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Optical properties of Ta(3-x)Zr(x)N(5-x)O(x) semiconductor pigments
Pergamon
Materials Research Bulletin 36 (2001) 1399 –1405
Optical properties of Ta(3-x)Zr(x)N(5-x)O(x) semiconductor pigments E. Guenthera, M. Jansenb,* a
Institut fu¨r Anorganische Chemie der Universita¨t Bonn, Gerhard-Domagk-Str.1, 53121 Bonn, Germany b Max-Planck-Institut fu¨r Festko¨rperforschung, Heisenbergstr. 1, 70569 Stuttgart, Germany (Refereed) Received 20 November 2000; accepted 5 February 2001
Abstract Tantalum zirconium oxynitride phases Ta(3-x)Zr(x)N(5-x)O(x) (0 ⱕ x ⱕ 0.66) having the Ta3N5 structure and Ta(1-x)Zr(x)N(1⫺x)O(1⫹x) (0 ⱕ x ⱕ 0.28) with the TaON structure have been prepared by solid state-gas syntheses. Oxide precursors have been prepared by a novel sol-gel process variant, using a mixture of acetic acid and acetic anhydride as solvent. The optical properties of the brilliantly coloured pigment quality Ta(3-x)Zr(x)N(5-x)O(x) (0 ⱕ x ⱕ 0.66) semiconducting oxynitrides have been examined by UV-VIS remission spectroscopy. Their applicability for an industrial scale production and a widespread use as brilliant, lightfast and inert inorganic colouring agents has been investigated. These pigments may be useful for colouring paints, plastics and medium temperature ceramic applications, replacing comtemporary toxic metal containing pigments. © 2001 Elsevier Science Ltd. All rights reserved. Keywords: A. Nitrides; A. Oxides; A. Semiconductors; B. Sol-gel chemistry
1. Introduction Semiconducting solids appear coloured when a selective absorption of visible light is related to an electronic interband transition. The brilliance and purity of a semiconductor’s colour is determined by the steepness of the absorption edge. By tuning the band gap of a semiconductor it is possible to determine its colour, and by optimizing the sharpness of the
* Corresponding author. Fax: ⫹49-711-689-1502. E-mail addresses: [email protected] (M. Jansen); [email protected] (E. Guenther). 0025-5408/01/$ – see front matter © 2001 Elsevier Science Ltd. All rights reserved. PII: S 0 0 2 5 - 5 4 0 8 ( 0 1 ) 0 0 6 3 2 - 8
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absorption edge an improved brilliance can be achieved. According to the concepts of Phillips-van Vechten [1,2] and Jørgensen [3] the width of a semiconductor’s band gap is connected with the difference of the electronegativities of its constituent atoms and an increasing amount of “softer” anions results in a bathochromic or red shift of the absorption edge. Thus, the colour of the extensively used cadmium sulfoselenide pigments CdSe(1-x)S(x) can be freely altered between brilliant yellow and red depending on the content of selenium, which is less electronegative than sulphur. The same holds true for the new system Ca(1-x)La(x)TaO(2-x)N(1⫹x) (0 ⱕ x ⱕ 1), [4,5,6,7] which offers the potential of replacing the cadmium sulphoselenide pigments presently under dispute for environmental and toxicological reasons [8,9,10,11]. By substituting the “softer” nitrogen atoms for the “harder” oxygen atoms, the band gap increases and the colour is shifted from red to yellow. Ta3N5 will be commercially produced in the near future [12,13,14] and an extension of the obtainable colours starting at the vermillion of pure Ta3N5 is desirable. For this purpose tantalum zirconium oxynitrides, which have been structurally investigated recently [15], are promising candidates. To this system the concepts of Phillips-van Vechten [1,2] and Jørgensen [3] are again applicable. By incorporating zirconium and oxygen into the Ta3N5 structure, the difference of the electronegativities increases, and the colour range of this pigment system is extended into the yellow-orange regime.
