Nitride Shaped

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In Situ Formation of Nanoparticle Titanium Carbide/Nitride Shaped Ceramics from Meltable Precursor Composition Teddy M. Keller,*,† Matthew Laskoski,† Andrew P. Saab,† Syed B. Qadri,‡ and Manoj Kolel-Veetil† †

Code 6120, Chemistry Division, and ‡Code 6360, Materials Science and Technology Division, Naval Research Laboratory, Washington, D.C. 20375, United States ABSTRACT: A new synthetic method has been developed for the in situ formation of TiC and TiN nanoparticles in a shaped solid from a meltable precursor composition. The reactants are titanium hydride (TiH2) and 1,2,4,5-tetrakis(phenylethynyl)benzene (TPEB), which interact above 600 °C to form TiC and TiN nanoparticles in the presence of argon and nitrogen, respectively. When the compacted powdered precursor composition is heated above 200 °C, the acetylenic units of TPEB react forming a networked polymer with the TiH2 homogeneously dispersed in the confines of the thermoset. During the carbonization process of 1,2,4,5-tetrakis(phenylethynyl)benzene, the carbon atom migrate into the interstitial space of the Ti lattice, resulting in stoichiometric TiC being formed. The novel reaction yields shaped solids of nanoparticle ceramics.

1. INTRODUCTION Refractory transition metal carbides and nitrides of group IV− VI have the highest known melting points (2600−3900 °C) and also outstanding hardness, chemical inertness, wear resistance, electrocatalytic activity, and neutron absorption.1,2 Films, fibers, and powders of these ceramics have been made from polymeric precursors, but large monolithic shapes elude the polymeric method.3 Refractory metal carbides have been traditionally prepared by powder metallurgy methods that include reaction of elemental transition metals or their oxides with solid carbon (graphite or amorphous carbon) in a reducing hydrogen atmosphere above 1700 °C, which is an expensive method to produce the refractory metal carbides.4−6 Metal carbides are typically produced by the powder process in which lattice interstices within the metals are not filled to a stoichiometric amount.7 Ordinarily, production of metal carbide ceramics by these techniques is energy and time intensive and result in brittle structural materials owing partly to the large granular particles and the inconsistency in the metal carbide particle sizes. The metal carbide ceramic parts, forms and other shapes are currently fabricated from transition metal carbide powders under extremely high pressure and at high temperatures (>2000 °C), a process known as hot press sintering. Extensive efforts have been made to produce the powdered refractory metal ceramics such as the carbides and nitrides at lower temperatures mainly by the carbothermal reduction process8−10 using metal oxides and polymeric materials as the carbon source. The use of polymers as the source of carbon allows a more energy efficient synthesis of the carbides at lower temperatures below 1700 °C but still affords powdered ceramic particles due to either low charred yields of the polymers or the polymer being a solid and not able to flow to a shaped form. © 2014 American Chemical Society

The carbothermal method is still high energy, is a timeconsuming process, and affords low yields due to residual impurities such as free carbon, oxides, and subcarbides. The current methods for the fabrication of shaped forms are limited to the formation of physically or chemically deposited films. There has been little success in the production of transition metal carbide fibers or other shaped structures from polymeric precursors. Powdered metal nitrides are also produced from metal oxides and carbon using carbothermal reduction in a flow of nitrogen, yielding carbon monoxide as byproduct and are formulated into shaped components under pressure and high temperatures, creating brittle granular materials.11−13 Transition metal nitride coatings are widely used in microelectronic, structural, tool, and ornamental applications.14−16 There is currently a great deal of interest in metal carbide nanopowders, which are mainly produced by mechanochemical reaction of metal powder and graphite after milling at temperatures between 1300 and 2100 °C for various periods of time under an argon atmosphere.17−23 Due to the high surface energy, the metal carbide nanoparticles tend to agglomerate during both the synthesis and the sintering into shaped components under high temperature and pressure. An advantage of nanosized metal carbide particles is a lower compaction temperature relative to microsized particles and less internal stresses within the sintered ceramics. The ability to retain nanosized metal carbide particles and to formulate fine grained composites from metal carbide nanopowders remains a major concern. Received: August 14, 2014 Revised: November 12, 2014 Published: November 14, 2014 30153

