Diamond and graphite

Key points: Graphite consists of stacked two-dimensional carbon sheets; oxidizing agents or reducing agents may be intercalated between these sheets with concomitant electron transfer. Diamond has the highest known thermal conductivity because its structure (shown in Fig. 14.1) distributes thermal motion in three dimensions very efficiently. The measurement of thermal conductivity is used to identify fake diamonds. Because of its durability, clarity, and high refractive index, diamond is one of the most highly prized gemstones. The ready cleavage of graphite parallel to the planes of atoms (as shown in Fig. 14.2) is largely due to the presence of impurities and accounts for its slipperiness. These graphene planes are widely separated from each other (at 335 pm), indicating that

there are weaker forces between them. These forces are some times, but not very appropriately, called ‘van der Waals forces’ (because in the common impure form of graphite, graphitic ox ide, they are weak, like intermolecular forces), and consequently the region between the planes is called the van der Waals gap. Unlike diamond, graphite is soft and black with a slightly metallic lustre; it is neither durable nor particularly attractive. The conversion of diamond to graphite at room temperature and pressure is spontaneous (∆_trs G^O =-2.90 kJ mol^-1) but does not occur at an observable rate under ordinary conditions: diamonds older than the solar system have been isolated from meteorites. Diamond is the denser phase (3.51 g cm^-3 instead of 2.26 g cm^-3), so it is favoured by high pressures, and large quantities of diamond abrasive are manufactured commercially by a d-metal-catalysed high-temperature, high-pressure process (Box 14.1). Thin films of boron-doped diamond are piezoresistive (their electrical resistance changes when pressure is applied) and are deposited on silica sur faces for use as high-temperature pressure sensors. The electrical conductivity and many of the chemical proper ties of graphite are closely related to the structure of its delocalized π bonds. Its electrical conductivity perpendicular to the planes is low (5 S cm^-1 at 25°C) and increases with increasing temperature, signifying that graphite is a semiconductor in that direction. The electrical conductivity is much higher parallel to the planes (30 kS cm1 at 25°C) but decreases as the temperature is raised, indicating that graphite behaves as a metal, more precisely a semimetal,2 in that direction. This effect is most striking

Figure 14.4 Potassium graphite compounds showing two types of alternation of intercalated atoms.

is subsequently removed at high temperatures, a highly flexible form of graphite is formed; this graphite tape is used to make sealing gaskets, valves, and brake linings. The halogens show an alternation effect in their tendency to form intercalation compounds with graphite. Graphite reacts with fluorine to produce ‘graphite fluoride’, a nonstoichiometric species with formula (CF)n (0.59<n<1). This compound is black when n is low in its range and colourless when n ap proaches 1. It is used as a lubricant in high vacuum applications and as the cathode in lithium batteries. At elevated temperatures the products of the reaction also include C2 F and C4 F. Chlorine reacts slowly with graphite to form C₈Cl and iodine does not re act at all. By contrast, bromine intercalates readily to give C₈ Br, C₁₆ Br, and C₂₀ Br in another example of staging.

اخر الاخبار

اخبار العتبة العباسية المقدسة

قسم شؤون المعارف ينظم دورة متقدمة في تحقيق المخطوطات لملاكاته

قسم الشؤون الفكرية يصدر العدد العشرين من مجلة دراسات إفريقية

المجمَع العلمي يكرم الأساتذة المشاركين في مشروع الدورات القرآنية الصيفية في كربلاء

العتبة العباسية المقدسة تختتم فعاليات البرنامج المركزي الأسبوعي لمنتسبيها

المزيد

الآخبار الصحية

تحذير من "عدوى بكتيرية" تسبب نوبات قلبية

ماذا يحصل لجسمك عند التوقف عن استخدام الزيت في الطهي؟

الأمل في كبسولة.. "فيتامين شهير" يبطئ التقدم في العمر

اشتهاء أطعمة بشكل مفاجئ.. ناقوس خطر للسرطان ؟!

المزيد

الآخبار التكنلوجية

زلزال أفغانستان يحصد أكثر من ألف قتيل.. ويدمر آلاف المنازل !

ناجٍٍ وحيد من انزلاق أرضي.. دمّر قرية سودانية بأكملها !

نهر الفرات.. مورد مائي بين الندرة والتقاسم

ما السر وراء مشية إيلون ماسك الغريبة والسريعة ؟

المزيد

مواضيع ذات صلة

Partially crystalline carbon

Fullerene metal complexes

Carbon clusters

Simple compounds

Occurrence and extraction

Thermodynamic considerations

The atom and its ions

Oxidation states

Occurrence

Enthalpies of atomization

Electronegativity

Ionization energies and electron affinities