The Synthesis And Characterization Of Functional Materials In The B-C-N Systen Under Extreme Conditions

With the development of modern science and technology, demandsfor materials with better capabilities and performances become moreand more urgent. As hardness is one of the most important andfundamental properties of materials, the synthesis, characterization andproperties of materials with super hardness become one of thehighlights in condensed matter physics and materials science andtechnology at present. The recent progresses in materials design, aswell as the discoveries of new phases, in the B-C-N system, hasaroused great interest among scientists to seek materials with hardnesscomparable to or even higher than diamond, both experimentally andtheoretically.Besides the potential super hardness, the compounds in the B-C-Nsystem are characteristic of their short, covalent bonds, which alsoendows them with desired physical, chemical, electronic andphotoelectronic properties, etc.. For example, diamon is a widebandgap semiconductor with the highest saturated eletron drift velocity,the lowest dielectric constant, excellent thermal conductivity andtransparence. Whereas cBN is a wide bandgap semiconductor withnegative electron affinity, chemical inertness,etc..Furthermore as materials with low densities, the compounds in theB-C-N system may find potential usages in industries that exert specialrequirements on densities of materials. For example, the developinggas storage techniques based on carbon nanomaterials and mesoporousmaterials may find their prosperity in future space science andtechnology, as well as the success of carbon-based mesoporousmaterials in gas seperation and sewage cleansing.In this thesis, exploring works in the following three aspects such asthe plasma induced phase transformation of BN in direct current arcdischarge, the synthesis and characterization of carbon-basedmesoporous materials and the characterization and the in situsynchrotron radiation X-ray diffraction under high pressures of asaturated CN phase synthesized via high pressure high temperaturetechnique are reported.(1)Initially cBN was synthesized in the pursuit for superhardmaterials to be used as cutting tools and protective coatings. Despite itsinferiority to diamond in hardness, its chemical inertness to ferrousmetal and alloys and thermal stability make it extraordinarilycompetitive in machining ferrous metals. In addition, cBN has beendeveloped and used in many fields such as high temperaturesemiconductor devices, heat sinks, passivation layers, lithographicalveils and field emitting electronic devices, etc..As a thoroughly manmade material, cBN was first fabricated underhigh pressure high temperature conditions with proper catalysts. Thetransformations among the phases of BN under high pressure and hightemperature conditions have been studied by Wentorf and Bundy et alin detail and the principles have been outlined. They have propose anequilibrium and reaction P-T phase diagram and pointed out that hBNshould be exposed to conditions well above the hBN/cBN equilibriumline in the phase diagram in order to acquire the phase transition tocBN. However, new techniques have been developed lately to fabricatecBN at moderate conditions with lower pressures and temperatures.Among them are the high pressure high temperature supercritical fluidstechnique invented by Solozhenko and Singh et al, the hydrothermalsolvothermal synthesis method developed by Hao and Jiang andco-workers, the laser-induced solid-liquid interface reaction methodused by Yang and co-workers, and high dosage radiation and highcurrent particle bombardment method carried out by Sokolowska andOlszyna. These remarkable successes have compelled the researchers toreconsider the synthesis of cBN, and what is more important, the phasetransition mechanisms between the BN phases.Using hBN powders as the starting material, we have successfullyfabricated nanocrystals of cBN under conditions with extremely hightemperatues generated in a direct current arc discharge with ammoniaas the working medium. It can be seen from our powder diffractionpatterns that the product is a mixture of nanocrystals of both hBN andcBN, which indicates that part of the iniatial hBN has transformed intocBN. In the fourier transform infrared spctra of the products, all thestrong absorption bands can be ascribed to the corresponding BNphases. Through a least square fitting of the characteristic bands ofhBN and cBN, the volume fraction of cBN in the final products isestimated to be 30?50%. Transmission electron microscopy has beenadopted to investigate the morphologies of the products. It can be seenthat the products are predominantly nanometer sized spherical particles,among them the particles with diameters lower than 10nm take up~50% of the total, the particles with diameters in the 10nm-20nm rangetake up ~30% and larger particles about ~10%. Through comparingthe X-ray photoelectron spetra of the starting hBN, bulk cBN producedvia the traditional HPHT method and our product, it is reinforced thatthe phase transition has occurred. In order to explain the phasetransition mechanism between hBN and cBN under such extremeconditions, we have proposed a vapor phase chemical transport model.Under the dynamic conditions generated in the plasma arc, hBN wasevaporated and volatile (BN)x clusters on the nanometer scale were produced.On adsorption of H, NH and NH2 radicles, the (BN)x species underwenttransitions from sp2 hybridization to sp3 hybridization. As those species wereaccelerated to high speed through bombardment of the radicles, they werequenched subsequently as soon as they escaped away from the arc region andthus the cubic phase was maintained. From a thermodynamic point of view,the nucleation of cBN was favored in our case once the effect of thenanosize-induced additional pressure on the Gibbs free energy was taken inaccount.