photo conductivity on photo current action.af

12
Effects of Bulk Photoconductivity on Photocurrent Action Spectra of Molecular p-n Heterojunction Solar Cells Oleg Shevaleevskiy, a,b,z Liudmila Larina, b Seung Yeop Myong, a and Koeng Su Lim a a Department of Electrical Engineering and Computer Science, Korea Advanced Institute of Science and Technology, Daejeon 305-701, Korea b Institute of Biochemical Physics RAS, 119991 Moscow, Russia We have investigated how bulk photoconductivity influences the photovoltaic parameters of zincphthalocyanine-fullerene ZnPc/C 60 p-n heterojunction solar cells. The results indicate that the photocurrent action spectrum of a cell depends strongly on the photoconductivity and spectral characteristics of each component material. We therefore propose a model that simulates the action spectra of the short-circuit photocurrent in the molecular organic solar cells, and our model is based on the assumption that the photocurrent action spectrum depends on the bulk layer photoconductivity. © 2005 The Electrochemical Society. DOI: 10.1149/1.2126576 All rights reserved. Manuscript submitted February 26, 2005; revised manuscript received June 30, 2005. Available electronically November 2, 2005. Molecular organic photovoltaic devices based on p-type semi- conductors, such as metal-phthalocyanines MPc where M = Zn, Cu, TiO, or H 2 , have recently attracted great interest due to the appearance of n-type fullerenes C 60 , which form photosensitive p-n junctions with MPc. 1-4 The efficiency of organic solar cells is still lower compared to traditional solid-state devices. 5 Initially, a relatively large power conversion efficiency for a p-n junction or- ganic photovoltaic cell based on p-type CuPc and n-type perylene derivative was realized by Tang. 6 Fullerenes were successfully ap- plied in different constructions of molecular photovoltaic cells for fabricating p-n heterojunctions, 1,2 improvement of organic cell performance by doping, 7,8 and for manufacturing plastic-type solar cells. 9 Recently, a record efficiency of around 4% was reported for p-i-n multilayer devices that used ZnPc and C 60 modified mo- lecular layers. 10 However, the power conversion efficiency of any molecular photovoltaic device is limited by poor charge carrier mobility 1 cm 2 /V s and small exciton diffusion length L D 10 nm in organic layers. 11,12 In condensed phthalocyanine lay- ers, exciton diffusion length may depend on the fabrication condi- tions, and the reported L D values vary from 10 to 30 nm for ZnPc 7,13 and from 12 to 60 nm for CuPc films. 14,15 The photoexcitations created in the bulk of a thick molecular layer do not reach the interface region of the p-n junction and therefore do not contribute to the photocurrent of the device. Thus, the carrier production re- gions are limited to the dimension of space-charge depletion layers that only extend over the film interfaces. Due to this the light illu- mination produces two kinds of photoeffects: photoconductivity in the bulk of the layer that decreases the cell serious resistance, and a photovoltaic effect in the interface region resulting in the generation of the photocurrent. Since Simon and Andre 16 reported significant information about photogenerated carriers at the p-n heterojunction, various models have been proposed that simulated the photocurrent of organic molecular solar cells. 17-20 However, the role of bulk layer photoconductivity is still being debated. We assume that the spectral dependence of series resistance in molecular organic cells should not be neglected because of its influ- ence on the cell photocurrent. Normally, the thickness of a bulk layer in organic solar cells several times exceeds the dimension of a depletion layer. Hence, the series resistance can severely limit the shape of photocurrent action spectrum and the conversion efficiency. To prove this supposition, we report on the performance of fabri- cated n-C 60 /p-ZnPc solar cells. We also propose a theoretical model that describes the behavior of photocurrent action spectra and effects of the bulk layer photoconductivity. Experimental Thin molecular layers were vacuum sublimated at a pressure of 10 -6 Torr from resistively heated quartz crucibles using conven- tional ZnPc and C 60 99.9% purity powder purchased from Kodak and MER Corp., respectively. Before the deposition, ZnPc powder was predominantly recrystallized under argon ambient by train sub- limation. Figure 1 shows the arrangement of the fabricated p-n hetero- junction solar cell produced by successive evaporation of C 60 and ZnPc. First, we deposited n-C 60 layers on Corning glass substrates coated with indium tin oxide ITO transparent front electrodes. After deposition, the samples were kept under an ultra- high vacuum of 10 -7 Torr during 24 h to reduce the initial oxy- gen content. Then, without exposure to air, the p-ZnPc layers were successively evaporated. Finally, the coplanar gold Au back contacts with a thickness of 50nm and an area of 2 10 mm were deposited on top of the device. Thus, we fabricated photo- voltaic cells with a structure of ITO/n-C 60 /p-ZnPc/Au. To provide a comparative study of organic layer thickness on cell performance, we have prepared three types of cells with the follow- ing parameters: A n-C 60 80 nm/p-ZnPc 120 nm, B n- C 60 400 nm/p-ZnPc400 nm, and C n-C 60 800 nm /p- ZnPc 800 nm. With the same deposition conditions, single layers of n-C 60 and p-ZnPc were prepared on ITO substrates for the UV- visible UV-vis optical spectra and photoconductivity measure- ments. Dark and photocurrent of single layers have been measured at constant dc electrical field 10 5 V/cm in sandwich cell con- figuration between ITO front and Au back electrodes. Action spectra of short-circuit photocurrent for C 60 /ZnPc cells were recorded under illumination from the ITO side. While measuring the photocurrent, we appropriately filtered and calibrated irradiation from a 500W xenon lamp through a monochromator to obtain a constant incident photon intensity P IN of 10 W/cm 2 in the wavelengths of 360– 850 nm. A Shimadzu UV-3101 PC spectrophotometer was used to record the optical absorption spectra. The current density–voltage J-V characteristics were measured under illumination of a 15 mW/cm 2 halogen lamp using an HP 1415B semiconductor pa- rameter analyzer. Results The dark conductivity of the n-C 60 layer D1 was measured just after its deposition to prevent the oxygen doping, while the p-ZnPc layer was exposed to air to induce the oxygen doping before we measure its dark conductivity D2 . Although the oxygen dop- ing significantly reduces the D1 value, it was reported that the doping process lasts longer than the time needed to take our measurements. 21 In contrast, a short exposure to air initiates a z E-mail: [email protected] Journal of The Electrochemical Society, 153 1 A1-A4 2006 0013-4651/2005/1531/A1/4/$20.00 © The Electrochemical Society, Inc. A1

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Page 1: Photo Conductivity on Photo Current Action.af

Journal of The Electrochemical Society, 153 �1� A1-A4 �2006� A1

Effects of Bulk Photoconductivity on Photocurrent ActionSpectra of Molecular p-n Heterojunction Solar CellsOleg Shevaleevskiy,a,b,z Liudmila Larina,b Seung Yeop Myong,a andKoeng Su Lima

aDepartment of Electrical Engineering and Computer Science, Korea Advanced Institute of Science andTechnology, Daejeon 305-701, KoreabInstitute of Biochemical Physics RAS, 119991 Moscow, Russia

We have investigated how bulk photoconductivity influences the photovoltaic parameters of zincphthalocyanine-fullerene�ZnPc/C60� p-n heterojunction solar cells. The results indicate that the photocurrent action spectrum of a cell depends strongly onthe photoconductivity and spectral characteristics of each component material. We therefore propose a model that simulates theaction spectra of the short-circuit photocurrent in the molecular organic solar cells, and our model is based on the assumption thatthe photocurrent action spectrum depends on the bulk layer photoconductivity.© 2005 The Electrochemical Society. �DOI: 10.1149/1.2126576� All rights reserved.

Manuscript submitted February 26, 2005; revised manuscript received June 30, 2005. Available electronically November 2, 2005.

0013-4651/2005/153�1�/A1/4/$20.00 © The Electrochemical Society, Inc.