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performed using a Micromeritics AccPyc 1330 Gas Displayment Pycnometry System. 2.3. Conversion of TPEB to Shaped Thermoset Polymer and Conversion to Carbon under Inert Condition. A sample of TPEB was measured into a TGA pan and heated at 10 °C min−1 to 1200 °C under an inert atmosphere (nitrogen or argon) resulting in a weight retention of 85% in argon. When the carbonization reaction was performed at lower heating rates of 1−5 °C min−1, the char yield improved up to 90%. 2.4. General Method for Formulation of TiH2 and Carbon Precursor Mixture. Various amounts of TiH2 and TPEB were thoroughly mixed by ball milling at room temperature for 1 min and used as precursor ceramic compositions. The preferred method was to melt TPEB above 200 °C and thermally advanced to a prepolymer intermediate in which the viscosity was more like a thick honey consistency and then use in the preparation of the ceramic precursor composition. 2.5. TGA/DTA Studies and Preparation of Consolidated Solid TiC Ceramic Sample under an Argon Atmosphere. A precursor powdered ceramic composition containing TiH2 and TPEB was added to a ceramic TGA pan and manually packed with a flat surface. In a typical experiment, the contents of the TGA pan were placed in the chamber of the TGA furnace and purged thoroughly with argon gas. The sample was then heated under a flow of argon at 10 °C min−1 to 250 °C and held for 1 h followed by heating at 1−3 °C min−1 to 1000−1300 °C and holding for 2−3 h. The TiC solid ceramic sample was cooled at 5 °C min−1 back to room temperature. 2.6. Preparation of Circular Pellet from TiH2 and TPEB Composition under Pressure and Conversion to Solid TiC Ceramic Sample. Measured amounts of TiH2 and TPEB were mixed and ball milled at room temperature for 1 min and used in the preparation of compacted pellets. Pellet sizes of 6 mm and 13 mm in diameter were fabricated under external pressures of 4000 and 12000 psi, respectively. 2.7. Conversion of Circular Pellets Containing TiH2 and TPEB Composition to Shaped Thermoset and Nanoparticle TiC-Carbon Composition. The circular pellets were placed in a furnace and degassed under an argon atmosphere for 30 min. The pellet was quickly heated at 20−50 °C min−1 to 250 °C and held for 1 h. The solid shaped pellets were then heated at 1−3 °C min−1 to 1000−1300 °C and held for 1−3 h, yielding the shaped solid TiC-carbon compositions. The ceramic sample was cooled at 5 °C min−1 back to room temperature. 2.8. TGA/DTA Studies and Preparation of Consolidated Solid TiC-TiN Ceramic Sample under a Nitrogen Atmosphere. A precursor powdered ceramic composition containing TiH2 and TPEB was added to a ceramic TGA pan and manually packed with a flat surface. In a typical experiment, the contents of the TGA pan were placed in the chamber of the TGA furnace and purged thoroughly with purified nitrogen gas. The sample was then heated under a flow of nitrogen at 10 °C min−1 to 250 °C and held for 1 h followed by heating at 1−3 °C/min to 1000−1300 °C and holding for 1−3 h. The TiCTiN ceramic sample was cooled at 5 °C min−1 back to room temperature.

A specific goal of our research is the development of meltable precursor compositions containing metal hydrides (TiH2, ZrH2, and HfH2) for the thermal conversion to shaped solid ceramic components. Our research centers on the use of 1,2,4,5tetrakis(phenylethynyl)benzene (TPEB)24 as the carbon source, which melts below 200 °C, cures to a shaped thermoset, and exhibits an extremely high char yield upon heating above 600 °C. The use of a meltable carbon precursor and conversion to a shaped thermoset offers several unique advantages including lower processing temperatures, an improvement in the stoichiometry and homogeneous dispersion of the metal carbides, and the ability to form shaped ceramic solids, fibers, and films. The encapsulation of metal carbides in a small amount of carbon could isolate the ceramic particles and eliminate or minimize agglomeration.25−29 In this paper, we report a new method for the in situ synthesis of nanoparticle TiC and TiN in a bulk solid shaped composition from pyrolysis of a precursor mixture of TiH2 and TPEB. The growth of the ceramic nanoparticles proceeds in the solid phase during a simple carbonization process. Shaped ceramic compositions can be readily fabricated by our novel method in one step. The precursor mixtures can be easily tailored to have varying amounts of nanoparticle ceramic embedded in a carbon matrix upon heat treatment to elevated temperatures.

