Synthesis of Group IVB Metals Oxicarbides by Carboreduction Reactions

The metals of the group IV B (Ti, Zr, Hf) present a series of carbides and oxicarbides with scientific and technological interest. Many of these compounds belong to the subsystem "MO - MC" of the pseudoternary "MO - MN - MC" system (where M = Ti, Zr or Hf). In this work carboreduction reactions of TiO2 and ZrO2 were performed in argon atmosphere, using temperatures from 1250° to 1650° and reaction times of 120 min. The oxicarbides obtained were in the range TiC0.16O0.84 to TiC0.73O0.27 and ZrC0.46O0.54 to ZrC0.90O0.10. respectively. The reaction products were characterized by X-ray diffraction (XRD), with the calculation of their cell constants by means of the Rietveld method. Scanning Electron Microscopy (SEM) was used in the characterization of powdered materials. Additionally, the carborreduction reaction was followed by weight loss.


Introduction
The group IV B metals, as titanium (Ti) and zirconium (Zr), present pseudoternary systems of the type "MO -MN -MC" (where M = metal).Both systems show substitutional solid solutions which correspond to extensive monophasic zones.The solutions, named M(C,N,O), have the same cubic crystalline structure (S.G.:Fm3m) that MN, MC and MO phases in the corners of diagrams.The unit-cell parameter (a o ) of M(C,N,O) phase depends on the MC, MO and MN content.
Figure 1 shows the pseudoternary phase diagram "TiO-TiN-TiC" proposed by Neumann et al. 1 .The monophasic zone covers the whole diagram at temperatures higher than 1100 °C.This zone corresponds to the Ti(C,N,O) phase.The cell parameter a o varies between 4.12 Å and 4.32 Å 1 .
The pseudoternary system "ZrO -ZrN -ZrC" was studied by Constant et al. 2 .These authors observed a considerable solubility of "ZrO" (added as an equimolar mix of ZrO 2 + Zr) in ZrC, ZrN and Zr(C,N).Figure 2 shows the limit of monophasic zone at 1600 °C.The a o parameter of the phase Zr(C,O,N) changes between 4.570 Å and 4.692 Å.
The monophasic zones cover in part the MO-MC axes in both systems.Thus, the subsystems "MO-MC" present oxycarbides M(C,O).These oxycarbides, Ti(C,O) and Zr(C,O), have the same cubic structure of M(C,N,O) with a o depending on MC and MO content.The values of a o increase with the MC content , the highest value corresponds to MC pure phase.
The synthesis of these compounds is usually obtained by TiO, TiC or ZrO 2, Zr, ZrC reactions in argon atmosphere or vacuum 3,4 .Carborreduction reactions of the MO 2 (TiO 2 -anatasa, ZrO 2 -badeleyite) oxides constitute an alternative method to obtain these phases.This method is within the present tendency to use abundant and cheap raw materials to obtain ceramic powders (oxides as well as carbides).Ceramics are manufactured starting from well characterized raw materials and then purified in order to control the degree of impurities.
The Rietveld method allows to properly characterize crystalline phases by x-ray diffraction (XRD).This method was developed by Hugo Rietveld in 1969 5 , in order to refine crystalline structures using neutron diffraction data.At the present time, it is also used to perform analysis of structure and crystalline defects, reticular parameter measurement and quantitative analysis in X-ray diffraction.
In this work, the carborreduction reactions of anatasa and badeleyite were studied by XRD.Reaction products were characterized by XRD (Rietveld) and scanning electron microscopy (SEM).The weight loss was also measured.

Materials And Methods
Raw materials used were badeleyite Riedel-de Haën from Germany with more than 99% (ZrO 2 + HfO 2 ) content, anatase Fluka from Switzerland with more than 99% TiO 2 content and carbon black (carbon 97%, ash 1%, volatiles 2% and a specific surface area BET = 45 m 2 /g).All powders have an average particle size lower than 5 mm.The argon used contains less than 5 ppm of O 2 and H 2 O.
Samples were prepared by wet mixing of the calculated amounts of regents, and then they were dried and pressed at 39 MPa into cylinders of 2 mm in height and 10 mm in diameter.Samples were called by a letter and the C/MO 2 relation.Letter A was used for samples prepared from anatase and letter B for samples from badeleyite.Thus, we obtain the samples A200 (C/TiO 2 = 0.200), A280, A360, A430 and A500 and the samples B180 (C/ZrO 2 = 0.180), B210, B240, B300 and B450.
Reactions were performed in a horizontal alumina reactor with argon flowing through it.The pressure used was 0.05 Mpa above the atmospheric pressure, and the flow was 1 l/min.Experiments were carried out within the range of 1250 to 1650 °C with reaction times of 120 min and the temperature slopes were 10 °C/min upward as well as downward.The Ar flow was kept during cooling up to 200 °C.
Crystalline phases were characterized by X-ray diffraction with a Philips 3020 Goniometer with a PW 3710 controller, Cu-Kα, Ni filter, 40 kV-20 mA.The scanning was made between 10° and 75° with step size of 0.02° and a step counting time of 2 sec.Some samples were scanned with 40 kV-30 mA and a step counting time of 4 sec.The set of divergence, receiving and scattering slits were 1°, 0.2°, 1° and no monochromator was used.The unit cell parameters were refined using the Rietveld refinement FULLPROF 6 program.The starting crystallographic data for each phase were taken from literature [7][8][9][10][11] .The refining sequence began with the adjustment of the shift in 2θ due to vertical sample displacement and the background.To do this, elemental Si was added to the samples as internal standard.Then, for each phase, the scale factors, the cell constants, the parameters for calculation of the full-width-athalf-maximum (FWHM), the profile (pseudo Voight) function coefficients, the preferred orientation parameter for the March function, were sequentially refined.Quantitative phase analysis was also performed by FULLPROF program based on the scale factors of each phase.
The Zr(C,O) and Ti(C,O) crystallite sizes were determined by the full-width-at-half-maximum of peak (200) for  obtained after refinement, using the Scherrer equation and the PC-APD (PW 1877) Analytical Powder Diffraction Software, Version 3.6.Weight losses were measured by weighing the samples before and after the reaction process.
The reaction products were also studied by scanning electron microscopy (SEM) using a Philips 505 equipment.

