Protecting Nanoscaled Non-oxidic Particles from Oxygen Uptake by

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Protecting Nanoscaled Non-oxidic Particles from Oxygen Uptake by Coating with Nitrogen-Containing Surfactants Rainer Mu¨ller,*,† Martin Knapp,† Klaus Heckmann,† Monika von Ruthendorf,‡ and Gottfried Boden‡ Laboratory of Interface Chemistry, University of Regensburg, Universita¨ tsstrasse 31, D-93053, Regensburg, Germany, and Fraunhofer Institute for Ceramic Technologies and Sintered Materials, Dresden, Germany Received August 28, 2003. In Final Form: January 20, 2004 To suppress the reactivity of nanoscaled non-oxidic powders of titanium nitride (TiN) and silicon carbonitride (SiCN) against hydrolysis and oxidation, chemical surface modification with nitrogen-containing surfactants was investigated. Among these surfactants, long-chain primary amines, ethylenediamines, guanidines, nitriles, isocyanates, and succinimides were examined. Thermogravimetry, elemental analysis, and behavior against the water-vapor adsorption of the modified particles were used as methods to estimate the protective capacity of the organic coating material. The best results were obtained by using the longchain amines and octadecylisocyanate, which were indicated by a significant shift of the powder oxidation toward the higher temperatures and an increase of the particle hydrophobicity. A long-chain succinimide was found to be the most effective in dispersing nanoscaled TiN in organic media. Preparation of a stable aqueous dispersion without significant changes in the elemental composition of the powder was achieved by the application of an ionic surfactant to the surface-modified particles.

Introduction In the development and production of high-performance ceramics, fine and finest powders with particle sizes far below 1 µm are required in an increasing manner because the properties of the ceramics depend on the microstructure, which is influenced by the particle size of the starting powders. Because of their high surface-to-volume ratio these particles possess a large excess of surface free energy. This results in a high intrinsic sintering activity of the particles, which should give a reason to reduce the temperatures for densification of the nanostructured ceramic green parts.1 Nanoscaled oxidic powders can be handled in the presence of air and water without major problems. On the contrary, the properties of the nanoscaled non-oxidic powders such as nitrides, carbides, or carbonitrides change drastically in the presence of air and water because of the oxygen uptake initiated by oxidation and hydrolysis.2 Spontaneous inflammation of the nanoscaled non-oxidic powders can be observed.3 If the chemical stability of these powders toward water and air could be modified, their handling and storage would be facilitated and an aqueous dispersion could be used for further colloidal processing. Some attempts were made to protect the surface of the non-oxidic powders from hydrolysis during processing in aqueous media. Aluminum nitride powders were coated with long-chain alcohols and long-chain carboxylic acids * Author to whom correspondence should be addressed. Laboratory of Interface Chemistry, University of Regensburg, Universita¨tsstrasse 31, D-93053 Regensburg, Germany. Phone: +49-941943-4521. Fax: +49-941-943-4686. E-mail: rainer.mueller@chemie. uni-regensburg.de. † University of Regensburg. ‡ Fraunhofer Institute for Ceramic Technologies and Sintered Materials. (1) Nass, R.; Albayrak, S.; Aslan, M.; Schmidt, H. Ceram. Trans. 1995, 51, 591-595. (2) Vassen, R.; Stoever, D. Mater. Sci. Eng., A 2001, 301, 59-68. (3) Gu¨nther, B.; Ko¨nig, T.; Meisel, R. L. 10th Hagener Symposium Pulvermetallurgie in Wissenschaft und Technik, Hagen, Germany, 1995; p 40-57.

from organic solvents such as benzene and cyclohexane.4-6 Adsorption isotherms showed a high affinity of the organic molecules toward the powder surface independent of the functional group. Protecting capacity, which was indicated by the suppression of the pH increase during the stirring of the modified powders in water, increased with the length of the hydrocarbon chains of the coating material. The best stability against hydrolysis was achieved by coating the powders with stearic acid, which is assumed to build up chemically bonded monomolecular layers onto the surface of the powders. These particles exhibit a highly hydrophobic surface, and they can be transferred into a stable aqueous dispersion by using anionic surfactants.7 Coating aluminum nitride powder with dicarboxylic acids such as sebacic acid from benzene resulted in the formation of a well-covered and hydrophilic surface. The particles were stable against hydrolysis and could be dispersed in aqueous media without further additives.8 Other authors propose the usage of short-chain organic compounds for building up stable and homogeneous aqueous dispersions of non-oxidic powders. Among these compounds, guanidine propionic acid9 or alkoxysilanes such as 3-aminopropyltriethoxysilane10 or 3-(triethoxysilyl)-2-methylpropylsuccinic acid11 were found. The silanes were proposed to bind chemically to the surface of a silicon nitride powder by condensation with surface-bound hydroxyl groups of the oxide layer. With this coating, (4) Egashira, M.; Shimizu, Y.; Takatsuki, S. J. Mater. Sci. Lett. 1991, 10, 994-996. (5) Egashira, M.; Shimizu, Y.; Takao, Y.; Yamaguchi, R.; Ishikawa, Y. J. Am. Ceram. Soc. 1994, 77, 1793-1798. (6) Hotza, D.; Sahling, O.; Greil, P. J. Mater. Sci. 1995, 30, 127-132. (7) Zhang, Y.; Binner, G. P. Int. J. Inorg. Mater. 1999, 1, 219-227. (8) Shimizu, Y.; Kawanabe, K.; Taki, Y.; Takao, Y.; Egashira, M. Ceram. Proc. Sci. Technol. 1995, 31, 403-407. (9) Nass, R.; Albayrak, S.; Aslan, M.; Schmidt, H. Ceram. Trans. 1995, 51, 591-595. (10) Buchta, M. A.; Shih, W. H. J. Am. Ceram. Soc. 1996, 79, 29402946. (11) Richter, H. J.; Breuning, U. 5th European Conference of Advanced Material Processing Applications, Maastricht, The Netherlands, 1999; p 2/277-2/280.