2. EXPERIMENTAL DETAILS 2.1. Chemicals. Titanium hydride (TiH2) powder was purchased from Strem Chemicals and used as received. 1,2,4,5Tetrakis(phenylethynyl)benzene (TPEB) was synthesized by a known published procedure.24 2.2. Materials Characterization. Thermogravimetric analysis (TGA) was performed on a TA SDT Q600 Simultaneous DTA-TGA module. For thermal analyses focused on examining the carbonization-ceramic formation process, samples were heated under both argon and nitrogen atmospheres and a flow rate of 100 cm3 min−1. Oxidation behavior was examined by heating at 10 °C min−1 under flowing oxygen (100 cm3 min−1). Room temperature X-ray diffraction scans were performed using a Rigaku 18 kW X-ray generator using CuKα radiation from a rotating anode X-ray source and a high resolution powder diffractometer. The high temperature XRD measurements were carried out on powder samples using a powder diffractometer attached with a high temperature stage. For high temperature diffraction scans, the diffractometer employed a 3 kW Cu source operating at 40 kV and 44 mA. The sample in the form of a pellet was held in place on a platinum holder and the temperature was maintained using a programmable temperature controller with a thermocouple attached inside the platinum sample holder. After stabilizing at the desired temperature, diffraction scans were collected at each temperature from 20° to 90° at the rate of 2° min−1 with a step size of 0.02°. In order to verify the accuracy of the experimental setup in terms of monitoring temperature, the thermal expansion coefficient for a platinum standard was measured as a function of temperature from room temperature to 773 K. The thermal expansion coefficient was found to be constant over this temperature range with α = 8.96 ppm K−1, which is in good agreement with the accepted value of 9.0 ppm K−1 for Pt. Scanning electron microscopy (SEM) studies were performed on a Zeiss Model Supra 55 electron microscope. The material density measurements were 30154

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3. RESULTS AND DISCUSSION 3.1. In Situ Synthesis of Nanoparticle TiC-Carbon Solid Compositions in One Step. Stoichiometric refractory TiC is formed from reaction of TiH2 with TPEB (mp ∼ 195 °C) as the carbon precursor. Previous thermal studies involving TPEB show the outstanding stability upon thermal treatment to temperatures in excess of 1000 °C.24 The TPEB contains four alkyne groups that readily react above 220 °C to form a networked or thermoset polymer with the TiH2 embedded homogeneously. In essence, the alkyne groups permit the carbon precursor to undergo conversion to the shaped polymeric solid. It is possible that, during this thermal conversion to the shaped TiH2-containing networked thermoset, the TiH2 commences to slowly thermally decompose into Ti atoms/nanoparticles and hydrogen. Thermal treatment of the thermoset above 500 °C results in total degradation of the TiH2 and carbonization of the carbon precursor, yielding carbon atoms that migrate in an argon atmosphere into the interstitial sites of the metal nanoparticles affording the metal carbide (TiC) nanoparticles, which are embedded in the excess carbon (Scheme 1). Carbon in excess of the stoichiometric

