Materials Science Of Carbides, Nitrides And Borides Download UPDATED

Materials Science Of Carbides, Nitrides And Borides Download

Recent advances in wide bandgap semiconductor-based gas sensors

F. Ren , S.J. Pearton , in Semiconductor Gas Sensors, 2013

five.3.3 TiB_ii ohmic contacts

Boride-based ohmic contacts on HEMTs show lower contact resistance than Ti/Al/Pt/Au after extended aging at 350°C ( Khanna et al., 2006).TiB_two-based ohmic contacts prove improved stability for long-term performance (Wang et al., 2007a). The structure of the ohmic contacts is Ti (200   Å )/Al (grand   Å )/ TiB_ii (400   Å )/Ti (200   Å )/Au (800   Å ). All of the metals were deposited past Ar plasma-assisted radio frequency (RF) sputtering at pressures of fifteen–40 mTorr and RF (thirteen.56   MHz) powers of 200–250   W. The contacts were annealed at 850°C for 45   s under a flowing N₂ ambient in a Heatpulse 610   T organization. Effigy five.13 shows the time dependence of forwards current at one.five   5 gate bias for devices with both types of ohmic contacts. These tests were carried out in the field, where temperature and humidity were not controlled. There are several features of annotation. First, the current is much higher in the diodes with TiB₂-based contacts because of their lower contact resistance (1.half-dozen   ×   ten^{– 6} Ω cmⁱⁱ vs vii. 5   ×   10^{– 6} Ω.cm² for the conventional Ti/Al/Pt/Au). Second, in that location is much ameliorate stability of the devices with TiB₂-based contacts. There is much less temperature dependence to the contact resistance of the boride contacts and this translates to less variation in gate electric current as the temperature cycles from day to night.

5.13. Variation in frontward current at fixed bias for diodes with boride-based ohmic contacts (upper trace) or conventional ohmic contacts (lower trace) as a function of time under field atmospheric condition where the temperature increases during the solar day and decreases at night.

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Group 13 (B, Al, Ga, In and Tl) Alkaline World Compounds

R.C. Ropp , in Encyclopedia of the Alkaline Earth Compounds, 2013

Barium Borides

Barium boride tin can exist formed straight from the elements. Ba melts at 727  °C and boron melts at 2076   °C. Therefore, if a vapor of Ba metal at >750   °C (cherry-red-heat) is passed over crystals of boron, a chemical reaction forms the desired boride. Nonetheless, to obtain stoichiometric compositions, it is better to heat the well-mixed powders of Ba and B to obtain specific compounds:

Ba   +   6B   ⇒   BaB_vi

BaB_six has the CAS number of 12046-08-1. Information technology was established that a barium hexaboride with a composition approaching theoretical tin can be obtained by a so-chosen borothermic technique at a temperature of 1600   °C from a charge with a 40% backlog of barium oxide. It is more economical to gear up BaB₆ by the reaction of barium carbonate with boron in ii stages (30   min at 900   °C and 60   min at 1500   °C).

Barium hexaboride has the aforementioned cubic structure as Ca²⁺ and Sr²⁺ hexaborides. The post-obit diagram shows 2 unit cells in the construction (Fig. 6.9).

Effigy 6.9.

Its melting betoken is 2543   °C and its density is 2.543   m/cc. Its molecular weight is 202.1973   chiliad/mol. Barium hexaboride, BaB₆, has been obtained in the class of single crystals. Its crystal structure was refined in space group Pmm (no. 221), a  =   4.2615   Å. The electronic situation of BaB_{half dozen} was discussed on the basis of different band construction calculations performed within the density functional theory (LMTO, aeroplane wave). A comparison with CaB_vi and the molecular anion [B₆H₆]²⁻ shows a similar band ordering. The different orbital contributions are strongly mixed and the inter-octahedral bonds are lower in energy than some of the intra-octahedral framework interactions.

Barium hexaboride has been considered equally a candidate for manufacture of a "hot-cathode" electron gun for utilise in diverse instruments. The following diagram shows some of the results of their usage in this application (Fig. 6.10).

Figure 6.ten.

Note that lanthanum hexaboride tin function up to 2200   °C and deliver a current as high as 75   A/cmⁱⁱ. SrB₆ has been measured as limited to 0.fourteen A/cm² and BaB₆ has a limit of 18 A/cm² because of losses as they approach their melting point (see diagram). Only CaB₆ with a melting point of 3162   °C tin can approach and exceed the performance of LaB₆. However, to reach the aforementioned level of amperage output, the CaB_vi cathode gun must attain a temperature of 2580   °C whereas LaB_six needs only a temperature of 2230   °C to do so. Thus, the CaB₆ electrode is not equally efficient as the LaB₆ ane. The deviation lies in the fact that the LaB₆ crystal exhibits metal conduction properties whereas the element of group i earths are semiconductors. The LaB₆ brightness is of the gild of 15 to 20 times that of tungsten operated at 40-h life conditions, and it was stated that single-crystal cathodes are more than reliable for producing loftier brightnesses of these electrodes.