10.1021/la0356046 CCC: $27.50 © 2004 American Chemical Society Published on Web 03/03/2004

Protecting Nanoscaled Non-oxidic Particles

the surface charge density of the powder suspended in water can be increased, and this allows a better electrostatic stabilization of the dispersion. The guanidine compound is able to adsorb through its amino groups to the surface of a titanium nitride (TiN) powder. In this way, a high density of negative charges is introduced to the surface of the particles, which allows the preparation of well-dispersed slips of high solid content from the nanoscaled powders. However, among these references, changes in the powder properties because of hydrolysis have not been specified, although they may occur. The aim of this paper arose from the question of why it is state of the art to suppress the oxygen uptake of non-oxidic powders by coating them just with oxygencontaining molecules such as alcohols or carboxylic acids? From this occasion, we investigated the capability of the long-chain molecules with polar headgroups, which consist mainly of nitrogen atoms. Here, we can be sure not to change the elemental composition of the surface significantly, only by adsorbing the surface modifiers. The second aim of the paper is to assess the possible use of the modified powder in aqueous suspensions. Experimental Section Powders. Nanoscaled particles of TiN were obtained from H.C. Starck (Laufenburg, Germany). Silicon carbonitride (SiCN) was synthesized via a two-step process.12 In the first step, volatile polysilazanes were prepared by ammonolysis of dimethyldichlorosilane in an organic aprotic solvent. These polysilazanes were finally pyrolyzed in a nitrogen- or ammonia-containing gas stream to give the SiCN powder in the submicrometer scale. The resulting powders were heat-treated at 1350 °C in a nitrogen atmosphere. The characterization included elemental analysis, BrunauerEmmett-Teller (BET) surface analysis, scanning electron microscopy (SEM), and IR spectroscopy. Storage of the particles was performed in a high-quality glovebox system under a nitrogen atmosphere. Organic Coating Material. Octadecylamine (I), octadecylnitrile (V), octadecylisocyanate (IV), the analogous decyl compounds, and stearic acid (VIII) were purchased from SigmaAldrich Chemie GmbH (Taufkirchen, Germany) in the highest purity and were used without further purification. A long-chain and highly branched alkyl succinimide containing about 95 carbon atoms per molecule was obtained from the University of Moscow (Russia). Cetylpyridinium chloride (CPC) was purchased from Merck KG (Darmstadt, Germany) and purified by fractional crystallization from ethanol. All of the other nitrogen-containing compounds were synthesized according to the following protocols. Each substance was characterized by elemental analysis, whereby data are displayed as the atomic weight percentages. Data of 1H NMR spectroscopy include the chemical shift (δ, ppm), type (s ) singlet, t ) triplet, m ) multiplet, and br ) broad), and intensity (number of protons) of the signals. Monooctadecylethylenediamine (II) was obtained by the reaction of 1-bromooctadecane with an 8-fold excess of ethylenediamine in refluxing ethanol for 24 h, analogous to the protocol of Linsker et al.13 After the evaporation of the solvent, the crude reaction product was suspended in water and extracted with diethyl ether. Purification was achieved by fractional crystallization from a mixture of methanol and acetone (10:1), which gave a colorless solid in 80% yield. Mp 62 °C. 1H NMR (CDCl3 + CD3OD) δ: 2.70 (m, 4H), 2.59 (t, 2H), 1.46 (m, 2H), 1.25 (br, 30H), 0.88 (t, 3H). Elem anal. Calcd for mono-octadecylethylenediamine: C, 76.89; H, 13.71; N, 8.13. Found: C, 76.85; H, 14.19; N, 8.96. Bisoctadecylethylenediamine (III) was synthesized according to the method of Mueller-Westerhoff et al.14 by heating 2 mol of octadecylamine with 1 mol of 1,2-dibromoethane in ethanol for (12) Boden, G.; Neumann, A.; Breuning, T.; Tschernikova, E.; Hermel, W. J. Eur. Ceram. Soc. 1996, 18, 1461-1469. (13) Linsker, F.; Evans, R. L. J. Am. Chem. Soc. 1945, 67, 15811582.