TiN formation with respect to typical reaction schemes. Moreover, by varying the amount of TiH2 relative to the carbon precursor, the amount of TiC or TiN can be changed with respect to the amount of carbon matrix in order to vary the properties of the resulting ceramic composite. The metal carbide or metal nitride carbon−matrix compositions are expected to show enhanced toughness, owing to the presence of the relatively elastic carbon, which exists in forms ranging from amorphous to nanotube to graphitic carbon as determined by XRD studies. 3.3. Nanoparticle Ceramic Formation in Excess Carbon Matrix. Nanoparticle ceramics have received a lot of attention and close scrutiny by scientists and engineers in recent years.17−23 The presence of an “elastic” carbon matrix has been observed to allow for toughening of the inherently brittle ceramics,25−29 and it may have the same effect in the present case. The carbon permits operation of the toughened ceramic at extremely high temperatures, owing to carbon’s high melting point (>3000 °C). Some studies have shown that a composition of metal carbides and amorphous carbon can increase the ductility and improve the adhesiveness of ceramic coatings due to the low friction coefficient of the carbon and the superior compatibility between carbon and the ceramic grains.29 Ceramic/carbon−matrix compositions are currently sought for these reasons, and the described procedure permits straightforward preparation of nanoparticle ceramic/carbon compositions in a single step, in contrast to the traditional means of first forming the ceramic as powder and then preparing the carbon−matrix components under sintering conditions. Also, the TPEB could improve the material structure and properties by assisting in the formation of crystalline nanoparticle sized ceramic. When an excess amount of the TPEB is used, the formation of the carbon matrix isolates the ceramic nanoparticles and hinders agglomeration to larger particles (see section 3.9). In addition, besides the simple production of pure TiC and TiN, the presence of the elastic carbon matrix may toughen the ceramic composition and afford new ultrahigh temperature nanoparticle ceramic material capabilities that do not exist today in current materials.25 By performing the reaction so that an excess of carbon is always present, a tough cemented refractory TiC- or TiN-based ceramic is formed and embedded or bound within a carbon matrix, which has temperature uses in excess of 3000 °C. Refractory Ti carbide or nitride ceramics fabricated by the sintered technique (high pressure and high temperature) of powdered Ti carbides or nitrides are very brittle due to the lack of elasticity or compliance within the sintered brittle products to dissipate mechanical energy. 3.4. Synthetic Strategies to Produce Refractory TiC or TiN Solids. Regardless of the ratio of TiH2 to carbon source (TPEB) used in the in situ synthesis of the TiC or TN−carbon matrix composites, the ceramic is produced as nanoparticles, as determined by X-ray diffraction studies. The TPEB has a char yield of 85−90% when heated under inert conditions.24 In the 10−15% volatiles, some will be carbon species, which could migrate into the interstitial sites of the Ti atoms. Two compositions were prepared and used in our studies, namely 1:1 and 1:1.15 atom ratios of TiH2 to TPEB, respectively. The TPEB has 38 carbon atoms to react with the 38 Ti atoms being formed from TiH2. Based on the projected char yield, ceramic TiC composition from the 1:1 ratio sample should have a slight excess of Ti metal remaining, whereas the 1:1.15 ratio sample will have a minute quantity of carbon available after formation

Scheme 1. Conversion of TiH2-TPEB Precursor Composition to Shaped Thermoset, Followed by Thermal Conversion to TiC

amount permits the formation of a carbon matrix in which the refractory TiC nanoparticle are bounded, or the reaction can be essentially conducted with a stoichiometric amount of carbon to yield the pure metal carbide. Carbon is believed to easily diffuse through the interstitial metal sites because it is much smaller than the metal. Moreover, carbon is known to enhance bonding at grain boundaries.25 The amount of TiC and carbon within the resulting composition can be varied based on the quantity of each individual component (TiH2 and TPEB) mixed in the preparation of the precursor ceramic composition. 3.2. In Situ Synthesis of Nanoparticle TiN-TiC Solid Composition. The carbon precursor (TPEB) contains only C and H to ensure that heteroatoms are not incorporated into the interstitial sites of the Ti nanoparticles during the reaction to produce the TiC and TiN. When the reaction is performed in a nitrogen atmosphere, the Ti nanoparticles preferentially react with the nitrogen relative to the carbon, affording the corresponding TiN in pure form, especially on the surface of the solid composition. The TiC or TiN forms above 600 °C under inert conditions from a reaction of the highly reactive Ti nanoparticles with either the carbon precursor (degradation above 500 °C) or nitrogen gas, respectively, but the reaction can be made to occur faster at higher temperatures. When an excess of the carbon precursor is used, the individual formed ceramic nanoparticles are glued or bound together with the resulting nanostructured or amorphous elastic carbon to afford structural integrity.25 We speculate that the high surface energy of the Ti nanoparticles are highly reactive with the carbon and nitrogen sources, thereby lowering the temperature of TiC or 30155