Molecular organic photovoltaic devices based on p-type semi-conductors, such as metal-phthalocyanines �MPc where M = Zn,Cu, TiO, or H2�, have recently attracted great interest due to theappearance of n-type fullerenes �C60�, which form photosensitivep-n junctions with MPc.1-4 The efficiency of organic solar cells isstill lower compared to traditional solid-state devices.5 Initially, arelatively large power conversion efficiency for a p-n junction or-ganic photovoltaic cell based on p-type CuPc and n-type perylenederivative was realized by Tang.6 Fullerenes were successfully ap-plied in different constructions of molecular photovoltaic cellsfor fabricating p-n heterojunctions,1,2 improvement of organic cellperformance by doping,7,8 and for manufacturing plastic-type solarcells.9 Recently, a record efficiency of around 4% was reportedfor p-i-n multilayer devices that used ZnPc and C60 modified mo-lecular layers.10 However, the power conversion efficiency of anymolecular photovoltaic device is limited by poor charge carriermobility ��1 cm2/V s� and small exciton diffusion length �LD

� 10 nm� in organic layers.11,12 In condensed phthalocyanine lay-ers, exciton diffusion length may depend on the fabrication condi-tions, and the reported LD values vary from 10 to 30 nm for ZnPc7,13

and from 12 to 60 nm for CuPc films.14,15 The photoexcitationscreated in the bulk of a thick molecular layer do not reach theinterface region of the p-n junction and therefore do not contributeto the photocurrent of the device. Thus, the carrier production re-gions are limited to the dimension of space-charge �depletion� layersthat only extend over the film interfaces. Due to this the light illu-mination produces two kinds of photoeffects: photoconductivity inthe bulk of the layer that decreases the cell serious resistance, and aphotovoltaic effect in the interface region resulting in the generationof the photocurrent. Since Simon and Andre16 reported significantinformation about photogenerated carriers at the p-n heterojunction,various models have been proposed that simulated the photocurrentof organic molecular solar cells.17-20 However, the role of bulk layerphotoconductivity is still being debated.

We assume that the spectral dependence of series resistance inmolecular organic cells should not be neglected because of its influ-ence on the cell photocurrent. Normally, the thickness of a bulklayer in organic solar cells several times exceeds the dimension of adepletion layer. Hence, the series resistance can severely limit theshape of photocurrent action spectrum and the conversion efficiency.To prove this supposition, we report on the performance of fabri-cated n-C60/p-ZnPc solar cells. We also propose a theoretical modelthat describes the behavior of photocurrent action spectra and effectsof the bulk layer photoconductivity.

z E-mail: [email protected]

Experimental

Thin molecular layers were vacuum sublimated at a pressure of10−6 Torr from resistively heated quartz crucibles using conven-tional ZnPc and C60 �99.9% purity� powder purchased from Kodakand MER Corp., respectively. Before the deposition, ZnPc powderwas predominantly recrystallized under argon ambient by train sub-limation.

Figure 1 shows the arrangement of the fabricated p-n hetero-junction solar cell produced by successive evaporation of C60and ZnPc. First, we deposited n-C60 layers on Corning glasssubstrates coated with indium tin oxide �ITO� transparent frontelectrodes. After deposition, the samples were kept under an ultra-high vacuum of 10−7 Torr during 24 h to reduce the initial oxy-gen content. Then, without exposure to air, the p-ZnPc layers weresuccessively evaporated. Finally, the coplanar gold �Au� backcontacts with a thickness of 50nm and an area of 2 � 10 mmwere deposited on top of the device. Thus, we fabricated photo-voltaic cells with a structure of ITO/n-C60/p-ZnPc/Au. To providea comparative study of organic layer thickness on cellperformance, we have prepared three types of cells with the follow-ing parameters: �A� n-C60�80 nm�/p-ZnPc �120 nm�, �B� n-C60�400 nm�/p-ZnPc�400 nm�, and �C� n-C60�800 nm�/p-ZnPc�800 nm�. With the same deposition conditions, single layersof n-C60 and p-ZnPc were prepared on ITO substrates for the UV-visible �UV-vis� optical spectra and photoconductivity measure-ments. Dark and photocurrent of single layers have been measuredat constant dc electrical field ��105 V/cm� in sandwich cell con-figuration between ITO front and Au back electrodes. Action spectraof short-circuit photocurrent for C60/ZnPc cells were recorded underillumination from the ITO side. While measuring the photocurrent,we appropriately filtered and calibrated irradiation from a 500Wxenon lamp through a monochromator to obtain a constant incidentphoton intensity �PIN� of 10 �W/cm2 in the wavelengths of 360–850 nm. A Shimadzu UV-3101 PC spectrophotometer was used torecord the optical absorption spectra. The current density–voltage�J-V� characteristics were measured under illumination of a15 mW/cm2 halogen lamp using an HP 1415B semiconductor pa-rameter analyzer.

Results

The dark conductivity of the n-C60 layer ��D1� was measuredjust after its deposition to prevent the oxygen doping, while thep-ZnPc layer was exposed to air to induce the oxygen doping beforewe measure its dark conductivity ��D2�. Although the oxygen dop-ing significantly reduces the �D1 value, it was reported that thedoping process lasts longer than the time needed to take ourmeasurements.21 In contrast, a short exposure to air initiates a

Page 2: Photo Conductivity on Photo Current Action.af

JOURNAL OF APPLIED PHYSICS 98, 054311 �2006� �� © 2005 American Institute of Physics98, 054311-1

Charge transport in hydrogenated boron-doped nanocrystallinesilicon-silicon carbide alloys

Seung Yeop Myong,a� Oleg Shevaleevskiy, and Koeng Su LimDepartment of Electrical Engineering and Computer Science, Korea Advanced Institute of Science andTechnology (KAIST), 373-1 Guseong-dong, Yuseong-gu, Daejeon 305-701, Republic of Korea

Shinsuke Miyajima and Makoto KonagaiDepartment of Physical Electronics, Tokyo Institute of Technology (TIT), 2-12-1 O-okayama, Meguro-ku,Tokyo 152-8552, Japan

�Received 25 January 2005; accepted 28 July 2005; published online 9 September 2005�

We have investigated the carrier transport mechanism of mixed-phased hydrogenated boron-dopednanocrystalline silicon–silicon carbide alloy �p-nc-Si-SiC:H� films. From temperature-dependentdark conductivity measurements, we found that the p-nc-Si-SiC:H alloys have two different carriertransport mechanisms: one is the thermally activated hopping between neighboring crystallites nearthe room-temperature region and the other is the band tail hopping below 150 K. © 2005 AmericanInstitute of Physics. �DOI: 10.1063/1.2037871�

I. INTRODUCTION

Thin films of hydrogenated amorphous silicon �a-Si:H� and silicon carbide �a-SiC:H� have attracted consid-erable research interest mainly due to their potential applica-tions in electronics, optical devices, and window layers inphotovoltaic thin-film solar cells. To achieve a high-efficiency thin-film solar cell, a window layer should have ahigh electrical conductivity and a wide optical band gap. Dueto the incorporation of carbon atoms, a-SiC:H has a widerband gap than a-Si:H. However, the incorporated carbonatoms limit the electrical conductivity of a-SiC:H.

We can improve the electrical conductivity of a-SiC:Hby an impurity doping. However, the impurity doping re-duces the optical band gap of films. In recent years, onepromising way to incorporate a wide band gap and a highconductivity has been proposed by producing the mixed-phase structure consisting of microcrystalline ��c− � ornanocrystalline �nc-� Si grains embedded in a-Si:H network.We firstly reported on the preparation of hydrogenated boron�B�-doped nc-Si–SiC:H �p-nc-Si−SiC:H� alloy films con-taining nc-Si grains embedded in a-SiC:H matrix via thephotodecomposition of C2H4.1 Its optical transmittance ismainly governed by the a-SiC:H matrix, while nc-Si grainsare responsible for the effective transport of charge carriers.2

This p-nc-Si−SiC:H alloy has a higher electrical con-ductivity, optical transmittivity, carrier mobility, and dopingefficiency than the conventional undiluted p-a-SiC:H. Basedon the deposition of p-nc-Si−SiC:H alloy,1–4 H2-dilutedp-a-SiC:H buffer layers of p-i-n-type a-Si:H or protocrys-talline silicon �pc-Si:H� solar cells were prepared.5–8 Wefound that the natural hydrogen treatment—etching the de-fective undiluted p-a-SiC:H window layer and improvingorder in the window layer—takes place just before the highlyconductive, low absorption, and well-ordered H2-dilutedp-a-SiC:H buffer layer deposition onto the undiluted

a�

FAX: �82-42-869-8530; electronic mail: [email protected]

Downloaded 24 May 2007 to 143.248.154.207. Redistribution subject t

p-a-SiC:H window layer.6 Due to the natural hydrogen treat-ment, we can effectively reduce the recombination at the p / iinterface, resulting in dramatic improvement of all solar cellparameters.

