Chem. Mater. 1990,2,241-244
24 1
ship. We thank David A Lange for the electron microprobe measurements. XPS and RBS analyses were carried out at the facilities of the Harvard Materials Research Laboratory, an NSF-funded facility (DMR-86-14003).
is removed cleanly in the precursor decomposition, probably via alkyl @-hydrogenactivation or homolytic Ti-C bond ~1eavage.l~
Acknowledgment. We are grateful to the National Science Foundation (DMR-8802306) for support of this work. R.M.F. thanks IC1 Americas for a summer fellow-
Registry No. 1, 3275-24-9; 2, 4419-47-0; 6, 70024-36-1; Ti-
(NMe2)&-Bu),126083-69-0;TiN, 25583-20-4.
Preparation and Pyrolysis of a Polymeric Precursor for the Formation of TiN-Tic Solid Solutions C . Russel Znstitut fur WerkstoffwissenschaftenZZZ (Glas und Keramik), Universitat Erlangen-Nurnberg, Erlangen, FRG Received June 27, 1989 Metallic titanium was anodically dissolved in an organic electrolytecontainingn-propylamine. A highly viscous solution was formed. Heating of the fluid led to the formation of an amorphous solid. Calcination of this precursor in an atmmphere of anhydrous ammonia resulted in the formation of a gold-colored titanium nitride-titanium carbide solid solution with a comparably low carbon content, about 5%. When the calcination was carried out in nitrogen, Ti(C,N) solid solutions containing large amounts of carbon were obtained. Calcining at comparably low temperatures led to products with an extremely small crystallite size.
Introduction
already been described in literature. The thermal decomposition of polymeric titanium precursors to TiN has also been described.8 However, the formation temperatures were at least 1100 "C, and additional phases containing oxygen, such as Ti305, occur up to temperatures of 1400 "C. It may be assumed that this effect is due to the use of a precursor, containing large amounts of oxygen. This paper introduces a method for the formation of TiN and TiN/TiC solid solutions by pyrolysis of a polymeric oxygen-free precursor at comparably low temperatures. A quite similar method has already been developed for the formation of aluminum nitride.9J0
Titanium nitride has some remarkable properties such as extreme hardness and excellent electrical conductivity.l3 At the moment, TiN is predominantly used as a cutting tool material. TiN coatings on metals, or even transition-metal carbides such as WC, are quite suitable as antiabrasive layers. For electronic applications, thin TiN layers are used as conductors. Thin TiN layers on various ceramic materials, including glass, might be used for optical and optoelectronical devices. TiN is cubic (a = 0.4239 nm)3and forms solid solutions with Tic, which also has a cubic lattice with a slightly larger lattice constant (a = 0.4330 nm). Over a wide range, T i 0 is soluble in TiN, Tic, or TiN/TiC solid solution^.^ A complete absence of carbon contamination is generally not required for all applications of TiN. For some special mechanical applications, TiN/TiC solid solutions are
Experimental Section Anodic Dissolution of Metallic Titanium. Metallic titanium was anodically dissolved in a purely organic electrolyte. It consists of a primary organic amine (e.g., n-propylamine),acetonitrile to increase the polarity, and tetrabutylammonium bromide as a supporting electrolyte to achieve sufficient electrical conductivity, necessary to reduce ohmic drops. Figure 1 shows the apparatus used for the electrolysis. A double-walled glass vessel contained the electrolyte and the electrodes. Sheets of metallic titanium (thickness 1 mm) were used for both the cathodes and anodes. The distance between two electrodes was also 1 mm. Alternate sheets were connected, and the polarity of the dc power supply was reversed every 15 min to achieve a uniform dissolution of all electrodes. A condenser,fixed to the top of the vessel, recovered solvent and excess amine, which were vaporized or carried along with the gas stream. The electrolysis was carried out without stirring in a volume of about 150 mL of electrolyte and a total electrode area of about 60 cm2. The current density was 67 mA/cm2 and the applied dc voltage in the range 5-6 V. For about 3 h, the current remained nearly constant and then decreased, due to an increasing viscosity of the solution. Then the electrolysis