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of the TiC. In the case of the 1:1.15 sample, a slight excess of carbon ensures that excess carbon atoms are available for migration into the interstitial sites of the metal nanoparticles to afford stoichiometric TiC and excess carbon matrix in which the refractory TiC nanoparticles are bounded. Moreover, depending on the application, the amount of carbon can be readily varied. The usage of meltable carbon precursors that carbonize at low temperatures combined with transition metal Ti compounds that decompose into Ti atoms/nanoparticles capable of reacting with carbon atom during the carbonization process results in the formation of shaped TiC ceramic−carbon compositions with structural integrity at temperatures in some instances below 1000 °C in an argon atmosphere (Figure 1). Figure 2. TGA-DTA thermogram of TiH2 and TPEB precursor composition to 1300 °C under a flow of argon.

Figure 1. Formation of solid nanoparticle TiC ceramic in one step from meltable precursor composition. Figure 3. TGA-DTA thermogram of TiH2 and TPEB precursor composition to 1200 °C under a flow of nitrogen.

When the reaction is performed in a nitrogen atmosphere, reaction of the nitrogen with the high surface area Ti particles occurs forming the TiN. The TiN is mainly formed on the exterior of the sample in contact with nitrogen especially in the pressure fabricated pellet samples. By varying the amount of TiH2 that forms the reactive Ti nanoparticles, which in turn either reacts with the carbon source or with the nitrogen gas, the amount of refractory Ti carbide or nitride may be easily varied to alter the properties of the resulting ceramic. 3.5. TGA-DTA Studies to Monitor Thermal Transition of TiH2-TBEB Reaction. Thermal analysis (TGA-DTA) studies of a TiH2-TPEB (1:1 atom ratio of Ti to carbon) precursor composition were used to monitor the thermal transitions occurring upon heating to 1300 °C and conversion to TiC and TiN under argon and nitrogen atmospheres, respectively (Figures 2 and 3). In a typical experiment, a sample of the precursor composition containing TiH2 and prepolymer TPEB was packed into a ceramic TGA pan. For the in situ formation of TiC nanoparticle carbon solid, the precursor mixture was quickly heated to 250 °C, resulting in the melting of the TPEB at about 195 °C and conversion into a thermoset polymer, as observed by an exothermic transition commencing above 220 °C. At this point, the TiH2 is homogeneously encapsulated within the domain of the solid thermoset. Further heating of the polymeric content resulted in two observed endothermic transitions peaking at about 430 and 560 °C, respectively. In addition, an exothermic transition was observed peaking at about 605 °C, which continued to 1300 °C. After heat treatment at 1300 °C, the sample retained about 93.6% weight.

Figure 3 shows the TGA-DTA thermogram of a similar composition with the study being performed in a nitrogen atmosphere with nitrogen atoms preferably reacting with the Ti and migrating or being incorporated into the interstitial sites of Ti atoms relative to carbon, especially in the exterior part of the sample exposed to nitrogen. At 505 °C, the weight retention was 98.9%, and at 510 °C there appeared to be a surge in heat and a rapid increase in weight to about 99.2%, followed by the typical weight loss observed during the carbonization of TPEB. At about 705 °C, the weight retention was about 93%, and the sample commenced to rapidly increase in weight due to the reaction of the Ti with nitrogen and the formation of TiN. For comparison when a similar precursor composition was heated under a flow of argon, the weight retention at 705 °C was 91.8%. An enhancement in weight to 103% was observed when this initial argon-heated sample was further heated under a flow of nitrogen at 1200 °C for 3 h. It was found from XRD analysis that TiN was mainly formed on the exterior portion of the solid ceramic composition exposed to nitrogen, while TiC was present in the interior part. Regardless of the reaction pathway, stoichiometric nanoparticle TiN and TiC were formed, as determined by the XRD studies. 3.6. Oxidative Stability. A nanoparticle-TiC solid sample containing a trace amount of extraneous carbon was exposed to air up to 1300 °C using TGA-DTA analysis to ascertain the oxidative properties (Figure 4). The ceramic sample was prepared by compacting a 1:1 atom ratio of TiH2-TPEB 30156

