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Fabrication and characterization of boron doped yttria-stabilized zirconia nanofibers.

INTRODUCTION

Polymer-based nanocomposites in the form of nano-powders, nanospheres, nanofibers, nanowires have gained much interest due to their ability to combine the advantages of both polymers and filler components. The nanocomposite form of polymers has attracted special attention in science and technology (1-5).

Nanostructure ceramic oxides, especially zirconium oxide or zirconia (Zr[0.sub.2]) gained importance due to complex structural aspects that can be achieved on a relatively small scale. Ceramic materials based on zirconium oxide (Zr[0.sub.2]), also referred to as zirconia because their excellent combination of electrical, thermal, and mechanical properties are of widespread application (6) including transparent optical devices and electrochemical capacitor electrodes (7), oxygen sensors (8), fuel cells (9), catalysts [10], and advanced ceramics (11).

Pure Zr[0.sub.2] exists in three polymorphs at low pressure (monoclinic, tetragonal, and cubic) and in an orthorhombic form at high pressure. Among them tetragonal (1175-2370[degrees]C) and cubic (2370-2680[degrees]C) phases are metastable forms but monoclinic phase is the stable form at room temperature (12).

The high temperature forms cannot be retained at room temperature. It has been found that the high temperature forms can partially or fully stabilized at room temperature by the addition of small amount of oxides (13). There are many materials such as magnesium, cerium, chromium, yttrium, and calcium oxides used as stabilizers for zirconia (14). Yttria ([Y.sub.2][O.sub.3]) stabilized zirconia (YSZ) is one of the most commonly used ceramic material with high ionic conductivity, thermal stability and excellent mechanical properties. Also, the introduction of yttrium produces defective oxides with oxygen vacancies in the crystal structure and the performance of devices based on stabilized zirconium oxide depends on the ability of these oxides to transport oxygen ions for example oxygen sensor in combustion engines, for controlling/optimizing fuel consumption, oxygen sensors in the steel industry, for monitoring oxygen content in molten steels during steel production, high temperature solid electrolytes in solid oxide fuel cell (SOFC) devices.

Electrospinning is a simple and versatile fabrication technique to produce nano and microfibrous materials. Recent advances in electrospinning have brought this already well established method back into focus of investigation as valuable method to produce ceramic nanofibers of various compositions through the use of sol-gels. Typical procedure for preparing nanofibers consists of three major steps: (i) preparation of an inorganic sol containing a matrix polymer together with a polymer precursor, (ii) electrospinning of the solution in a well controlled environment at room temperature. (iii) calcination, sintering or chemical conversion of the precursor into the desired ceramic at elevated temperature, with the removal of organic components from the precursor fibers (15). Recent study has shown that YSZ can be successfully electrospun from a sol-gel precursor and that annealing at 1500[degrees]C for 1 h results in a nanolibrous structure of pure zirconia (16).

Sintering temperature and densification of YSZ is an important economical aspect. Additives of low melting point oxides such as boron oxide ([B.sub.2][O.sub.3]), which melts at 460[degrees]C can be used for lowering the sintering temperature and improve the densification of YSZ. It has been shown that introduction of [B.sub.2][O.sub.3] to calcia stabilized zirconia (CSZ) and yttria stabilized zirconia with different amounts of boron oxide improve densification [17, 18].

This study is similar to our recent publication appeared at [19] where we prepared Boron doped poly(vinyl) alcohol/zirconium acetate (PVA/Zr) nanofibers. In this study, we wanted to improve the properties of the composite material by using both boron oxide as a sintering aid to improve the densification of zirconia, to lower the sintering temperature and to increase the thermal shock resistance of the composite structures and, the yttrium oxide as a stabilizer to produce defective oxides with oxygen vacancies in the crystal structure.

In this work, electrospinning was used for the preparation of YSZ nanofibers. The effect of calcinations' temperature on morphology and crystal structure was investigated at 250, 500, and 800[degrees]C. The study also establishes the effect of boron doping on the morphology of YSZ nanofibers at various calcinations' temperatures.

EXPERIMENTAL

Materials

Granular polyvinyl alcohol (PVA, average molecular weight 72,000 g [mo1.sup.(-1)], Aldrich Chemicals) was used as the polymeric component of all the composites fabricated in this study. Boric acid ([H.sub.2][BO.sub.4], 99.9% purity) was supplied by Merck. Zirconium acetate, solution in dilute acetic acid (Zr, 15-16%) and yttrium acetate was obtained from Sigma Aldrich. Deionized water was used as a solvent.

Preparation of Boron Doped PVA/Zirconiurn Acetate Composite Fibers

Aqueous PVA solution was prepared by dissolving the granular PVA in deionized water under constant and vigorous stirring at 80-90[degrees]C for 2 h to give 10% (w/w) PVA solution. PVA solution was then cooled to room temperature and stirred for 2 h. About 2.0 g of zirconium acetate and 2.0 g yttrium acetate with 0.2 g boric acid were added to 50.0 g aqueous PVA solution at 60[degrees]C separately and drop by drop to obtain homogeneous hybrid polymer composite solution. The solution was stirred for 1 h at this temperature and stirring was continued for 2 11 at room temperature. Thus, a viscous gel of PVA/Zr-Y acetate solutions doped and undoped with boron were obtained. Undoped PVA/Zr-Y acetate solution was prepared in the same way without the addition of boric acid.