The B doping of nc-Si–SiC:H alloys considerably en-hances their dark conductivity ��D�. At the same time, thedoping level may influence the degree of the structural dis-order, film crystallinity, and defect density distribution. Vari-ous carrier transport mechanisms have been reported for a-Si:H,9–11 �c-Si:H,12,13 and nc-C:H films.14 In this paper, weinvestigated the electric transport of p-nc-Si−SiC:H alloys.

II. EXPERIMENT

Films were deposited by the Hg-sensitized photoassistedchemical-vapor deposition �photo-CVD� technique using themixture of SiH4, H2, B2H6, and C2H4 reactant gases. A low-pressure Hg lamp with resonance lines of 184.9 and 253.7nmwas used as an UV light source to dissociate the mixturegases. In all depositions, the hydrogen dilution ratio�H2/SiH4�, ethylene gas flow ratio �C2H4/SiH4�, chamberpressure, substrate temperature, and Hg bath temperaturewere kept at 20, 0.07, 0.46 Torr, at 250 and 20 °C, respec-tively. We deposited about 150–170-nm-thick films on Corn-ing 7059 glass substrates with varying boron doping ratio�B2H6/SiH4� from 1000 to 8000 ppm.

We performed Raman spectroscopy and dynamic forcemicroscopy �DFM� to inspect the structural change of thefilms. Raman spectra were measured by using JASCO Corp.,NRS-1000 system. The wavelength of Ar laser is 532 nm.We used phase-modulated spectroscopic ellipsometer �JobinYvon, UNISEL� in order to measure the film thickness andabsorption coefficient. We measured the direct current �dc��D via coplanar Al contacts �gap:1 mm� formed by thermalevaporation. The temperature dependence of �D was mea-sured using a closed-cycle He cryostat with a proportional-integral-derivative �PID� temperature controller. Since �D

measurements performed in a coplanar configuration are sen-15

sitive to the presence of surface adsorbates, we maintained

o AIP license or copyright, see http://jap.aip.org/jap/copyright.jsp

Page 3: Photo Conductivity on Photo Current Action.af

JOURNAL OF APPLIED PHYSICS 99, 033520 �2006�

A cubic phase of C3N4 synthesized in the diamond-anvil cellL. C. Minga� and P. ZininHawaii Institute of Geophysics and Planetology, University of Hawaii at Manoa, Honolulu, Hawaii, 96822

Y. MengHPCAT, APS, Argonne National Laboratory, 9700 South Cass Avenue, Argonne, Illinois 60439

X. R. Liu and S. M. HongLaboratory of High Pressure Physics, Southwest Jiaotong University, Chengdu, Sichuan, China 610031

Y. XieStructure Research Laboratory and Department of Chemistry,University of Science and Technology of China, Hefei, Anhui, China 230026

�Received 1 September 2005; accepted 15 December 2005; published online 14 February 2006�

A cubic phase of C3N4 was discovered. It was recovered at ambient conditions from thegraphite-like C3N4 �g-C3N4� phase subjected to pressures between 21 and 38 GPa in adiamond-anvil cell, laser heated to temperatures between 1600 and 3000 K. The x-ray-diffractiondata of the phase are best explained by a cubic unit cell with the lattice parameters a=3.878±0.001 Å. With an assumption of 1 molecule/unit cell �Z=1� for the cubic phase, the molarvolume of the cubic phase is 35.126 cm3/mol and the density is 2.62 g/cm3. The density of thecubic phase is less than that which was predicted for the high-pressure phases but is 12% denserthan the low-pressure graphitic phase ��=2.336 g/cm3�. The cubic phase has not been predictedtheoretically and represents an unknown structure in C3N4. © 2006 American Institute of Physics.�DOI: 10.1063/1.2168567�

I. INTRODUCTION

The prediction by Liu and Cohen1 of the existence of a�-C3N4 phase with a bulk modulus and hardness similar todiamond has fostered significant research efforts to synthe-size this material �see review papers2–4� Various reportsclaim the synthesis of carbon nitride C3N4: however, “noneof the studies provide a comprehensive characterization of asingle phase material.”3 The data on the synthesis of denseC3N4 phases reported by different authors to date “have yetto present unambiguous evidence for the crystallization ofcarbon nitrides with the proposed structures.”4 It was alsoargued from chemical considerations that “extensive networkwith C–N single bonding has never been documented andonly very local C–N bonds are known as in amino acids.”5

Recent studies of the phase transformation of the turbo-stratic carbon nitride �t-CN� under high pressure and tem-perature showed that at 4.7 GPa, the thermal decompositionof t-CN starts at 990 K and forms a disordered graphite. Witha pressure increase up to 17.8 GPa, the onset temperature ofthe decomposition increases to 1850 K, and the process isaccompanied by the formation of diamond.6 It was con-cluded that the low-compressibility forms of carbon nitridecould not be obtained in a large volume press at pressures upto 20 GPa and temperatures up to 2100 K.

In this study, we present evidence for an unquenchablehigh-pressure phase at a pressure above 20 GPa and at tem-perature above 1600 K, which converts into a cubic phaseupon the decompression to ambient conditions.

a�

Electronic mail: [email protected]

0021-8979/2006/99�3�/033520/6/$23.00 99, 03352

II. EXPERIMENTAL METHODS

The pressure- and temperature-induced phase transfor-mation of C3N4 was studied up to 38 GPa using laser-heateddiamond-anvil cells and angle-dispersive powder x-ray dif-fraction �ADXD� at Advanced Photon Source �APS�, Ar-gonne National Laboratory. A Mao-Bell-type diamond anvilcell �DAC� and a symmetrical DAC were used in this study.Each cell has a pair of type I brilliant-cut diamond anvils of0.25–0.3 carat and a culet of 350–500 �m. A stainless-steel,fully hardened 250-�m-thick gasket was first indented to40 �m, and then a hole of 90–130 �m in diameter wasdrilled, which served as the sample chamber. The experi-ments were performed using a monochromatic synchrotron-radiation source ��=0.3876 or 0.46047 Å� at the 16-IDBbeamline of the High Pressure Collaborative Access Team�HPCAT� facility at the Advanced Photon Source �APS�. Themonochromatic x-ray beam was focused down to 30�30 �m2, using multilayer bimorph mirrors in aKickpatrick-Baez configuration, similar to that described ear-lier for the GeoSoilEnviroConsortium for Advanced Radia-tion Studies �GSECARS� facility.7 Diffraction images wererecorded for 30–300 s with an on-line image-plate fast-scandetector and were integrated and corrected for distortionsusing FIT2D software.8 The distance between the sample andthe charge-coupled device �CCD� detector was either347.964 mm �for �=0.38760 Å� or 347.818 mm �for �=0.46047 Å�. Pressure was determined off line by the rubyfluorescent method and also by the internal NaCl using thediffraction peak �200� with the established equation of state.9

To achieve high-pressure and high-temperature condi-

tions, the sample in a diamond-anvil cell was heated by a

© 2006 American Institute of Physics0-1

Page 4: Photo Conductivity on Photo Current Action.af

evier.com/locate/diamond

Diamond & Related Materials

Combined FIB technique with acoustic microscopy to detect steel–DLC

interface defects

Karin Bernland a, Bernd Kohler a, Pavel V. Zinin b,*, Dong Fei c, Douglas A. Rebinsky c

a Fraunhofer IZFP-D, 01326 Dresden, Germanyb School of Ocean and Earth Science and Technology, University of Hawaii, Honolulu, HI 96822, USA

c Technical Center, Caterpillar Inc., Peoria, IL 61656, USA

Received 18 March 2005; received in revised form 1 September 2005; accepted 25 October 2005

Available online 4 January 2006

Abstract

In this work, focused ion beam (FIB) technique was employed to characterize subsurface defects in chromium-containing DLC (Cr-DLC)

coatings. Subsurface defects as small as one micron were successfully detected in a flat Cr-DLC coated steel coupon by scanning acoustic

microscopy (SAM). The nature of the subsurface defects in the coating was investigated by repeated FIB milling and a scanning electron

microscopy (SEM) imaging technique where element analysis was done by Energy Dispersive X-ray spectroscopy (EDX). It has been found that

defects are located mostly at the DLC steel interface. A model for the bump-like defects is proposed based on the SAM, EDX and FIB

measurements. Model is that of a dilamination/void between Cr layer and steel substrate. The existence of these defects at the interface leads to the

defects visible on the surface of the DLC coating; bumps or pits which are larger in diameter compared to the subsurface defects.