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confirmed the formation of a film of TiO2 on the exterior of sample with TiC below the surface. 3.7. Scanning Electron Microscopy (SEM) Studies. SEM studies were performed on some preliminary samples in an effort to understand how to obtain densified solid ceramics by the synthetic method. Solid circular samples were prepared by compacting a 1:1.5 atom ratio of precursor milled composition of TiH2:TPEB up to 10000 psi. After releasing the pressure, the circular samples were then thermally converted to the thermoset with the TiH2 embedded within the domain of the polymer. The polymeric samples were heated at 2−3 °C min−1 to 1200 °C and used for the high resolution image studies. Figures 5 and 6 show several SEM images of pellets compacted from a precursor composition at 10000 psi, cured thermally to a shaped polymer, and converted directly to the solid nanoparticle TiC and TiC-TiN ceramics, respectively, in argon and nitrogen atmospheres at elevated temperatures. The compacted samples contained microscopic voids, which indicate the potential necessity for preprocessing vacuum drying and the fabrication of the pellet or any other shaped component under vacuum and pressure to eliminate issues with air pockets before conversion to the shaped thermoset and ceramic nanoparticle composition. It may also be desirable to heat the compacted sample totally under some external pressure and vacuum to consolidate the ceramic nanoparticle in intimate contact during the thermal treatment to the shaped ceramic composition. 3.8. Density of the TiC in Carbon Matrix. The true or absolute density of various TiC compositions was measured using a Micromeritics AccuPyc 1330 in the presence of a helium flow. The small samples were prepared by packing the individual TiH2-TPEB precursor compositions into a ceramic TGA pan with a flat surface, curing to the thermoset, and heating at 3 °C min−1 up to 1400 °C and holding for 3 h under a flow of argon. As expected, the density was dependent on the

Figure 4. Oxidative study of TiC ceramic composition to 1400 °C.

precursor composition into a ceramic TGA pan, converting to the solid thermoset, and heating at 1300 °C for 3 h under a flow of argon. Upon cooling, the sample was heated at 10 °C min−1 in a flow of air and showed stability to about 400 °C with an exothermic transition occurring and peaking at about 645 °C. The sample took on a small amount of weight (0.4%) up to 585 °C with a weight retention of 99.5% at 645 °C. During this initial exothermic transition, excess carbon in which the TiC is embedded on the surface of the sample would be oxidized. Further heating shows a rapid uptake of weight up to 121% attributed to conversion of unreacted Ti to TiO2 with no additional weight change being observed. Upon cooling and reheating to 1400 °C, no further weight loss occurred, indicating that a passivating layer of TiO2 had formed on the exterior part of the sample protecting the interior portion against additional oxidation. XRD analysis of the sample

Figure 5. Various SEM images of nanoparticle TiC ceramic solid fabricated from thermal treatment of pellet to 1200 °C under an argon atmosphere. 30157

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Figure 6. Various SEM images of nanoparticle TiC-TiN (outer surface) ceramic solid fabricated from thermal treatment of pellet to 1200 °C under an nitrogen atmosphere.

