Int. Journal of Refractory Metals & Hard Materials 27 (2009) 764–767

Contents lists available at ScienceDirect

Int. Journal of Refractory Metals & Hard Materials

journal homepage: www.elsevier .com/locate / IJRMHM

Heat treatment of molybdenum under vacuum conditions

Gábor Dobos a,*, Katalin V. Josepovits a, Ágoston Böröczki b, István Csányi b, György Hárs a

a Budapest University of Technology and Economics, Department of Atomic Physics, Budafoki út 8, H-1111 Budapest, Hungaryb GE Consumer and Industrial, Váci út 77, H-1340 Budapest, Hungary

a r t i c l e i n f o

Article history:Received 30 July 2008Accepted 26 December 2008

Keywords:MolybdenumOxideReductionDissociationBinding states

0263-4368/$ - see front matter � 2009 Elsevier Ltd. Adoi:10.1016/j.ijrmhm.2008.12.010

* Corresponding author. Tel.: +36 1 463 4210; fax:E-mail address: dobosg@eik.bme.hu (G. Dobos).

a b s t r a c t

Oxide coverage of molybdenum plays an important role in several applications, for example in lightingindustry. Surface conditioning procedures were simulated in an XPS instrument by in situ heat treat-ments while monitoring the surface composition and changes in the chemical states of molybdenum.Heat treatments have been made at different temperatures between 435 and 690 �C under vacuum con-ditions. It has been observed that during heating the molybdenum test samples the native MoO3 layer onthe surface dissociates, and a layer of suboxides forms on the surface. This layer hinders the furtherreduction of the surface, thus reaction speed decreases after the initial phase. It has been established thatin the second phase of the heat treatment the activation energy of the process is 1.1 ± 0.2 eV. Reduction ofMoO3 to elemental molybdenum runs through two intermediate states: Mo6+ ? Mo5+ ? Mo4+ ? Mo0.

� 2009 Elsevier Ltd. All rights reserved.

1. Introduction

In quartz HID lamps thin molybdenum foils serve as electrodefeed-throughs. The wall of the fused silica arc tube is pinched ontothe foils producing vacuum-tight bonds. The quality of the bond isstrongly influenced by the surface composition of molybdenum:while organic contaminations weaken the bond, oxides have animportant role in forming it. (Practical experience shows that ifthe oxide is completely removed form the surface before the seal-ing process, bond quality deteriorates.) Manufacturers often de-posit oxides of other metals to the surface to enhance the bond[1,2].

In practice, a heat treatment is generally used to clean the sur-face of the foils before the sealing process is started. This treatmentmay also affect the oxide coverage of the foil. Molybdenum mayform different oxides of different properties [3]. The native oxideis molybdenum trioxide (MoO3). However, the properties ofMoO2 seem to be more favorable. It is known from the literaturethat MoO2 is much less volatile than MoO3 [3]. This is an importantproperty, since the parts of the electrical feed-through are heatedto a very high temperature during the sealing process. If the oxidelayer disappears from the surface before the wall of the envelope ispinched onto it, it cannot assist the formation of the glass-to-metalbond. Another advantage of MoO2 is that it solves metallic molyb-denum [3], which may enhance the bond considerably. In case ofMoO3 we have not found any reference in the literature to such

ll rights reserved.

+36 1 463 4357.

behavior. Besides these, the density of MoO2 is higher than thatof MoO3, which is also of advantage if mechanical stability of thestructure is considered.

During the heat treatment the native oxide may evaporate,dissociate (auto-reduction), or it may be reduced by the metal tobecome a compound of lower oxide state. Because of these, it isvery important to understand the processes taking place duringthe heat treatments, and establish their time and temperaturedependence, in order to find the optimal conditions of the heattreatments.

2. Experimental

Heat treatments were carried out in the analytical chamber ofthe XPS instrument in our laboratory. Molybdenum foils wereheated by direct resistance heating in the measurement position.