The synthesis and germination mechanism of BaB_six powder past the reaction of BaCO₃ with B₄C and carbon were investigated systematically. The influences of heating temperature and holding fourth dimension on the reaction products were studied by X-ray diffractometry, and the morphologies of BaB₆ were investigated by scanning electron microscopy (SEM). The interaction in the BaCO₃–B₄C–C system by which BaB_{half dozen} is formed was found to exist a solid-phase diffusion process that passes through Ba₃B_iiO₅ and BaB₂O₄ transition phases. The optimal weather for BaB_{half dozen} synthesis are a holding time of ii   h at 1400   °C, under a vacuum of 10⁻ⁱⁱ  Pa of pressure.

Barium hexaboride (BaB₆) crystals can likewise be synthesized electrochemically using a molten salt technique. Barium carbonate (BaCO₃) and boron trioxide (B_iiO_iii) were used as reactants. Lithium fluoride (LiF) was used as the supporting electrolyte. Small single crystals could exist separated from the molten mixture.

Single crystals of alkaline globe hexaborides, CaB₆, SrB₆ and BaB_vi, take been grown from a molten aluminum metallic flux. The size of crystals produced was up to v   mm for CaB₆, 3   mm for SrB_half-dozen and 1   mm for BaB₆ in the longest dimension. The color of single crystals is blackness for CaB₆, dark blue for SrB_half-dozen and dark green for BaB_half-dozen. The optical reflectance of single crystals was measured over ane.v–5.v   eV photon energy range.

Barium hexaboride is available in small to medium lots commercially worldwide. It is also offered equally single-crystal "whiskers" for sale for research into its properties.

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Silicon

In Chemical science of the Elements (Second Edition), 1997

nine.three.1 Silicides^(26,27)

As with borides ( p. 145) and carbides (p. 297) the formulae of metal silicides cannot be rationalized by the application of unproblematic valency rules, and the bonding varies from substantially metallic to ionic and covalent. Observed stoichiometries include Thou₆Si, M₅Si, Thousand₄Si, K₁₅Si₄, M_threeSi, M₅Si₂, M₂Si, M₅Si₃, M₃Si₂, MSi, M₂Si₃, MSi_ii, MSi₃ and MSi₆. Silicon, like boron, is more electropositive than carbon, and structurally the silicides are more closely related to the borides than the carbides (cf. diagonal relation, p. 27). However, the. covalent radius of Si (118 pm) is appreciably larger than for B (88 pm) and few silicides are really isostructural with the respective borides. Silicides have been reported for virtually all elements in Groups one–10 except Exist, the greatest range of stoichiometries being shown past the transition metals in Groups iv–ten and uranium. No silicides are known for the metals in Groups 11–xv except Cu; near form unproblematic eutectic mixtures, but the heaviest post-transition metals Hg, Tl, Lead and Bi are completely immiscible with molten Si.

Some metal-rich silicides have isolated Si atoms and these occur either in typical metal-similar structures or in more polar structures. With increasing Si content, there is an increasing trend to catenate into isolated Si_two or Si₄, or into chains, layers or 3D networks of Si atoms. Examples are in Table 9.three and farther structural details are in refs. 24, 26 and 27.

Table nine.3. Structural units in metallic suicides

Unit	Examples
Isolated Si	CuvSi (β-Mn construction)		Metallic structures (good electric conductors)
	1000iiiSi (β-W structure) M = 5, Cr, Mo
	Atomic number 263Si (FeiiiAl superstructure)
	Mn3Si (random bcc)
	10002Si (anti-CaFii); M = Mg, Ge, Sn, Atomic number 82		Not-metalstructures (non-conductors)
	M2Si (anti-PbCl2); M = Ca, Ru, Ce, Rh, Ir, Ni
Siii pairs	U3Si2 (Si–Si 230pm), also for Hf and Thursday
Si4 tetrahedra	KSi (Si–Si 243pm), i.e. [1000+]iv[Si4]iv− cf. isoelectronic P4 (G = Li, Grand, Rb, Cs; also for MfourGeiv)
Sichains	USi (FeB structure) (Si–Si 236pm);also for Ti, Zr, Hf, Th, Ce, Pu CaSi(CrB structure)(Si–Si 247pm);also for Sr, Y
Plane hexagonal Si nets	β-USiii (AlB2 construction) (Si–Si 222–236pm); too for other actinoids and lanthanoids
Puckered hexagonal Si nets	CaSi2 (Si-Si 248pm) — every bit in "puckered graphite" layer
Open 3D Si frameworks	SrSi2, α-ThSi2(Si-Si 239pm; closely related to AlBtwo),α-USi2

Silicides are usually prepared by directly fusion of the elements but coreduction of SiO₂ and a metal oxide with C or Al is sometimes used. Heats of formation are like to those of borides and carbides but mps are substantially lower; e.g. TiC 3140°, TiB₂ 2980°, TiSi_ii 1540°; and TaC 3800°, TaB₂ 3100°, TaSi_two 1560° C. Few silicides cook as high as 2000–2500°, and in a higher place this temperature merely SiC is solid (decomp ∼2700° C).

