# Large Thermal Expansion LTCC System for Cofiring with Integrated Functional Ceramics Layers
## 3 Results and Discussion
The microstructure of a sintered CT708 sample (sintered at 900 \({}^{\circ}\)C, heating rate 2 K/min) was studied using SEM (Figure 4). The SEM micrograph (BSE mode) documents the presence of multiple crystalline phases embedded in a glassy matrix. In a smaller section of the SEM image (Figure 4, upper right) the presence of a significant amount of quartz crystals (dark particles in BSE) is clearly documented. This is corroborated by EDX analysis (individual element scans) from the same selected sample  
Figure 3: X-ray diffraction patterns before (**a**) and after (**b**) sintering at 900 \({}^{\circ}\)C (2 h) of CT708 laminates (heating rate 2 K/min). appear with high concentrations of Si and O in the EDX scans. Celsian (Ba/Sr)Al\({}_{2}\)Si\({}_{2}\)O\({}_{8}\) is found to exist as second major crystalline phase (gray crystals in BSE, simultaneous presence of Ba, Sr, Al, and Si in EDX). The appearance of Ba and Sr in the same areas in the corresponding EDX scans confirms the simultaneous presence of both in the observed celsian phase, as already suggested from XRD. A third phase (bright crystals in BSE) with the simultaneous presence of Sr, Ti, and Ba (and Si as well) in the EDX maps is also found. This demonstrates (consistent with XRD) the crystallization of a Sr-containing fresnoite phase (Ba,Sr)\({}_{2}\)TiSi\({}_{2}\)O\({}_{8}\). Ca and Mg EDX maps suggest that the main part of both ions is located at the residual glass phase. Co and Al appear mostly at the same positions in their respective EDX scans (CoAl\({}_{2}\)O\({}_{4}\) as small crystallites). The Al EDX map also shows few Al-rich crystallites (not together with Co), representing the presence of few corundum (Al\({}_{2}\)O\({}_{3}\)) particles in the sintered CT708 as well. The permittivity of sintered CT708 samples was measured at room temperature at 1 kHz and from 10 MHz-1 GHz. A frequency scan between 10 MHz and 1 GHz shows a permittivity \(\varepsilon_{\mathrm{r}}\) = 6 and a small tan \(\delta\) = 0.0004 (Figure 5a). No significant effect of the heating rates on the dielectric properties is observed (Figure 5b), confirming that all samples exhibit the same phase composition regardless the heating rate. The permittivity and the loss tangent measured at 1 kHz and 1 GHz are listed in Table 1. Measurements with a different setup at 1 kHz result in slightly increased permittivity of about 6.5. The measured values are in good agreement with specifications provided by the manufacturer [17]. The low permittivity of the quartz filler [16], as well as of the crystallized celsian and fresnoite phases contribute to the low permittivity of the composite. Therefore, CT708 tape is a low-k and low-loss GCC-type LTCC system which might be applied for high-frequency electronic applications. The thermal expansion behavior of CT708 was measured using dilatometry (Figure 6). The heating and cooling curves of CT708 exhibit a hump at about 575 \({}^{\circ}\)C signaling an expansion/contraction due to the \(\alpha\rightleftarrows\beta\) quartz phase transformation. The resulting CTE of CT708 amounts to 10.6 ppm/K (140 \({}^{\circ}\)C-740 \({}^{\circ}\)C); above the quartz transformation temperature the CTE seems to be smaller (7.3 ppm/K). As a GCC-type LTCC tape system with a significant concentration of ceramic quartz filler, CT708 exhibits a large CTE as compared with most other LTCC materials systems. This large CTE of CT708 is comparable  
Figure 4: SEM micrographs (BSE mode, upper line) and individual EDX element maps (lower two lines) at the same position as the BSE micrograph (upper right) of CT708 sintered at 900 \({}^{\circ}\)C.  
to that of many functional ceramics. As an example, the thermal expansion of a Sc-substituted hexagonal ferrite (ScM) with a CTE = 10.8 ppm/K is included in Figure 6.  
The large CTE of CT708 allows for cofiring with ferrite tapes.