refer able.^
At the moment, TiN is produced either by direct nitridation of metallic titanium or by the reaction of titanium halides of hydride with anhydrous ammonia or nitrogen/hydrogen mixturesa6 The temperatures required for the formation of TiN are usually higher than 1000 "C. Presently, TiN coatings are formed by sputter or chemical vapor deposition (CVD) techniques. It should be noted that a method for the preparation of TiN by thermal decomposition of gaseous titanium tetrakis(dialky1amides): which allows much lower formation temperatures, has (1) Grewe, H.; Koloska, J. Hartatoffe. In Handbuch der Keramik; Verlag Schmidt: Freiburg, 1983; Vol. I1 K3, pp Iff. (2) Toth, L. Transition Metal Carbides and Nitrides; Academic Press: New York, 1971; pp 176ff. (3) Christensen, A. J . Cryst. Growth 1976, 33, 99. (4) Duwez, P.; Odell, F.J. Electrochem. SOC.1950,97, 299. (5) Schedler, W. Hartmetall far den Praktiker; VDI-Verlag: Dbseldorf, 1988; p 224ff. (6) Powell, C. F.;Oxley, J. H.; Blocker, J. M. Vapor Deposition; Wdey New York, 1966. (7) Sugiyama, K.; Pac, S.; Takahashi, Y.; Motojima, S. J.Electrochem. SOC.1975,122, 1545.
~~~~~~~~~~~~
(8) Kuroda, K.; Tanaka, Y.; Sugahara, Y.; Kato, C. Better Ceramics Through Chemistry 111. Mater. Res. SOC.Symp. R o c . 1988, 121, 575. (9) Seibold, M.; Riissel, C. Better Ceramics Through Chemistry 111. Mater. Res. SOC.Symp. Proc. 1988; 121,477. (IO) Seibold, M.; Rbsel, C. J. Am. Ceram. SOC.1989, 72, 1503.
0897-4756/90/2802-0241$02.50/0 0 1990 American Chemical Societv , , I
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Russel
242 Chem. Mater., Vol. 2, No. 3, 1990
h0
Figure 1. Apparatus for the anodic dissolution of metallic titanium: l, glass vessel; 2, electrolyte; 3, titanium electrodes; 4, connections to the power supply; 5, condenser. was stopped, and the dark brown electrolyte was drawn off in an inert-gas atmosphere to another gas tight vessel. Drying of the Precursor Solution. A vacuum of about 10 mbar was applied to the electrolyte. Subsequent heating caused the evaporation of excess amine and solvent. At a temperature of about 70 "C,a liquid of high viscosity was obtained,which could be transformed into a solid of gellike consistency. On further heating, a foam full of gas bubbles is obtained. Calcination. All organic compounds must be removed during calcining. The pyrolysis is carried out in nitrogen, argon, or anhydrous ammonia atmospheres over the temperature range 600-1200 O C (heating rate, 200 OC/h; soak, 1 h; gas flow, 60 mL/min).
Results and Discussion At the anode, metallic titanium is dissolved. At the cathode NH2R is reduced to gaseous hydrogen (gas bubbles), and its corresponding anion is formed. To investigate the valency of titanium after dissolution, ESR spectra (Brucker Spectrospin 414) of the precursor solution were collected. A strong paramagnetic signal at g = 1.99, typical for Ti3+l1 was observed, proving that at least part of the titanium occurs as Ti3+, which might be formed either directly at the electrodes or by the reduction of Ti4+by hydrogen. The assumed chemical reactions at the electrodes are described in eq 1. anodic reaction: Ti TiX+ xecathodic reaction: xNHR- + (x/2)H2 xNH2R + xe-
Ti
+ xNH2R
+
Ti(NHR), I
+ (x/2)H2
800
1200
1600
-41°CFigure 2. TGA profile of the dried precursor; heating rate 5 O C / min.
-4
-
400
(1)
(11)Kurkjian, C.R.;Peterson, G. E. Phys. Chem. Glasses 1974, 15, 12.