Silicides of groups 1 and 2 are more often than not much more than reactive than those of the transition elements (cf. borides and carbides). Hydrogen and/or silanes are typical products; e.g.:

$\begin{array}{l}{\text{Na}}_{\text{2}}{\text{Si\hspace{0.5em}+\hspace{0.5em}3H}}_{\text{ii}}\text{O}\underset{\text{complete}}{\overset{\text{rapidand}}{\to }}{\text{Na}}_{\text{two}}{\text{SiO}}_{\text{3}}{\text{+\hspace{0.5em}3H}}_{\text{2}}\\ {\text{Mg}}_{\text{ii}}{\text{Si\hspace{0.5em}+\hspace{0.5em}2H}}_{\text{ii}}{\text{SO}}_{\text{four}}\left(\text{aq}\right)\to {\text{2MgSO}}_{\text{4}}{\text{+\hspace{0.5em}SiH}}_{\text{four}}\end{array}$

Products also depend on stoichiometry (i.e. structural type). For example, the polar, non-conducting Ca₂Si (anti-PbCl₂ structure with isolated Si atoms) reacts with h2o to give Ca(OH)₂, SiO_ii (hydrated), and H₂, whereas CaSi (which features zigzag Si chains) gives silanes and the polymeric SiH₂. By contrast CaSi₂, which has puckered layers of Si atoms, does not react with pure water, only with dilute muriatic acid it yields a yellow polymeric solid of overall composition Si₂H₂O. Transition element silicides are usually inert to aqueous reagents except HF, but yield to more aggressive reagents such as molten KOH, or F₂ (Cl₂) at crimson heat.

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Chemistry of nonmetallic elements I. Hydrogen, boron, oxygen, and carbon

James E. House , in Inorganic Chemistry (Third Edition), 2020

thirteen.2.3 Borides

Boron forms one or more borides when it reacts with well-nigh metals. For example, the reaction between magnesium and boron produces magnesium boride, Mg _iiiB₂.

(13.36) $3\text{Mg}+2\text{B}\to {\text{Mg}}_3{\text{B}}_{\mathrm{two}}$

This product reacts with acids to produce diborane, B_twoH_half-dozen.

(13.37) ${\text{Mg}}_3{\text{B}}_{\mathrm{two}}+6{\text{H}}^{+}\to {\text{B}}_2{\text{H}}_{\mathrm{six}}+3{\text{Mg}}^{2+}$

Although BH₃ may be the expected product, it is not stable as a detached, monatomic unit. It is stable as an adduct with several Lewis bases. Some metals react with boron to class borides containing the hexaboride group, B₆ ²⁻. An case of this type of chemical compound is calcium hexaboride, CaB₆. In general, the structures of compounds of this type incorporate octahedral B₆ ²⁻ ions in a cubic lattice with metal ions. Most hexaborides are refractory materials having melting points over 2000°C.

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Boron

In Chemical science of the Elements (Second Edition), 1997

half dozen.three.1 Introduction

The borides comprise a group of over 200 binary compounds which evidence an amazing diversity of stoichiometries and structural types; e.k. Thou _vB, M₄B, M₃B, 1000_vB₂, G_viiB_three, 1000_iiB, M₅B₃, G₃B₂, M₁₁B₈, MB, M₁₀B₁₁, M_iiiB_iv, Thou_iiB_three, M₃B_five, MB_ii, Yard_iiB_five, MB₃, MB₄, MB_six, M_twoB₁₃, MB_x, MB₁₂, MB₁₅, MB₁₈ and MB₆₆. There are as well numerous nonstoichiometric phases of variable composition and many ternary and more than complex phases in which more than one metallic combines with boron. The rapid advance in our understanding of these compounds during the past few decades has been based mainly on X-ray diffraction analysis and the work has been stimulated not merely past the inherent bookish challenge implied by the existence of these unusual compounds simply too by the extensive industrial involvement generated by their unique combination of desirable physical and chemical properties (run across Console).

Backdrop and Uses of Borides

Metal-rich borides are extremely hard, chemically inert, involatile, refractory materials with mps and electrical conductivities which often exceed those of the parent metals. Thus the highly conducting diborides of Zr, Hf, Nb and Ta all accept mps > 3000°C and TiB₂ (mp 2980°C) has a conductivity 5 times greater than that of Ti metal. Borides are normally prepared as powders but can exist fabricated into the desired form by standard techniques of pulverization metallurgy and ceramic engineering science. TiB₂, ZrB_two and CrB₂ find awarding as turbine blades, combustion chamber liners, rocket nozzles and ablation shields. Ability to withstand attack by molten metals, slags and salts accept commended borides or boride-coated metals every bit high-temperature reactor vessels, vaporizing boats, crucibles, pump impellers and thermocouple sheaths. Inertness to chemical assail' at high temperatures, coupled with first-class electrical electrical conductivity, suggest application equally electrodes in industrial processes.