2. Experimental In order to obtain active precursors for the preparation of tantalum zirconium oxynitrides, tantalum zirconium oxide gels were prepared by a novel sol-gel process [14] using a mixture of acetic acid and acetic anhydride as a solvent to which metal alcoxides were added. This approach allows for a slow gelation process which insures an optimal distribution of the metal atoms within the resulting precursor tantalum zirconium oxide powders. The solvent consists of 90% (270 ml) absolute acetic acid and 10% (30 ml) acetic anhydride. This mixture keeps the water, which is promoting the condensation reaction of metal alcoxides at a low and constant concentration, thus allowing for a slow and well controlled condensation. Acetic anhydride acts as a buffer, which binds the water produced by esterification and prevents the condensation from proceeding too rapidly. In addition, acetic acid acts as a chelating agent, which surpresses premature coagulation of the evolving sol particles. When metal alcoxides are added to the acetic acid/acetic anhydride solution, a variety of reactions, especially ligand exchange and esterification takes place, thus producing in situ the amount of water needed for the hydrolysis of the alcoxide monomers. A mixture of 50 g tantalum pentaethoxide and the required amount of a 70% solution of zirconium tetra-n-propoxide in n-propanol [Table 1] was added to the solvent. These mixtures were kept at 50°C for 48 h. The tantalum zirconium oxide gels obtained were then dried in vacuum at 100°C. In order to remove all traces of solvent, the gels were then heated to 250°C in an argon flow. Finally, remaining organic residues were burned out by calcining the gels in an oxygen atmosphere at 600°C. The fine tantalum-zirconium-oxide powders obtained were still amorphous, which was checked by x-ray powder diffractometry. The Ta(3-x)Zr(x)N(5-x)O(x)- and Ta(1-x)Zr(x)N(1-x)O(1⫹x)-phases
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Table 1 Compositions of starting mixtures for the preparation of oxide gel precursors for the oxynitrides Ta(3-x)Zr(x)N(5-x)(O(x)(0 ⱕ x ⱕ 0.66) x
Ta(OEt)5
70% Sol. of Zr(OPr)4
Zr(OPr)4
Target Phase A3X5
0.25 0.50 0.66
50g 50g 50g
5.234 g 11.5151g 16.2393g
3.6639g 8.0606g 11.3675g
Ta2.75Zr0.25O0.25N4.75 Ta2.50Zr0.50O0.50N4.50 Ta2.34Zr0.66O0.66N4.34
were prepared by nitridation of the amorphous tantalum zirconium oxide powders with flowing ammonia at 950°C–1000°C at various flow rates during 36 h. The nitrididation products were examined by x-ray powder diffractometry using a STOE STADI P diffractometer. The inert samples were applied on aluminium sample holders under a Mylar foil and measured in transmission with Cu-K␣1 radiation. A step size of 0.02° in the 2 range of 15°–100° was employed to collect data over a 6 h period. The resulting profiles were of good quality and allowed for a determination of the cell parameters by Rietveld refinements. The program FULLPROF [16] was used for this purpose. UV-VIS remission spectra of the oxynitrides were measured with a VARIAN CARY 2400 spectrophotometer using an Ulbricht chamber and evaluated according to Kubelka and Munk [17,18]. The chemical and thermal stability of the oxynitrides was tested by Difference-Thermo-Analysis DTA, using a NETZSCH STA429. For this purpose, samples were heated to 1000°C in an oxygen athmosphere in order to determine their resistance against oxidation at elevated temperatures. The ignition point temperature sets the limit for applications at high temperatures. To examine the applicability of the oxynitride Ta(3-x)Zr(x)N(5-x)O(x)-phases as commercial pigments samples of 1 g were embedded in 3 g of PVC and extruded to foils. These foils were used to record UV-VIS remission spectra which allowed for the determination of CIELab [19] colour coordinates.
