Metal junctions

An interesting aspect of friction is the manner in which the area of contact changes as sliding occurs. This change may be measured either by conductivity, proportional to if, as in the case of metals, it is limited primarily by a number of small metal-to-metal junctions, or by the normal adhesion, that is, the force to separate the two substances. As an illustration of the latter, a steel ball pressed briefly against indium with a load of IS g required about the same IS g for its subsequent detachment [37]. If relative motion was set in, a value of S was observed and, on stopping, the normal force for separation had risen to 100 g. The ratio of 100 IS g may thus be taken as the ratio of junction areas in the two cases. 

Fig. 13. Schematic of a microscopic view of contact interface where constriction resistance originates in the constriction of current flow through the touching metallic junctions (a spots) of the mating surfaces. The arrows and lines indicate the flow of current.

The thermocouple is made by welding together two wires of metals 1 and 2 in such a manner that a segment of metal 1 is connected to two terminal wires of metal 2. One junction between metals 1 and 2 is heated by the infrared beam, and the other junction is kept at constant temperature small changes in ambient temperature are thus minimised. To avoid losses of energy by convection, the couples are enclosed in an evacuated vessel with a window transparent to infrared radiation. The metallic junctions are also covered with a black deposit to decrease reflection of the incident beam... 

The optical properties of electrodeposited, polycrystalline CdTe have been found to be similar to those of single-crystal CdTe [257]. In 1982, Fulop et al. [258] reported the development of metal junction solar cells of high efficiency using thin film (4 p,m) n-type CdTe as absorber, electrodeposited from a typical acidic aqueous solution on metallic substrate (Cu, steel, Ni) and annealed in air at 300 °C. The cells were constructed using a Schottky barrier rectifying junction at the front surface (vacuum-deposited Au, Ni) and a (electrodeposited) Cd ohmic contact at the back. Passivation of the top surface (treatment with KOH and hydrazine) was seen to improve the photovoltaic properties of the rectifying junction. The best fabricated cell comprised an efficiency of 8.6% (AMI), open-circuit voltage of 0.723 V, short-circuit current of 18.7 mA cm, and a fill factor of 0.64. 

These prepared wires are very useful for sealing through Pyrex and Phoenix. Care must be taken to see that potassium or sodium nitrite does not come into contact with the metal junctions or the tungsten... 

In principle there are two types of solid state devices (i) pn-photocells and (ii) Schottky type cells. The first one consists simply of a pn-junction whereas the other of a semi-conductor-metal junction. The energy schemes of these cells are given in Fig. 1 a and b. The current-potential dependence of both types of cells is given by (see e.g. ) ... 

The forward current at a semiconductor-metal junction is mainly determined by a majority carrier transfer i.e. electrons for n-type, as illustrated in Fig. 1 d. In this majority carrier device the socalled thermionic emission model is applied according to which all electrons reaching the surface are transferred to the metal. In this case we have ... 

Polymeric (isocyanide)gold(i) aryls ( gold oligo-phenylene-ethynylene-isonitriles ) were tested as electrical conductors at metal-molecule-metal junctions (7r-conjugated molecular wires), but the preparation, structure, and properties of the materials were not fully disclosed (Scheme 52).218... 

The use of the thermoelectric power of metallic junctions presents the following advantages ... 

Beebe JM, Kim B-S, Gadzuk JW, Frisbie CD, Kushmerick JG (2006) Transition from direct tunneling to field emission in metal-molecule-metal junctions. Phys Rev Lett 97 026801... 

Kergueris C, Bourgoin J-P, Palacin S, Esteve D, Urbina C, Magoga M, Joachim C (1999) Electron transport through a metal-molecule-metal junction. Phys Rev B59 12505-12513... 

Chabinyc ML, Chen X, Holmlin RE, Jacobs H, Skulason H, Frisbie CD, Mujica V, Ratner MA, Rampi MA, Whitesides GM (2002) Molecular rectification in a metal-insulator-metal junction based on self-assembled monolayers. J Am Chem Soc 124 11731-11736... 

Gayathri SS, Patnaik A (2006) Electrical rectification from a fullerene[60]-dyad based metal-organic-metal junction. Chem Commun 1977-1979... 

Active and Non-Active Large-Area Metal-Molecules-Metal Junctions... 

Nijhuis CA, Reus WF, Whitesides GM (2009) Organometallic molecular rectification in metal-SAM-metal junctions. J Am Chem Soc 131 17814-17827... 

Mativetsky JM, Pace G, Elbing M, Rampi MA, Mayor M, Samori P (2008) Azobenzenes as light-controlled molecular electronic switches in nanoscale metal-molecule-metal junctions. J Am Chem Soc 130 9192-9193... 

Tran E, Duati M, Whitesides GM, Rampi MA (2006) Gating current flowing through molecules in metal-molecules-metal junctions. Faraday Discuss 131 197-203... 

Liu Y, Offenhausser A, Mayer D (2010) Rectified tunneling current response of biofunctionalized metal-bridge-metal junctions. Biosens Bioelectron 25 1173-1178... 

Guerin D, Merckling C, Lenfant S, Wallart X, Pleutin S, Vuillaume D (2007) Silicon-mo-lecules-metal junctions by transfer printing chemical synthesis and electrical properties. J Phys Chem C 111 7947-7956... 

Duati M, Grave C, Tcbeborateva N, Wu J, Mullen K, Shaporenko A, Zhamikov M, Kriebel JK, Whitesides GM, Rampi MA (2006) Electron transport across hexa-peri-hexabenzo-coronene units in a metal-self-assembled monolayer-metal junction. Adv Mater 18 329-333... 

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