Langmuir, Vol. 20, No. 7, 2004 2599 24 h. The hydrobromide salt precipitated upon cooling and was filtered. For deprotonation, a suspension of this salt in dichloromethane was treated with an aqueous solution of potassium hydroxide. The bisalkylated ethylenediamine was isolated from the organic phase and purified by fractional crystallization from ethanol as a colorless solid in 65% yield. Mp 73 °C. 1H NMR (CDCl3) δ: 2.71 (s, 4H), 2.59 (t, 4H), 1.69 (s, 2H), 1.47 (m, 4H), 1.26 (br, 60H), 0.88 (t, 6H). Elem anal. Calcd for bisoctadecylethylenediamine: C, 80.18; H, 13.59; N, 4.57. Found: C, 80.77; H, 14.27; N, 4.96. The synthesis of octadecylguanidine (VII) was performed by heating an equimolar mixture of guanidinium rhodanide and 1-octadecylamine at 150 °C until the development of ammonia stopped. The reaction mixture was dissolved in methanol and acidified with nitric acid, whereby the precipitation of octadecylguanidinium nitrate occurred.15 To obtain the free guanidine, a suspension of the salt in dichloromethane was treated with an aqueous solution of potassium hydroxide. The pure product was isolated from the organic phase as a colorless powder in 40% yield. Mp 65 °C. 1H NMR (CDCl3) δ: 7.5-6.5 (br, 4H), 3.07 (m, 2H), 1.44 (br, 2H), 1.26 (br, 30H), 0.85 (t, 3H). Elem anal. Calcd for octadecylguanidine: C, 72.55; H, 13.33; N, 12.78. Found: C, 73.25; H, 13.26; N, 13.49. Octadecylsuccinimide (VI) was synthesized by melting octadecylsuccinic acid, which was purchased from Lancaster (Eastgate, England), with an excess of urea at 180 °C in an inert atmosphere until the development of gaseous byproducts stopped. The crude reaction product was suspended in water and extracted with chloroform. Fractional crystallization from petrolium ether gave the pure succinimide as a colorless powder in 55% yield. Mp 100 °C. 1H NMR (CDCl3) δ: 7.98 (b, 1H), 2.88 (m, 2H), 2.44 (q, 1H), 1.90 (m, 2H), 1.54 (m, 2H), 1.30 (br, 30H), 0.88 (t, 3H). Elem anal. Calcd for octadecylsuccinimide: C, 75.10; H, 11.56; N, 3.72. Found: C, 75.15; H, 11.75; N, 4.00. A long-chain alkylamine containing approximately 50 carbon atoms was synthesized via a three-step protocol. In the first step, polyethylene monoalcohol (MW ∼ 700 g/mol), which was purchased from Sigma-Aldrich (Taufkirchen, Germany) was treated with an 10:1 mixture of hydrobromic acid (47% aqueous solution) and sulfuric acid at 100 °C for 24 h. The resulting polyethylene bromide was suspended in boiling water to remove excess acid and was recrystallized from petrolium ether (80:110). The bromide was then heated with an equimolar amount of potassium phthalimide in dimethylformamide at 130 °C for 16 h.16 After the addition of water, the precipitation of the N-alkyl phthalimide occurred. The product was washed several times with water and dried. In the last step, the phthalimide was treated with a 2-fold amount of hydrazinium hydrate in n-butanol at 120 °C for 3 days.17 The filtered precipitate was extracted with petrolium ether in a Soxhlet apparatus to give pure polyethylene amine as a colorless solid in 55% yield. Mp 96 °C. Elem anal. Calcd for the long-chain alkylamine: C, 82.77; H, 14.60; N, 1.41. Found: C, 83.68; H, 14.37; N, 1.95. For the synthesis of the bifunctional compound aminodecyltrimethylammonium bromide (IX), 1,10-dibromodecane was refluxed with half of the equivalent amount of potassium phthalimide in acetone for 24 h.18 The filtrate of the cooled, resulting mixture was evaporated to dryness, and the purification of the 10-bromodecylphthalimide was performed by chromatography with the mixtures of CH2Cl2 and heptane. The purified intermediate was treated with a 2-fold excess of NMe3 in refluxing ethanol for 12 h. After cooling, ether was added to the solution to precipitate the crude phthalimidodecyltrimethylammonium bromide. The product was suspended in ethanol, and the nondissolved material was removed by filtration. Evaporation of the filtrate provided the purified product. Hydrazinolysis of the resulting phthalimide was performed in n-butanol as described (14) Mueller-Westerhoff, U. T.; Zhou, M. J. Org. Chem. 1994, 59, 4988-4992. (15) Rust, J. B. U.S. Patent 2,400,786, 1946. (16) Sheehan, J. C.; Bolhofer, W. A. J. Am. Chem. Soc. 1950, 72, 2786-2788. (17) Osby, J. O.; Martin, M. G.; Ganem, B. Tetrahedron Lett. 1994, 25, 2093-2096. (18) Mary, A.; Renko, D. Z.; Guillou, C.; Thal, C. Bioorg. Med. Chem. 1998, 6, 1835-1850.

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Table 1. Physical and Chemical Properties of the Nonmodified Powders powder

nitrogen (%)

carbon (%)

oxygen (%)

chlorine (%)

BET surface (m2/g)

particle size (SEM) (nm)

IEP (pH)

