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Int. Journal of Refractory Metals and Hard Materials 38 (2013) 60–66

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Int. Journal of Refractory Metals and Hard Materials

j ourna l homepage: www.e lsev ie r .com/ locate / IJRMHM

A novel approach to synthesis of scandia-doped tungsten nano-particles forhigh-current-density cathode applications

R.K. Barik a, A. Bera a, A.K. Tanwar b, I.K. Baek b, S.H. Min b, O.J. Kwon b, W.S. Lee c, G.-S. Park a,b,d,⁎a School of Electrical Engineering and Computer Science, Seoul National University, Seoul, Koreab Department of Physics and Astronomy, Center for THz-Bio Application Systems, and Seoul Teracom, Seoul National University, Seoul, Koreac Agency for Defense Development, Daejeon, Koread Advanced Institute of Convergence Technology, Suwon-si, Gyeonggi-do, Republic of Korea

⁎ Corresponding author at: Seoul National University, SeTel.: +82 2 880 7749; fax: +82 2 882 9374.

E-mail address: [email protected] (G.-S. Park).

0263-4368/$ – see front matter © 2012 Elsevier Ltd. Allhttp://dx.doi.org/10.1016/j.ijrmhm.2012.12.009

a b s t r a c t
a r t i c l e i n f o
Article history:Received 1 July 2012Accepted 23 December 2012

Keywords:Tungsten nano-particleNano-particle synthesisSol–gel process

A novel synthesis approach for scandia-doped tungsten nano-powder using a sol–gel method is developed. Itinvolves dissolving tungsten oxide at 300 °C in the presence of a mixture of nitric acid, citric acid and ammonia.The dissolved tungsten oxide reacts with an aqueous solution of scandium nitrate in the liquid–liquid phase,which results in the homogeneous mixing of tungsten and scandium particles. A spherical shape particle wasobtained due to the dissolving of tungsten oxide in the solution. Citric acid enhances the mixing of ions at theatomic scale, which affects the hydrolysis reactions and leads to the formation of the phase pure nano-particle.The synthesized nano-powder was characterized by SEM (Scanning Electron Microscopy), EDS (Energy-dispersiveX-ray spectroscopy), TEM (Transmission ElectronMicroscopy), and XRD (X-ray Diffraction) analyses. The sphericalmorphology was observed via a SEM analysis and a narrow particle size distribution was noted by means of a TEManalysis. The XRD analysis of the powder showed the complete formation of the phase pure nano-particle with anaverage diameter of 50 nm without any contamination by other materials.

© 2012 Elsevier Ltd. All rights reserved.

1. Introduction

At present, high-power terahertz vacuumdevices arewidely used invarious fields, such as the medical, communication, space application,and security fields. A high-current-density stable and uniform emissiondispenser cathode is the key requirement for a terahertz vacuumdevice. The development of such a cathode will enhance terahertzresearch and applications. It was found in previous work [1–4] thatthe emission properties of the cathode are affected by raw powderproperties such as the morphology, particle size, size distribution andphase composition. Owing to the spherical particle shape, the diffusionof doped material from the bulk to the emission surface is enhanced,which enhances the emission [5]. A uniform grain size distribution,which enhances the inter-pore connectivity and pore uniformity, willenhance the life of the cathode [6]. The homogeneous mixing of thedoping material, enhances the emission uniformity of the cathode [7],and the emission current density of the cathode can be enhanced by1.3 times when using a pure-phase material [8]. As a result, synthesisof the spherical shape, a uniform grain size distribution, homogeneousmixing and a pure phase of scandium-doped tungsten nano-particlesare the critical requirements for the development of high-current-density cathodes to apply to THz devices.

oul 151-742, Republic of Korea.

rights reserved.

Tungsten nano-particle powders can be prepared by various process-es, including physical and chemical methods. Physical methods such aswire electrical exploration [9–11], spray drying [12], microwave plasmasynthesis [13–16] and thermal decomposition [17–19] can be used, allof which are not suitable for homogeneous doping with other materials.Chemical methods, such as the sono-electrochemistry method, produceimpure-phase particles which are not suitable for cathode applications[20,21]. Combustion synthesis [22] produces a wide particle size distri-bution, which affects the pore uniformity and inter-pore connectivityof the emission pellet. Solvothermal decomposition, as proposed bySahoo et al. [23], produces an arbitrary shape of the particle, whichreduces the diffusion of the doping particle and which is not suitablefor cathode applications. Recently Wang et al. [24] adopted a sol–gelprocess, followed by high-temperature reduction in a hydrogen environ-ment. Initially, they prepared Sc2O3-doped tungsten oxide powder usinga sol–gel process. Then, the doped tungsten oxide powder was reducedto metallic tungsten nano-powder in two steps using dry hydrogen ata high temperature. However, during this process, the particle sizedepends on the amount of scandium present. Also during this process,controlling the particle size is critical. At the same time, it is difficult tocontrol the phase of the material.

