Microstructure and Thermal Conductivity

As mentioned in Chapter 1, thermal conductivity depends on microstructure. Anything that changes the local atomic arrangement or induces elastic strains in the lattice will decrease 1. 

026Wm K, significantly less than most crystalline ceramics. Consequently ceramics con-taiiung a high volume fraction of porosity have 

FIGURE 34.5 Schematic of the laser flash method used for measuring thermal conductivity of ceramics. 

FIGURE 34.6 The thennal conductivities of solid solutions in the MgO-NiO system. 

Most practical ceramics are multiphase materials in which each constituent will, in general, have a different k. The value of k for the heterogeneous material is dependent upon the amount and distribution of the different phases. 

---

EFFECT OF DEPOSITION RATE ON MICROSTRUCTURE AND THERMAL CONDUCTIVITY... 

Effect of Deposition Rate on Microstructure and Thermal Conductivity of YSZ Films... 

In the nuclear collision region, amorphisation of SiC occurs for fluences larger than 5.10 ions/cm. The Raman spectroscopy results are in good agreement with thermal conductivity measurements. It seems that both microstructure and thermal conductivity reach a saturated state beyond 5.10 ions/cm, that corresponds to SiC amorphisation. 

Despite these difficulties, connections can be made between the microstructure and the physical and sensory properties. We have already discussed some of these, for example, the correlation between ice crystal size and sensory smoothness shown in Figure 7.5. Analysis of physical and sensory data by PCA and other statistical methods is an important tool for example, we saw in Figure 6.18 how changes in formulation and storage conditions (which alter the microstructure) affect the sensory attributes. Another example is the relationship between the ice and air microstructure, the thermal conductivity and the perception of coldness in the mouth. The high thermal conductivity of water ice means that heat is rapidly removed from your mouth when you eat the product. This is one of the reasons why an ice lolly feels colder than an ice cream even if they are actually at the same temperature. Table 7.3 summarizes established links between microstructure, physical and sensory properties. 

Aluminum possesses good electrical and thermal conductivity as well as its durability and availability makes it a preferable material for various microelectronics and MEMS applications. EMM of patterned A1 and Al-based alloys can be utilized in a wide range of applications, which includes energy storage devices microanalytical systems and microfluidic, optical, and microelectronic devices. EMM is economical and scalable to large geometric areas platform for microstructuring of A1 substrates. 

The poor corrosion resistance of borides may be partially overcome in composites with SiC if the microstructure allows passive oxidation kinetics. In combination with SiC the borides retain their high electrical and thermal conductivity and thus suitable thermal shock resistance. SiC-TiB2 composites have been extensively developed for wear parts in machinery such as sliding rings, valves, valve seats, roller and ball bearings, plungers, and rocker arm pads. 

The interest in development of SiC/Si3N4 composites stems from the fact that (a) SiC and Si3N4 are thermodynamically compatible and stable at temperatures to 1700°C, (b) silicon nitride processing temperature can be tailored to avoid fiber degradation, (c) silicon nitride matrix microstructure can be controlled to inprove composite properties such as matrix cracking strength and thermal conductivity. 

As noted in the previous section, oxides are generally added as sintering additives for densifying AIN, and will react with any alumina impurity in an AlN powder. After sintering, the secondary phases of aluminate compounds will then remain in the microstructure. The thermal conductivity of the aluminates may be as low as a few W m K, this value being about two orders lower than that of AlN [23]. If the secondary oxide phases are discretely distributed within the AlN matrix, the secondary phase is of minor importance, whereas if the AlN grains are covered with the low-thermal conductivity secondary phase the material s thermal... 

---