TiN SiCN

21.50 21.75

0.02 13.93

1.93 1.96

0.80

42 33

30-40 50-500

5.5 6.0

above, whereby the isolation of the product was achieved by precipitation with ether. Repeated precipitation with ether from ethanol solutions gave aminodecyltrimethylammonium bromide as a highly hygroscopic colorless solid in 31% yield. Mp 154 °C. 1H NMR (DMSO-d ) δ: 3.32 (q, 2H), 3.07 (s, 9H), 2.73 (t, 2H), 6 1.65 (m, 2H), 1.53 (m, 2H), 1.25 (s, 12H). Elem anal. Calcd for aminodecyltrimethylammonium bromide: C, 49.86; H, 11.56; N, 8.38; Br, 26.72. Found: C, 52.85; H, 10.58; N, 9.53; Br, 27.04. Coating the Particles from Organic Medium. Hexane was used as an inert organic solvent for coating the particles with nitrogen-containing compounds. It was purchased in the highest purity from Merck KG (Darmstadt, Germany) and was additionally deoxidized by sonication and saturation with dry argon gas. The long-chain molecules were dissolved to a maximum concentration of 10 mM. Stirring 200 mg of powder in 10 mL of an organic solution for 2 h was established as the standard coating procedure. The coating temperature varied between room temperature as the standard and boiling temperature of the solvent. The modified powder was centrifuged, washed twice with pure hexane under the assistance of ultrasonic agitation for 15 s to remove the nonadsorbed surfactant molecules, and finally dried under vacuum. For the determination of the adsorption isotherms, the powders were coated from solutions of different concentration using the standardized procedure. Equilibrium concentrations of the surfactants in the organic solvent were measured by gas chromatography (GC). Dispersing of the Particles. To estimate the dispersing properties of the coated particles in organic media, the resulting slurry of the coating procedure was transferred into a graduated glass tube and then sonicated twice for 30 min. For dispersing in an aqueous media, 100 mg of the modified powder was sonicated twice in 10 mL of pure or ionic-surfactant-containing water again for 30 min. In both cases, the rate of sedimentation was recorded as the settling distance versus time. For comparison, in some cases, the sedimentation equipment Turbiscan MA 1000 of Formulaction (Toulouse, France) has been used. Characterization of the Modified and Nonmodified Particles. Thermogravimetric analysis was performed with a TGA 7 from Perkin-Elmer Corp. (Norwalk, CT). A total of 10 mg of the powder sample was heated (rate ) 10 °C/min) in a synthetic air atmosphere until the oxidation of the powder was indicated by a significant increase of the sample mass. The temperature of the beginning oxidation was evaluated by onset analysis for each type of modified particle and compared with the noncoated reference powder. A shift from the point of the starting oxidation toward higher temperatures indicated a certain corrosion inhibition capacity of the organic coating. Heating the powder samples in an inert atmosphere enabled the estimation of the amount of adsorbed organic material because of the weight loss caused by the desorption or cracking of the molecules. Adsorption of water vapor was performed on an adsorption balance, which was designed at the University of Bochum (Germany). A total of 100 mg of the powder sample was placed on the balance inside the sealable sample compartment and was exposed to a nitrogen atmosphere, which was moistened to about 60% relative humidity at 20 °C. The increase in the sample weight, which belonged to the water adsorption, was recorded versus time until an equilibrium was reached. The amount of adsorbed water in the equilibrium state was determined, and this value was used to compare the coated specimen with the noncoated reference. Analysis of the surface area of the powders was performed according to the method from BET using the accelerated surface area and porosimetry system, ASAP 2010, from Micromeritics Instrument Corp. (Norcross, GA). A total of 100 mg of the powder sample was analyzed using nitrogen as the adsorbate. IR spectra were recorded with a Fourier transform infrared spectroscope, (FTIR)-610, from Jasco Corp. (Tokyo, Japan) using the KBrpellet technique. Elemental analysis was performed by a nitrogen/ oxygen determinator TC 436 from Leco Corp. (St. Joseph, MI)

Figure 1. IR spectra of TiN and SiCN as received. by heating the powder samples up to 2900 °C. Isoelectric points (IEPs) were determined by electroacoustic spectrometry with an ESA-8000 of Matec Applied Sciences (Northborough, MA). For this purpose, the dispersions with a volume fraction of 2% of the powders in an aqueous potassium chloride solution were prepared using an Ultraturrax T50 from IKA-Labortechnik (Staufen, Germany). The pH of the dispersions was adjusted by titration with hydrochloric acid or sodium hydroxide, and the ESA signal was transformed into ζ potential by the system software. The pH value where the sign of the ζ potential changed was defined as the IEP of the powder. Primary particle sizes were estimated with SEM. Actual particle sizes were determined by photon correlation spectroscopy (PCS) using the Zetasizer Z3000 from Malvern Instr. (Malvern, U.K.). Here, the highly diluted dispersions of the coated and noncoated powders in chloroform were analyzed. Particle coating was visualized with a transmission electron microscope (TEM), JEM-2010FEF, from JEOL (Tokyo, Japan).

Results Characterization of the Noncoated Particles. Table 1 summarizes the results of the analysis of the powders as received from the purchaser or the synthesis apparatus. TiN exhibited an inferior amount of chlorine and carbon. Additionally, both of the powders showed an oxygen content of about 2%. IR spectroscopy confirmed the presence of hydroxyl groups for TiN with the signals at 3434 and 1634 cm-1. On account of the bands at 2323 and 2850 cm-1, the presence of hydrocarbon species at the surface of TiN could be shown additionally. The signal at 801 cm-1 represents the Ti-N vibration. In the spectrum of the SiCN signals, the O-H and C-H vibrations were almost absent. The broad signal around 924 cm-1 resulted in a combination of Si-C, Si-N, and Si-O vibration peaks with a predominance on Si-N (Figure 1). SEM analysis displayed a narrow particle size distribution for TiN between 30 and 40 nm and a very broad particle size distribution for SiCN from 50 to 500 nm, shown in Figure 2. According to the smaller primary particle size of TiN, a higher value for the specific surface was measured by BET in contrast to SiCN. For both of the powders, the IEPs were fixed in the acidic pH range, which means that the surfaces exhibit an excess of negative charges. Coating the TiN Particles from the Apolar Organic Solvent. Chart 1 shows the chemical structures of the organic compounds used for coating from hexane. Significant differences in the solubility of these molecules