Fig. 1 shows a sketch drawing of the heating equipment. Thesample (1) was mounted on stainless steel consoles (2), held by amolybdenum support (5). (The steel consoles and the molybdenumsupport were electrically isolated by ceramic rings (3).) Four iso-lated molybdenum electrical connectors (male parts) (4) were in-stalled to the support and connected to the consoles holding thesample. The counterparts of the electrodes (female parts) (7) weremounted on the sample holder (6). These connectors were stainlesssteel coils, positioned in holes drilled into a ceramic cylinder (8)around the sample holder. (Male and female parts of the connec-tors were made of different materials in order to avoid theirfusion.) This way the sample (together with the molybdenum sup-port and the steel consoles) can easily be replaced, and the electri-cal connections can be made without venting up the vacuum

Fig. 1. Sketch drawing of the heating equipment.

Fig. 2. Typical Mo 3d spectra recorded before (A) and during (B, C and D) heating amolybdenum foil at 595 �C. Spectrum (B) was recorded immediately after the startof heating, spectrum (C) and (D) 810 and 8910 s later, respectively.

G. Dobos et al. / Int. Journal of Refractory Metals & Hard Materials 27 (2009) 764–767 765

system. The molybdenum foil samples were 20 lm thick, 1.5 mmwide, and their free length between the consoles were 19 mm.

At higher temperatures an IRCON MODLINE 5 infrared pyrome-ter was used to accurately measure temperature of the samples.Below the measurement range of the pyrometer, temperature set-tings were estimated from the heating current by extrapolating theI–T curves recorded at higher temperatures.

During XPS measurements, atoms of the sample were excitedby Mg Ka X-ray radiation from a VG Microtech XR3E2 X-raysource. A VG Microtech CLAM2 truncated hemispherical analyser,having an energy resolution of 0.3 eV, was used for electron energyanalysis. The analyser has collected the photoelectrons from a4 � 4 mm area. The background pressure in the analytical chamberwas below 3 � 10�7 Pa.

As a result of the heat loss through the steel consol holding thesample, the ends of the sample were considerably colder than itsmiddle part. Despite of this, the temperature distribution in themiddle region was more or less homogenous according to thepyrometric measurements: in the middle part of 5–6 mm lengththe temperature inhomogeneity was below 6–7%. Thus the area,seen by the energy analyser, was uniformly heated.

The samples were heated to 435, 480, 595, and 690 �C, respec-tively, and their surface composition was continuously monitoredby XPS. A series of spectra was recorded in every 135 s, allowingus to follow the process of reduction. The spectra were decom-posed to Gauss (70%) – Lorentz (30%) mixture peaks using theCasaXPS v2.19 software [4].

3. Results and discussion

Fig. 2 shows four typical Mo 3d spectra recorded during heatinga molybdenum foil at 595 �C. The first spectrum (A) was recordedbefore, the second (B), immediately after the start of heating. Spec-trum (C) was recorded 810 s, and spectrum (D), 8910 s later, at theend of the experiment.

According to spectrum ‘‘A” on Fig. 2, the surface was covered byMoO3 and some MoO2. In addition, metallic, elemental bindingstate of molybdenum can also be detected, probably because theoxide layer was thinner than the information depth of XPS. Usingthe XPS MultiQuant software [5] the oxide thickness was esti-mated to be 11 nm. (Note that this is only a lower estimation be-cause of the roughness of the surface.)

After the start of heat treatment another binding state appears(spectrum ‘‘B” in Fig. 2). The spectra recorded after the start of theheating can be resolved into four doublet peaks: Mo0, Mo4+, Mo6+

and an intermediate state between Mo4+ and Mo6+. Table 1 showsthe binding energies and doublet splitting parameters of the peaks.The Mo0, Mo4+ and Mo6+ binding states were identified based onliterature data for elemental molybdenum, MoO2 and MoO3 takenfrom [6].