Nuclear applications plow on the very loftier absorption cross-department of ¹⁰B for thermal neutrons (p. 144) and the fact that this property is retained for loftier-free energy neutrons (10⁴-10⁶ eV) more effectively than for whatsoever other nuclide. Another advantage of ¹⁰B is that the products of the (n,α) reaction are the stable, non-radioactive elements Li and He. Accordingly, metal borides and boron carbide have been used extensively equally neutron shields and control rods since the get-go of the nuclear power industry. More dramatically, following the disaster at Chernobyl in the early hours of 26 April 1986, some 40 tonnes of boron carbide particles were dumped from helicopters onto the stricken reactor to prevent further delinquent fission occurring. (In addition there were 800 tonnes of dolomite to provide a CO_two gas coating, 1800 t of clay and sand to quench the fires and filter radionuclides, plus 2400 t of lead to absorb estrus past melting and to provide a liquid layer that would in fourth dimension solidify and seal the top of the core of the vault.)

The principal non-nuclear industrial apply of boron carbide is equally an abrasive grit or powder for polishing or grinding; it is also used on restriction and clutch linings. In add-on, there is much electric current interest in its utilize as lite-weight protective armour, and tests have indicated that boron carbide and glucinium borides offering the best choice; applications are in bullet-proof protective wear and in protective armour for aircraft. More elegantly, boron carbide can at present be produced in fibre grade by reacting BCl₃/H_two with carbon yarn at 1600–1900°C:

${\text{4BCl}}_{\text{3}}{\text{+6H}}_{\text{2}}\text{+C}\left(\text{fibres}\right)\to {\text{B}}_{\text{4}}\text{C}\left(\text{fibres}\right)\text{+12HCl}$

Fibre curling tin can exist eliminated by heat treatment under tension most the mp, and the resulting fibres have a tensile forcefulness of 3.5 × ten^five psi (1 psi = 6895 Due north grand⁻ⁱⁱ) and an rubberband modulus of 50 × 10⁶ psi at a density of two.35 g cm⁻³; the form was 1 ply, 720 filament yarn with a filament bore of 11–12 μm. The fibres are inert to hot acid and alkali, resistant to Cl_ii upwards to 700° and air up to 800°C.

Boron itself has been used for over two decades in filament form in various composites; BCl_three/H₂ is reacted at 1300° on the surface of a continuously moving tungsten fibre 12 μm in bore. Us production capacity is about xx tonnes pa and the price in about $800/kg. The primary employ so far has been in military aircraft and infinite shuttles, merely boron fibre composites are also being studied every bit reinforcement materials for commercial aircraft. At the domestic level they are finding increasing application in golf shafts, tennis rackets and bicycle frames.

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Transition Elements, Lanthanides and Actinides

A. Simon , H. Mattausch , in Comprehensive Inorganic Chemistry II (Second Edition), 2013

2.twenty.7.3 Boride Carbides of Lanthanoids

The structural chemistry of borides and carbides of the lanthanoids distinctly differs in the general feature that extended B–B bonding contrasts with C–C bonding in quasi-molecular units. The combination of B and C results in an enormously rich chemistry between these extremes. It is once again helpful to first the give-and-take with lanthanoid halide boride carbides whose structures contain discrete or low-dimensional anionic species and therefore offer an access to the chemical bonding from a viewpoint of molecular chemistry. They as well provide topologies of typical coordination polyhedra around B and C which frequently occur in three-dimensional frameworks like the trigonal prism for B and the tetragonal pyramid for C (encounter Figure 47 ). ²²⁰

The different beliefs of B and C is particularly obvious in the case of La₄I₅B₂C which represents an ordered intergrowth of chains that are the only components in the ternary compounds Ln₄X₅(C₂) and Ln₄10₅(B₄), respectively (see Figure 62 ). ^{221 , 222} The close fit between the two types of bondage may propose that an intergrowth series (La₄I₅(C₂)) _grand (La_ivI₅(B₄)) _n exists.

Figure 62. Comparing of the crystal structures (from left to right) of La₄I_five(C_ii), La₄I₅(B_iv), and La₈I_ten(C_ii)(B₄). ^{221 , 222}

The separation of B and C is rather the exception, and B–C bonding frequently occurs. In the structure of Ce₃Br₃BC and the isotypic chloride and bromide, compounds of Ce and La ²²⁰ units of limerick (BC) with brusque B–C distance, 157   pm, occur which can exist described equally (BC^{five   −}) anions derived from a methyl borane molecule. These units are continued via weak B–B bonds, to form a ribbon with a zigzag spine of B atoms at a distance of 218   pm ( Figure 63 ). The B and C atoms accept the characteristic coordination polyhedra mentioned earlier, trigonal prism and quadratic pyramid, respectively.

Figure 63. (Top) Chain of limerick (BC) in the crystal structure of Ce_threeBr₃BC (bottom). ²²⁰

There is a close relationship to the ThBC structure; indeed, Ce₃Br₃BC may be discussed equally a dispersion of ane-dimensional fragments of the ThBC-type structure in a halogen atom matrix. These one-dimensional fragments characterized past B–B bonding tin be combined with only carbon-containing fragments as in the first chosen case. In the structure of Gd₄Br₃(BC)C shown in Effigy 64 ²³⁹ (BC) ribbons occur besides discrete C atoms in a layered arrangement. A whole series of phases can result from a combination of the structural elements at variable ratio which nevertheless have to exist identified. Our noesis is bars to a few examples out of a presumably big family of compounds where the ratio of dissimilar building units is one variable likewise the variation of the edifice units themselves. The structure of Ce₆Br₃(CB₂C)C ( Effigy 64 ) ²³⁹ serves as an example.