Figure 2. Scanning electron micrographs (100 000 magnification) of TiN (left) and SiCN (right) as received.

were observed, especially for the octadecyl compounds. Succinimide and guanidine showed with a maximum concentration of 10-5 mol/L an inferior solubility in hexane compared to the other molecules, which were soluble up to 10-2 mol/L. For the cationic amine, a likewise poor solubility was ascertained. Table 2 gives a summary of the protecting properties of the surfactants as resulted from the screening-methods thermogravimetry and watervapor adsorption. It was found that the oxygen uptake of the treated powders was influenced by the number of carbon atoms of the nitrogen-containing surfactants. The lowest oxidation occurred by using substances containing an octadecyl chain. Only the succinimides showed a different behavior, which is based on the very low solubility of octadecyl succinimide in hexane compared to the C10 and C95 analogues. Moreover, significant differences in the protection efficiency according to the polar headgroups of the surfactants were observed. Thermal stability increased for the C18 compounds within the sequence: succinimide < nitrile and guanidine < stearic acid < isocyanate and amines. This is indicated by the shift of the weight increase, which is due to the oxidation of the powders toward higher temperatures (Figure 3, left). The most effective was the coating with double-substituted ethylenediamine, which shifted the onset of the oxidation temperature by 93 °C. The second best results were obtained by using amine and isocyanate. For these three substances, the coated powders exhibited the highest organic contents with 5.7 to 8.4% (Figure 3, right). The lowest efficacy was shown for the succinimide, the thermogravimetric curve of which does not differ significantly from that of the noncoated reference. Coating with stearic acid and cationic amine effected a medium protection efficiency by shifting the oxidation temperature by 64 and 41 °C, respectively. For

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these substances, the adsorbed amounts were rather low compared with those of the amines. PCS measurements displayed no significant changes in the particle sizes after being coated with the organic compounds. Only with amine and isocyanate, a slight increase in the mean particle size was found. The mean particle size in the chloroform dispersion was about 10 times higher than the primary particles size, which was estimated by SEM. Powders coated with amines, stearic acid, and guanidine showed a low uptake of water vapor and therefore a high hydrophobicity compared to that of the nonmodified titanium nitride. The amount of adsorbed water was reduced from 43 mg/g to a minimum of 8 mg/g (18%). Coating with nitrile led to a reduction of the hydration to a value of 16 mg/g (38%), whereas coating with succinimide and isocyanate caused no considerable hydrophobization of the powder (86%, Figure 4). Figure 5 shows the adsorption isotherms of the good soluble organic compounds on the surface of TiN from hexane at room temperature. For amine and ethylenediamine, a high affinity toward the particle surface was observed. A slight increase in the concentration of the hexane solution led to an intense improvement in the surface adsorption. Both of the substances reached a maximum in surface coverage, which was 4.5 µmol/m2 for amine and 2.5 µmol/m2 for ethylenediamine. For isocyanate, a high adsorption tendency was found too. In this case, no maximum in the surface coverage was observed. Carboxylic acid also showed high affinity for the powder surface, but the maximum coverage reached with 1.9 µmol/ m2 was only less than half of the values determined for the amines. Contrary, nitrile showed poor adsorption properties in the presence of the nanoscaled TiN powder. Only high solution concentrations caused improvement in surface adsorption. Dispersing TiN in the Organic Solvent. Figure 6 presents the sedimentation curves of the differently modified TiN powders in hexane. The noncoated powder settled immediately after the termination of the ultrasonic treatment. Using one of the different amines did not stabilize the dispersion significantly. After 5 min of dispersing, already most of the powder settled to the ground of the glass tube. The powder, which was modified with C95 succinimide, gave a stable dispersion over a period of several days. A combination of long-chain succinimide and the best protective compound (III) in a

Chart 1. Chemical Structures of the Compounds Used: Amine (I), Monoalkylated Ethylenediamine (II), Bisalkylated Ethylenediamine (III), Isocyanate (IV), Nitrile (V), Succinimide (VI), Guanidine (VII), Stearic Acid (VIII), and Cationic Amine (IX)

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Figure 3. Thermogravimetric analysis of coated and noncoated TiN (heating rate ) 10 °C/min). Left ) a synthetic air atmosphere. Right ) an inert nitrogen atmosphere. Table 2. Solubility of the Coating Substances in Hexane and Properties of the Modified TiN Powders Resulting from the Screening Methods Thermogravimetry, Adsorption of the Water Vapor, and PCS substance, chain length

solubility of coating substance concd max. (mol/L)

oxidation temp (°C)

shift in oxidation temp (°C)

organic content (%)

noncoated I, C10 I, C18 I, C50 II, C18 III, C10 III, C18 IV, C18 V, C18 VI, C10 VI, C18 VI, C95 VII, C18 VIII, C18 IX, C10

>10-2 >10-2 10-2 >10-2 >10-2 >10-2 >10-2 10-4 10-5 >10-2 10-5 >10-2 10-4

370 421 457 372 450 427 463 452 405 388 377 435 397 434 411

+51 +87 +2 +80 +57 +93 +82 +35 +18 +7 +65 +27 +64 +41

1.04 3.74 5.71 1.09 5.76 4.59 6.12 8.36 5.19 1.43 1.30 4.44 1.91 3.92 2.38

adsorbed water (mg/g of powder)

reduction of water adsorption (%)

43.2 9.1 9.3

21 21

9.2 9.1 11.7 36.7 16.3 11.1 37.0 27.0 8.0 12.1 18.7

21 21 27 85 38 26 86 63 18 28 43

mean particle size (PCS) (nm) 324 ( 4 355 ( 6

309 ( 4 358 ( 5 326 ( 7 309 ( 2 310 ( 3

Figure 4. Adsorption of the water vapor on coated and noncoated nanoscaled TiN.