3. Results and discussion Phases of the composition Zr(x)Ta(1-x)(O,N)y (0 ⱕ x ⱕ 1) have recently been described by Grins et al. who focused mainly on zirconium rich phases having the baddeleyite type Table 2 Absorption edges, band gaps and lattice parameters as determined by Rietveld analyses of the oxynitride phases Ta(3-x)Zr(x)N(5-x)O(x) with Ta3N5-structure [20] [21] (Spacegroup Cmcm) Lattice constants
x ⫽ 0.00
x ⫽ 0.25
x ⫽ 0.50
x ⫽ 0.60
a [Å] b [Å] c [Å] Abs. Edge [nm] Gap-Energy [eV]
3.8870 (2) 10.2143 (7) 10.2642 (6) 581 2.130
3.8985 (3) 10.2348 (8) 10.2900 (8) 570 2.175
3.9027 (2) 10.2445 (7) 10.3025 (6) 560 2.214
3.9083 (2) 10.2500 (5) 10.3127 (4) 555 2.234
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Table 3 Absorption edges, band gaps and lattice parameters as determined by Rietveld analyses of the oxynitride phases Ta(1-x)Zr(x)N(1-x)O(1⫹x) with TaON structure [22] [23] (Spacegroup P21/c) Lattice constants
x ⫽ 0.000
x ⫽ 0.0833
x ⫽ 0.1667
x ⫽ 0.28
a [Å] b [Å] c [Å]  [°] Abs. Edge [nm] Gap-Energy [eV]
4.9682 (3) 5.0368 (3) 5.1849 (3) 99.602 (3) 465 2.666
4.9789 (2) 5.0459 (2) 5.1923 (2) 99.599 (2) 461 2.690
4.9924 (3) 5.0597 (3) 5.2046 (3) 99.559 (3) 456 2.720
5.0080 (8) 5.0723 (8) 5.2171 (8) 99.576 (7) 451 2.743
structure [15]. Tantalum rich single phase products with Ta3N5 structure, too, have been briefly mentioned but a closer evaluation of their colouristic properties was not undertaken. We have reinvestigated the tantalum rich zirconium tantalum oxinitrides with Ta3N5structure [20,21], and have found that over a considerable homogenity range a solid solution exists. There are two crucial preconditions to be considered during synthesis of the oxynitride phases Ta(3-x)Zr(x)N(5-x)O(x) (0 ⱕ x ⱕ 0.60): 1. a homogenous distribution of tantalum and zirconium in the oxidic precursor and 2. a sufficiently high flow of ammonia during nitridation. Otherwise the products will consist of less nitrided, pale yellow phases of the composition Ta(1-x)Zr(x)N(1-x)O(1⫹x) (0 ⱕ x ⱕ 0.28) having TaON- [22,23] or m-ZrO2structure [24]. These observations again emphasize the dependence of the nitridation power of ammonia on its flow rate during nitridation [14,25,26]. If the tantalum zirconium oxide precursors have a zirconium content x exceeding the limits given in the compositions above, this excessive amount of zirconium will be deposited after nitridation as monoclinic ZrO2 (Baddeleyite), besides the oxynitride phase. The cell parameters of both oxynitride phases Ta(3-x)Zr(x)N(5-x)O(x) (0 ⱕ x ⱕ 0.60) and Ta(1-x)Zr(x)N(1-x)O(1⫹x) (0 ⱕ x ⱕ 0.28) increase linearly with growing zirconium content x according to Ve´gard’s rule [Table 2, Table 3]. This indicates miscibility of m-ZrO2 and Ta3N5 or TaON respectively, within the boundaries found. Taking into account that the crystal structures of TaON and m-ZrO2 are closely related to each other, it is surprising that the amounts of zirconium and oxygen which can be incorporated into the TaON-structure phases Ta(1-x)Zr(x)N(1-x)O(1⫹x) are limited to the small amounts found. This might be due to the ordered anionic sublattice of TaON, which consists of alternating layers of oxygen and nitrogen atoms perpendicular to (100), which might only allow for a limited substitution of nitrogen by oxygen at the respective crystallographic sites. These findings are different from those of Grins et al. [15] who have reported on a complete miscibility of m-ZrO2 and TaON. In contrast, we always have identified m-ZrO2 (Baddeleyite) besides the maximum zirconium containing oxynitride phase Ta0.72Zr0.28N0.72O1.28, when oxide gels with a content of zirconium content exceeding 0.28 were nitrided in weak ammonia flows. The powder diffraction lines caused by the excess m-ZrO2 can be clearly distinguished from those of the isotypic Ta0.72Zr0.28N0.72O1.28 because of their lower 2 values. The band gaps of both oxynitride phases increase linearly with the zirconium content [Table 2, Table 3, Figure 1, Figure 2]. Due to the fact, that the Ta(1-x)Zr(x)N(1-x)O(1⫹x)-phases all have a pale yellow colour, this tendency cannot be detected by the naked eye, however, has been confirmed by UV-VIS-spectroscopy. On the contrary, the colour shift with increas-
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Fig. 1. Linear relation between the zirconium content x and the Gap-Energy Eg [eV] within the oxynitride semiconducting pigments Ta(3-x)Zr(x)N(5-x)O(x).
Fig. 2. Linear relation between the zirconium content x and the Gap-Energy Eg [eV] within the oxynitride semiconducting pigments Ta(1-x)Zr(x)N(1-x)O(1⫹x).
