INTRODUCTION

Getters are materials, that have the ability to adsorb(chemi-sorption) residual gas molecules from a sample

surface. Such getters are categorized into two classes,namely evaporable and non-evaporable getters (NEGs)depending on methods used for the sample surfacetreatment. Barium and Titanium are the most common

ABSTRACT

To investigate the temperature dependence of a synthesized Zr57V36Fe7 non evaporable vacuum gettermaterial, the in-situ temperature x-ray photoelectron spectroscopy (in-situ XPS) were performed in a UHVchamber equipped with a programmable ceramic sample heating system. The surface and bulk compositionof Zr, V, and Ti was determined in the as-received state and after in-situ heating from 100 ℃ to 600 ℃ at 100℃ per step. The peak fitting results for O 1s, C 1s, Zr 3d, V 2p, and Fe 2p high resolution spectra wereacquired and the chemical state of the elements were then characterized as a function of heating temperature.In-situ XPS investigations showed that oxide reduction proceeds via the formation of sub-oxides with thesimultaneous formation of carbides in the region near the surface. The activation temperature for completionof the Zr57V36Fe7 alloy, which approximates the XPS peaks changed from oxide to metallic state (20 % of theoxide peak), was determined around 480 ℃ The findings suggest that the in-situ temperature XPS techniqueis a useful analytical tool for evaluating activation characteristics of NEG materials.

Key words: in-situ temperature XPS, non evaporable vacuum getter, Zr57V36Fe7 alloy

An In-situ XPS Study of Non-evaporable ZrVFe Getter Material

Jang-Hee Yoon,1 Won Baek Kim,2 Jong Sung Bae,1 Jong Phil Kim,1

J. K. Kim,2 Byoung Seob Lee,1 Mi-Sook Won1*

1Busan Center, Korea Basic Science Institute, Busan 609-735, Republic of Korea 2Korea Institute of Geoscience & Mineral Resources, Daejeon 305-350, Republic of Korea

*Corresponding author:Mi-Sook Won, Tel: +82-51-510-2986, Fax: +82-51-517-2497, E-mail: mswon@kbsi.re.kr

Journal of Analytical Science & Technology (2010) 1 (1), 61- 65Technical Note www.jastmag.orgDOI 10.5355/JAST.2010.61

62 Journal of Analytical Science & Technology (2010) 1 (1), 61- 65

evaporable getters [1]. Evaporable getters function byevaporating metal atoms which are chemicallycombined with residual gas molecules and are attachedto the inside surfaces of a vacuum chamber. However,in the case of NEG, the cleaning process occurs whenoxygen, generated from the passivation of surfaceoxidized materials by a heating process under thevacuum, diffuses into the getter [2]. In this case, themetallic components are exposed to the surface and thecleaning process is referred to as activation [3]. NEGscan be used when there is insufficient surface areaavailable to support sublimation or when damage mightoccur, if EGs were to be used.

Lowering the activation temperature is an importantstrategy for extending the range of NEGs, in practicaluse. Many getter materials are comprised of only puremetals (Ti, Ba, Ta, etc.), but binary and ternary alloysare frequently used, to achieve a lower activationtemperature [4]. However, a getter composed of onlypure Zr has not been developed because such a materialcould only have getter characteristics if it were presentin the form of an alloy. Therefore, the preparation ofalloys of Zr metal, which has excellent hydrogenadsorption behavior, have attracted considerableattention in recent years because of their potential foruse as getterings [3-4]. The activation temperature of aZr(V1-XFeX)2(0.16≤X≤0.18) ternary(400~500℃) islower than that for a Zr-Al(St 101) binary(700~900℃),and exhibits excellent adsorption properties for mostactivated gases [5-13]. In a Zr-V-Fe alloy, the order ofaffinity for an activated gas, such as H2, is Zr>V metal.This indicates that the each indicidual element has aspecific role in the alloy. Most alloys that contain Vshow the low activation temperatures. The Zr57V36Fe7

alloy is composed of hcp-Zr and cubic-Zr(V, Fe)2. Thehcp-Zr phase plays a role of the main sorption site forH2 , while the cubic-Zr(V, Fe)2 phase functions reducethe activation temperature.

