# Large Thermal Expansion LTCC System for Cofiring with Integrated Functional Ceramics Layers
## 3 Results and Discussion  
The initial CT708 powder has a particle size of \(d_{50}\) = 1.6 \(\upmu\)m and a specific surface area of about \(S\) = 8 m\({}^{2}\)/g. A single-phase ferrite powder was obtained after calcination at 1300 \({}^{\circ}\)C (XRD results not shown here). After milling, the powder has a mean particle size of \(d_{50}\) = 0.7 \(\upmu\)m and a specific surface of \(S\) = 5 m\({}^{2}\)/g. Thermal analysis of the CT708 green tape shows a mass loss starting at 100 \({}^{\circ}\)C and ending at 450 \({}^{\circ}\)C complemented by a broad exothermic DTA signal, indicating binder burnout (Figure 1). A DSC curve of the initial CT708 powder (Figure 1, inset) exhibits a series of thermal effects starting with an endothermic signal at 570 \({}^{\circ}\)C up to a broad exothermic peak at about 820 \({}^{\circ}\)C.  
Dilatometry measurements with a push-rod dilatometer were performed on CT708 green tapes with different heating rates (Figure 2a-c). A small shrinkage of about 1% with a shrinkage rate peaking at around 275 \({}^{\circ}\)C is observed in all samples reflecting binder burnout. The main shrinkage sets in at around 800 \({}^{\circ}\)C, however, a significant effect of the heating rate on the onset of shrinkage (\(T_{\text{on}}\)), as well as on the temperature of maximum shrinkage rate (\(T_{\text{MSR}}\)) is found. For a slow heating rate of 2 K/min, \(T_{\text{on}}\) is at 793 \({}^{\circ}\)C, and \(T_{\text{MSR}}\) is found at 836 \({}^{\circ}\)C. The onset \(T_{\text{on}}\) is shifted to 811 \({}^{\circ}\)C and 822 \({}^{\circ}\)C for heating rates of 4 K/min and 8 K/min, respectively. Correspondingly, the \(T_{\text{MSR}}\) shifts to 850 \({}^{\circ}\)C and 863 \({}^{\circ}\)C for heating rates of 4 K/min and 8 K/min, respectively. The total shrinkage of 22% (z-direction) is reached at the end of the heating segment for a small heating rate of 2 K/min (Figure 2a). In the case of a larger heating rate (8 K/min), some residual shrinkage proceeds after the sintering temperature of 900 \({}^{\circ}\)C has been reached (Figure 2c), and slightly larger total shrinkage (23%) is found. Note, that all measurements were performed with the smallest possible contact force of the pushrod of 0.01 N. Preliminary tests with larger forces of \(F\) = 0.05 N or \(F\) = 0.1 N resulted in significantly larger total shrinkage. This is a typical signature of glass ceramics composite LTCC material which is compacted upon heating under load. Consequently, dilatometry curves of LTCC laminates or compacts should be performed with small or without contact force, and values of absolute shrinkage should be  
Figure 1: Thermal analysis (TG and DTA) of CT708 green tape (heating rate 1 K/min); inset: DSC curve of CT708 powder (heating rate 5 K/min). considered with care. The results of shrinkage measurements on macroscopic (real size) test samples are summarized in Table 1. The _xy_-shrinkage amounts to 16.5(5)% independent of heating rate. The _z_-shrinkage of the test samples slightly increases with heating rate up to 20.7% at 8 K/min which agrees well with the trend of _z_-shrinkage observed in dilatometry measurements (Figure 2). All CT708 laminates exhibit a good densification after sintering of at 900 \({}^{\circ}\)C for 2 h, and a sintered density of 2.82(2) g/cm\({}^{3}\) is reached. Correspondingly, little porosity of about 0.8(1)% remains. No significant effect of the heating rate on total shrinkage and density is