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Table 4 CIELab colour coordinates of the pigment system Ta(3-x)Zr(x)N(5-x)O(x). The CIELab values of industrially produced cadmium sulfoselenide pigment cadmium orange is given for comparison [27] CIELab-coordinates
x ⫽ 0.00
x ⫽ 0.25
x ⫽ 0.50
Cd-orange [27]
L* (Lightness) a* (⫺): green, (⫹): red b* (⫺): blue, (⫹): yellow
39.55 ⫹42.69 ⫹50.09
46.97 ⫹44.02 ⫹58.03
55.59 ⫹49.08 ⫹69.88
40 ⫹55.00 ⫹57.00
ing x of the orange Ta(3-x)Zr(x)N(5-x)O(x)-phases is clearly visible. The CIELab colour values [19] of the pigment quality oxynitrides Ta(3-x)Zr(x)N(5-x)O(x) were extracted from the remission spectra [Table 4]. The CIELab values [19] of commercial cadmium orange pigment are given for comparison [27]. For the yellow to orange regime, the coloristic properties of the tantalum zirconium oxynitrides are comparable to those of the established and widely used cadmium sulfoselenide pigments. Thus, these oxynitride pigments have the potential for replacing cadmium orange pigments. The particle size distribution in samples of the pigment quality oxynitrides phases Ta(3-x)Zr(x)N(5-x)O(x) (0 ⱕ x ⱕ 0.66) obtained by solid state - gas synthesis has been investigated and shows a distribution with 90% of the particles being smaller than 45.14 m, 50% smaller 12.7 m and 10% smaller 0.4 m. The colour of these samples did not change significantly after reducing the particle size by grinding in a ball mill. Thus, an extension of the colour range into the yellow regime by decreasing the particle size of yellow-orange Ta2.34Zr0.66N4.34O0.66 seems to be unlikely. The colour properties of the pale yellow oxynitrides Ta(1-x)Zr(x)N(1-x)O(1⫹x) were not further investigated due to their obvious poor coloristic qualities. The oxynitride pigments Ta(3-x)Zr(x)N(5-x)O(x) (0 ⱕ x ⱕ 0.66) can withstand temperatures up to 600°C in air without being oxidized. Thus, these pigments are applicable in glass colours, which are processed at temperatures around 600°C. Below this temperature, the pigments are inert, even in an oxygen atmosphere. Consequently, a variety of applications such as the colouring of thermoplasts, rubber, paints and enamels can be envisaged.
4. Conclusion The semiconducting pigment system Ta(3-x)Zr(x)N(5-x)O(x) (0 ⱕ x ⱕ 0.66) offers good prospects to become an established and widely used colouring agent class due to its good coloristic, manufacturing and processing qualities. Regarding the increasing concerns [8,9, 10,11] about toxic metal containing semiconductor pigments these new oxynitride pigments might be an environmentally sound solution.
Acknowledgments The authors are grateful to Dr. H.P. Letschert of DMC2 at Hanau-Wolfgang, Germany for conducting the CIELab value and particle size measurements and his valuable practical support.
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References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27]
J.C. Phillips, Science 169 (1970) 1035. J.A. van Vechten, J.C. Phillips, Phys. Rev. B2 (1971) 2160. C.K. Jørgensen, Oxidation Numbers and Oxidation States, Springer, Ch. 7 (1969). M. Jansen, H.P. Letschert, D. Speer, European Patent: 627382, Oxonitrides of the Formula LnTaON2 with enhanced Brightness and a Process for their Production and Use, 1995. M. Jansen, H.P. Letschert, European Patent: 697373, Oxonitrides with adjustable Colour, a Process for their Production and their Use, 1995. M. Jansen, H.P. Letschert, Nature 404 (2000) 980 –982. H.P. Letschert, Dissertation, Bonn, 1995. EEC 1983, Council Directive, EEC 83/514, Sept. 26, 1983, On the Limits for Cadmium in Waste Water. EEC, Council Directive, EEC 84/500/EC, 1984. EEC, Council Directive, EEC 91/338, 1991. European Commission, On the Use of Colourants in Plastic Materials coming in Contact with Food, Resolution AP (89) 1, Sept. 13th, 1989. M. Jansen, E. Guenther, H.P. Letschert, German Patent: 199 07 618.9, Verfahren zur Herstellung von gelben und roten Pigmenten auf der Basis von Nitriden und Oxidnitriden, 1999. M. Jansen, H.P. Letschert, D. Speer, European Patent: 592876, Method of Producing Tantalum(V) Nitride and its Use, 1993. E. Guenther, Dissertation, Bonn, 2000. J. Grins, P.O. Ka¨ll, G. Svensson, J. Mater. Chem. 4 (8) (1994) 1293. J. Rodriguez-Carvajal, FULLPROF©, Ver. 3.1, Jul 1995, LLB-JRC, 1995. P. Kubelka, F. Munk, Z. Tech. Phys. 12 (1931) 512. P. Kubelka, J. Opt. Soc. Amer. 38 (1948) 448. M. Richter, Einfu¨hrung in die Farbmetrik, Walter de Gruyter, Berlin-New York, 1976. J. Stra¨hle, Z. Anorg. Allg. Chemie 402 (1973) 47. N.E. Brese, M. O’Keefe, Acta Cryst. B47 (1991) 192–197. B. Armitage, B.E.F. Fender, Acta Cryst. B30 (1974) 8. R. Weishaupt, J. Stra¨hle, Z. Anorg. Allg. Chem. 429 (1977) 26. A. Gualtieri, P. Norby, J. Hanson, J. Hriljac, J. Appl. Cryst. 29 (1996) 707–713. I. Elphinstone, A. Hendry, British Ceramic Proceedings (Fabr. Technol.), 45 (1989) 15. M. Katsura, J. Alloys Comp. 182 (1992) 91. R.A. Eppler, Ceram. Bull. 6 (11) (1987) 1600 –1604.