D 2005 Elsevier B.V. All rights reserved.

Keywords: Acoustic microscopy; Focused ion beam; Subsurface defects; Diamond-like carbon

1. Introduction

DLC (diamond-like-carbon) coatings have been widely used

in tribological applications because of their distinct tribological

properties such as high wear, pitting and scuffing resistance,

and low friction [1–6]. DLC coatings are typically produced

using physical vapor deposition (PVD) processes. Defects

including voids, scratches, flaking, cracks, and macroparticles

often occur in the coating, both at and below the surface. These

defects can adversely affect the performance of the coating,

leading to unpredicted premature failure of the coated

component. To our knowledge defects at the DCL coating/

steel interface have not been investigated.

Acoustic microscopy studies conducted with a high-

frequency (1 GHz) acoustic microscope showed that most of

the defects are located 2–3 Am below the top surface of the

coatings [7,8]. However, acoustic microscopy cannot deter-

mine the exact size, depth, and composition of the defects.

While conventional optical microscopy or scanning electron

0925-9635/$ - see front matter D 2005 Elsevier B.V. All rights reserved.

doi:10.1016/j.diamond.2005.10.070

* Corresponding author.

E-mail address: [email protected] (P.V. Zinin).

microscopy (SEM) can give this information, both techniques

have the limitation in that only surface defects can be

examined. The purpose of this paper is to characterize the

subsurface defects (location of the defect and its composition)

in chromium-containing DLC (Cr-DLC) coated components by

using a focused ion beam technique FIB/SEM imaging system

and scanning acoustic microscopy (SAM). An ion sputtering

process was applied from above the specimen to do fine

milling of the coating. The specimen was then imaged with

high resolution by SEM. With repeated sputtering and imaging,

the 3-D microstructure of the coating was examined without

removing or repositioning the specimen. We demonstrated that

a combination of the FIB/SEM and SAM techniques is a

powerful tool for studying defects at the DLC/steel interface.

2. Deposition process

The Cr-DLC coating was produced in Caterpillar’s in-house

closed-field unbalanced magnetron sputtering system

(CFUMS). The CFUMS is a 1/4 industrial scale chamber.

The system incorporates two opposing rectangular cathodes

each having a Cr target with a purity of 99.95%. Argon (Ar) is

15 (2006) 1405 – 1411

www.els

Page 5: Photo Conductivity on Photo Current Action.af

evier.com/locate/diamond

Diamond & Related Materials

Combined FIB technique with acoustic microscopy to detect steel–DLC

interface defects

Karin Bernland a, Bernd Kohler a, Pavel V. Zinin b,*, Dong Fei c, Douglas A. Rebinsky c

a Fraunhofer IZFP-D, 01326 Dresden, Germanyb School of Ocean and Earth Science and Technology, University of Hawaii, Honolulu, HI 96822, USA

c Technical Center, Caterpillar Inc., Peoria, IL 61656, USA

Received 18 March 2005; received in revised form 1 September 2005; accepted 25 October 2005

Available online 4 January 2006

Abstract

In this work, focused ion beam (FIB) technique was employed to characterize subsurface defects in chromium-containing DLC (Cr-DLC)

coatings. Subsurface defects as small as one micron were successfully detected in a flat Cr-DLC coated steel coupon by scanning acoustic

microscopy (SAM). The nature of the subsurface defects in the coating was investigated by repeated FIB milling and a scanning electron

microscopy (SEM) imaging technique where element analysis was done by Energy Dispersive X-ray spectroscopy (EDX). It has been found that

defects are located mostly at the DLC steel interface. A model for the bump-like defects is proposed based on the SAM, EDX and FIB

measurements. Model is that of a dilamination/void between Cr layer and steel substrate. The existence of these defects at the interface leads to the

defects visible on the surface of the DLC coating; bumps or pits which are larger in diameter compared to the subsurface defects.

D 2005 Elsevier B.V. All rights reserved.

Keywords: Acoustic microscopy; Focused ion beam; Subsurface defects; Diamond-like carbon

1. Introduction

DLC (diamond-like-carbon) coatings have been widely used

in tribological applications because of their distinct tribological

properties such as high wear, pitting and scuffing resistance,

and low friction [1–6]. DLC coatings are typically produced

using physical vapor deposition (PVD) processes. Defects

including voids, scratches, flaking, cracks, and macroparticles

often occur in the coating, both at and below the surface. These

defects can adversely affect the performance of the coating,

leading to unpredicted premature failure of the coated

component. To our knowledge defects at the DCL coating/

steel interface have not been investigated.

Acoustic microscopy studies conducted with a high-

frequency (1 GHz) acoustic microscope showed that most of

the defects are located 2–3 Am below the top surface of the

coatings [7,8]. However, acoustic microscopy cannot deter-

mine the exact size, depth, and composition of the defects.

While conventional optical microscopy or scanning electron

0925-9635/$ - see front matter D 2005 Elsevier B.V. All rights reserved.

doi:10.1016/j.diamond.2005.10.070

* Corresponding author.

E-mail address: [email protected] (P.V. Zinin).

microscopy (SEM) can give this information, both techniques

have the limitation in that only surface defects can be

examined. The purpose of this paper is to characterize the

subsurface defects (location of the defect and its composition)

in chromium-containing DLC (Cr-DLC) coated components by

using a focused ion beam technique FIB/SEM imaging system

and scanning acoustic microscopy (SAM). An ion sputtering

process was applied from above the specimen to do fine

milling of the coating. The specimen was then imaged with

high resolution by SEM. With repeated sputtering and imaging,

the 3-D microstructure of the coating was examined without

removing or repositioning the specimen. We demonstrated that

a combination of the FIB/SEM and SAM techniques is a

powerful tool for studying defects at the DLC/steel interface.

2. Deposition process

The Cr-DLC coating was produced in Caterpillar’s in-house

closed-field unbalanced magnetron sputtering system

(CFUMS). The CFUMS is a 1/4 industrial scale chamber.

The system incorporates two opposing rectangular cathodes

each having a Cr target with a purity of 99.95%. Argon (Ar) is

15 (2006) 1405 – 1411

www.els

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ISSN 0020-1685, Inorganic Materials, 2006, Vol. 42, No. 5, pp. 528–531. © Pleiades Publishing, Inc., 2006.Original Russian Text © A.V. Shlyakhtina, I.V. Kolbanev, O.K. Karyagina, 2006, published in Neorganicheskie Materialy, 2006, Vol. 42, No. 5, pp. 587–590.

528

INTRODUCTION

The (

Gd

1 –

x

Ca

x

)

2

Ti

2

O

7

(

x

= 0.1) pyrochlore titanatewas reported to possess high ionic conductivity,

~5

×

10

–2

S/cm at 1000

°

C [1].