Figure 64. Projections of the crystal structures of Gd_fourBr₃(BC)C (left) and Ce₆Br₃(CB₂C)C (right). ²³⁹

Discrete (B₂C_ii) units alternating with single C atoms in a metal atom matrix separated by Br atoms. Again the close similarity with three-dimensional networks is obvious: formally, the Thursday₃B₂C₃ (=   Th_iiiC(BC)₂) structure is cut into two-dimensional fragments by inserting layers of Br atoms. Last but not least, the structures of La₃Br_ii(CBC) and La₉Br₅(CBC)_iii present withal another type of quasi-molecular units (CBC), in both cases equally the only anionic species besides Br. ²³⁹ The short B–C altitude of 150   pm and the bent shape lead to an assignment as (BC₂)⁷⁻ corresponding to isoelectronic and neutral SO₂. This assignment must exist taken as a crude approximation because backbonding from the antibonding π_u into empty Ln d states will reduce the charge. ²⁴⁰

When it comes to the ternary boride carbides, the structural complexity is greatly increased. In an early stage, the cardinal rules have been established ²⁴⁰ : how the number of electrons provided past the metal atoms determines the anionic boron–carbon system. The dimensionality of the nonmetal substructure is closely related to the valence electron concentration (VEC) in agreement with the Zintl–Klemm approach. As a starting time approximation, the dimensionality of the B _y C _z system is the lower, the higher VEC. Table two presents a compilation of structural information known at that time. Of course, the general trend is obvious, as the metal-valence electrons enter antibonding states in the B _y C _z organisation and disrupt bonds in the anionic sublattice. The approach is rather qualitative in grapheme, because lanthanoid boride carbides can incorporate one- and goose egg-dimensional B _y C _z subsystems simultaneously, for example, in Gd₄B₃C_iv. ²⁴¹ Boosted parameters such equally lanthanoid cantlet sizes are manifestly important to determine the dimensionality of the B _y C _z networks. One-dimensional C-branched zigzag chains of B atoms occur with the after lanthanoids Dy, Ho, and Er, and finite chains are found with the larger early lanthanoids La to Sm at identical compositions LnBC. ^{242 , 243}

Table ii. Different Ln _x B _y C _z rare-earth metal borocarbides which are structurally characterized. The VEC is calculated with northward  =   3, in brackets for north  =   4; for references, see Bauer et al. ²⁴⁰

Structural type		VEC	B–C network	(y+ z)/10
Two-dimensional networks
LaB2C2	(Y to Lu)	four.25	2/∞-(2B2C)·(4B4C)	4.0
ScB2Ctwo		4.25	2/∞-(2B3C)·(4B3C)	iv.0
YBiiC	(Sc, Tb to Lu)	4.33	2/∞-(6B)·(6B(3C))	iii.0
ThBtwoC	(Ce, U, Np, Pu)	iv.33 (4.67)	2/∞-(6B(2C))	3.0
UBtwoC		4.33 (four.67)	ii/∞-(2B2C)·(4B4C)	iii.0
Gd2B3C2		4.sixty	2/∞-(8B4C)	2.five
Infinite branched chains
YBC	(Dy, Ho, Er)	5.00	1/∞-[B2C2]	2.0
ThBC		five.00 (v.50)	one/∞-[B2Cii]	two.0
UBC	(Np, Pu)	v.00 (5.fifty)	1/∞-[B2C2]	2.0
UB0.78C1.22		5.11 (5.61)	1/∞-[B2C2]	2.0
TbthreeB2C3		five.40 (half-dozen.00)	[C]-1/∞-[B2Cii]	ane.67
Finite linear chains
La15B14C19		4.94	[B4C7] [B5Chalf-dozen]	2.2
Ce10B9C12	(La, Nd)	v.00	[BvC8] [BivC4]	two.i
CevB4C5		5.22	[B4C4] [B3C3] [BC2] [C]	i.8
Ce5BtwoCvi	(La, Gd, Ho)	5.63	[BCtwo] [C2]	1.vi
Sc2BC2		5.67	[BC2]	1.5
Gd5B2Cfive	(Sm)	5.86	[BC2] [C]	1.4

In spite of the rather straightforward simple approach, the predictability is quite impressive. The values of VEC shown in Tabular array 2 are based on the number of valence electrons provided by the metal atoms. If there is an backlog of electrons, these stay dominantly in metal–metallic-bonding states, and many of the boride carbides are (marginally) low-valence compounds with metallic properties for all of them.

A plethora of lanthanoid boride carbides exists, and its number is still increasing. A common structural principle is illustrated with La₅B₂C₆ in Figure 65 . ²⁴⁴ About planar square nets of La atoms are stacked in a style that the La–La distances are approximately the same in and between the nets. The resulting tetragonal structure offers two kinds of voids to be occupied past nonmetal atoms. The octahedral void results from a stacking of Ln nets which position Ln atoms above and below the center of a square, and the larger void is an antiprism terminated by Ln atoms above and below. This large void can easily be extended by repeated antiprismatic stacking as shown subsequently.

