Aluminum nitride , material properties

Table 3 summarizes the properties of the so-called nonmetallic hard materials, including diamond and the diamondlike carbides B C, SiC, and Be2C. Also iacluded ia this category are comadum, AI2O2, cubic boroa nitride, BN, aluminum nitride, AIN, siUcon nitride, Si N, and siUcon boride, SiB (12). 

Heat dissipation can be effectively dealt with by using substrate materials such as aluminum nitride, beryllia and, more recently, diamond which combine electrical insulation with high thermal conductivity. The relevant properties of these three materials are shown in Table 14.1. 

A GaN substrate would be a help in this respect but it would need to be semi-insulating. In addition, GaN has a poor thermal conductivity and is not very suitable due to this negative material property. Aluminum nitride substrates may become the substrate of choice for GaN high-frequency applications. It has a reasonable thermal conductivity and is intrinsically semi-insulating but only time will tell. 

Thermal Evaporation The easiest way of evaporating metal is by means of resistance evaporators known commonly as boats . Boats, made of sintered ceramics, are positioned side by side at a distance of approximately 10 cm across the web width (Fig. 8.1). Titanium boride TiB2 is used as an electrically conductive material with boron nitride BN (two-component evaporator) or BN and aluminum nitride AIN (three-component evaporator) as an insulating material [2]. By combination of conductive and insulating materials, the electrical properties of evaporators are adjusted. 

There are other insulating materials that can be used instead of silicon dioxide. Silicon nitride, alumina, and aluminum nitride are a few that are often used. The selection of a proper insulation layer is based on the specific needs and the properties of selected materials. 

In the best-case situation, both types of microcantilever sensors would be grouped in an array to provide a cross platform of sensitive and selective explosive detection system. Additional co-funded (TSA/ATF) work is eurrently on going in materials development for novel microeantilevers. This involves R D of silicon carbide (SiC) based cantilevers, for improvements in material properties (e.g., less fragile eompared to silicon) and to provide a platform for wide band gap type materials, like aluminum nitride (AIN). With an AlN/SiC based cantilever, the sensor can now work in the piezoeleetric resonator mode, providing enhance response and henee sensitivity to the analyte, along with a direct measurement by frequeney/resistance response, versus the more complex optieal deteetion... 

Other filler materials are hydroxyapatite, aluminum oxide and aluminum nitride. In addition, nanofillers are used. PEEK polymer filled with nano sized silica or alumina fillers of 15-30 nm exhibit an improvement of the mechanical properties by 20-50%. The agglomeration tendency can be somewhat dimiiushed by a modification of the surface of the fillers with stearic acid. °... 

Aluminum nitride is an attractive material for microelectronics substrates due to its high thermal conductivity, good dielectric properties, and thermal expansion coefficient comparable with silicon. The thermal conductivity of approximately 300 W m is measured for pure single crystal of aluminum nitride. 

The most efficient alloying elements for improving oxidation resistance of iron in air are chromium and aluminum. Use of these elements with additional alloyed nickel and silicon is especially effective. An 8% Al-Fe alloy is reported to have the same oxidation resistance as a 20% Cr-80% Ni alloy [51]. Unfortunately, the poor mechanical properties of aluminum-iron alloys, the sensitivity of their protective oxide scales to damage, and the tendency to form aluminum nitride that causes embrittlement have combined to limit their application as oxidation-resistant materials. In combination with chromium, some of these drawbacks of aluminum-iron alloys are overcome. 

Aluminum nitride (AlN) has interesting properties, such as a high thermal conductivity (70-210 W m for the polycrystalline material, and up to 285 W m for single crystals), a high volume resistance, and moderate dielectric properties. The thermal expansion coefficient of AlN is close to that of silicon, and it is one of the most mechanically strong and thermally stable ceramics. These excellent attributes make AlN a useful material for many applications [160, 161]. 

Aluminum nitride may be used in composite structures containing aluminum for either structural or electronic applications, due to its attractive thermal, electronic, and mechanical properties [176-178]. AlN ceramics are also known to have a sufficiently high-temperature compatibility with refractory metals. Finally, AlN is an ecologically safe material. The structure of AlN as a ceramics layer of the multilayer Al/AlN composites has been investigated to only a limited degree [179]. 

The previous chapter was a review of the structure and composition of the three refractory covalent nitrides boron nitride, aluminum nitride, and silicon nitride. This chapter is an assessment of the properties and a suimnary of the fabrication processes and applications of these three materials. 

Aluminum nitride is a highly stable material with the unusual combination of high thermal conductivity comparable to diat of metals and high electrical insulation comparable to the best insulators. Its characteristics and properties are summarized in Table 13.7 (see Ch. 12 for structural data). The reported property values often vary considerably and the values given here are a general average.t K K ... 

HTCC is an all-inclusive term to describe ceramic substrates that are consolidated at temperatures above about 1000°C. Applied to electronic packaging, this descriptor includes aluminum oxide, aluminum nitride (AIN), and a variety of other developmental or seldom-used materials. Until recently, discriminating between HTCC and low-temperatme cofired ceramics (LTCC) was elementary, as the firing temperatures differed by roughly 600°C. To confoimd that difference, an intermediate-firing multilayer ceramic, or medimn-temperature cofired ceramic (MTCC), has recently been introduced. Details on the processing and properties of this material will be discussed in Section 6.2 and Section 6.4. 

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