Figure 5. Adsorption isotherms of the coating substances on the nanoscaled TiN from hexane.

7:3 weight ratio resulted in the generation of a short-time stable dispersion. Increasing the content of C95 succinimide of up to 50% improved the stability of the resulting dispersion of up to several days. Exposure of TiN to Air and Water. In Figure 7, the results of a long-time aging experiment are displayed where noncoated and ethylenediamine-coated TiN was exposed to air (room conditions) and double-distilled water (pH 6). The oxygen content of the nonmodified particles increased from 2 to 4.5% (water) and 5.2% (air) within the first hour and to 5.1% (water) and 6.5% (air) during the 10 day exposure, respectively. Modification with ethylenediamine reduced the oxygen uptake from air to one-

third and from water to two-thirds compared with that of the reference. Dispersing TiN in Water. Noncoated and ethylenediamine- and cationic amine-coated nanoscaled TiN were dispersed in pure water (pH 6). Additionally, the ethylenediamine-coated powder was suspended in the presence of the water-soluble surfactant CPC, which was applied in concentrations (ionic strength) of 10-2 and 10-3 mol/L. Both of the solutions showed a pH of about 6 because of some dissolved carbon dioxide from the air. Figure 8 displays the rate of sedimentation of TiN in dependence of the surface conditioning. Noncoated particles settled almost completely within 4 days to the ground, while


Figure 6. Rate of sedimentation of the TiN powder in hexane.

Figure 7. Oxygen content of water- and air-exposed ethylenediamine-coated and noncoated TiN.

Figure 8. Rate of sedimentation of the coated and noncoated TiN powder in water.

ethylenediamine-coated powder remains at the top of the water column because of its highly hydrophobic surface. In the presence of the cationic surfactant CPC, a stable dispersion was formed. Within 14 days, the settling distance of the powder reached 30% of the length of the sedimentation tube. Modification of the powder with cationic amine enabled the generation of a stable dispersion for up to 1 week. Figure 9 shows the influence of the time of dispersing on the oxygen content of the particles. For the noncoated powder, a rapid oxygen uptake was observed. Coating with ethylenediamine reduces the hydrolyzation of the powder significantly. The presence of CPC in the dispersion slightly weakens the barrier function of the coating. The

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Figure 9. Changes in the oxygen content of the TiN powder during the dispersing in water.

Figure 10. Oxygen content of water- and air-exposed ethylenediamine-coated and noncoated SiCN

oxygen content of these powders slightly increases compared to the coated powders in the absence of CPC but stays clearly beyond the value for the noncoated particles. For cationic amine, no inhibition properties were detected. Coating the SiCN Particles from the Apolar Organic Solvent. The coating of the SiCN powder was performed as an in situ conditioning during its synthesis. That means that the powder was passed through the inhibitor containing organic solution directly after the vapor-phase pyrolysis of the polysilazane precursor. Figure 10 shows the changes in the oxygen content of the ethylenediamine-coated and noncoated SiCN particles during the 10 day exposure to air and water. The results demonstrate that oxidation and hydrolysis of the particles have been clearly suppressed by the organic coating. The oxygen content of the modified SiCN powder was about one-half lower after the exposure to air and water than that of the noncoated reference. Figure 11 displays the transmission electron micrographs of the SiCN particles in different states of conditioning. The picture of a noncoated particle (A) exhibits a sharp phase boundary and a smooth surface. After exposure in water (B), the surface of particles became rough because of the formation of oxidation products. The picture of ethylenediamine-coated SiCN (C) displays the appearance of a light-gray border of about 10 nm thickness, which gives evidence for an entire coverage of the particles. Discussion Ammonia treatment of dimethyldichlorosilane gave a liquid silazane precursor, which was used to produce

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Figure 11. Transmission electron micrographs of the SiCN particles: (A) noncoated (500 000 magnification), (B) noncoated and exposed to water (1 000 000 magnification), and (C) ethylenediamine-coated after dispersing in the CPC-containing aqueous solution (500 000 magnification).