In a simple synthesis process, scandium-doped tungsten nanoparticles were prepared by dissolving tungsten oxide at 300 °C inthe presence of an optimum mixture of nitric acid, citric acid andammonia. The advantage of citric acid in a sol–gel process is that itenhances the mixing of ions on the atomic scale [25]. As a result,

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61R.K. Barik et al. / Int. Journal of Refractory Metals and Hard Materials 38 (2013) 60–66

the phase purity and particle size were influenced by the citric acidconcentration during the gelling process. It was found that theparticle size can be easily controlled and that the morphology of theparticle is spherical, as it dissolves into the solvent. At the sametime, it was found that the scandium, when homogeneously mixedwith tungsten nano-particle, will enhance the emission uniformity.The effect of the citric acid concentration on the phase purity andthe particle size is described in this study.

2. Powder preparation

A schematic flow diagram of the scandium-doped tungstennano-particle preparation is shown in Fig. 1. Coarse-grade tungstenoxide powder (99.9% purity) with an average particle size of about80 μmwas used as the starting material. Sc2O3 doped tungsten powderwas prepared using a sol–gel process by dissolving tungsten oxide in amixture of nitric acid, citric acid and ammonia at 300 °C. An initial 3%aqueous solution of scandium nitrate (Sc(NO3)3) and an 8% aqueoussolution of citric acid were added to 50 ml of nitric acid. Then, 2 g oftungsten oxide was added to the solution, which was stirred continu-ously for 2 min. Ammonia was then added slowly to maintain therequired pH of the solution. Stirring of the solution continued, withheating to 300 °C for 1 h. A hydrolysis reaction occurred at 300 °C;this temperature termed the reaction temperature. The mixture wasdried at 400 °C for 30 min under continuous stirring. During thisprocess, the sol becomes a gel. After the formation of the gel, the solutionwas allowed to dry at the same temperature to remove residualorganics and water molecules. After that, the powder was collectedand grind in agate mortar. The sample was then calcined at 500 °C for16 h, after which it became a yellow tungsten trioxide nano-powder.Finally, tungsten metal nano-powder was produced by reducing theoxides with dry hydrogen at 700 °C for 1 h. We refer to this tempera-ture as the reduction temperature. It was found that the particle sizeof the nano-Sc2O3-addedW powder can be controlled by these processparameters.

Mix in tungsten oxide

Prepare solution of HNO3+NH3+CH4

Add 3% aqueous solution of Sc(NO3)3

Stir and Heat to 300 0C for 1 hour

Dry the solution at 400 0C

Calcine the powder at 500 0C for 16 hours

Fire in dry hydrogen at 700 0C for 1 hour

Fig. 1. Schematic illustration of the synthesis processes to prepare Sc2O3-doped metallictungsten nano-particles.

3. Powder characterization

Several characterization tools were used to determine the particlesize, phase and the morphology of the nanoparticles. The particle sizeand phase of the powder were examined at room temperature byHigh-Resolution X-ray Diffraction (HRXRD) using CuKα radiation. AXRD analysis was carried out over the 2θ range of 10°–90°. Themorphology and microstructure of the powder were examined usingUltra-High-Resolution Thermal FE-SEM and High-Resolution TEM(HRTEM). The FE-SEM equipped with an EDS apparatus used in thisstudywas the “HITACHI S-4300”. The TEMstudy of powderswas carriedout using the “JEM-3010 TEM”. The samples for the analysis were pre-pared by dispersing nanoparticles in methanol and Di water at a ratioof 1:1. This was ultrasonically treated for 15 min and drop-castedonto a carbon-coated copper grid, which was followed by dying in airat room temperature.