The compound Ti(NHR), is expected to be formed as an intermediate, which undergoes further chemical reactions. In the course of the electrolysis, the viscosity of the electrolyte increases. CHN analysis of the dried precursor resulted in an empirical formula of TiC3.94H8.24N1.14. ESR spectra of the dried precursor showed the same paramagnetic signal as already observed for the viscous solution. As previously described in literature,12J3the reaction of tetrakis(dialky1amino)titanium with primary amines results in the formation of oligomeric compounds, if residual dialkylamino groups act as chain stoppers. If the dialkylamino groups are completely substituted, linear polymers are formed, in which each unit of the polymer is joined by a double nitrogen bridge12J3 (see 11).
I1
Therefore, the monomeric compound Ti(NHR)4should polycondense to the polymeric compound 11. Thus, the dried precursor is assumed to be polymeric too, although the condensation chemistry is certainly more complicated, because at least part of the titanium occurs as Ti"'. Further evidence of the occurrence of a polymeric compound is shown by the inability of the dried precursor to dissolve inorganic solvents or to be evaporated without decomposition. Figure 2 shows a TGA profile of the polymeric precursor previously dried at 200 "C. It was carried out in dry nitrogen. A steep decrease at comparably low temperatures was observed. At a temperature of 300 "C, 40 wt % was already evaporated. At higher temperatures, the weight loss approaches an almost constant slope down to a value of 8 wt 9o at about 950 OC. Upon further increase of the temperature, the weight remained nearly constant. The ceramic yield of about 20 wt % is low compared to that for the formation of aluminum nitride by a similar route (42 wt 5%; see ref 9 and 10). Comparison of this with the empirical formula shows that a considerable number of titanium compounds are also vaporized. The FTIR spectrum (Polaris, Mattson Inst.) of the viscous solution (see Figure 3) shows strong absorption in the N-H regions (3000-3400 and 1500-1600 cm-l) and the C-H region (2800-2900 cm-l). A small, very sharp peak D.C.;Torrible, E. G. Can. J. Chem. 1963,41, 134. Neese, H.-J. Chimia 1970, 24, 209. (13) Burger, H.;
(12) Bradley,
TiN-Tic Solid Solutions
Chem. Mater., Vol. 2, No. 3, 1990 243
~
60
50
40
30
28 (CuKlxI/"
3600
2800
2000
1600
1200
800
Figure 3. FTIR spectra of (a) the highly viscous solution, (b) the dried precursor and (c) a sample calcined at 700 O C . at 2250 cm-', typical for the C=N bond in acetonitrile, indicates that a considerable amount of solvent was still present. The FTIR spectrum of the polymeric precursor, dried at 200 "C, shows a remarkable decrease in absorption in the C-H region, due to the evaporation of the solvent and the propylamine. Absorption in the C=N region at 2250 cm-' cannot be observed, and therefore the solvent (acetonitrile) can be assumed to be totally removed at 200 "C. The strong absorption in the 3000-3400-cm-' region indicates that N-H bonds are still present. Figure 3 shows another FTIR spectrum, recorded from a sample calcined at 700 "C. Although significant absorption in the region of C-H and N-H stretching vibrations cannot be observed, moderate absorption in the region from 1000 to 1600 cm-' indicates that the organic compounds were not completely removed. FTIR spectra of samples calcined at temperatures higher than 950 "C match IR spectra recorded from conventionally produced TiN powders. This compares well with the TGA profile (see Figure 2), where a weight loss occurs up to temperatures of 950 "C. X-ray diffraction data show that the dried precursor is completely amorphous before calcination. During calcination in an atmosphere of dried ammonia, amorphous products are obtained up to a temperature of about 600 "C (see Figure 4). Already at 700 "C, a temperature at which considerable weight loss still occurred, as shown by the TGA profile, characteristic peaks of the desired product, titanium nitride, are visible. Very broad peaks indicate extremely small crystallite sizes. With increasing calcination temperatures the peaks become sharper. When the calcination atmosphere consisted of dried argon, amorphous products were obtained up to a calcination temperature of 700 "C. The first (extremely broadened) peaks, related to Ti(N,C),were visible after calcination at 800 "C (see Figure 5). Although increased temperatures led to better pronounced peaks, calcination in an argon atmosphere resulted in smaller crystallites than calcination in anhydrous ammonia. From XRD line broadening, the
Figure 4. XRD patterns of the calcination products after heat treatment in the range 600-1100 O C in anhydrous ammonia flow.