submicrometer-scaled non-oxidic particles. Pyrolysis of this liquid precursor in an ammonia-containing atmosphere resulted in the formation of an amorphous and nitrogen-rich SiCN powder. On the basis of the elemental analysis, the molecular ratio between nitrogen and carbon was calculated to 0.57:0.43. The IR spectrum (Figure 1) showed a broad signal between 700 and 1200 cm-1. The occurrence of this band has been interpreted as a combination of the stretching vibrations of Si-C (820 cm-1), Si-N (950 cm-1), and Si-O (1085 cm-1), whereby the main component influences the position of the maximum of the signal. 19,20 In our spectrum, the maximum was observed at 924 cm-1, which refers to a nitrogen-rich powder and therefore coincides with the elemental analysis. The light shoulder of this signal on the side of the higher wavenumbers and the oxygen content of about 2% point to a partial oxidation of the powder surface. In distinction to the IR spectra of the powders from the cited literature, our spectrum misses signals in the wavenumber area of 3430 and 1635 cm-1, which implies that there is no adsorbed water at our SiCN particles. This is a result of the heat treatment, which was performed immediately after the powder synthesis. TiN exhibited slight amounts of chlorine and carbon, which may indicate the presence of residual byproducts from the production process. Additionally, the powder showed an oxygen content of about 2%. From the IR spectrum, the oxygen content can be explained by the presence of adsorbed water. With this technique, the presence of hydrocarbon impurities at the particle surfaces could be shown likewise. Heating of the TiN powder in an inert atmosphere to 500 °C resulted in the removal of the adsorbed molecules, which was indicated by the reduction of the peak intensity of the C-H and O-H vibration signals. The polar headgroups of the long-chain nitrogencontaining molecules should attach to the surface of the particles when they are offered from hexane solution. Under ideal conditions, the covering should be built up without defects and the hydrocarbon chains of the surfactants should act as a steric and hydrophobic barrier. For this reason, the diffusion rate of oxygen and water toward the particle surface should be minimized and oxidation or hydrolysis of the non-oxidic powder should be suppressed. These hypotheses have been investigated (19) Baraton, M. I.; Chang, W.; Kear, B. H. J. Phys. Chem. 1996, 100, 16647-16652. (20) Chen, Y.; Liang, Y.; Zheng, F.; Zhou, R.; Feng, Z. Ceram. Int. 2001, 27, 73-79.

by thermogravimetric analysis and by the determination of water-vapor adsorption of the coated particles. Thermogravimetry was previously introduced by our group as a screening method to characterize the protection of airsensitive powders by an organic coating.21 Both the length of the hydrocarbon chain and the species of the polar headgroup influenced the coating of the particles from hexane solution. First of all, both of the parameters defined the solubility of the molecules in the organic solvent (Table 2), which can be classified into two groups. Succinimide, guanidine, long-chain amine (C50), and cationic amine, with a maximum solubility at room temperature of less than 10-4 mol/L, belong to the group of poorly soluble molecules. Coatings with molecules of this group could not protect the TiN particles from oxidation in a sufficient manner as shown by thermogravimetry. Additionally, the measured low organic content of the powders coated with these molecules gave evidence for the presence of defect-rich layers at the particle surfaces. Within this group, variations of the standardized coating procedure by increasing the coating temperature or enhancing the solution-to-powder ratio showed no positive results within the technically practicable limits. Therefore, the group of molecules with a poor solubility was excluded from further investigations. Amines (C10 and C18), isocyanate, nitrile, and stearic acid could be dissolved up to the maximum concentration of 10-2 mol/L at room temperature, and therefore, they were classified as well soluble. Within this group, an increase of the chain length from 10 to 18 carbon atoms resulted in a higher protection capacity against the powder oxidation. The thermogravimetrically measured oxygen uptake of TiN was shifted toward higher temperatures by 40%. Much more than the chain lengths, the polar headgroups of the coating molecules were responsible for the significant differences in the protection capacity as determined by the screening methods. Only amines and isocyanate were able to build up dense barriers, which reduced the rate of the oxygen diffusion toward the particle surface significantly. The values in the organic content of the amine-coated powders were within the range of 5.76.1%. From the adsorption isotherms, a strong affinity of the amines toward nanoscaled TiN was evaluated. The data follow a Langmuir-type adsorption behavior, where a maximum coverage is attained at a defined solution (21) Mu¨ller, R.; Heckmann, K.; Habermann, M.; Paul, T.; Stratmann, M. J. Adhes. 2000, 72, 65-83.


concentration. If the cross-sectional areas of amine and ethylenediamine are assumed as 2522 and 50 Å2, the complete surface coverage by monolayer distribution would be 6.6 and 3.3 µmol/m2, respectively. With the observed maximum values of about 4.5 and 2.5 µmol/m2, respectively, a coverage of approximately 75% of the powder surface could be assumed. For carboxylic acid, the adsorption maximum was found at 1.9 µmol/m2, which implies a powder coverage of about 30%. These results are in good accordance with the data previously published by Wang et al.,23 where the adsorption of benzylamine and benzoic acid to the silicon nitride powder from cyclohexane resulted in a surface coverage of 5 and 1.5 µmol/m2, respectively. The distinct behavior in the adsorption ability of the molecules with either a basic or acidic headgroup can be explained by the IEP of the powder. The IEP of TiN used in our paper was measured at pH 5.5, while that of the silicon nitride of the cited reference was measured at pH 4.4. Therefore, both of the powders must exhibit a distinct excess of acidic groups at their surface. From this point of view, it seems clear that the molecules with a basic group are able to adsorb higher amounts at the powder surface than the molecules with an acidic group. The different protection capacity of both kind of molecules is the direct consequence of this phenomenon. Nevertheless, the molecules with a neutral headgroup also showed different behavior in protecting the non-oxidic particle surface. For nitrile, the most deficient protecting capacity of the examined molecules with good solubility was ascertained by thermogravimetry. The organic content of the nitrile-modified powder did not significantly differ from those conditioned with the amines. From the adsorption isotherm, it can be seen that the affinity of the nitrile toward the surface of TiN is very weak but no saturation of surface coverage could be observed. Both of the aspects can be interpreted by the formation of a defect rich multilayer during the adsorption of the nitrile molecules from hexane. The adsorption isotherm of isocyanate likewise lacks a saturated coverage, but in contrast, it demonstrated a high affinity of the molecules toward the powder surface. This caused the highest amount of adsorbed material (∼8.5%) determined in this paper. The builtup multilayer seems to be defectless, which was demonstrated by the good protection of the isocyanatecoated powder during the heating in an air atmosphere. One reason for the high affinity of isocyanate to the particle surface could be their ability of forming covalent bindings with the surface hydroxyl and amino groups.24 Hydrophobicity of the coated particles, which was estimated by the adsorption of water vapor, correlated well with their resistance against oxidation. Coatings of the molecules, which were well-soluble or showed high adsorption affinity, resulted in low water uptake of the powder. The coatings were dense enough to suppress the penetration of the small molecules toward the particle surface. Only for the isocyanate, a contrary behavior was found. The presence of highly reactive groups in the adsorbed multilayer led to the binding of significant amounts of water from the environment. The resulting amino and urea groups kept the particle surface more hydrophilic compared to the coatings with amines where the hydrocarbon groups remain at the surface. (22) Sasaki, T.; Muramatsu, M. Bull. Chem. Soc. Jpn. 1953, 26, 96100. (23) Wang, L.; Sigmund, W.; Aldinger, F. J. Am. Ceram. Soc. 2000, 83, 691-696. (24) Breuning, T.; Neumann, A.; Boden, G. Solid State Ionics 1997, 101-103, 811-817.