A number of studies dealing with the pumpingactivity of NEGs for different gases have been reported[14]. However, our knowledge of the getteringmechanism and its relation to surface reactions remainlimited [15-17]. X-ray photoelectron spectroscopy(XPS) is an appropriate tool for use in these studies dueto its sensitivity to surface environments [15, 17-18].

In this study, to develop new low-temperature NEGmaterials, we synthesized Zr57V36Fe7 alloys andinvestigated the temperature dependence of the detailedchemical state of each element for NEG materials at asurface using XPS. The objective of the study was tounderstand the activation characteristics of a Zr57V36Fe7

alloy as an NEG material. The findings indicate thatXPS is a fast and efficient technique for investigatingthe activation process for a new getter material.

MATERIALS AND METHODS

The Zr57V36Fe7 getter alloy used in this work wassynthesized, in the form of a pellet, using arc-melting-furnace methods (5 melts). The alloy was processedusing the hydride-dehydride process to obtain brittlealloys that contained high levels of hydrogen. For theXPS measurements, an ESCALAB 250 (ThermoFisherCo., UK. KBSI-PA311) XPS spectrometer equippedwith a ceramic heating stage suitable for use in hightemperature experiment and an Al KαX-ray source wasused. Before each measurement, the sample wasmaintained for 10 min in a UHV chamber after thesample reached its target temperature. The temperaturewas controlled by means of a programmable heatingsystem. For consistency, the chamber pressure wasmaintained below 1 x 10-7 Torr during the measurementsregardless of the sample temperature. To monitor anyabnormality in the sample and chamber, a full surveyspectrum and high resolution XPS spectrameasurements were made. The increment of the sampletemperature was 100 ℃ steps. The XPS spectra werecarefully examined to eliminate any artifacts in thepeak analysis. The peak area of the each element wasdivided by the corresponding sensitivity factors andplotted as a function of activation temperature.

RESULTS and DISCUSSION

In-situ XPS sepctra for Zr57V36Fe7 alloyThe Zr57V36Fe7 getter alloy was investigated by in-situ

XPS experiments. The air-exposed Zr57V36Fe7 alloy wasfirst degassed in a vacuum at 100 ℃ for 30 min. Thethermal activation process was then carried out in six

Jang-Hee Yoon et al. 63

consecutive 100 ℃ heating steps. Fig.1 shows theevolution of XPS spectra for the components ofZr57V36Fe7 NEG during the activation process at varioustemperatures. As shown in Fig.1(A), the O1s peak iscomposed of three elemental intensities that areassigned to an O-H group at 532.8 eV, and a metallicoxide at 530.1 eV, respectively [14]. The intensity ofall three peaks decrease with increasing temperatureowing to the desorption and degassing of the Zr57V36Fe7

alloy. The peak at 532.8 eV disappeares as dehydratingof the surface at a temperature 100 ℃ occurs. Themetallic oxide peak at 530.1 eV decreases as theactivation progresses, then, completely disappeares at

temperatures above 500 ℃. This suggests that themetallic oxide diffused into the getter at thistemperature. The C1s peaks of the Zr57V36Fe7 alloy arereduced with increasing sample temperature. However,the small C1s peak at 285.0 eV still remained at atemperature of 600 ℃ due to carbide formation (Dataare not shown) [15]. The C 1s peak at 285.0 eV,obtained after in-situ XPS maintained in the vacuum atroom temperature, with increasing time.

The series of Fe 2p spectra are plotted in Fig. 1(B).The Fe 2p peaks are too low to permit their resolutionin the temperature range studied. However, an analysis

(A) (B)

(C) (D)

Figure 1. Evolution of XPS spectra for the components of the Zr57V36Fe7 NEG during the activation process. (A) O1s, (B) Fe 2p, (C) V 2p,

and (D) Zr 3d, respectively.