ratio of the two reactants in the precursor composition and thermal conditions for conversion to the TiC. Two samples, prepared from a 1:1 atom ratio of TiH2 and TPEB precursor composition, packed into a ceramic TGA pan and heated at 250 °C for 2 h, then at 1 °C/min to 1400 °C, and isothermally soaked for 4 and 6 h at 1400 °C, respectively, displayed density values of 5.534 and 5.025 g·cm−3 compared to a density value of 4.93 g·cm−3 for pure TiC. The ceramic synthesized from this precursor composition has been shown to have a small excess of Ti metal, so it is not surprising that the density is slightly higher than the theoretical value. The high pressure compacted circular samples or pellets appeared to react more readily due to the intimate contact of the Ti particles with the developing carbonaceous matrix and the carbon atoms. For example, a 1:1.15 atom ratio sample of TiH2 and TPEB precursor composition, pressed into a small pellet at 4000 psi, was heated at 5 °C min−1 to 1200 °C and isothermed for 3 h, followed by cooling back to room temperature. The ceramic TiC carbon matrix composition exhibited a measured density of 4.29 g·cm−3, which is slightly below the known density of TiC. XRD studies showed that all of the Ti had completely reacted and that the TiC nanoparticles were embedded in a minute quantity of graphitic or nanostructured carbon. The particle size of the TiC was 33 nm and the solid ceramic had a strain of 0.4% attributed to compaction and thermal strain. The lattice parameter was calculated to be 4.300 Å compared to pure stoichiometric TiC value of 4.330 Å.30 3.9. XRD Studies on TiC and TiN in Trace Amount of Carbon Matrix. Figure 7 shows the XRD scan of the TiC sample prepared from the 1:1 atom ratio sample of TiH2 and TPEB precursor composition. All the peaks have been indexed based on FCC lattice and the intensities of the peaks are consistent with 1:1 stoichiometry of Ti/C atomic ratios. The lattice parameter was determined to be 4.338(1) Å by using least-squares refinement of the observed reflections. The lattice

Figure 7. 2θ/ω scan of TiC taken with Cu Kα1 radiation showing a single phase of stoichiometric TiC.

parameter is in reasonable agreement with the bulk lattice parameter of 4.318 reported by Elliot and Kempter for the khamrabaevite phase with a space group of Fm3̅m.30 The crystallite size was determined to be 40 nm using HalderWagner analysis of the observed XRD peaks and correcting for instrumental broadening using the diffraction peaks of an external standard of Si.31 Figure 8 shows the XRD scan of a TiN sample prepared by thermal treatment of precursor composition to 1200 °C under flow of pure nitrogen. All the peaks have been identified as those coming from TiN with a Osbornite phase with a space group of Fm3̅m and the intensities are consistent with a 1:1 stoichiometry of Ti/N atomic ratios. The lattice parameter was determined to be 4.255(2) Å by using least-squares refinement of the observed reflections and in general agreement of 4.244 Å reported by Bannister.32 The crystallite size was determined to 30158

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Figure 8. 2θ/ω scan of TiN taken with Cu Kα1 radiation showing a single phase of stoichiometric TiN.

Figure 10. 2θ/ω scans of TiH2 + TPEB taken with Cu Kα1 radiation from 900 °C up to 1200 °C showing increasing TiC phase with a concomitant decrease in α-Ti phase. At 1200 °C, the TiO2 phase is formed at the expense of α-Ti phase as the air was allowed to leak in, while the TiC phase remained invariant and resistant to oxidation.

be 22 nm based on Halder-Wagner analysis of the observed peaks after correcting for instrumental broadening in their full width at half maximums (FWHMs). In order to examine the evolution of the TiC khamrabaevite phase from the TiH2-TPEB precursor composition, the in situ XRD patterns were monitored as a function of temperature from room temperature up to 1200 °C. The temperature was ramped to the desired setting at a rate of 10 °C min−1. Once the temperature was reached and maintained for 10 min, the XRD scan was taken at the rate of 2 °C min−1 for a total scan time of 30 min. This procedure was repeated for each temperature until the final temperature of 1200 °C was reached (Scheme 2). The sample was then slowly cooled for 3−4 h to