Figure 65. Crystal construction of La₅B_iiC_six formed by stacking of quadratic nets of La atoms with cut-out coordination polyhedra for the anions. ²⁴⁴

The minor octahedral voids can be left empty or may be (partially) filled with single C atoms or (C₂) units. The larger voids are completely occupied past either (CBC) or (CBCC) units. In the instance of the early lanthanoids La, Ce, Pr, and Nd, the longer (CBCC) unit is observed which occurs in twofold disorder, (CBCC) and (CCBC), and may be formulated equally (CBCC⁵⁻). With these metals, a variable occupation of the octahedral voids by C and/or (C_ii) is the origin of a broad range of homogeneity, the low limit of which is corresponding to Ln_five(CBCC)_ii  =   Ln₅B_twoC₆. In the case of the late lanthanoids Dy to Tm with smaller atomic sizes, the disorder is less pronounced, and the composition changes toward a lower C content within the same Ln atom stacking sequence. ²⁴⁵ (CBC) units are found in the larger voids, and the octahedral voids are nearly completely filled with either C or (C_ii) which splits the broad range of homogeneity observed with the early lanthanoids into 2 narrow ones effectually the limiting compositions Ln_vC(CBC)₂  =   Ln₅B_twoC₅ and Ln_v(C₂)(CBC)₂  =   Ln₅B₂C_vi, respectively. ²⁴⁶

As discussed above, the variation of the stacking of foursquare nets of Ln atoms opens the style to create tubular voids of face-sharing quadratic antiprisms which tin be occupied by chain-like B _y C _z anions. Figure 66 depicts a few of them ²⁴² : (C–B–B–C–B–C) and (C–B–C–B–B–C–B–C) in La₅B₄C_v, (C–B–C–B–C–B–B–C–B–C) in LaBC, (C–C–B–C–B–C–B–C–B–C–C) and (C–B–C–B–C–B–C–B–C–B–C) in La_fifteenB₁₄C₁₉, and (C–C–C–B–B–C–B–C–B–B–C–C–C) in La₁₀B_nineC₁₂. The germination of infinite chains and finally layers as a consequence of decreasing VEC is well documented in Ref. 240 and has been verified by many more recent findings. In particular, the simultaneous occurrence of zip- and 1-dimensional B _y C _z regimes provides a finer grid than in the earlier addressed example of Gd_ivB₃C_four (come across Effigy 67 ). ²⁴¹

Figure 66. (a, b) B₃C_three and B₄C₄ entities in La₅B₄C₅, (c) B₅C₅ in LaBC, (d, e) B₄C₇ and B₅C_vi in La₁₅B₁₄C₁₉, and (f) B₅C_viii in La_tenB₉C₁₂. ²⁴²

Figure 67. Crystal structure of Gd_fourB₃C_four. ²⁴¹

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Corrosion behaviour of nitrided, nitrocarburised and carburised steels

H.-J. Spies , in Thermochemical Surface Engineering of Steels, 2015

6.12 Determination

Iron nitride and atomic number 26 boride are passivatable. Boriding can considerably enhance the corrosion resistance of ferrous materials in non-oxidising dilute acids and alkali media. Compared to the unnitrided condition, nitrided components from unalloyed and low alloyed steels have an increased resistance against atmospheric corrosion and corrosion in neutral aqueous media.

The corrosion resistance of nitride layers is very dissimilar and increases in the social club of γ′-nitride, ε-nitride, ε-carbonitride. ε-carbonitride with a concentration of at least 8.six% of nitrogen and carbon interstitials has an optimum resistance against pitting corrosion. The corrosion behaviour of nitrided layers can be improved by postal service-oxidation. Oxidised nitride layers exceed the corrosion resistance of hard chromium and have proven every bit a substitute for these layers. A farther increase in resistance is possible past means of a concluding post-impregnation by immersion into polymeric oil.

For the production of ε-carbonitride layers with an optimum corrosion behaviour, apart from bath nitrocarburising both gas and plasma nitrocarburising are available today. Owing to the development of controlled nitriding, it is now possible with these processes to generate ε-carbonitride layers with a high corrosion resistance past defined concentrations of nitrogen and carbon.

The corrosion behaviour of thermochemical treated stainless steels is determined by the temperature and time dependent chromium depletion of the matrix. Nitriding and nitrocarburising in the conventional temperature range above 500°C lead to precipitation of chromium nitrides and carbides in the case of treated components. The resulting chromium depletion of the matrix causes a significant loss of the corrosion resistance. After a depression temperature handling the passivation behaviour is comparable with untreated material, and the resistance against pitting corrosion is shifted in the noble direction. The microstructure of the example, supersaturated with nitrogen and/or carbon, is more stable in austenitic stainless steels than in ferritic and martensitic stainless steels.

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Plasma-Enhanced Chemical Vapor Deposition of Functional Coatings

L. Martinu , ... J.Due east. Klemberg-Sapieha , in Handbook of Degradation Technologies for Films and Coatings (Third Edition), 2010

9.six.two.i Metal-Based Tribological Coatings

Binary metal nitrides, carbides, borides, and oxides are frequently used in unlike tribological applications including cutting tools, protection of dissimilar components of automobiles, aircrafts, and consumer products. Their about attractive characteristic is a combination of hardness with high clothing, erosion and corrosion resistance, while some of them provide attractive colors and other advantageous functional characteristics. Such coatings are traditionally made by CVD and PVD methods, simply PECVD is increasingly taking its place because of the possibility of benefiting from the plasma chemical reactions, plasma–surface interactions, and interface applied science at depression temperature.