Materials Research Bulletin 36 (2001) 1399 –1405
Optical properties of Ta(3-x)Zr(x)N(5-x)O(x) semiconductor pigments E. Guenthera, M. Jansenb,* a
Institut fu¨r Anorganische Chemie der Universita¨t Bonn, Gerhard-Domagk-Str.1, 53121 Bonn, Germany b Max-Planck-Institut fu¨r Festko¨rperforschung, Heisenbergstr. 1, 70569 Stuttgart, Germany (Refereed) Received 20 November 2000; accepted 5 February 2001
Abstract Tantalum zirconium oxynitride phases Ta(3-x)Zr(x)N(5-x)O(x) (0 ⱕ x ⱕ 0.66) having the Ta3N5 structure and Ta(1-x)Zr(x)N(1⫺x)O(1⫹x) (0 ⱕ x ⱕ 0.28) with the TaON structure have been prepared by solid state-gas syntheses. Oxide precursors have been prepared by a novel sol-gel process variant, using a mixture of acetic acid and acetic anhydride as solvent. The optical properties of the brilliantly coloured pigment quality Ta(3-x)Zr(x)N(5-x)O(x) (0 ⱕ x ⱕ 0.66) semiconducting oxynitrides have been examined by UV-VIS remission spectroscopy. Their applicability for an industrial scale production and a widespread use as brilliant, lightfast and inert inorganic colouring agents has been investigated. These pigments may be useful for colouring paints, plastics and medium temperature ceramic applications, replacing comtemporary toxic metal containing pigments. © 2001 Elsevier Science Ltd. All rights reserved. Keywords: A. Nitrides; A. Oxides; A. Semiconductors; B. Sol-gel chemistry
1. Introduction Semiconducting solids appear coloured when a selective absorption of visible light is related to an electronic interband transition. The brilliance and purity of a semiconductor’s colour is determined by the steepness of the absorption edge. By tuning the band gap of a semiconductor it is possible to determine its colour, and by optimizing the sharpness of the
* Corresponding author. Fax: ⫹49-711-689-1502. E-mail addresses: [email protected] (M. Jansen); [email protected] (E. Guenther). 0025-5408/01/$ – see front matter © 2001 Elsevier Science Ltd. All rights reserved. PII: S 0 0 2 5 - 5 4 0 8 ( 0 1 ) 0 0 6 3 2 - 8
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E. Guenther, M. Jansen / Materials Research Bulletin 36 (2001) 1399 –1405
absorption edge an improved brilliance can be achieved. According to the concepts of Phillips-van Vechten [1,2] and Jørgensen [3] the width of a semiconductor’s band gap is connected with the difference of the electronegativities of its constituent atoms and an increasing amount of “softer” anions results in a bathochromic or red shift of the absorption edge. Thus, the colour of the extensively used cadmium sulfoselenide pigments CdSe(1-x)S(x) can be freely altered between brilliant yellow and red depending on the content of selenium, which is less electronegative than sulphur. The same holds true for the new system Ca(1-x)La(x)TaO(2-x)N(1⫹x) (0 ⱕ x ⱕ 1), [4,5,6,7] which offers the potential of replacing the cadmium sulphoselenide pigments presently under dispute for environmental and toxicological reasons [8,9,10,11]. By substituting the “softer” nitrogen atoms for the “harder” oxygen atoms, the band gap increases and the colour is shifted from red to yellow. Ta3N5 will be commercially produced in the near future [12,13,14] and an extension of the obtainable colours starting at the vermillion of pure Ta3N5 is desirable. For this purpose tantalum zirconium oxynitrides, which have been structurally investigated recently [15], are promising candidates. To this system the concepts of Phillips-van Vechten [1,2] and Jørgensen [3] are again applicable. By incorporating zirconium and oxygen into the Ta3N5 structure, the difference of the electronegativities increases, and the colour range of this pigment system is extended into the yellow-orange regime.
