 1300 °C, were denoted SA1275 and SA1300, respectively. The geometrical density of the ceramic samples was calculated from the mass and dimensions of the pel- lets. The ceramics were analyzed by X-ray powder diffrac- tion (XRD) at room temperature using a diffractometer (PANalytical, X’Pert PRO MPD, The Netherlands). The data were collected in the 2 θ range from 20 ° to 50 ° in steps of 0.034 °, with an integration time of 100 s. The phases were identified by software X-Pert High Score using the PDF-2 database [10]. The particle size distribution of the powders was meas- ured using a static light-scattering particle size analyser (Microtrac S3500). K. Makovšek et al; Informacije Midem, Vol. 46, No. 2(2016), 100 – 105 102 For the microstructural analysis the scanning electron microscope (SEM, JSM-5800 JEOL, Japan) equipped with energy dispersive spectroscopy (EDXS, Tracor- Northern) was used. For standardless analysis, the Tra- cor SQ standardless analysis program, using multiple least-squares analysis and a ZAF matrix correction pro- cedure, was used. The samples were analyzed using an accelerating voltage of 20 kV and a spectra acquisition time of 100 s. The oxygen content in the samples was obtained by difference and is considered in the matrix correction calculations. The estimated error for EDXS analysis is up to 10 % for major elements and up to 30 % for minor elements. For dielectric investigations, the gold paste (ESL 8884 G) was screen-printed on top- and bottom- surfaces of the samples having a diameter of 11 mm and a thick- ness of 3 mm. The paste was fired at 900 °C for 10 min- utes. The dielectric constant (e) and dielectric losses (tan δ) were measured at room temperature and fre- quency of 1 MHz with a Novocontrol Alpha High Reso- lution Dielectric Analyzer. The amplitude of the prob- ing AC electric signal was 1 V. 3 Results and discussion The morphology of the spray-dried powders prepared from suspension without any PAA (denoted G) and with PAA (denoted G PAA) is shown in Figure 1 a and b, respectively, The morphology of both powders was similar. The granules were roughly spherically shaped with sizes between a few and about 20 µm. The mean granule size dv50 measured by the particle size analyzer was 10 µm and 9.9 µm for G and G PAA, respectively. The dv90 for both powders was 20 µm. The XRD spectra of the SA1275, SA1300, S1275 and S1300 ceramic samples are shown in Figure 2. Figure 2: Diffraction spectra of S1275, S1300, SA1275 and SA1300 steatite ceramic. o-protoenstatite, #-cristo- balite All the spectra were similar with the diffraction peaks that corresponded to the orthorhombic protoenstatite phase (PDF 76-1806) and the tetragonal SiO2 (crysto- balite, PDF 76-0941). The main phase was protoen- statite, while the SiO2 was present in a low quantity. The relatively high background of the spectra indicat- ed that the ceramic contained also some amorphous phase. The intensity of the diffraction peaks and the background were comparable for all the samples, therefore we assume that the amounts of the phases in all the samples were similar. The cell parameters of the protoenstatite in the S1275, S1300, SA1275 and SA1300 steatite ceramics are shown in Table 1. Figure 1: SEM image of the spray-dried powder pre- pared from the suspension a) without any PAA (G) and b) with PAA (G PAA) 20 25 30 35 40 45 50 0 2500 5000 7500 10000 12500 15000 17500 20000 o o o o o o ooo o o o o o o o o o o # ### # S1300 S1275 SA 1300 In te ns ity (a .u .) 