In order to augment the range of the pic properties suitable for different applications, and especially to benefit from synergistic furnishings, ternary metal compounds (mostly including metallic carbon nitrides), nanocomposites, and nanolaminate structures (multilayers formed by several to several tens of nanometer thick individual layers) appear to satisfy the ever increasing requirements.

In the context of aerospace and avionics applications, commercial, armed services, rescue or humanitarian aid aircraft or helicopters oft operate in a hostile environment, in high humidity and marine common salt concentrations. They may operate in the presence of ambitious chemicals, in heavily polluted regions, where the air can carry dust, sand particles, volcanic ash, and other compounds. Such conditions contribute to increased erosion and corrosion of aircraft engine components. This generally leads to decreased efficiency and increased fuel consumption, and can eventually result in catastrophic failure. Therefore, frequent inspections and parts replacements are necessary. These contribute to downtime and increased machine functioning costs. Awarding of loftier-functioning erosion-, habiliment-, and corrosion-resistant protective coatings tin significantly extend the lifetime and performance of aircraft engine components and helicopter parts and thus lead to major reductions in maintenance costs, and to improved safety.

Examples of the wearable properties of TiN-based coatings deposited onto martensitic 410 stainless steel are shown in Figure 9.30(a) . In order to heighten adhesion, the hardness of the steel (H  =   5   GPa) has been gradually increased at the interface by nitriding the near-surface region (to a depth of several μm, H  =   15   GPa), followed by the degradation of 1   μm thick TiN (H  =   24   GPa), on which four   μm thick nc-TiN/SiN, nc-TiCN/SiCN coatings have been prepared (H  =   35–42   GPa; triplex process) [237].

Figure ix.30. Tribological properties of Tin can and nanocomposite nc-TiN/SiN_ane.3 and nc-TiCN/SiCN films deposited at T _s of 673   1000 and 773   K, compared to: (a) the vesture coefficient, K, and the coefficient of friction, μ (after [237]); (b) solid particle erosion charge per unit of the coatings deposited on 410 stainless steel (Al_iiO_three particles, diameter 50   μm, speed 80   m/south) (afterwards [243]).

The figure shows that nc-TiCN/SiCN exhibits superior tribological properties compared to Tin and to nc-TiN/SiN coatings, such every bit depression friction (μ  =   0.17 compared to 0.five for the sometime two coatings) and low habiliment rate (K  =   1.6   ×   10⁻⁷  mmⁱⁱⁱ/Nm compared to 9.5   ×   x⁻⁶  mm^three/Nm for the former coatings). The nc-TiCN/SiCN thus allows i to reduce wear past a factor of ∼   600 compared to bare SS410. It has been proposed that this performance is due to higher hardness, college elastic rebound, and lower friction due to the presence of carbon-containing, flexible, Si–C and C–N bonds in the tribolayer [237]. In addition, the presence of carbon leads to attractive colour changes compared to nc-TiN/SiN and to the traditional aureate color of TiN [167].

The significant result of applying nc coatings onto aerospace components has also been found past evaluating the erosion resistance (ER), following the ASTM G76 standard. As illustrated in Figure nine.30(b), application of PECVD Tin can, nc-TiN/SiN, and nc-TiCN/SiCN films resulted in an increment of ER by a factor of 20, compared to uncoated SS410 substrates.

As discussed in a higher place, assessment of the tribological properties of coatings with respect to their elastoplastic characteristics opens the possibility of better prediction and optimization of their performance. In this context, the 1000 and ER values of the Can-containing coatings from Figure 9.30 are related to the H ³/E ⁱⁱ ratio in Effigy 9.31. Information technology has been concluded that a substantial comeback in the motion picture'south tribological behavior (e.g. erosion and wear) occurs for H ³/E ²  >   0.5   GPa (or H/Due east  >   0.15–0.twenty) [237, 243].

Figure 9.31. Tribological characteristics of Tin and nanocomposite nc-TiN/SiN_1.3 films equally a function of the H ³/E ^two ratio: (a) wearable coefficient (subsequently [237]); (b) erosion rate (afterward [243]); (c) book removed nether single particle erosion (finite element modeling) (afterward [242]).

Recently, the H ³/E ²  >   0.5   GPa condition has also been predicted for such a tribological situation (Al₂O₃ erodent particles, fifty   μm size, 85   m/s velocity) based on finite element calculations using a model considering tensile and shear stress, and yielding criteria for failure upon particle bear upon [244]. In other words, there is at present growing evidence that a critical H ³/E ⁱⁱ ratio (or H/E ratio) has to be satisfied for the coating to provide appropriate tribological protection. It likewise means that for successful tribological applications the H ³/E ² and H/E ratios of the used materials should exist maximized. In reality, it is generally important to ensure that the blanket hardness is sufficiently higher than that of the eroding or wearing medium, but the Due east value should exist minimized. Therefore, film microstructure should exist adjusted to satisfy such conditions: in this respect, PECVD metal-based nc coatings are very suitable candidates for many surface engineering and tribological solutions.