2. Experimental In order to obtain active precursors for the preparation of tantalum zirconium oxynitrides, tantalum zirconium oxide gels were prepared by a novel sol-gel process [14] using a mixture of acetic acid and acetic anhydride as a solvent to which metal alcoxides were added. This approach allows for a slow gelation process which insures an optimal distribution of the metal atoms within the resulting precursor tantalum zirconium oxide powders. The solvent consists of 90% (270 ml) absolute acetic acid and 10% (30 ml) acetic anhydride. This mixture keeps the water, which is promoting the condensation reaction of metal alcoxides at a low and constant concentration, thus allowing for a slow and well controlled condensation. Acetic anhydride acts as a buffer, which binds the water produced by esterification and prevents the condensation from proceeding too rapidly. In addition, acetic acid acts as a chelating agent, which surpresses premature coagulation of the evolving sol particles. When metal alcoxides are added to the acetic acid/acetic anhydride solution, a variety of reactions, especially ligand exchange and esterification takes place, thus producing in situ the amount of water needed for the hydrolysis of the alcoxide monomers. A mixture of 50 g tantalum pentaethoxide and the required amount of a 70% solution of zirconium tetra-n-propoxide in n-propanol [Table 1] was added to the solvent. These mixtures were kept at 50°C for 48 h. The tantalum zirconium oxide gels obtained were then dried in vacuum at 100°C. In order to remove all traces of solvent, the gels were then heated to 250°C in an argon flow. Finally, remaining organic residues were burned out by calcining the gels in an oxygen atmosphere at 600°C. The fine tantalum-zirconium-oxide powders obtained were still amorphous, which was checked by x-ray powder diffractometry. The Ta(3-x)Zr(x)N(5-x)O(x)- and Ta(1-x)Zr(x)N(1-x)O(1⫹x)-phases
E. Guenther, M. Jansen / Materials Research Bulletin 36 (2001) 1399 –1405
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Table 1 Compositions of starting mixtures for the preparation of oxide gel precursors for the oxynitrides Ta(3-x)Zr(x)N(5-x)(O(x)(0 ⱕ x ⱕ 0.66) x
Ta(OEt)5
70% Sol. of Zr(OPr)4
Zr(OPr)4
Target Phase A3X5
0.25 0.50 0.66
50g 50g 50g
5.234 g 11.5151g 16.2393g
3.6639g 8.0606g 11.3675g
Ta2.75Zr0.25O0.25N4.75 Ta2.50Zr0.50O0.50N4.50 Ta2.34Zr0.66O0.66N4.34
were prepared by nitridation of the amorphous tantalum zirconium oxide powders with flowing ammonia at 950°C–1000°C at various flow rates during 36 h. The nitrididation products were examined by x-ray powder diffractometry using a STOE STADI P diffractometer. The inert samples were applied on aluminium sample holders under a Mylar foil and measured in transmission with Cu-K␣1 radiation. A step size of 0.02° in the 2 range of 15°–100° was employed to collect data over a 6 h period. The resulting profiles were of good quality and allowed for a determination of the cell parameters by Rietveld refinements. The program FULLPROF [16] was used for this purpose. UV-VIS remission spectra of the oxynitrides were measured with a VARIAN CARY 2400 spectrophotometer using an Ulbricht chamber and evaluated according to Kubelka and Munk [17,18]. The chemical and thermal stability of the oxynitrides was tested by Difference-Thermo-Analysis DTA, using a NETZSCH STA429. For this purpose, samples were heated to 1000°C in an oxygen athmosphere in order to determine their resistance against oxidation at elevated temperatures. The ignition point temperature sets the limit for applications at high temperatures. To examine the applicability of the oxynitride Ta(3-x)Zr(x)N(5-x)O(x)-phases as commercial pigments samples of 1 g were embedded in 3 g of PVC and extruded to foils. These foils were used to record UV-VIS remission spectra which allowed for the determination of CIELab [19] colour coordinates.