The position of the intermediate state was changing during theheating. (If its position was fixed to any energy between Mo4+ andMo6+, the curve fitting has failed on some of the spectra.) In the

Table 1Chemical binding states of molybdenum observed during the experiments.

Binding state Mo 3d5/2

position (eV)Chemicalshift (eV)

Doubletsplitting (eV)

Mo0 227.7 0 3.15Mo4+ 229.0 1.3 3.2Mo5+ 230.5–231 2.8–3.3 3.2Mo6+ 232.2 4.5 3.2

Fig. 4. Variation of the average oxidation state of molybdenum as a function of timeat different temperatures.

766 G. Dobos et al. / Int. Journal of Refractory Metals & Hard Materials 27 (2009) 764–767

beginning, the intermediate state appeared 3.3 eV away from theelemental peak, and its binding energy continuously decreased un-til it settled at a value 2.8 eV away from the elemental peak. Thisenergy is almost precisely halfway between that of the Mo4+ andMo6+ peaks. This suggests that the final position is associated toMo5+, while the initial one is a slightly more oxidised state.(According to [7] the Mo 3d peak position is approximately a linearfunction of the partial charge of the molybdenum atoms in thecompound.) On the other hand, literature [8] and [9] suggest thatMo5+ appears at higher binding energies, approximately 3.8 eVaway from the elemental peak. Due to the lack of measurementdata on pure Mo2O5, the intermediate state cannot be unambigu-ously associated to Mo5+, but it is obvious, that it is approximatelyhalfway between Mo4+ and Mo6+, and it is slightly reduced duringthe heating. (In the following part of the text we are going refer thisstate as Mo5+.)

Fig. 3 shows the changing of the relative intensities of the fourbinding states as a function of time, in case of the sample that washeated to 480 �C. (The shapes of the curves are very similar forother temperatures too, even though the rates of changes are dif-ferent.) In the first 5 min a rapid change can be observed. Theintensity of the Mo6+ sharply decreases, while Mo5+ (and someMo4+) appears on the surface. The intensity of Mo5+ starts to de-crease right after this initial step, while Mo4+ keeps rising. This sug-gests that the MoO3 transforms to MoO2 through an intermediatestate, which is close to Mo5+.

Fig. 4 shows the change of the average oxidation state of molyb-denum as a function of time for all samples. (The average oxidationstate is calculated by weighting the oxidation states by the area ofthe peak associated with them.) Since the information depth of ourmeasurements is higher than the thickness of the oxide layer, theaverage oxidation state of molybdenum has a strong connection

Fig. 3. Changing of the relative intensities of the different binding states ofmolybdenum as a function of time, in case of the sample that was heated to 480 �C.

to the total amount of oxygen in the oxide. (Note that in the O 1speak other components may appear, such as the oxygen adsorbedon the surface of the samples. Because of this, the average oxida-tion state of molybdenum is a better indicator of the amount ofoxygen in the oxide, than the O 1s peak.)

It can be seen that all of the curves in Fig. 4 show a sharp drop atthe first few measurement points. This means that in this initialphase of the treatment the oxide layer has quickly lost some oxy-gen either by the dissociation, reduction or by the sublimation ofMoO3.

After the quick changes in the first few minutes the reactionslows down, but still proceeds. (Except for the highest temperatureheat treatment: in this case the changes are so fast even after thisinitial step, that the change in the reaction rate cannot be seen.)The estimated evaporation rate of MoO3 (based on literature datafrom [3]) exceeds the observed rate of changes in the second phase.This means that during this second phase the surface of the foils isnot covered by MoO3, but some other, less volatile material. This isprobably a suboxide of molybdenum, formed by the dissociation ofMoO3 during the initial step.

Fig. 5. Reduction rate of molybdenum as a function of temperature on a log – 1/Tscale.