3.1. FE-SEM analysis

The powderwas observedwith FE-SEMequippedwith EDS. Fig. 2(a)shows a FE-SEM image of the tungsten oxide precursor powder used inthis study. The tungsten oxide powder was extremely coarse, with anaverage particle size of approximately 80 μm. Fig. 2(b) shows themetal powder obtained from the sol–gel synthesis. It shows an averageparticle size of 50 nm, a uniform size distribution and a spherical shape.

Fig. 2. SEMmicrograph of a tungsten particle: a) Starting tungsten oxide precursor andb) metal tungsten nano-particle obtained from the sol–gel synthesis.

62 R.K. Barik et al. / Int. Journal of Refractory Metals and Hard Materials 38 (2013) 60–66

The composition of the nanoparticles was analyzed by an energy-dispersive X-ray (EDX) spectroscope attached to the FE-SEM device.Fig. 3(a–d) depicts the EDS spectrum of a nano-particle taken fromfour different sample positions. The figure indicates that only W–Scand O are present, no other element was detected, and the scandiumis uniformly distributed with the tungsten.

3.2. Transmission electron microscopy (TEM)

The morphology and size of the nanoparticles were examinedby high-resolution transmission electron microscopy (HRTEM), as

Fig. 3. EDS spectra and corresponding SEM ima

shown in Fig. 4. The figure shows that the tungsten particles have avery uniform and spherical shape. The powder was prepared atdifferent reduction temperatures of 1000, 800 and 700 °C. The corre-sponding measured average grain sizes were 70, 50, and 36 nm. ATEM image and corresponding particle size distribution are shownin Fig. 4a–c.

3.3. X-ray diffraction (XRD)

The prepared nano-particles were characterized by XRD using aBruker D8 DISCOVER (Germany) device. The diffraction patterns

ge taken from different sample positions.

image of Fig.�3

Fig. 4. TEM images and particle distribution of various tungsten nano-particles after reduction in hydrogen prepared under different temperatures with different average particlesizes: (a) 70 nm, (b) 50 nm, and (c) 36 nm.


were recorded from 10°–90° at a scan rate of 0.02° per step and at 5 sper point. The average particle sizes were estimated from diffractionpeak broadening using Scherrer's equation,

D ¼ kλ=βcos θð Þ; ð1Þ

where D is the average grain size, k the shape factor (0.89), λ is thewavelength of the X-ray radiation (1.5406 Å), β is the full-width-at-half- maximum (FWHM), and θ is the diffraction angle. The resultsof the XRD patterns at different reduction temperatures are illustratedin Fig. 5a–c. The average particle size obtained for the reduction

Fig. 5. X-ray patterns of tungsten nano-particles at different reduction temperatures andcorresponding particle size: a) 1000 °C, 74 nm, b) 800 °C, 44 nm, and c) 700 °C, 41 nm.

temperatures of 1000, 800 and 700 °C were 74, 44 and 41 nm, respec-tively. The result indicates that, the particle size can be preciselycontrolled by varying reduction temperature.

3.4. Effect of citric acid

The advantage of citric acid in a sol–gel process is that it enhancesthe mixing of ions at the atomic scale. The concentration of the citricacid in the aqueous solution can affect the hydrolysis reactions, which

Fig. 6. XRDpatterns of tungstennano-particles at different citric acid aqueous concentrationsand corresponding particle size: a) 4%, 76 nm, b) 8%, 68 nm, and c) 12%, 50 nm.

image of Fig.�4image of Fig.�5image of Fig.�6

Table 1Citric acid aqueous concentrations and particle sizes.

Sample Citric acidconcentration

1st peak/2ndpeak

1st peak/3rdpeak

Particle size(nm)

(a) 4% 6.87 3.68 76(b) 8% 7.07 7.07 68(c) 12% 7.56 4.27 50


leads to the formation of many types of complexes. In addition, thephase purity and particle size are influenced by the acid concentra-tion during the gelling and calcination processes. Therefore, in thispart of the study, the effect of the citric acid concentration was eval-uated. The aqueous concentration of citric acid was within the rangeof 4–12%. The samples were prepared according to the processshown in Fig. 1 and were calcined at 500 °C for 16 h. XRD patternsof the powders are shown in Fig. 6. For sample (a), a 4% aqueoussolution was used, while for sample (b), an 8% aqueous solutionwas used and for sample (c) a 12% aqueous solution of citric acidwas used. The crystallite size of the powder was determined usingScherrer's formula. Table 1 shows the effect of the citric acid concen-tration for the three different samples. This table shows that as the