(2)Compared with the traditional porous materials, carbon-basedmeso-and micro-porous materials are characteristic of lower densities,higher surface areas, chemical inertness and mechanical strengths, etc..Thus in many fields of modern industry such as gases seperation, gasesstorage at low pressures, fuel cells, catalysts supports, highperformance electrochemical double layers, etc. they are deemed aspotential candidate materials and have attracted increasing interest ofboth technology and science researchers. Generally speaking, in order todevelop porosity in carbon, a proper physical or chemical activation process isneeded. In a physical activation process, carbonization of a carbonaceousprecursor is carried out followed by activation of the resulting char with thehelp of active agents such as steam, CO2, etc. Whereas in the case of achemical activation process, the precursors are preimpregnated with activeagents such as KOH, ZnCl2 and H3PO4, H2SO4 etc and then exposed to heattreatments. Various precursors such as coals, petroleum cokes, coconuts shells,etc have been subjected to one or both of these activation processes to produceporous carbons with success. It is generally accepted that surface functionalgroups or complexes are of particular importance to the commonfunctionalities such as adsorption, electrochemical properties, acidity andbasicity, redox properties, hydrophilicity and other properties of porouscarbons. Thus the chemical modification of the surface of the inner pores hasgained considerable attention.The starting materials we used are melamine powders with chemicalpurity. In the direct current arc discharge with argon, nitrogen andammonia as the working media, respectively, melamine undergoespyrolysis followed by quenching process and powders with mesoporousstructures are formed. Typical chemical elemental analyses of theproducts show that they are mainly composed of carbon, whosecontents hardly vary with respect to the media. The nitrogen contentsare relatively low. The noticeable contents of oxygen and hydrogen arethought to originate from the surface oxidation and hydrolyzation.Taking the intrinsic porosity of the products into accounts, they canalso be ascribed to the adsorbed ammonia and water. Through indexingthe typical XRD patterns of the products, we consider them to beturbostratic carbon with a certain degree of disorder. By adoptingSherer's equation, the characteristic dimension of the consecutivemicro-domain of aromatic layers is estimated to be 10nm-20nm.Analyzing the fourier transform infrared spectra and the X-rayphotoelectron spectroscopy of the products in parallel, we find the finalpowders are abound in radical groups such as C?H, O?H, ?NH2, C orCN aromatic rings, C?O, C–C, C?N, C?O?C, C?N=C, N=N, C=C,C=N, C=O, O?C?O, COOH and O=C?O. Transmission electronmicroscopy investigations show that the products are composed oflarge amounts of curved scale-like lamellar flakes conglomerating withone another. Larger flakes are 100nm-200nm in diameter and20nm-30nm in thickness. Smaller flakes are 20nm-50nm in diameterand 3nm-5nm in thickness. These nanometer sized flakes conglomeratewith one another and form sponge-like porous structures. As to theformation of the special struture, we attribute it to the pyrolysis andquenching processes melamine undergoes in the direct current arcplasma. Being heated and bombarded by the plasma, melamineevaporates instantly and decomposes completely into radicals such as C,CN and CNH. Those radicals react with each other and aromatic layerscome into being. In the dynamic circumstance of the arc plasma, thegrowth of graphitic layers are restrained and twisting and interlinkingof the layers form turbostratic carbon. The stress caused by the effectsof the hot plasma, the interactions of the surface functional groups andthe lattice defects are thought to be the origin of the curving of thelayers. As the starting materials and the media are oxygen free, theoxygenous groups are thought to originate from the surface oxidationand hydrolyzation.(3)Various characterizing techniques have been used in parallel toinvestigate the properties of the three CN compounds that had beenfabricated under different high pressure high temperature conditions.X-ray photoelectron spectra studies show that the samples, after beingwashed with hydrochloric acid and distilled water repeatedly to removethe ferrous compounds related with the Fe catalyst, are predominantlycomposed of C and N, with a certain amount of O contaminationswhich are thought to be due to the surface oxidation and hydrolyzation.The typical bond structures between C and N atoms are thosed relatedwith one N atom bonded to two or three adjacent C atoms, respectively.Powder X-ray diffaction studies show that besides the impuritiesintroduced by the Fe catalyst such FeCl2 and Fe2N, the samples containmainly graphitic C3N4 and an unidentified orthogonal CN material withits lattice parameters to be a=6.508 ?, b=4.597 ?, c=3.459 ?. The fouriertransform infrared spectra studies show that the samples are characteristic offunctional groups such as C─N, C═N, R─NH─R, R3N and aromatic CNheterocycles. Comparing the testing results of the three samples wefind that 5.8 GPa and 1100 oC may well be close to the optimum conditionsto produce graphitic C3N4. It is also found that with increasingtemperature the contents of C═N double bonds in the samples decreasewhile the contents of C─N single bonds increase, indicating that higherpressures and temperatures are required to produce the hypothetical superhardmetastable phases of saturated C3N4.In situ synchrotron radiation X-ray diffaction studies under highpressures and room temperature on the sample produced under 1200 oCand 6.0 GPa conditions have been carried out via the diamond anvil cell(DAC) technique. The results show that at room temperature and up to44 GPa, both graphitic C3N4 and the unidentified orthogonal CN phaseare stable. Obvious evidences of a phase transition have not been found.P~V data of both the two materials have been fitted to Murnaghanequation of state by using the least square method. Thus the bulkmodulus and its first order derivative at zero pressure of graphitic C3N4is shown to be B0=163±14 GPa and B'0=5.5±0.8, indicating graphitic C3N4is very compressible. Likewise, the bulk modulus and its first orderderivative at zero pressure of the unidentified orthogonal CN phase isshown to be B0=47.4±0.4 GPa and B'0=13.5±0.9, indicating that thisorthogonal CN phase is even compressible and its bulk modulusincrease drastically with pressure. Thus a reasonable speculation ismade as to the orthogonal CN phase that it may be a kind of organicpolymeric macromolecule that is formed by the starting materials suchas melamine and cyanuric chloride under high pressure andtemperature.