9 00 OC
I
I
L
60
50
40 2e(cu~)/o
-
Figure 5. XRD patterns of the calcination products after heat treatment in the range 800-1100 "C in anhydrous argon flow. mean crystallite sizes can be calculated by the aid of a Scherrer equation (9,where B is the line broadening, D = o . ~ ~ x /cos ( Be) (2) characterized by half-width; t9 is the Bragg angle; X is the wavelength of the radiation (Cu K a = 0.154 nm); and D is the mean crystallite size. Figure 6 shows the mean crystallite size (corrected to a Si standard) after calcination in anhydrous ammonia or an argon atmosphere. It should be noted that other effects such as lattice distortions also may influence X-ray line broadening and that additional amorphous phases cannot be seen in X-ray patterns and therefore cannot be ex-
Russel
244 Chem. Mater., Vol. 2, No. 3, 1990
1 E
/
/
/
6o
.
~~
1 0
20
0
700
800
900
1000
Table I. Oxygen, Nitrogen, and Carbon Content of Samples Obtained by Calcining at 1200 OC in Different Gas Atmospheres nitrogen carbon oxygen content, wt content, content, wt calcination atmos wt % % % 4.75 31 5.9 argon 11.7 10.5 2.2 nitrogen 15.4 5.1 1.6 ammonia
1100
W"CFigure 6. Mean crystallite size as a function of the calcination temperature: (0) calcination atmosphere, anhydrous ammonia; ( 0 )calcination atmosphere, dried argon.
cluded. A t a calcination temperature of 800 "C, mean crystallite sizes of 9.7 nm in anhydrous ammonia and of 4.9 nm in dry argon were calculated. The mean crystallite sizes increased with increasing temperature. At lo00 "C, values of 32 nm in ammonia and of 8.3 nm in argon were obtained. Calcining at 1100 "C resulted in mean crystallite sizes of 73 nm in ammonia and 43 nm in argon. The observed XRD lines can be related to the cubic phase of TiN or Ti(C,N): 20 = 37" ( l l l ) , 43" (200), 62" (220). Additional peaks could not be observed at any temperature or calcination atmosphere. The color of the products obtained was strongly dependent on the calcination conditions. Black or grayish samples were obtained up to temperatures of 950 "C in all calcination atmospheres and at higher temperatures if the calcination was carried out in argon. After calcining in anhydrous ammonia at 10oO and 1100 "C, copper brown- and gold-colored samples, typical
of TiN, were respectively obtained. After calcining in nitrogen at 1200 "C, a copper brown product was also obtained, while calcining at lower temperatures led to black or grayish products. Table I shows the oxygen, nitrogen, and carbon content of samples calcined at 1200 "C in different atmospheres. The gold-colored product calcined in anhydrous ammonia had the lowest oxygen (1.6 wt 9%) and carbon (5.1 wt %) contents, but the highest nitrogen content (15.4 w t %). The oxygen content of the sample calcined in nitrogen was also low, but the carbon content was rather high and nearly equal to the nitrogen content. The high carbon content of the sample calcined in argon was quite insufficient because excess carbon obviously contaminates the product. It is assumed that the comparably small crystallite sizes (see Figure 5) are due to the carbon contaminations,which are expected to hinder the crystal growth.
Conclusions It was possible to prepare a polymeric titanium organic precursor by anodic dissolution in an organic electrolyte. Calcining of this precursor led to Ti(C,N) solid solutions. Depending on the calcination atmosphere, different compositions could be achieved. A complete absence of carbon could not be obtained even when the samples were calcined in ammonia. But for most technical applications this is not required. Further studies should investigate the preparation of thin coatings on various substrates by the same polymeric route. Registry No. Ti, 7440-32-6; propylamine, 107-10-8; titanium carbide nitride, 12347-09-0; acetonitride, 75-05-8; tetrabutylammonium bromide, 1643-19-2.