Gd

2

(Ti

1 –

x

Zr

x

)

2

O

7

disorderedpyrochlores (1000

°

C ionic conductivity of

~

1

×

10

2

S/cm) [2, 3] and

Gd

2

((GaSb)

1

x

Zr

x

)

2

O

7

[4] werealso shown to be good high-temperature oxygen-ionconductors. Recently, the family of pyrochlore-like titan-ates with high ionic conductivity has been extendedowing to the synthesis of new materials with the generalstoichiometry

Ln

2 +

x

Ti

2 –

x

O

7 –

x

/2

(Ln = Dy–Lu,

x

=0

0.44) (LANTIOX) [5–11]. Among these materials,the highest ionic conductivity,

~5

×

10

–3

to 10

–2

S/cm at740

°

C, is offered by Ln

2

Ti

2

O

7

and

Ln

2 +

x

Ti

2 –

x

O

7 –

x

/2

with

x

= 0.096 synthesized at 1600–1670

°

C [5–11].The high-temperature ionic conductivity of Yb

2

Ti

2

O

7

(

~5

×

10

–3

to 10

–2

S/cm at 740

°

C) prepared throughmechanical activation or hydroxide coprecipitation fol-lowed by firing at 1600

°

C was studied in detail byShlyakhtina et al. [5–7] and Abrantes et al. [10, 11].

As reported by Kramer and Tuller [1], heterovalentsubstitution of calcium for gadolinium in the pyro-chlore titanate Gd

2

Ti

2

O

7

(

(Gd

1 –

x

Ca

x

)

2

Ti

2

O

7

,

x

= 0.1)increases its ionic conductivity by three orders of mag-nitude: from 2

×

10

–5

to 2

×

10

–2

S/cm at 740

°

C. It isreasonable to expect that calcium doping will alsocause a significant increase in the ionic conductivity ofYb

2

Ti

2

O

7

.

In this paper, we report the synthesis of

(Yb

1

x

Ca

x

)

2

Ti

2

O

7

and

(Yb

1 –

x

Ba

x

)

2

Ti

2

O

7

(

x

= 0, 0.05,0.1) samples using coprecipitation or mechanical acti-vation and their electrical conductivity.

EXPERIMENTAL

(Yb

1 –

x

Ca

x

)

2

Ti

2

O

7

(

x

= 0.05, 0.1) samples were pre-pared via coprecipitation of Yb(III), Ca(II), and Ti(IV)hydroxides with NH

4

OH and (NH

4

)

2

CO

3

at pH 10.4from appropriate chloride solutions. A sulfate test forCa

2+

in the mother solution showed that all of the cal-cium was precipitated. The precipitate was separatedfrom the solution by centrifugation and then washedseveral times with water until free of chloride ions.Next, the precipitate was dried at 105

°

C for 24 h. Afterprefiring at 650

°

C for 2 h, the resultant powder waspressed at 10 MPa into disks 10 mm in diameter and2 mm in thickness, which were sintered at 1400

°

C for4 h and then furnace-cooled.

We also used mechanical activation of Yb

2

O

3

+TiO

2

, CaO + Yb

2

O

3

+ TiO

2

, and Yb

2

O

3

+ TiO

2

+ BaO

2

mixtures in an eccentric vibratory mill [12] to prepare

Yb

2

Ti

2

O

7

, (Yb

1 –

x

Ca

x

)

2

Ti2O7, and (Yb1 – xBax)2Ti2O7 (x =0.05, 0.1) samples. It is well known that mechanicalactivation of starting mixtures allows one to vary thestructural perfection and, accordingly, the conductivityof the resulting mixed oxides [13, 14]. After mechani-cal activation, the mixtures were pressed at 26 MPa into

Effect of Heterovalent Substitution on the Electrical Conductivity of (Yb1 – xMx)2Ti2O7

(M = Ca, Ba; x = 0, 0.05, 0.1) A. V. Shlyakhtinaa, I. V. Kolbaneva, O. K. Karyaginab

a Semenov Institute of Chemical Physics, Russian Academy of Sciences, ul. Kosygina 4, Moscow, 119991 Russia

b Emanuel Institute of Biochemical Physics, Russian Academy of Sciences, ul. Kosygina 4, Moscow, 119991 Russia

e-mail: [email protected] December 8, 2005

Abstract—(Yb1 – xëax)2Ti2O7 and (Yb1 – xBax)2Ti2O7 (x = 0, 0.05, 0.1) have been synthesized using hydroxidecoprecipitation and mechanical activation of oxide mixtures, and their electrical conductivity has been mea-sured from 350 to 1000°C. The pyrochlore titanate (Yb0.9Ca0.1)2Ti2O7 synthesized at 1400°C from a mechani-cally activated oxide mixture has the highest conductivity, ~0.1 S/cm at 1000°C, among the oxygen-ion-con-ducting pyrochlores studied so far. The (Yb0.95Ca0.05)2Ti2O7 and (Yb0.9Ca0.1)2Ti2O7 samples prepared by react-ing coprecipitated powder mixtures at 1400°C have a lower conductivity, as do the (Yb1 – xBax)2Ti2O7 (x = 0.05,0.1) samples prepared using mechanical activation.

DOI: 10.1134/S0020168506050141

Page 7: Photo Conductivity on Photo Current Action.af

ISSN 0020-1685, Inorganic Materials, 2006, Vol. 42, No. 5, pp. 528–531. © Pleiades Publishing, Inc., 2006.Original Russian Text © A.V. Shlyakhtina, I.V. Kolbanev, O.K. Karyagina, 2006, published in Neorganicheskie Materialy, 2006, Vol. 42, No. 5, pp. 587–590.

528

INTRODUCTION

The (

Gd

1 –

x

Ca

x

)

2

Ti

2

O

7

(

x

= 0.1) pyrochlore titanatewas reported to possess high ionic conductivity,

~5

×

10

–2

S/cm at 1000

°

C [1].

Gd

2

(Ti

1 –

x

Zr

x

)

2

O

7

disorderedpyrochlores (1000

°

C ionic conductivity of

~

1

×

10

2

S/cm) [2, 3] and

Gd

2

((GaSb)

1

x

Zr

x

)

2

O

7

[4] werealso shown to be good high-temperature oxygen-ionconductors. Recently, the family of pyrochlore-like titan-ates with high ionic conductivity has been extendedowing to the synthesis of new materials with the generalstoichiometry

Ln

2 +

x

Ti

2 –

x

O

7 –

x

/2

(Ln = Dy–Lu,

x

=0

0.44) (LANTIOX) [5–11]. Among these materials,the highest ionic conductivity,

~5

×

10

–3

to 10

–2

S/cm at740

°

C, is offered by Ln

2

Ti

2

O

7

and

Ln

2 +

x

Ti

2 –

x

O

7 –

x

/2

with

x

= 0.096 synthesized at 1600–1670

°

C [5–11].The high-temperature ionic conductivity of Yb

2

Ti

2

O

7

(

~5

×

10

–3

to 10

–2

S/cm at 740

°

C) prepared throughmechanical activation or hydroxide coprecipitation fol-lowed by firing at 1600

°

C was studied in detail byShlyakhtina et al. [5–7] and Abrantes et al. [10, 11].

As reported by Kramer and Tuller [1], heterovalentsubstitution of calcium for gadolinium in the pyro-chlore titanate Gd

2

Ti

2

O

7

(

(Gd

1 –

x

Ca

x

)

2

Ti

2

O

7

,

x

= 0.1)increases its ionic conductivity by three orders of mag-nitude: from 2

×

10

–5

to 2

×

10

–2

S/cm at 740

°

C. It isreasonable to expect that calcium doping will alsocause a significant increase in the ionic conductivity ofYb

2

Ti

2

O

7

.

In this paper, we report the synthesis of

(Yb

1

x

Ca

x

)

2

Ti

2

O

7

and

(Yb

1 –

x

Ba

x

)

2

Ti

2

O

7

(

x

= 0, 0.05,0.1) samples using coprecipitation or mechanical acti-vation and their electrical conductivity.

EXPERIMENTAL

(Yb

1 –

x

Ca

x

)

2

Ti

2

O

7

(

x

= 0.05, 0.1) samples were pre-pared via coprecipitation of Yb(III), Ca(II), and Ti(IV)hydroxides with NH

4

OH and (NH

4

)

2

CO

3

at pH 10.4from appropriate chloride solutions. A sulfate test forCa

2+

in the mother solution showed that all of the cal-cium was precipitated. The precipitate was separatedfrom the solution by centrifugation and then washedseveral times with water until free of chloride ions.Next, the precipitate was dried at 105

°

C for 24 h. Afterprefiring at 650

°

C for 2 h, the resultant powder waspressed at 10 MPa into disks 10 mm in diameter and2 mm in thickness, which were sintered at 1400

°

C for4 h and then furnace-cooled.