64 Journal of Analytical Science & Technology (2010) 1 (1), 61- 65

of the Fe 2p3/2 peaks indicated the presence of bothoxide and metallic states for iron in the system,corresponding to the binding energy due to oxide at711.6 eV, 708.6 eV (Fe 2p3/2) and a metallic peak at706.7 eV [20]. As shown in Fig. 1(B), the metallic Fepeak increased with increasing temperature. The V 2pspectra for the thermal activation are presented in Fig.1(C). The V 2p3/2 peak is composed of both an oxidestate both at 514.2 eV (VO) and 516,6 eV (VO2), and ametallic state at 512.0 eV(V) [19]. During theactivation process, the chemical shift (514.2 eV→512.0 eV‚ △=-2.2 eV) of the V 2p3/2 peak was observedabove 300 ℃. Above 400 ℃, only the metallic state ofV2p3/2 peak is seen in the spectra. Fig. 1(D) shows dataon the temperature dependence of the Zr 3d5/2 peaks.The Zr 3d5/2 spectrum was decomposed into severalelementary peaks. These peaks can be attributed toZrO2 at a binding energy of 185.6 eV, sub-oxide ZrO(182.8 eV), and metallic Zr (3d5/2 at 179.0 eV, and 3d3/2

at 181.4 eV).[20] The reduction of surface oxidesduring thermal activation is illustrated in Fig. 1-a by theappearance of metallic Zr (179.0 eV). The highlyoxidized Zr 3d5/2 (183.3 eV ) shifts to a lower bindingenergy characteristic of lowly the oxidized state Zrn+(n<4)or metallic Zr state (Zr0) after the activation from 100 ℃ to400 ℃. These findings indicate that the highly oxidized Zrgradually changes to sub-oxidized Zrn+ (n<4) and metallicZr with an increase in activation temperature.

Activation temperature for the Zr57V36Fe7 alloyThe quantitative results for the elements contained in the

sample are summarized in Fig. 2-a. The peak area of eachelement is divided by the corresponding sensitivity factorsand plotted as a function of the activation temperature. Thequantities of carbon and oxygen are substantially reducedwith increasing sample temperature and became negligibleat a temperature above 500 ℃. The vanadium contentincreased with increasing temperature. However, theamounts of Fe and Zr remain constant at temperaturesabove 300 ℃ . From the results, it can be seen theactivation process of Zr57V36Fe7 NEG ends at 500 ℃.

The dependence of the metallic state and the oxide stateon temperature for all metallic components of theZr57V36Fe7 NEG are shown in Fig. 2-b. The high-resolutionspectra of Zr57V36Fe7 NEG were decomposed to the oxide

state and the metallic state, then, the area of each peak isdivided by the corresponding sensitivity factors and plottedas a function of activation temperature.

The activation of Zr is complete during the 500 ℃activation step, however, the oxide states of V and Fe werepartially observed even at a temperature of 500 ℃. Theactivation temperature for the complete formation of theZr57V36Fe7 alloys, which approximates the XPS peaks forchanging from the oxide to metallic of the Zr57V36Fe7 alloy(20 % of the oxide peak) on temperature, was determinedto be about 480℃.

The findings reported herein indicate that the in-situXPS technique is a useful analytical tool for evaluating the

(A)

(B)

Figure 2. The variations in the Zr57V36Fe7 NEG vs. temperature.(A) for all components, and (B) for the chemical states.

Jang-Hee Yoon et al. 65

activation characteristics of NEG materials. The activationprocess of ternary Zr57V36Fe7 NEG, identified as a metallicreduction, starts at 280℃ and is complete at 480℃.During the thermal activation, surface regions becomemetal-rich due to the removal of oxygen. Only the oxygenpeak of adsorption was present in the vacuum chamber atroom temperature following the high temperaturetreatment. This indicates that the activation of theZr57V36Fe7 alloy was successfully achieved by the hightemperature treatment.

ACKNOWLEDGEMENTSThis work was supported by the NCRCP, No. R15-2006-02201002-0, Korea.

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