G. Dobos et al. / Int. Journal of Refractory Metals & Hard Materials 27 (2009) 764–767 767

In this second phase of the process the intensity of Mo0 in-creases, while the intensity of Mo6+ and Mo5+ decreases. In thebeginning the Mo4+ intensity increases too, but after reaching amaximum (spectrum ‘‘C” in Fig. 2), it starts to fall. (In case of thelower temperature heat treatments the process was so slow thatthe maximum was not reached during the time span of the exper-iment.) In this region the average oxidation state is a linear func-tion of time, which means that the rate of oxygen loss is constant.

If the rate of oxygen loss would be determined by a diffusionprocess, a constant rate on a certain temperature would mean thatthe concentration gradient is also constant. In case of a chemicalreaction a constant rate on a certain temperature means, that theconcentrations are constant. In both cases the rate of oxygen lossis proportional to exp(�E/kT), where E is the activation energy ofthe process, k is the Boltzmann constant and T is the absolute tem-perature. Fig. 5 shows the gradients as a function of temperatureon a log – 1/T scale. From this, the activation energy of the processis 1.1 ± 0.2 eV.

In case of the highest temperature heat treatment (where theprocess is the fastest) it can also be seen, that after the averageoxidation state falls below 2 and the relative intensity of Mo5+

and Mo6+ decreases to (or below) 10%, the process slows further.The reason of this might be that the main source of oxygen hasalmost disappeared. Without the presence of molybdenumtrioxide, further oxygen can only be extracted from the suboxides,which is considerably harder since these compounds already havea lack of oxygen. Thus the further reduction of the oxide layerbecomes increasingly harder, and even after several hours of heattreatment some MoO2 is still presented on the surface (SpectrumD in Fig. 2).

4. Conclusion

Based on the XPS measurements it has been established, thatduring the heat treatment of molybdenum foils, the native oxidedissociates, which leads to the formation of a layer of suboxideson the surface. After the formation of this layer, further reactionis hindered either by the diffusion of oxygen or by the rate of achemical reaction. The activation energy of this process is1.1 ± 0.2 eV. It has also been established that the reduction ofMoO3 runs through two intermediate states: Mo6+ ? Mo5+ ?Mo4+ ? Mo0.

References

[1] Leichtfried G, Thurner G, Weirather R. Molybdenum alloys for glass-to-metalseals. Int J Refract Met Hard Mater 1998;16:13–22.

[2] Gritsch M, Piplits K, Hutter H, Wilhartitz P, Wildner H, Martinz HP.Investigations on the oxidation behavior of technical molybdenum foils bymeans of secondary-ion mass spectrometry. Surf Sci 2000;454–456:284–8.

[3] Gmelin Handbuch der Anorganischen Chemie, 8. Auflage, Molybdän,Ergänzungsband, Teil B1. Berlin: Springer-Verlag; 1975.

[4] Casa Software Ltd, CasaXPS, <http://www.casaxps.com/>.[5] Mohai M. XPS MultiQuant for Windows, 1999–2005, <http://www.chemres.hu/

aki/XMQpages/XMQhome.htm>.[6] Wagner CD, Riggs WM, Davis LE, Moulder JF, Muilenberg GE. Handbook of X-ray

photoelectron spectroscopy. Perkin-Elmer; 1979.[7] Grim SO, Matienzo LJ. Inorg Chem 1975;14(5).[8] Sunu SS, Prabhu E, Jayaraman V, Gnanasekar KI, Seshagiri TK, Gnanasekaran T.

Electrical conductivity and gas sensing properties of MoO3. Sens Actuators B:Chem 2004;101(1–2):161–74.

[9] Ruiz Frine, Benzo Zully, Quintal Manuelita, Garaboto Ángel, Albornoz Alberto,Brito Joaquín L. X-ray photoelectron spectroscopy study of pyrolytically coatedgraphite platforms submitted to simulated electrothermal atomic absorptionspectrometry conditions. Appl Surf Sci 2006;252(24):8695–701.