We also used mechanical activation of Yb

2

O

3

+TiO

2

, CaO + Yb

2

O

3

+ TiO

2

, and Yb

2

O

3

+ TiO

2

+ BaO

2

mixtures in an eccentric vibratory mill [12] to prepare

Yb

2

Ti

2

O

7

, (Yb

1 –

x

Ca

x

)

2

Ti2O7, and (Yb1 – xBax)2Ti2O7 (x =0.05, 0.1) samples. It is well known that mechanicalactivation of starting mixtures allows one to vary thestructural perfection and, accordingly, the conductivityof the resulting mixed oxides [13, 14]. After mechani-cal activation, the mixtures were pressed at 26 MPa into

Effect of Heterovalent Substitution on the Electrical Conductivity of (Yb1 – xMx)2Ti2O7

(M = Ca, Ba; x = 0, 0.05, 0.1) A. V. Shlyakhtinaa, I. V. Kolbaneva, O. K. Karyaginab

a Semenov Institute of Chemical Physics, Russian Academy of Sciences, ul. Kosygina 4, Moscow, 119991 Russia

b Emanuel Institute of Biochemical Physics, Russian Academy of Sciences, ul. Kosygina 4, Moscow, 119991 Russia

e-mail: [email protected] December 8, 2005

Abstract—(Yb1 – xëax)2Ti2O7 and (Yb1 – xBax)2Ti2O7 (x = 0, 0.05, 0.1) have been synthesized using hydroxidecoprecipitation and mechanical activation of oxide mixtures, and their electrical conductivity has been mea-sured from 350 to 1000°C. The pyrochlore titanate (Yb0.9Ca0.1)2Ti2O7 synthesized at 1400°C from a mechani-cally activated oxide mixture has the highest conductivity, ~0.1 S/cm at 1000°C, among the oxygen-ion-con-ducting pyrochlores studied so far. The (Yb0.95Ca0.05)2Ti2O7 and (Yb0.9Ca0.1)2Ti2O7 samples prepared by react-ing coprecipitated powder mixtures at 1400°C have a lower conductivity, as do the (Yb1 – xBax)2Ti2O7 (x = 0.05,0.1) samples prepared using mechanical activation.

DOI: 10.1134/S0020168506050141

Page 8: Photo Conductivity on Photo Current Action.af

006) 1149–1155www.elsevier.com/locate/ssi

Solid State Ionics 177 (2

Synthesis and electrical transport properties ofLu2+xTi2−xO7−x/2 oxide-ion conductors

A.V. Shlyakhtina a,⁎, J.C.C. Abrantes b, A.V. Levchenko c, A.V. Knot'ko d,O.K. Karyagina e, L.G. Shcherbakova a

a Semenov Institute of Chemical Physics RAS, Kosygina str. 4, 119991 Moscow, Russiab Polytechnic Institute of Viana do Castelo, ESTG, 4900 Viana do Castelo, Portugal

c Institute of Problems of Chemical Physics RAS, Chernogolovka, 142432 Moscow reg., Russiad Moscow State University, Chemical Department, Leninskie gory, 119992, Russia

e Emanuel Institute of Biochemical Physics RAS, Kosygina str. 4, 119991 Moscow, Russia

Received 24 November 2005; accepted 12 April 2006

Abstract

The Lu2+xTi2−xO7−x/2 (x=0; 0.052; 0.096; 0.286; 0.44; 0.63; 33.3–49 mol% Lu2O3) nanoceramics with partly disordered pyrochlore-typestructure are prepared by sintering freeze-dried powders obtained by a co-precipitation technique with 1600 °C annealing. Similar to pyrochlore-like compositions in the zirconate system, some of the new titanates are good oxide-ion conductors in air. The new solid-state electrolytes haveoxide-ion conductivity in the interval of 1.0×10−3–2.5×10−3 S/cm at 740 °C in air. This value of conductivity is comparable with that of ZrO2/Y2O3 ceramics. The conductivity of Lu2+xTi2−xO7−x/2 depends on the chemical composition. The highest ionic conductivity is exhibited by nearlystoichiometric Lu2+xTi2−xO7−x/2 (x=0.096; 35.5 mol% Lu2O3) material containing ∼ 4.8 at.% LuTi anti-site defects.© 2006 Elsevier B.V. All rights reserved.

Keywords: Disordered pyrochlore; Anti-site defect; Oxide-ion conductor

1. Introduction

Many solid-state electrolytes have a crystal structure of thefluorite type, CaF2. Fast oxide- or fluorine-ionic transport inthese structures is usually related to presence of numerousanionic vacancies. High oxide-ion conductivity, according tothe vacancy mechanism, has been shown to exist in the cubicphases of ZrO2 (9 mol% Y2O3, Sc2O3), CeO2, ThO2 and δ-Bi2O3 [1,2].

Double pyrochlore-like oxides Ln2M2O7 (Ln-rare-earthelements, M=Hf, Zr, Ti) are very close structurally to oxy-gen-deficient fluorites. Ln2M2O7 often demonstrate a tendencyto order–disorder (pyrochlore–fluorite) transformation at ele-vated temperatures. Order–disorder phase transitions of thistype are known for Gd2Hf2O7 (TP→F=2150 °C) and Tb2Hf2O7

⁎ Corresponding author. Tel.: +7 495 137 83 03; fax: +7 495 242 02 53.E-mail addresses: [email protected], [email protected]

(A.V. Shlyakhtina).

0167-2738/$ - see front matter © 2006 Elsevier B.V. All rights reserved.doi:10.1016/j.ssi.2006.04.022

(TP→F=2350 °C) [3]. As for the zirconates, the pyrochlore–fluorite transformations exist in Nd2Zr2O7, Sm2Zr2O7 (TP→F=2000 °C), and Gd2Zr2O7 (TP→F=1530 °C) [4]. It was found thatthe pyrochlore–fluorite transformations greatly influence theiroxide-ion conduction properties, so the high-temperature modi-fications possess pronounced ionic conductivity while pureelectronic transport dominate in the low-temperature ones.

It is also known that the structural type of Ln2Zr2O7 (Ln-rare-earth elements) compounds is clearly defined by the Ln-element in its formula. The existence of pyrochlore-type struc-tures was shown for the rare-earth zirconates with Ln=La–Sm,whereas the zirconates with Ln=Tb–Lu have fluorite-typestructures. Sm2Zr2O7, Eu2Zr2O7, and Gd2Zr2O7 are located nearthe phase boundary between the pyrochlores and the fluorites.One of these compounds (Eu2Zr2O7) is known to possess thehighest oxide-ion conductivity in the series, σ=8.3×10−3 S/cmat 800 °C [5].

Among rare-earth titanates, some Ln2Ti2O7 pyrochlorecompositions have already proved their ability for oxide-ion

Page 9: Photo Conductivity on Photo Current Action.af

New Oxide-Ion Conductors Ln2+xTi2-xO7-x/2

(Ln = Dy – Lu; x=0.096)

A. V. Shlyakhtina1,a, J.C.C. Abrantes2,b, A.V. Levchenko3, c , S. Yu. Stefanovich4,d, A.V. Knot,ko4, d , L.L. Larina 5, e

1Semenov Institute of Chemical Physics RAS; Kosygina str., 4, 119991, Moscow, Russia;

2Polytechnic Institute of Viana do Castelo, ESTG, 4900 Viana do Castelo, Portugal;

3Institute of Problems of Chemical Physics RAS; Chernogolovka, Moscow reg., 142432,

Russia; 4Moscow State University, Chemical Department; Leninskie gory, 119992, Russia;

5Emanuel Institute of Biochemical Physics RAS; Kosygina str., 4, 119991, Moscow,

Russia [email protected],

[email protected],

[email protected]

Keywords: electrolyte, ionic conductivity, ion blocking, disordered pyrochlore

Abstract. New oxide-ion conductors Ln2+xTi2-xO7-x/2 (Ln = Dy-Lu, x = 0.096; 35.5 mol. % Ln2O3·

64.5 mol.% TiO2)-LANTIOX with a disordered pyrochlore structure are obtained by coprecipitation

with next thermal annealing at 1600ºC. Their ionic conductivity in air at 740°C attains 5·10-3

S/cm

for Ln2.096Ti1.904O6.952 (Ln= Lu, Yb), 10-3

S/cm for Ln2,096Ti1,904O6,952 (Ln=Tm, Er, Ho) and 10-

4S/cm for Dy2,096Ti1,904O6,952 due to the presence of defects in both the cation and anion sublattices.