3. Results and discussion Phases of the composition Zr(x)Ta(1-x)(O,N)y (0 ⱕ x ⱕ 1) have recently been described by Grins et al. who focused mainly on zirconium rich phases having the baddeleyite type Table 2 Absorption edges, band gaps and lattice parameters as determined by Rietveld analyses of the oxynitride phases Ta(3-x)Zr(x)N(5-x)O(x) with Ta3N5-structure [20] [21] (Spacegroup Cmcm) Lattice constants
x ⫽ 0.00
x ⫽ 0.25
x ⫽ 0.50
x ⫽ 0.60
a [Å] b [Å] c [Å] Abs. Edge [nm] Gap-Energy [eV]
3.8870 (2) 10.2143 (7) 10.2642 (6) 581 2.130
3.8985 (3) 10.2348 (8) 10.2900 (8) 570 2.175
3.9027 (2) 10.2445 (7) 10.3025 (6) 560 2.214
3.9083 (2) 10.2500 (5) 10.3127 (4) 555 2.234
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Table 3 Absorption edges, band gaps and lattice parameters as determined by Rietveld analyses of the oxynitride phases Ta(1-x)Zr(x)N(1-x)O(1⫹x) with TaON structure [22] [23] (Spacegroup P21/c) Lattice constants
x ⫽ 0.000
x ⫽ 0.0833
x ⫽ 0.1667
x ⫽ 0.28
a [Å] b [Å] c [Å]  [°] Abs. Edge [nm] Gap-Energy [eV]
4.9682 (3) 5.0368 (3) 5.1849 (3) 99.602 (3) 465 2.666
4.9789 (2) 5.0459 (2) 5.1923 (2) 99.599 (2) 461 2.690
4.9924 (3) 5.0597 (3) 5.2046 (3) 99.559 (3) 456 2.720
5.0080 (8) 5.0723 (8) 5.2171 (8) 99.576 (7) 451 2.743
structure [15]. Tantalum rich single phase products with Ta3N5 structure, too, have been briefly mentioned but a closer evaluation of their colouristic properties was not undertaken. We have reinvestigated the tantalum rich zirconium tantalum oxinitrides with Ta3N5structure [20,21], and have found that over a considerable homogenity range a solid solution exists. There are two crucial preconditions to be considered during synthesis of the oxynitride phases Ta(3-x)Zr(x)N(5-x)O(x) (0 ⱕ x ⱕ 0.60): 1. a homogenous distribution of tantalum and zirconium in the oxidic precursor and 2. a sufficiently high flow of ammonia during nitridation. Otherwise the products will consist of less nitrided, pale yellow phases of the composition Ta(1-x)Zr(x)N(1-x)O(1⫹x) (0 ⱕ x ⱕ 0.28) having TaON- [22,23] or m-ZrO2structure [24]. These observations again emphasize the dependence of the nitridation power of ammonia on its flow rate during nitridation [14,25,26]. If the tantalum zirconium oxide precursors have a zirconium content x exceeding the limits given in the compositions above, this excessive amount of zirconium will be deposited after nitridation as monoclinic ZrO2 (Baddeleyite), besides the oxynitride phase. The cell parameters of both oxynitride phases Ta(3-x)Zr(x)N(5-x)O(x) (0 ⱕ x ⱕ 0.60) and Ta(1-x)Zr(x)N(1-x)O(1⫹x) (0 ⱕ x ⱕ 0.28) increase linearly with growing zirconium content x according to Ve´gard’s rule [Table 2, Table 3]. This indicates miscibility of m-ZrO2 and Ta3N5 or TaON respectively, within the boundaries found. Taking into account that the crystal structures of TaON and m-ZrO2 are closely related to each other, it is surprising that the amounts of zirconium and oxygen which can be incorporated into the TaON-structure phases Ta(1-x)Zr(x)N(1-x)O(1⫹x) are limited to the small amounts found. This might be due to the ordered anionic sublattice of TaON, which consists of alternating layers of oxygen and nitrogen atoms perpendicular to (100), which might only allow for a limited substitution of nitrogen by oxygen at the respective crystallographic sites. These findings are different from those of Grins et al. [15] who have reported on a complete miscibility of m-ZrO2 and TaON. In contrast, we always have identified m-ZrO2 (Baddeleyite) besides the maximum zirconium containing oxynitride phase Ta0.72Zr0.28N0.72O1.28, when oxide gels with a content of zirconium content exceeding 0.28 were nitrided in weak ammonia flows. The powder diffraction lines caused by the excess m-ZrO2 can be clearly distinguished from those of the isotypic Ta0.72Zr0.28N0.72O1.28 because of their lower 2 values. The band gaps of both oxynitride phases increase linearly with the zirconium content [Table 2, Table 3, Figure 1, Figure 2]. Due to the fact, that the Ta(1-x)Zr(x)N(1-x)O(1⫹x)-phases all have a pale yellow colour, this tendency cannot be detected by the naked eye, however, has been confirmed by UV-VIS-spectroscopy. On the contrary, the colour shift with increas-
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Fig. 1. Linear relation between the zirconium content x and the Gap-Energy Eg [eV] within the oxynitride semiconducting pigments Ta(3-x)Zr(x)N(5-x)O(x).
