Solid state bonding

Four main types of crystalline solid may be specified according to the method of bonding in the solid state, viz. ionic, covalent, molecular and metallic. There are materials intermediate between these classes, but most crystalline solids can be classified as predominantly one of the basic types. 

Metallic crystals (e.g. copper) comprise ordered arrays of identical cations. The constituent atoms share their outer electrons, but these are so loosely held that they are free to move through the crystal lattice and confer metallic properties on the solid. For example, ionic, covalent and molecular crystals are essentially non-conductors of electricity, because the electrons are all locked into fixed quantum states. Metals are good conductors because of the presence of mobile electrons. 

Semiconducting crystals (e.g. germanium) are usually covalent solids with some ionic characteristics, although a few molecular solids (e.g. some polycyclic aromatic hydrocarbons such as anthracene) are known in which under certain conditions a small fraction of the valency electrons are free to move in the 

The electrical conductivity of a semiconductor can be profoundly affected by the presence of impurities. For example, if v silicon atoms in the lattice of a silicon crystal are replaced by v phosphorus atoms, the lattice will gain v electrons and a negative (n-type) semiconductor results. On the other hand, if X silicon atoms are replaced by v boron atoms, the lattice will lose v electrons and a positive (p-type) semiconductor is formed. The impurity atoms are called donors or acceptors according to whether they give or take electrons to or from the lattice. 

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Problems of this nature, however, can be somewhat alleviated by the availability of complementary gas phase electron diffraction or micro-wave data, from which values for the crucial structural parameters uninfluenced by solid-state bonding and packing effects can be evaluated. Changes in these reference values coupled with a suggestive stereochemistry between the donor-acceptor components of the coordinate bond are often sufficient to confirm bonding. The utility of combined microwave spectroscopic and X-ray diffraction methods is amply illustrated by the cyano derivatives of di- and trimethylgermane. 

Larger systematic differences between gas-phase and solid-state bond distances may be caused by vibrational effects. 

To return to our discussion of molecular and solid state bond formation, let s pursue the trivial chemical perspective of the beginning of this section. The guiding principle, implicit in 80, is to maximize bonding. There may be impediments to bonding. One such impediment might be electron repulsion, another steric effects, i.e., the impossibility of two radicals to reach within bonding distance of each other. Obviously, the... 

Coord. No. Solid state Bond length (A) Closest polyhedron Gas phase Coord. No. Bond length (A)... 

Trimethylthallium is monomeric in the gas phase, in the melt, and in solution. However, it forms a polymeric framework, with weakly bridging methyls, in the solid state. Bonded Tl-C lengths are 219.6-221.6 pm and nonbonded Tl- -C distances are 324.3-336.4pm. The dissociation energy of the first Tl-C bond in trimethylthallium... 

Finally, the structural modifications of elemental boron exhibit complex extended lattices of cages in the solid state, whereas those of metals possess much simpler close-packed atomic lattices. These differences are a direct reflection of atomic properties and result in the respective nonmetallic and metallic behavior. However, boron combines with most other elements including metals. There are a wide range of metal borides known with stoichiometric as well as nonstoi-chiometric atomic ratios. The amazingly varied interpenetration of the two characteristic structural motifs and the subtly balanced competition between the two modes of solid state bonding found in the metal borides constitutes further justification of our theme. This is discussed in some detail in Section II,C. 

We have already pointed out that the most stable forms of the solid state bonding of elemental boron and metals differ in an essential aspect. Hence, in the solidification of a melt containing a random mixture of metal and boron atoms the observed structure will be determined by a balance between the tendencies for boron to form a covalently bound network and the metal to form a close-packed lattice. Among other things, this competition will depend on relative metal and boron concentrations and one expects in proceeding from the metal-rich to the boron-rich borides that the B-B bonded network will become more extensive and dominant. 

There are many examples of compounds with Sn-Si, Sn-Ge, Sn-Sn and Sn-Pb bonds which have been prepared in the last decade, and the majority has been characterized by Sn NMR (Table 8). More examples became also available for compounds with Sn-B, Sn-Li and even Sn-K (in the solid state) bonds (Table 8). 

It is instructive to consider Monte Carlo calculations of ion/solid interactions. Ishitani et al. (23) have performed calculations for 5 keV Ar" ions incident on elemental silicon. The minimum displacement energy for a silicon atom was taken to be 25 eV and the effects of solid state bonding were taken... 

Solid state bonding Direct joining by eutectics M - X (Cu - O)... 

Solid state bonding Metallizing + brazing Active brazing Ceramic frit process... 

Mas] Masahashi, N., Hanada, S., Mierostmeture Evolution Mechanism on Iron Aluminides/ CrMo Steel Composite Prepared by Solid State Bonding , ISIJ Int., 44, 878-885 (2004), (Phase Relations, Experimental, Kinetics, 19)... 

Fig. 5.43 The three mechanisms of grain-shape accommodation and neck growth during solution reprecipitation controlled densification of liquid-phase sintering a contact flattening, b dissolution of small grains, and c solid-state bonding. Reproduced with permission from [73]. Copyright 2009, Springer...

EXW uses explosive charge to supply energy for a cladding sheet-metal to strike the base sheet-metal causing plastic flow and a solid state bond. Bond strength is obtained from the characteristic wavy interlocking at the Joint face. Can also be used for tube applications. 

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