2 Theta (degree) SA1275 # o K. Makovšek et al; Informacije Midem, Vol. 46, No. 2(2016), 100 – 105 103 Table 1: Cell parameters of protoenstatite MgSiO3 in the S1275, S1300, SA1275 and SA1300 steatite ceramic a [nm] b [nm] c [nm] SA 1275 0.9246(1) 0.8746(2) 0.5319(1) SA 1300 0.9251(1) 0.8750(1) 0.5322(1) S 1275 0.9247(1) 0.8745(2) 0.5319(1) S 1300 0.9250(1) 0.8750(2) 0.5321(1) The cell parameters of the protoenstatite phase in ce- ramics prepared from the granulated powder without PAA and with PAA depended on the processing tem- perature and were similar for the samples sintered at 1275 °C (compare S1275 and SA1275) and 1300 °C (compare S1300 and SA1300), respectively. They were in agreement with the protoenstatite unit cell reported in literature [11]. The cell parameters of the protoen- statite sintered at 1300 °C were larger than those of the sample sintered at 1275 °C. The microstructures of the SA1275, SA1300, S1275, and S1300 ceramic samples are shown in Figure 3. The mi- crostructure of all the samples contains a bright phase (denoted S) and a dark, glassy phase (denoted LP) in addition to pores (P) which is in agreement with the li- terature [1]. Pores with sizes up to about ten microme- ters are homogeneously distributed in the microstruc- ture. The geometrical density of the ceramic was similar for all the samples, 2.77 ± 0.01 g/cm3. It is evident that the grains of the bright phase (S) are distributed in the dark, glassy phase The size of the grains ranged from submicrometre to about 5 µm. In some grains we observed cracks. By SEM/EDXS analysis we confirmed that some of the grains with the cracks were SiO2-rich. It is known that crystaoballite can trans- form during cooling to quartz with corresponding change in unit cell volume that leads to the formation of intragranular cracks [12]. With XRD analysis we con- firmed the presence of cristobalite phase in all ceramic Figure 3: Microstructure of ceramic samples a) SA1275; b) SA 1300; c) S1275; d) S1300. S = bright phase (steatite), LP = dark phase, P: pores. K. Makovšek et al; Informacije Midem, Vol. 46, No. 2(2016), 100 – 105 104 samples. The characteristic quartz (101) diffraction peak with the highest intensity at 2 Theta of 26.2 ° was not detected in any of the samples. It is possible that the amount of quartz was below the detection limit of XRD analysis, which is about 2 wt. %. The formation of cracks could also be a consequence of protoenstatite to clinoenstatite phase transformation during cooling and corresponding difference in the unit cell volume [2,13]. However we have not detected any monoclinic clinoenstatite phase by XRD. The characteristic clinoen- statite (-2 2 1) diffraction peak with the highest intensi- ty at 2 Theta of 29.75 ° (PDF 75-1406) was not detected in any of the samples (see Figure 2). It is possible that the amount to clinoenstatite was below the detection limit of XRD analysis. We performed the EDXS analysis of the bright phase (S) and the dark phase (LP) in all the samples. We found out that the composition of the selected phase in the ceramic prepared from the granulated powder without PAA and with PAA was similar, but it varied with the pro- cessing temperature. The results of the analysis of the samples SA1275 and SA1300 are presented in Table 2. Table 2: The composition of the dark (LP) and bright (S) phases in the samples SA1275 and SA1300. SA 1275 SA 1300 LP [wt. %] S [wt. %] LP [wt. %] S [wt. %] MgO 14.4 ± 0.7 29.6 ± 1.5 12.8 ± 0.6 31.1 ± 1.5 Al2O3 3.6 ± 0.3 3.6 ± 0.3 3.9 ± 0.6 2.8 ± 0.4 SiO2 79.3 ± 4 64.4 ± 3 80.4 ± 4 63.9 ± 3 CaO 1.9 ± 0.3 1.3 ± 0.2 2.1 ± 0.3 1.3 ± 0.2 Fe2O3 0.80 ± 0.12 1.10 ± 0.16 0.80 ± 0.10 0.90 ± 0.13 In the dark and in the bright phases we identified the following elements: Mg, Al, Si, Ca, Fe and O. Concern- ing the sample sintered at 1275 °C, it is evident that the bright phase (S) contained about 65 % of SiO2, about 30 % of MgO, 3.6 % of Al2O3, 1.3 % of CaO and about 1 % of Fe2O3 (all in wt. %). The ratio between SiO2, MgO and Al2O3 corresponded well to that of steatite. On the basis of XRD and EDXS analyses we concluded that the bright phase crystallized in the orthorhombic structure and contained Si-Mg-Al-O protoensteatite