Fig. 2. Linear relation between the zirconium content x and the Gap-Energy Eg [eV] within the oxynitride semiconducting pigments Ta(1-x)Zr(x)N(1-x)O(1⫹x).
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Table 4 CIELab colour coordinates of the pigment system Ta(3-x)Zr(x)N(5-x)O(x). The CIELab values of industrially produced cadmium sulfoselenide pigment cadmium orange is given for comparison [27] CIELab-coordinates
x ⫽ 0.00
x ⫽ 0.25
x ⫽ 0.50
Cd-orange [27]
L* (Lightness) a* (⫺): green, (⫹): red b* (⫺): blue, (⫹): yellow
39.55 ⫹42.69 ⫹50.09
46.97 ⫹44.02 ⫹58.03
55.59 ⫹49.08 ⫹69.88
40 ⫹55.00 ⫹57.00
ing x of the orange Ta(3-x)Zr(x)N(5-x)O(x)-phases is clearly visible. The CIELab colour values [19] of the pigment quality oxynitrides Ta(3-x)Zr(x)N(5-x)O(x) were extracted from the remission spectra [Table 4]. The CIELab values [19] of commercial cadmium orange pigment are given for comparison [27]. For the yellow to orange regime, the coloristic properties of the tantalum zirconium oxynitrides are comparable to those of the established and widely used cadmium sulfoselenide pigments. Thus, these oxynitride pigments have the potential for replacing cadmium orange pigments. The particle size distribution in samples of the pigment quality oxynitrides phases Ta(3-x)Zr(x)N(5-x)O(x) (0 ⱕ x ⱕ 0.66) obtained by solid state - gas synthesis has been investigated and shows a distribution with 90% of the particles being smaller than 45.14 m, 50% smaller 12.7 m and 10% smaller 0.4 m. The colour of these samples did not change significantly after reducing the particle size by grinding in a ball mill. Thus, an extension of the colour range into the yellow regime by decreasing the particle size of yellow-orange Ta2.34Zr0.66N4.34O0.66 seems to be unlikely. The colour properties of the pale yellow oxynitrides Ta(1-x)Zr(x)N(1-x)O(1⫹x) were not further investigated due to their obvious poor coloristic qualities. The oxynitride pigments Ta(3-x)Zr(x)N(5-x)O(x) (0 ⱕ x ⱕ 0.66) can withstand temperatures up to 600°C in air without being oxidized. Thus, these pigments are applicable in glass colours, which are processed at temperatures around 600°C. Below this temperature, the pigments are inert, even in an oxygen atmosphere. Consequently, a variety of applications such as the colouring of thermoplasts, rubber, paints and enamels can be envisaged.
4. Conclusion The semiconducting pigment system Ta(3-x)Zr(x)N(5-x)O(x) (0 ⱕ x ⱕ 0.66) offers good prospects to become an established and widely used colouring agent class due to its good coloristic, manufacturing and processing qualities. Regarding the increasing concerns [8,9, 10,11] about toxic metal containing semiconductor pigments these new oxynitride pigments might be an environmentally sound solution.
Acknowledgments The authors are grateful to Dr. H.P. Letschert of DMC2 at Hanau-Wolfgang, Germany for conducting the CIELab value and particle size measurements and his valuable practical support.
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