Results And Discussion
The following theoretical equations describe the reactions carried out in these systems: Table 1 shows the results obtained in the TiO 2 carborreduction.Estimated standard deviations of weight percents were derived from the estimated standard deviations on individual scale factors for the respective phases, and other error contributions were not included.
The Ti (C,O) content increase with the temperature and carbon content being the unique product obtained at high temperatures and high C content.
Table 2 shows the values of a o obtained by the Rietveld method for the Ti(C,O) phases.Ranges of values appear in some cases due to the necessity of using two or more Ti(C,O)   phases in order to obtain a good fitting of peak widths.4).This curve, from Neuman et al. 1 , was determined from samples prepared by reaction of pure TiO and TiC.
The values of a o tend to be higher with the increase of reaction temperature.It is not possible to determine a defined tendency of a o values changes with carbon content.This little a o variation is due to the low slope in Fig. 4 where big changes in composition produce little shift of the unit cell parameter.Some titanium sub-oxides were also detected.These  12 .These oxides become more important at low temperature and in samples with low carbon content.In Table 3 the refined unit-cell parameters of these phases are in good agreement with published data [8][9][10][11] .
The weight losses showed in Table 4 allow us to see that the reaction progresses with the increase of carbon content and temperature, being the obtained tendency consistent with the detected crystalline phases, within the margin of experimental error.
Badeleyite carborreduction results (reaction B) for the 5 studied samples are shown in Tables 5 and 6.All samples have enough carbon to obtain an oxicarbide as unique phase.Theoretically oxicarbide compositions would be between ZrC 0.45 O 0.55 for B180 and ZrC for B300.
The zirconium phases obtained were: Zr(C,O) phase, monoclinic ZrO 2 (badeleyite) and tetragonal ZrO 2 in lower proportions.Figure 5a  Tetragonal zirconia content was between 1 and 3 wt.% in all samples containing ZrO 2 presenting similar behavior to the badeleyite.
Zr(C,O) increases with the increase of temperature and carbon content.It is the principal phase at a temperature of 1600° or higher.

Figure 1 .
Figure 1.Pseudoternary diagram of the TiN-TiC-TiO system at 1100 °C (from Neumann et al.) 1 .The cell parameters a o of Ti(C,N,O) are indicated.The lines separate zones of similar a o .

Figure 2 .
Figure 2. Pseudoternary diagram of the ZrN -ZrO -ZrC system at 1600 °C (from Constant et al.) 2 .The unit-cell parameters a o of Zr(C,N,O) are indicated monophase zone and polyphase zone.
where a, b, c and d are stoichiometric coefficients (0 ≤ a ≤ 1, 1 ≤ b ≤ 2, 0.40 ≤ c ≤ 1 and 1.4 ≤ d ≤ 2).The phases {TiC a O 2-b }and {ZrC c O 2-d } are metal oxicarbides belonging to the MC-MO edge of the MC-MO-MN system.These phases are called Ti(C,O) and Zr(C,O) respectively.

*
Values in parenthesis represent estimated standard deviations in the last quoted place.
Figure 3a shows the final Rietveld plot for sample A360 treated at 1450 and Fig. 3b is part of the same graphic amplified to show the overlap of Ti(C,O) peaks belonging to different phases.In Fig. 3b, individual calculated profiles for both Ti(C,O) phases are also shown.Within the temperature range used in these experiments, the Ti (C, O) cubic phase is throughout the TiO-TiC axis (Fig. 4) as well as the Ti (C, N, O) cubic phases cover the triangular diagram completely (Fig. 1).The carbon content of samples A would allow to obtain the complete composition range (100% TiO to 100%TiC).In Table 2, a o values of Ti(C,O) are between 4.234 Å and 4.308 Å.These values correspond to a composition range between TiC 0.16 O 0.84 and

Figure 4 .
Figure 4. Unit-cell parameter a o vs. Ti(C,O) composition taken from Neumann et al. 1 .
shows the final Rietveld plot for sample B450 treated at 1600 °C and Fig. 5b is part of the same graphic amplified to show the overlap of Zr(C,O) peaks belonging to different phases.In Fig. 5b, individual calculated profiles for all Zr(C,O) phases are also shown.Badeleyite is the principal phase at temperatures lower than 1600°.The amount of this phase decreases with the increase of temperature and carbon content, being the minority phase at higher temperatures.Mean unit-cell parameters refined for badeleyite were: a = 5.151(3) Å, b = 5.210(5) Å, c = 5.318(5) Å and b = 99.27(4)°Å.
Figure 6.Unit-cell parameter a o vs. Zr(C,O) composition taken from Kosolapova et al.4.

Table 6 .
Weight loss % in ZrO 2 samples at different temperatures.

Table 5 .
Unit-cell parameter a o and weight % of Zr(C,O) for ZrO 2 samples.

Table 3 .
Mean unit-cell parameters of titanium oxides obtained in the refinement

Table 4 .
Weight loss % in TiO 2 samples at different temperatures.