What Is Silicon Carbide?

Silicon carbide, more commonly referred to as Carborundum, is an extremely hard chemical compound with properties of both metal and insulator at different temperatures. Used in industrial applications requiring long lasting applications.

Green SiC is an extremely hard and corrosion resistant ceramic material first synthesized by Edward Acheson in 1891 through the combined heating of silica sand and petroleum coke in a special furnace. It boasts outstanding properties including good resistance to corrosion, high mechanical strength, low thermal expansion and remarkable thermal shock resistance.

Intrinsic Conductivity

Silicon carbide (SiC) is an outstanding material characterized by superior strength and hardness, chemical inertness, thermal shock resistance and wide bandgap properties that make it suitable for many demanding industrial applications such as slide bearings, wear parts, crucibles, sintering aids semiconductor components as well as burner nozzles.

SiC acts like an insulator when produced pure; but with controlled addition of impurities or dopants it can exhibit semi-conducting properties. Aluminum, boron or gallium dopants produce P-type semiconductors; adding phosphorus or nitrogen dopants create N-type semiconductors. Their ability to alter electron and hole concentration – the difference between their number in conduction and valence bands – directly correlates to conductivity.

The intrinsic conductivity of a semiconductor is determined by several factors, including its Fermi energy, height of its valence and conduction bands, mobility of electrons in its conduction band and charge per electron. When more atoms in the valence band are bound to their parent atomic orbitals they have lower Fermi energies that make them less likely to be promoted into conduction via thermal vibrations of SiC lattice (phonon emission). At room temperature the intrinsic charge carriers are evenly distributed across both bands with equal numbers of electrons and holes per band – making a better conductor overall.

However, when heated to elevated temperatures, its valence band becomes partially empty as silicon atoms no longer experience sufficient thermal vibrations to excite their vibrational modes sufficiently. This results in some of the valence electrons crossing over into its conduction band and significantly increasing conductivity.

As temperature continues to rise, electrons and holes recombine until an equilibrium point is reached at the characteristic temperature of a semiconductor. Conductivity can further be increased using electric currents or electromagnetic radiation to stimulate generation and recombination of electron-hole pairs allowing electronic devices such as diodes and transistors to operate at higher voltages and frequencies without compromising reliability.

Conductivity at Grain Boundaries

Silicon carbide possesses an intricate crystal structure with numerous polytypes. A particular polytype can be identified by its number and location of carbon atoms in its layers; each stacking sequence generates unique orientation combinations due to energy considerations (lateral translations and rotations are both possible), leading to hundreds of possible configurations per layer in an experimental sample of SiC.

To better comprehend the conductivity of SiC, it is necessary to consider all possible directions of charge transport. In order to do so, complex impedance measurements were carried out on samples whose grain boundaries (GBs) had been identified via EBSD analysis and fitted into a model which considered both bulk and GB conductivities; results show that gB conductivity is the dominant force determining overall electrical transport properties at elevated temperatures.

gB conductivity depends heavily on temperature and grain size, with cooling procedure and impurities present at the grain boundary also having an influence. Figure 5a illustrates this relationship by comparing grain ionic conductivity obtained through this work to literature values for melt-cast ceramics; you can see that its fit with literature values for melt-cast ceramics is very close; any variances reported likely stemming from differences in pellet sample preparation method used for measurement, inaccurate impedance fitting or incorrect calculations of equivalent circuits.

Typically, the gB conductivity decreases with increasing temperature and impurity content; this effect is much less pronounced for pure materials containing significant quantities of second phases than with those that contain significant amounts. As temperature rises, electron mobility declines; they become more likely to become trapped at gB structures hindered by structural defects – thus explaining why high purity SiC conductivity levels tend to be lower than commercially available forms; to compensate for this, electrically conducting second phase particles at low temperature can be added and eventually reduce gB conductivity to levels that meet practical application needs.

Conductivity at Grain Surfaces

Silicon carbide, commonly referred to as SiC, refers to an extremely diverse group of materials ranging from ceramics fabricated from impure SiC crystallites bonded together using various biners under high temperature and pressure through to industrial wafers produced via chemical vapor deposition or vacuum growth of SiC crystallites. Each type of silicon carbide exhibits distinct physical properties such as electrical conductivity that make predicting its performance for specific applications more challenging.

Conductivity is a property of atomic structure, determined by material’s composition and grain size. A silicon carbide bicrystal’s conductivity can be affected by grain boundary composition and structure as well as formation method; for instance, its n-type conductivity may depend on oxygen impurities at its interface with native oxide – something observed using scanning electron microscopy and scanning nonlinear dielectric microscopy (SNDM).

Similar to SiC, semiconducting SiC can be made to display p-type characteristics by doping with aluminium, boron, gallium or nitrogen doping – while doping it with nitrogen or phosphorus results in n-type characteristics. Increased doping increases electrical conductivity but must account for increasing surface area when making predictions regarding overall conductivity of materials.

A grain boundary’s resistance is determined by its atomic structure, since fluctuations in periodic atomic potential from adjacent crystals cause electrons to scatter along its boundary and decrease resistivity. As atoms tend to cluster more densely at grain boundaries due to closer spacing at boundaries, its resistance tends to be greater than interior resistivities; this closeness contributes to oxide molecules being formed which reduce conductivity further; but this effect can be minimized by designing them so as to be as smooth and dense as possible.

Conductivity at Grain Intervals

Silicon carbide (SiC) has seen wide use in electronic devices due to its higher conductivity, electron mobility and reduced power loss at high temperatures. Due to this property it enables devices such as Schottky diodes and transistors which amplify, switch or convert electrical signals in electronic circuits to be constructed more easily than with other materials.

SiC can suffer significant conductivity degradation due to grain boundary resistance, caused by changes in its periodic atomic potential relative to that of bulk SiC lattice. As electrons cross multiple boundaries within this region, changes in potential can induce electron scattering that significantly increases resistance compared to that found within bulk material.

Researchers have investigated the effects of carbon on the conductivity of SiC polycrystals and bicrystals to understand this phenomenon. As part of their experiment, they studied a polished bicrystal that contained 5 weight percent carbon additive, using scanning probe microscopy-nanoindentation-diffraction measurement to analyze surface energy measurements of both this specimen and a p-type single crystal from SiC. They also performed EBSD-resolved grain mapping, topographic carrier type images as well as topographic carrier type/concentration images on each one for further clarity.

Topographically, the surface of a bicrystal appeared flat; however, carrier type and concentration images revealed a dark area near its grain boundary caused by carrier depletion layers caused by Sc substitution in Si sites of its lattice of SiC additive composition during sintering.

EBSD analysis confirmed the existence of a depletion layer and showed that its composition included SiAlON and b-Si3N4 particles in its grain boundaries (GBs). Furthermore, their s values mirrored those found for bulk SiC. Furthermore, low s values indicate that most conductivity comes from phonon scattering rather than free electrons; which fits in well with temperature dependence of thermal conductivity for both bodies pristine and C-SiC bodies.

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