When Wolfgang Amadeus Mozart was 25 years old he wrote a theme and 12 variations piece called, “Ah, vous dirai-je Maman.” (The traditional French tune is widely recognized in the English-speaking world as “Twinkle, twinkle little star.”) The simple structure of the melody gave the composer the freedom and structure to explore the music and see how much he could get the melody to give. Joseph DuBose, on classicalconnect.com writes,
The French tune is stated in simple two-part harmony, allowing ample room for Mozart’s imagination to run free. Throughout each of the succeeding twelve variations, the harmony is enriched through the introduction of suspensions and chromatic chords. The variations also maintain the tune’s twenty-four-measure structure. In some, the melody itself is embellished, such as Variations I or III; in others, the tune is set against an embellished countermelody, such as Variation II or VI.
When you hear the piece (or better yet, watch it), you can almost hear Mozart thinking, “What happens if I let the left hand show off? What about the right? What if I tiptoe around the melody? How does it sound with both hands pounding and trilling?”
The feature article in the March issue of the Journal of the American Ceramic Society brought this piece of music to mind. The article is a review titled, “Optical properties of (oxy)nitride materials: A review,” by Rong-Jun Xie and Hubertus T. Hintzen. The reviewers focus only on the optical properties because, as the authors say in the paper, “The optical properties of these (oxy)nitrides, in conjunction with their excellent mechanical strength, thermal properties, and chemical stability, enable (oxy)nitrides to be used in a variety of industrial fields… .”
Nitrogen is parked between carbon and oxygen on the Periodic Table, so some nitrides have characteristics similar to carbides, while some behave more like oxides. The authors identify two categories of nitrides compounds based on bonding character: transition-metal nitrides and ionic-covalent nitrides. The authors describe the nature of the bonding thus:
Nitrogen is interstitial in the metal atom arrangement in transition-metal nitrides in which the metal-metal bonds are dominant. On the other hand, nitrogen-(non)metal bonds are common in ionic-covalent nitrides, and (non)metals are interstitial in a nitrogen array.
This means that transition-metal nitrides have crystal structures and properties that are similar to carbides. Exploring compositions across the transition-metal series of the Periodic Table is like letting the piano left hand show off. Compounds can be refractory (TiN, ZrN, TaN), magnetic (FeN, CoN, CrN, MnN), superconducting (NbN, MoN, HfN), or catalytic (Ta3N5, TaON, TiO2-xNx).
The ionic-covalent family of nitrides behaves more like oxides (see what the right hand can do!). These compounds offer interesting properties such as ionic conductivity (Li3N), thermomechanical (Si3N4, BN), optoelectronic (GaN, InAlGaN, AlN, BN), and luminescence (α-sialon, β-sialon, M2Si5N8, CaAlSiN3).
The properties of oxynitride compounds change depending on the oxygen-to-nitrogen ratio. The authors say, “The chemical and physical properties of (oxy)nitrides are greatly connected with the composition of materials, typically the O/N ratio.” They continue, “Even at a doping level, the incorporation of nitrogen into an oxidic framework will make changes in the properties.”
This means that optical properties can be tuned in variety of materials for a wide range of applications, similar to the subtle interplay and balance between the lower registers and upper registers of the piano keyboard.
The review begins with brief definitions of key optical properties: refraction, reflection, absorption, transmission, scattering, and luminescence. In depth consideration is giving for both types of (oxy)nitride compounds to the antireflection and solar selectivity properties of thin films, band gap and absorption edge properties, photoluminescence, and transmittance. As always with ceramics, processing matters, and the authors include process influences along with their discussion on crystal structure and chemical composition.
The range of applications for (oxy)nitride materials is vast. Applications for the transition-metal (oxy)nitride group include antireflection coatings; heat mirror coatings for energy efficient architectural windows, aircraft, solar collectors, and lighting; mid- and high-temperature solar absorbers for water heating, space heating and cooling refrigeration, industrial process heat, desalination, solar thermal power systems; photocatalysis for water splitting, water and atmospheric purification, antifouling, demisting, and deodorizing. An interesting application is as eco-friendly pigments. The colorful transition-metal oxynitride compounds, such as (Ca,La)Ta(O,N)3, could replace heavy-metal based pigments.
Applications for the ionic-covalent (oxy)nitride group includes armor (see, for example, the article on AlON in the March 2013 issue of the ACerS Bulletin); transparent windows, plates, domes, etc.; semiconductor devices; solid-state LED lighting for general illumination, vehicle headlamps, liquid crystal display backlighting; and field-emission displays. When compounded with rare earths, these oxynitrides may find applications as ecological pigments, too.
The variations on the theme of optical properties of (oxy)nitride materials appear to be infinite.
The paper is “Optical properties of (oxy)nitride materials: A review,” by Rong-Jun Xie and Hubertus T. (Bert) Hintzen, JACerS. (doi:10.111/jace/12197).
As always, ACerS members have free access to JACerS and ACerS other two journals, the International Journal of Applied Ceramic Technology and the International Journal of Applied Glass Science. Full access, 24/7 is only one membership enrollment away!