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Chemical Vapor Deposition

Milton Ohring , in Materials Scientific discipline of Sparse Films (2d Edition), 2002

6.ii.4 COMPOUND FORMATION

A variety of carbide, nitride, boride, etc., films and coatings can be readily produced by CVD techniques. What is required is that the compound elements exist in a volatile grade and be sufficiently reactive in the gas phase. Examples of commercially important reactions for the deposition of difficult, wear-resistant surface coatings include:

(half dozen-9) ${\text{SiCl}}_{\mathrm{four}(\mathrm{thou})}+{\text{CH}}_{\mathrm{four}(g)}\to {\text{SiC}}_{(\mathrm{southward})}+4{\text{HCl}}_{(\mathrm{1000})}\begin{array}{cc}& ({1400}^{\circ }\text{C})\end{array}.$

(6-ten) ${\text{TiCl}}_{4(\mathrm{thou})}+{\text{CH}}_{4(\mathrm{one\; thousand})}\to {\text{TiC}}_{(s)}+4{\text{HCl}}_{(g)}\begin{array}{cc}& ({\mathrm{m}}^{\circ }\text{C})\end{array}.$

(6-11) ${\text{BF}}_{3(g)}+{\text{NH}}_{3(g)}\to {\text{BN}}_{(\mathrm{due\; south})}+3{\text{HF}}_{(m)}\begin{array}{cc}& ({1100}^{\circ }\text{C})\end{array}.$

Films and coatings of compounds can generally exist produced using a variety of precursor gases and reactions. For instance, in the much-studied SiC system, layers were commencement produced in 1909 through reaction of SiCl_four + C_half-dozenH₆ (Ref. 11). Subsequent reactant combinations over the years have included SiCl₄ + C₃H₈, SiBr_four + C_twoH₄, SiCl₄ + C₆H₁₄, SiHCl_three + CCl_iv, and SiCl₄ + C₆H₅CH_iii, to name a few, likewise as volatile organic compounds containing both silicon and carbon in the same molecule (due east.k., CH_threeSiCl₃, CH_iiiSiH_three, (CH₃)_twoSiCl_two). Although the deposit is nominally SiC in all cases, resultant properties generally differ because of structural, compositional, and processing differences.

Impermeable insulating and passivating films of Si_threeN₄ are required to hermetically seal integrated circuits. Although they can be deposited at 750°C by the reaction

(6-12) ${\text{3SiCl}}_2{\text{H}}_{2(g)}{\text{+4NH}}_{\mathrm{three}(\mathrm{grand})}\to {\text{Si}}_3{\text{N}}_{\mathrm{iv}(s)}+6{\text{H}}_{2(\mathrm{chiliad})}+6{\text{HCl}}_{(g)},$

the necessity to eolith silicon-nitride films at lower temperatures has led to alternate processing involving the utilize of plasmas. Films tin can be deposited below 300°C with SiH_iv and NH_three reactants, only considerable amounts of hydrogen are incorporated into the deposits.

The very important and apace growing metalorganic CVD (MOCVD) processes, used to eolith contrasted epitaxial compound-semiconductor films, as well fit under the present category of reactions. As the name implies, volatile organic precursor-compounds such equally trimethylgallium (TMGa or (CH₃)₃Ga), trimethylindium (TMIn), etc., are employed. They are reacted with group V hydrides to form the semiconductor compound, east.1000.,

(half-dozen-xiii) ${({\text{CH}}_3)}_{\mathrm{iii}}{\text{Ga}}_{(g)}+{\text{AsH}}_{3(g)}\to {\text{GaAs}}_{(s)}+3{\text{CH}}_{4(g)}.$

Similar MOCVD reactions are exploited in producing a wide assortment of complex oxides and semiconductors, and these processes will be treated further in Sections 6.six.4 and 8.v.3.1, respectively.

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Ceramic Matrix Materials

R. Morrell , in Reference Module in Materials Science and Materials Engineering science, 2016

6.8 Other Non-oxides

There is an enormous range of other metallic borides, carbides, and nitrides from which ceramic materials, and hence in principle CMC matrices, tin can be made. The metal species include peculiarly Ti, Zr, and Hf, simply too transition metals and rare earths. However, mostly these compounds are hard, are difficult to densify other than by hot-pressing or spark plasma sintering, or require the use of large amounts of other phases, and practice not have peculiarly proficient long-term oxidation resistance. In CMCs, these non-oxides are thus likely to have limited general value, just at that place is currently significant research towards ultra-high temperature materials, for example, Hu et al. (2015) study on the employ of C interfaces on SiC fiber composites with infiltrated particulate ZrB₂ and and then with SiC deposited by a CVI process. Silvestroni et al. (2013) employed HfB₂ with ZrSi_ii or Si₃N_four every bit sintering aids, and Spencer et al. (2011a,b) and Guo et al. (2015) who used a and so-called MAX phases, for case, Ti₃AlC, and hot pressing to consolidate composites with SiC fibers.

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