phase with minor amounts of Ca and Fe. The presence of CaO and Fe2O3 is related to impurities originated from the natu- ral raw materials. The dark phase (LP) contained a higher amount of SiO2, about 80 % and a lower amount of MgO, about 14 %, while the amounts of Al2O3, CaO and Fe2O3 were similar. The chemical composition of the bright phase slightly changed after sintering at 1300 °C. The amount of MgO increased, while the amounts of Al2O3 and SiO2 de- creased. A higher amount of Mg2+ with ionic radius r = 0.072 nm [14] and a lower amount of Si4+ (r = 0.026 nm) [12] and Al3+ (r = 0.039 nm) [12] may result in larger unit cell at 1300 °C which is consistent with the calculated lattice parameters of the protoensteatite (Table 1). The dark phase (LP) contained in a sample sintered at 1300 °C less MgO and more SiO2, while the amounts of Al2O3, CaO and Fe2O3 were similar to that in sample sin- tered at 1275 °C. The dielectric constant e and the dielectric losses of the S1275, S1300, SA1275 and SA1300 samples measured at room temperature are presented in Table 3. Table 3: Dielectric constant e and the dielectric losses tan δ of the S1275, S1300, SA1275 and SA1300 e (1 MHz) tan δ (1 MHz) SA1275 8.42 0.002 SA1300 5.5 0.003 S1275 7.74 0.001 S1300 4.6 0.002 The dielectric constants of the samples sintered at 1275 °C were higher than of those sintered at 1300 °C. This could be related to the chemical composition of the steatite ceramic which depends on the processing temperature. The dielectric constant of the steatite MgSiO3 with the density of 93 % is 6.2 [15]. The dielectric constants of MgO and Al2O3 are similar, 9.8 and 10, respectively, while the dielectric constant of SiO2 is 4. It was report- ed that the properties of the steatite ceramic depend mostly on the composition of the glassy phase [1]. From the results it is evident that at 1300 oC the liq- uid phase contained more SiO2 and less MgO than at 1275 oC. A higher amount of SiO2 with a low dielectric constant and a low amount of MgO with a high dielec- tric constant decreased the dielectric constant of the glassy phase. Consequently the dielectric constant of the steatite ceramic processed at 1300 oC was lower than that at 1275 °C. 4 Conclusions Aqueous suspensions of talc/clay/dolomite mixtures stabilized electrostatically and electrosterically with polyacrylic acid at pH 9 have been prepared in a solid load of 33 wt. %. After spray-drying the powder con- tained spherically shaped granules with sizes between a few and about 20 µm. The powder compacts were K. Makovšek et al; Informacije Midem, Vol. 46, No. 2(2016), 100 – 105 105 sintered at 1275 °C and 1300 °C. The microstructures of all the steatite ceramics were homogeneous and con- tained protoenstatite grains surrounded with a glassy phase together with a low amount of crystobalite and some pores. The dielectric constant was about 5 and about 8 for the ceramics sintered at 1300 oC and 1275 °C, respec- tively, while the dielectric losses were between 0.001 and 0.003. The values of the dielectric constant were related to the chemical composition of the glassy phase that contained more SiO2, less MgO and similar amounts of Al2O3, CaO and Fe2O3 after sintering at 1300 °C. A higher amount of SiO2 with the dielectric constant of about 4 and a lower amount of MgO with the dielectric constant of about 10 contributed to a lower dielectric constant of the steatite ceramics. 5 Acknowledgments Ministry of Education, Science and Sport of Slovenia is acknowledged for the financial support through the project Raziskovalci na začetku kariere-2013-IJS-726 (OP13.2.1.8.01.0014). This work was supported also by the Slovenian Research Agency (P2-0105). The authors would like to thank to Jana Cilenšek and Silvo Drnovšek for technical assistance and dr. 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