The materials contain ~4.8-10.6% LnTi anti-site defects. Ln2.096Ti1.904O6.952 ceramics are shown to

be stable in the temperature range of 1400 to 1700ºC. 1400ºC-samples Ln2+xTi2-xO7-x/2 have lower

ionic conductivity then 1600ºC and 1690ºC samples.

Introduction

Oxygen-ion conductors are the subject of extensive studies as potentially attractive solid electrolytes

for use in high-temperature fuel cells, oxygen sensors for the automobile industry, and oxygen-ion-

conducting membranes for catalytic synthesis [1].

In 2003, a new family of ion-conducting pyrochlores with the general formula Ln2+xTi2-xO7-x/2 (Ln =

Tm-Lu; x = 0-0.63; 33.3-49 mol% Ln2O3) was discovered [2, 3]. The high-temperature phases of

the stoichiometric compounds Ln2Ti2O7 (Ln = Tm-Lu) have a distorted pyrochlore structure, form at

1600-1670ºC, i.e., near their melting points (1670ºC for Lu2Ti2O7, 1690ºC for Yb2Ti2O7 and

Tm2Ti2O7), and at 740ºC have an ionic conductivity of 10-3

to 10-2

S/cm. Ion-conducting

nonstoichiometric Lu2+xTi2-xO7-x/2(x = 0.052-0.63; 34.5- 49 mol% Lu2O3) were sintered in a broader

temperature range : 1400-1700ºC [3].

According to the Ln2O3 –TiO2 (Ln = Tm-Lu) phase diagrams, the compounds Ln2Ti2O7 in these

systems have broad homogeneity ranges, from 33.3 to 50 mol % Ln2O3 [3,4]. The phase diagrams of

the Er2O3 –TiO2, Ho2O3 –TiO2, and Dy2O3 –TiO2 systems differ from those of Ln2O3 –TiO2 with

Ln = Tm-Lu [4], however the pyrochlore phases of Ln2Ti2O7 in the former systems have 33.3-50

mol% homogeneity ranges when Ln is Er or Ho. Dy2O3-TiO2 system has narrower homogeneity

range of pyrochlore Dy2Ti2O7.

In this paper, we report the synthesis and electrical conductivity of Ln2+xTi2-xO7-x/2 (Ln = Dy-Lu, x =

0.096) materials (compositions falling within the homogeneity range of Ln2Ti2O7).

Materials Science Forum Vols. 514-516 (2006) pp. 422-426online at http://www.scientific.net© (2006) Trans Tech Publications, Switzerland

Page 10: Photo Conductivity on Photo Current Action.af

(2007) 59–66www.elsevier.com/locate/ssi

Solid State Ionics 178

Effect of non-stoichiometry and synthesis temperature on the structure andconductivity of Ln2+xM2−xO7−x/2 (Ln=Sm–Gd; M=Zr, Hf; x=0–0.286)☆

A.V. Shlyakhtina a,⁎, A.V. Knotko b, M.V. Boguslavskii b, S.Yu. Stefanovich b,I.V. Kolbanev a, L.L. Larina c, L.G. Shcherbakova a

a Semenov Institute of Chemical Physics, Russian Academy of Sciences, Kosygina str. 4, 119991 Moscow, Russiab Department of Chemistry, Moscow State University, Leninskie gory, 119992 Moscow, Russia

c Emanuel Institute of Biochemical Physics, Russian Academy of Sciences, Kosygina str. 4, 119991 Moscow, Russia

Received 31 May 2005; received in revised form 19 July 2006; accepted 2 November 2006

Abstract

The effect of non-stoichiometry on the crystal structure and total conductivity of Ln2+xZr2−xO7−x/2 (Ln=Sm–Gd; x=0–0.286) was investigated.The intensity of the pyrochlore superlattice reflections from Ln2+xZr2−xO7−x/2 is shown to decrease with increasing Ln concentration. Within thehomogeneity range of the pyrochlore phase of Ln2+xZr2−xO7−x/2 (Ln=Sm–Gd), the activation energy of high-temperature conduction in samplesprepared by 1600 °C sintering of mechanically activated oxide mixtures (Ln2O3 and ZrO2) is ∼0.87–1.04 eV. The highest conductivity among theLn2+xZr2−xO7−x/2 (Ln=Sm–Gd) materials is offered by stoichiometric Ln2Zr2O7 samples with a pyrochlore structure, which contain 5–8.1% LnZr+ZrLn anti-structure pairs, except for Gd2Zr2O7 (∼22%). The crystal structure of Ln2+xHf2−xO7−x/2 (Ln=Sm–Gd) is investigated after sintering at1000–1670 °C. The compounds Ln2.096Hf1.904O6.9 (Ln=Eu, Gd) prepared by 1200 °C sintering of mechanically activated oxides (Ln2O3 and HfO2)undergo a fluorite-type to pyrochlore phase transition above 1200 °C. The conductivity of Gd2Hf2O7 and Sm2.096Hf1.904O6.952 sintered at 1600 °Cseems to be ionic above 780 °C, with an activation energy of 0.77 and 0.82 eV, respectively. In this work, using mechanical activation of startingmixtures, the conductivity of the Ln2+xHf2−xO7−x/2 (Ln=Sm–Gd) hafnates was raised close to the level of Ln2+xZr2−xO7−x/2 (Ln=Sm–Gd). Thehafnates synthesized by the procedure in question are similar in structural disorder to Ln2+xZr2−xO7−x/2 (Ln=Sm–Gd), and the disorder ensures highoxygen ion mobility and, accordingly, significant high-temperature conductivity.© 2006 Elsevier B.V. All rights reserved.

Keywords: Pyrochlore; Fluorite; Oxide-ion conductor; Ceramics

1. Introduction

Pyrochlore oxides have recently been the subject of extensivestudies as potentially attractive dielectric materials [1], catalysts,solid electrolytes [2,3] and radiation absorbers [4]. The Gd2Zr2O7,Sm2Zr2O7, Gd2Hf2O7 and Sm2Hf2O7 pyrochlores are known toundergo a high-temperature (1530–2400 °C) order–disorder(pyrochlore–fluorite) phase transition, which is, typically accom-panied by a sharp rise in ionic conductivity [5–8]. Very recently, theLn2Ti2O7 (Ln=Tm–Lu) pyrochlores have also been found toundergo a high-temperature order–disorder transformation [9–11].

☆ Presented at the 15th Int. Conf. on Solid State Ionics, July 17–22, 2005,Baden-Baden, Germany.⁎ Corresponding author.E-mail address: [email protected] (A.V. Shlyakhtina).

0167-2738/$ - see front matter © 2006 Elsevier B.V. All rights reserved.doi:10.1016/j.ssi.2006.11.001

Among the rare-earth titanates, zirconates and hafnates with thepyrochlore structure, the highest temperatures of the pyrochlore–fluorite phase transition are observed for the hafnates. The ionicconductivity of these pyrochlores can be increased by thermaltreatment or appropriate doping. In particular, a high ionicconductivity was found in Gd2(Ti1−xZrx)2O7 with x≥0.4 [12]and (Gd1−xCax)2Ti2O7 with x=0.1 [13].