Langmuir, Vol. 20, No. 7, 2004 2605

Aliphatic hydrocarbons especially hexane and cyclohexane are often used for handling non-oxidic powders because of their chemical inertness. On the other hand, these organic solvents show only poor-dispersing performance, whereby high agglomeration rates of the particles were found.20,25 One reason for this behavior is the presence of polar surface groups, which make the powders incompatible with liquids with low dielectric constants. Although highly hydrophobic surfaces were achieved by coating the powders with amines (C18), the dispersion behavior of these particles in hexane remained very poor. Even an elongation of the hydrocarbon chain of the monoamine from 18 to 50 carbon atoms did not result in a higher dispersion quality. Only the highly branched 95 carbon atom succinimide was able to build up stable dispersions because of sufficient steric hindrance. Nevertheless, the protection capacity of this molecule is very low. For the achievement of both protection and dispersion qualities, mixtures of ethylenediamine (C18) and succinimide (C95) have to be used. Elemental analysis of the coated and noncoated powder after the exposure to air and water furnished evidence of the protecting capacity of ethylenediamine (C18). Oxygen uptake was significantly reduced in the case of both of the powders and both of the aging conditions. Additionally, only on the surface of the nonprotected particles, oxidation products, which were formed during contact with water, could be visualized by a TEM. The ethylenediamine coating acts as a compact barrier, which is also expressed by the TEM micrograph. Dispersing of the nanoscaled particles in aqueous media is an important condition for the fabrication of ceramic materials. Nonmodified TiN or SiCN could be dispersed in water for up to 3 days but exhibited a significant change in the surface composition because of oxidation and hydrolysis. Coating the particles with long-chain molecules suppressed the oxygen uptake, but the hydrophobic surface did not allow the formation of a stable dispersion in water. Water-soluble anionic surfactants have already been used for dispersing hydrophobized aluminum nitride particles in water.7 The role of the surfactant has been identified as a deflocculant and a hydrolysis depressor. It should bind to the hydrocarbon regions of the coating and introduce electric charges to the surface, which should cause repulsion forces to stabilize the aqueous dispersion of the powder. With cetylpyridinium chloride, a cationic surfactant was used in our paper, which guaranteed the maintenance of a stable dispersion for up to 2 weeks. Additionally, the oxygen uptake remained reduced compared to the nonmodified powder. An attempt to combine the protective capacity of long-chain amines with the dispersing capacity of a cationic group by coating the particles with bifunctional amine failed. The aqueous dispersion remained stable for up to 1 week, but the oxygen content was higher compared to the noncoated reference. Therefore, a two-step conditioning, the first in the organic solvent and the second in the surfactant-containing aqueous media, is necessary to prepare the processible dispersions of the non-oxidic nanoscaled ceramic powders. Conclusion Coating the nanoscaled non-oxidic powders with longchain amines and isocyanates from the organic solvent significantly reduces the oxygen uptake during the handling in water and air. This has been demonstrated with TiN and SiCN, whereby the usage of N,N′-bisoctadecylethylenediamine showed the best results. By the (25) Bleier, A. J. Am. Ceram. Soc. 1983, 66, C79-C81.

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addition of an ionic surfactant to the precoated powder, a stable aqueous dispersion can be obtained. This is an important condition for the further processing of the nonoxidic particles in water, for example, for the mixing with additives and for the drying and granulation. Acknowledgment. The authors thank the Deutsche Forschungsgemeinschaft (DFG) for supporting the work

Mu¨ ller et al.

by a grant (HE 378/28 and BO 1525/2) within the program “Handling of highly disperse powders”. Additional thanks to Martina Kreuzer and Ahmed Eljaouhari for excellent technical support, Rudolf Vasold for GC measurements, and Christiane Oestreich and Martina Mangler (both TU Freiberg) for examinations with the TEM. LA0356046

Protecting Nanoscaled Non-oxidic Particles from Oxygen Uptake by

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