Fig. 7. SEM images of Sc2O3 doped tungsten nano-particle pellet sintered at different temperdistribution of the sintered pellet.

citric acid concentration increases, the full-width-at-half-maximum(FWHM) band width (β) increases at a fixed diffraction angle (θ).This indicates a reduction of the particle size from 76 nm to 50 nm.It also shows that, as the citric acid concentration increases, thepeak-to-peak ratio increases, which produce a phase pure material.This indicates that the particle size and phase purity can be controlledby aqueous concentration of the citric acid in the solution.

4. Sc2O3-W matrix characterization

The Scandium doped Tungsten nano-powders with average particlesize 300 nm were prepared by above sol–gel synthesis process. Thepowders were die-pressed into porous matrices using a die-punch.The porous matrices were then sintered at five different temperatures1000 to 1500 °C for 15 min in dry hydrogen environment. The five pel-lets were then analyzed by XPS and SEM in order to observe scandiumdiffusion and agglomeration tendency of the tungsten nano-particle.

Fig. 7(a–e) shows the SEM image of the five pellets sintered atdifferent temperatures. As the sintering temperature increases theparticle size also increases which indicates the growth of the particleor agglomeration of the particles. Note that pore size uniformity is animportant requirement for high current density uniform emission

atures: (a) 1000 °C, (b) 1200 °C, (c) 1300 °C, (d) 1400 °C, (e) 1500 °C and (f) pore size

image of Fig.�7

Fig. 8. X-ray photoelectron spectra of Sc2O3 doped tungsten nano-particle pellet sintered at different temperatures: (a) 1000 °C, (b) 1200 °C, (c) 1300 °C, (d) 1400 °C, and (e) 1500 °C.


cathode. Fig. 7(f) shows the pore size distribution of the sinteredpellets. It can be seen that, there is only one sharp peak that wasobtained which implies uniform pore distribution. Though the parti-cles are agglomerated at high temperature the pores are uniformlydistributed over the matrix. In future we shall optimize sinteringtemperature in order to obtain 20–25% porosity of the pellet matrix.

Fig. 8 shows the XPS data of the five pellet matrices for differentsintering temperatures and the corresponding surface atomic concen-tration of W, O and Sc is shown in Fig. 9. It was found that atomic con-centration of W, O and Sc remains almost unchanged at the surfaceduring sintering up to 1200 °C. However after 1200 °C the scandiumconcentration decreases slowly. This implies that at high temperaturescandium oxide may reduce with hydrogen which diffuses into thematrix. This phenomenon can be confirmed by observing the oxygenatomic concentration in Fig. 8 which shows that the oxygen atomicconcentration reduces with sintering temperature and at the sametime tungsten atomic concentration increases.

5. Conclusion

Nano-sized scandium-doped tungsten powder was prepared by asol–gel method and was characterized using SEM, TEM, XRD and EDS.

Fig. 9. Atomic concentrations of Sc,W andO at the surface of pellet changeswith sinteringtemperature.

The SEM and TEM results show a spherical shape and a homogeneousparticle size distribution with an average grain size of 50 nm for thetungsten nano-particles. The EDS results show that the scandium isuniformly distributedwith the tungsten nano-particles. The XRD resultsshow that broadening of the peaks occurred with a lower reductiontemperature, which indicates a reduction of the particle size. This canbe attributed to the fact that at high temperatures, the particles diffuseto each other and coagulate, which increases the grain size. Also, with adecrease in the reduction temperature, the intensity of the first peak in-creases, indicating a pure phase material. Hence, it is possible to controlparticle size and phase purity precisely by controlling the reductiontemperature and the citric acid concentration in the solution. Thus thespherical shape, homogeneously sized and phase pure scandia-dopedtungsten nano-particles as obtained by the sol–gel method describedhere are the most suitable material for high-current-density cathodeapplications.

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A novel approach to synthesis of scandia-doped tungsten nano-particles for high-current-density cathode applications1. Introduction2. Powder preparation3. Powder characterization3.1. FE-SEM analysis3.2. Transmission electron microscopy (TEM)3.3. X-ray diffraction (XRD)3.4. Effect of citric acid

4. Sc2O3-W matrix characterization5. ConclusionReferences

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