The Ln2O3–ZrO2, Ln2O3–HfO2 (Ln=Sm–Gd) and Ln2O3–TiO2 (Ln=Dy–Lu) phase diagrams are similar in that each ofthem has a significant range of pyrochlore-like solid solutions.At 1600 °C, the homogeneity range of the pyrochlore phase is33.3–49 mol% Ln2O3 in the Ln2+xTi2−xO7−x/2 (Ln=Ho–Lu)systems, ∼26–40 mol% Ln2O3 in Ln2±xZr2−±xO7±x/2 (Ln=Sm–Gd), and ∼30–40 mol% Ln2O3 in Ln2+xHf2−xO7−x/2 (Ln=Sm–Gd). Most of these pyrochlore solid solutions can be regarded asdoped Ln2M2O7 (M=Ti, Zr, Hf) pyrochlores, i.e., solid

Page 11: Photo Conductivity on Photo Current Action.af

Order–disorder phase transitions and high-temperature oxide ion

conductivity of Er2+xTi2�xO7�d (x = 0, 0.096)

A.V. Shlyakhtina a,*, A.V. Levchenko b, J.C.C. Abrantes c, V.Yu. Bychkov a,V.N. Korchak a, V.A. Rassulov d, L.L. Larina e, O.K. Karyagina e, L.G. Shcherbakova a

a Semenov Institute of Chemical Physics, Russian Academy of Sciences, ul. Kosygina 4, Moscow 119991, Russiab Institute of Problems of Chemical Physics, Russian Academy of Sciences, Chernogolovka, Moscow Region 142432, Russia

c UIDM, ESTG, Instituto Politecnico de Viana do Castelo, Apartado 574, 4901-908 Viana do Castelo, Portugald All-Russia Research Institute of Mineral Resources, Staromonetnyi per. 31, Moscow 119017, Russia

e Emanuel Institute of Biochemical Physics, Russian Academy of Sciences, ul. Kosygina 4, Moscow 119991, Russia

Received 24 January 2006; received in revised form 9 June 2006; accepted 19 July 2006

Available online 22 August 2006

Abstract

The Er2+xTi2�xO7�d (x = 0.096; 35.5 mol% Er2O3) solid solution and the stoichiometric pyrochlore-structured compound

Er2Ti2O7 (x = 0; 33.3 mol% Er2O3) are characterized by X-ray diffraction (phase analysis and Rietveld method), thermal analysis

and optical spectroscopy. Both oxides were synthesized by thermal sintering of co-precipitated powders. The synthesis study was

performed in the temperature range 650–1690 8C. The amorphous phase exists below 700 8C. The crystallization of the ordered

pyrochlore phase (P) in the range 800–1000 8C is accompanied by oxygen release. The ordered pyrochlore phase (P) exists in the

range 1000�1200 8C. Heat-treatment at T � 1600 8C leads to the formation of an oxide ion-conducting phase with a distorted

pyrochlore structure (P2) and an ionic conductivity of about 10�3 S/cm at 740 8C. Complex impedance spectra are used to

separately assess the bulk and grain-boundary conductivity of the samples. At 700 8C and oxygen pressures above 10�10 Pa, the

Er2+xTi2�xO7�d (x = 0, 0.096) samples are purely ionic conductors.

# 2006 Elsevier Ltd. All rights reserved.

Keywords: A. Ceramics; A. Oxides; B. Chemical synthesis; C. Electrochemical measurements; C. Impedance spectroscopy

1. Introduction

A2B2O7 pyrochlore-structured oxides have recently been the subject of intense attention owing to their unusual

electrical, magnetic, dielectric, optical and catalytic properties [1–3], which make them attractive materials for solid

oxide fuel cells, oxygen sensors, thermistors, resistors and switches. High-temperature oxide-ion conduction has been

recently found in Ca-doped Gd2Ti2O7 [4] and disordered Gd2(Ti1�xZrx)2O7 and Y2(Ti1�xZrx)2O7 pyrochlores [5,6].

Ionic conduction in the zirconate titanates is due to the formation of Frenkel defects in their oxygen sublattice,

accompanied by anti-site cation disordering [7]. Since reducing the difference in ionic radius between the A and B

cations in the pyrochlore structure favours cation disordering, Yb2Ti2O7 and Lu2Ti2O7 are likely to have disordered

www.elsevier.com/locate/matresbu

Materials Research Bulletin 42 (2007) 742–752

* Corresponding author. Tel.: +7 495 137 83 03; fax: +7 495 242 02 53.

E-mail addresses: [email protected], [email protected] (A.V. Shlyakhtina).

0025-5408/$ – see front matter # 2006 Elsevier Ltd. All rights reserved.

doi:10.1016/j.materresbull.2006.07.011

Page 12: Photo Conductivity on Photo Current Action.af

(2007) 353–357www.elsevier.com/locate/jmicmeth

Journal of Microbiological Methods 69

Novel insoluble dye-labeled substrates for screeninginulin-degrading microorganisms

Leonid N. Ten a, Wan-Taek Im b, Zubair Aslam b, Luidmila Larina c, Sung-Taik Lee b,⁎

a Department of Biology & Medicinal Science, Pai Chai University, 14 Yeonja-1-Gil, Seo-Gu, Daejeon, 302-735, Republic of Koreab Department of Biological Sciences, Korea Advanced Institute of Science and Technology, 373-1, Guseong-dong, Yuseong-gu, Daejeon 305-701, Republic of Korea

c Institute of Biochemical Physics, Kosigin st. 4, 119991 Moscow, Russia

Received 22 August 2006; accepted 6 February 2007Available online 28 February 2007

Abstract

Inulin hydrogels were used to prepare insoluble colored substrates for screening of inulin-degrading microorganisms. Methycrylated inulin,synthesized with glycidyl methacrylate, was crossed-linked by free radical polymerization producing an insoluble hydrogel. The inulin hydrogelswere colored with one of three covalent dyes (Reactive blue 4, Reactive orange 14, Reactive red 120). The efficacy of the colored inulin hydrogelswas determined by comparing their performance to insoluble, colored amylose, xylan and HE-cellulose in screening assays. Novel substrates(alone or in combination with other insoluble colored substrates) were successfully used for screening inulin-degrading microorganisms fromsludge samples. Selected strains were identified using 16S rDNA gene partial sequencing; all of them belong to the genus Bacillus.© 2007 Elsevier B.V. All rights reserved.

Keywords: Insoluble colored substrates; Inulin; Inulin-degrading microorganisms

1. Introduction

Microorganisms are the most convenient sources forproduction of polysaccharide-degrading enzymes such asamylase, xylanase and inulinase (Chaudhary et al., 1997;Pandey et al., 1999). The most widely used plate screeningmethods for the detection of polysaccharide- and protein-degrading microorganisms are based on the use of insolubledye-labeled substrates (Caplan and Fahey, 1982; Safarik, 1987;Lee and Lee, 1997; Ruijssenaars and Hartmans, 2001).Combinations of insoluble substrates, each individually col-ored, allow simultaneous screening for multiple polysaccharide-and/or protein-degrading activities (Ten et al., 2004, 2005).However, existing methods for synthesis of these substrates arenot acceptable for low molecular weight polysaccharides suchas inulin that consists of linear chains of β-(2,1)-linkedfructofuranose molecules (Van Loo et al., 1995). Inulin is oneof the main sources for production of functional sweeteners;

⁎ Corresponding author. Department of Biological Sciences, Korea AdvancedInstitute of Science and Technology, 373-1 Kuseong-Dong, Yuseong-Gu,Daejeon 305-701, Republic of Korea. Tel: +82 42 869 2617; fax: +82 42 8635617.

E-mail address: [email protected] (S.-T. Lee).

0167-7012/$ - see front matter © 2007 Elsevier B.V. All rights reserved.doi:10.1016/j.mimet.2007.02.007

internal hydrolysis of this polyfructan by microbial endoinuli-nase yields inulo-oligosaccharides that possess various health-promoting properties (Yun, 1996; Kim et al., 1997; Sangeethaet al., 2005). Selection of endoinulinase-producing microorgan-isms as sources of the enzyme is necessary for industrialproduction of functional food ingredients. It can be achievedstraightforwardly at primary screening only by using insolublecolored inulins. Taking into account the abovementionedlimitations of existing methods a new approach is required tosynthesize these practically important substrates.

The aims of this study are: (i) to design a novel method forproduction of insoluble diversely colored inulin hydrogels, and(ii) to use the resulting substrates for screening of inulin-degrading microorganisms.

2. Materials and methods

2.1. Chemicals

Reactive orange 14, Reactive red 120, inulin fromchicory root,Cibacron blue 3GA, ammonium persulfate, N,N,N′,N′-tetra-methylethylenediamine (TMEDA), 4-dimethylaminopyridine,and N,N-dimethylformamide (DMF, HPLC grade) were