different temperatures. The TiH2 phase persists until 500 °C and then it decomposes into β-Ti and α-Ti phases with β-Ti being more than 95%. Although the α-phase is more stable than the β-phase, the β-phase is probably formed first due to the presence of hydrogen in the sample from thermal degradation of the TiH2 to Ti and H2.33 At 700 °C, only α-Ti is observed as hydrogen has escaped from the sample. Once the α-Ti is formed, it reacts with the carbon component of the TPEB, first forming cubic Ti2C at 800 °C, which then transforms to TiC khamrabaevite phase at 900 °C. As the temperature is increased from 900 to 1200 °C, more of TiC phase is formed with a concomitant decrease in the α-Ti phase. At 1200 °C, air was allowed into the sample chamber to examine the oxidative stability of the TiC phase. This resulted in the conversion of any unreacted α-Ti part of the sample to TiO2 phase, whereas the TiC phase remained intact. This observation shows the stability of the TiC phase against oxidation at high temperature. Table 1 shows the lattice parameters and crystallite sizes of different phases as a function of temperature. The data indicate that nanoparticle-sized TiH2 forms the nanoparticle β-Ti and αTi phases, which do not agglomerate into larger particle sizes as the temperature is increased. The reaction of nanoparticle Ti with the carbon atoms, being formed from the carbonization of the TPEB, occurs from migration of the carbon atoms into the interstitial sites of the nanoparticle Ti atoms, yielding the stoichiometric TiC as nanoparticles. These results show that the ceramic composition can be tailored as a function of the reaction temperature.

Scheme 2. In Situ X-ray Diffraction Patterns was Monitored as a Function of Temperature

room temperature, and a final XRD scan was recorded. Figures 9 and 10 show an overlay of diffraction scans taken at the

4. CONCLUSION The discovery and use of processable TPEB as the carbon source revolutionize or serve as a superior polymeric method for preparing ceramic materials due to the potential for reductions in manufacturing costs attributed to shorter processing time, energy savings, and the relatively simple equipment required. Moreover, synthesis by the meltable polymeric precursor route can be ceramic ingredient-designable and achieves high purity shaped ceramic products relative to other conventional methods necessitating the sintering

Figure 9. 2θ/ω scans of TiH2 + TPEB taken with Cu Kα1 radiation from room temperature up to 800 °C showing the sequence of transformations from the outer surface of stoichiometric TiC. 30159

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Table 1. Lattice Parameters and Crystallite Sizes of the Different Phases as a Function of Temperature, as Determined by XRD temperature (°C) 25 300 400 500 600 700 800 900 1000 1100 1200 25 (aft. cool)

phase

lattice parameters (Å)

crystallite size (nm)

TiH2 TiH2 TiH2 TiH2 β-Ti α-Ti α-Ti α-Ti Ti2C α-Ti α-Ti TiC α-Ti TiC α-Ti TiC TiC

4.454(2) 4.461(3) 4.458(4) 4.418(4) 3.337(6) a = 2.979(2); c = 4.787(1) a = 2.974(3); c = 4.742(7) a = 2.972(2); c = 4.746(5) 8.638(2) a = 2.996(8); c = 4.76(2) a = 2.998(5); c = 4.815(6) 4.336(2) a = 3.010(5); c = 4.847(2) 4.341(3) a = 3.015(7); c = 4.870(3) 4.353(2) 4.316(2)

8.7 18.9 21.5 24.5 16.9 22.0 16.7 24.0 30.0 13.0 19.0 20.0 27.5 20.0 25.0 25.6 26.2

consolidation of the powdered ceramic. The ready availability of the carbon atoms on conversion of the thermoset to carbon above 600 °C for migration into the interstitial sites of the Ti is the key to the formation of stoichiometric TiC. More importantly, the XRD temperature studies show that the TiC does not agglomerate into larger particles with time due to minute quantities of carbon being available to isolate the individual TiC nanoparticles. The chemistry of the reaction to the ceramic materials is understood. Similar studies are being performed for ZrH2 and HfH2, yielding stoichiometric ZrC and HfC, respectively. Future studies are designed to minimize or eliminate the presence of voids by performing experiments under pressure and vacuum to determine the mechanical and electrical properties and hardness of selected samples.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: 202-767-3095. Fax: 202-767-0594. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors wish to thank the Office of Naval Research (ONR) for financial support of this work. REFERENCES

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