Preparation of Nanolibers

The suspension was poured in a syringe and the needle (18 gauge) being connected to the positive terminal of a high-voltage supply (Gamma High Voltage Research) able to generate DC voltages up to 40 kV. It was delivered to the needle by a syringe pump (New Era Pump Systems, USA). The distance between the tip of the needle and the aluminum collector (diameter 10 cm) was fixed at 11-13 cm. The following operative parameters were chosen: flow rate 0.8 ml [h.sup.(-1)], applied voltage 18 kV. The fibers thus formed were dried for 12 h at 70[degrees]C. Free nanofiber mats were obtained by peeling off from the aluminum foil. Nanofiber mats were heat treated at 250. 500, and 800[degrees]C.

Measurement and Characterization

pH values of polymer solutions were determined with a pH meter (WTW 315 I Set Sentix 41 electrode, Wissen-schaftlich-Technische Werkstatten GmbH, Weilheim, Germany). Solution viscosity and conductivity measurements were performed with SV-10 viscometer (A&D Company, Tokyo, Japan). The surface tension of the polymer solution was measured by using K-100 model (KROSS, GmbH, Hamburg, Germany) manual measuring system. All measurements were made at room temperature.

The Fourier transform infrared (FTIR) spectra obtained on a Bruker Vertex 70 (Bruker Optics, Ettlingen, Germany) spectrometer equipped with a diamond protected attenuated total reflectance (ATR) crystal unit. The FTIR spectra were acquired in the range of 4000-500 [cm.sup.(-1)], and 50 scans were obtained and averaged to a resolution of 4 [cm.sup.(-1)]. Differential scanning calorimetry (DSC) measurements were carried out on DSC-60 (Shimadzu, Kyoto, Japan) equipment by using nitrogen as the purge gas. The temperature was raised from room temperature to 200[degrees]C then cooled to room temperature and the sample was heated again to 500C at a rate of 10[degrees]C [min.sup.(-1)]. Fiber formation and morphology of the electrospun boron doped and undoped YSZ samples were determined by SEM (QUANTA 400, FEI Company, Eindhoven, Netherlands) at an accelerating voltage of 10 kV. The samples were sputter coated with Au-Pd alloy before examination.

Fiber diameters were measured using ImageJ software (Image Pro-Express, Version 5.0.1.26, Media Cybernetics), calibrated using an image of known diameter. The crystal structures of the Zr[0.sub.2] powders calcined at 250, 500, and 800[degress]C were investigated by means of XRD with Ultima-IV XRD (Rigaku, Tokyo, Japan) with Cu Kcc radiation at 40 kV and 30 mA. The samples were scanned from 2[theta] = 5[degress] to 80[degress] with a scanning rate of 1 deg [min.sup.(-l)] and a step size of 0.04[degress]. X-ray photoelectron spectroscopy (XPS) analysis was performed on a SPECS XPS (Berlin, GERMANY) instrument with unmonochromatized Mg Ka radiation (1253.6 eV) as an X-ray anode. Survey and high-resolution spectra were collected using 144 and 48 eV pass energy, respectively. The X-ray gun was operated at 10 kV and 20 mA. The pressure inside the analyzer was maintained at [10.sup.(-9)] torr. The binding energy scale was referenced by setting the C--H peak maximum in the C1, spectrum to 285.0 eV and the atomic composition estimated using SpecsLab software.

RESULTS AND DISCUSSION

Solution properties have been found to affect the morphology of the fibers. The pH, viscosity, conductivity and the surface tension of the polymer solutions before the electrospinning experiment are given in Table 1. One of the major parameter influencing the fiber diameter is the solution viscosity. A higher viscosity results in a large fiber diameter. In addition, viscosity has a significant effect on whether the electrospinning jet breaks up into small droplets or whether the resulting electrospun fibers contain beads (15), (20). The conductivity of the polymer solution is important to initiate the electrospinning process. As the electrical conductivity of the solution increases, the diameter of the electrospun nanofibers decreases (15). As we introduce boron source to the polymer system, it produces crosslinks between the polymer chains that may lead to increased conductivity.

TABLE 1. Physical properties of fiber solutions before and after boron
doping.

Polymer        Conductivity           Viscosity         Surface tension
solution  (mS [cm.sup.(-1)])  (mPa [s.sup.(-1)])   PH  (mN [m.sup.(-1)])

PVA/Zr-Y                2.43               1.60  4.71                51
acetate

PVA/Zr-Y                2.63               1.62  4.80                53
acetate

B doped


FTIR spectra for the boron doped and undoped PVA/ Zr-Y acetate nanofiber samples are shown in Fig. 1. FTIR characterization was done on heat-treated ones to see if the all organics such as acetate or PVA molecules removed from the heat treated nanofibers.

As can be observed, both boron doped and undoped samples have approximately the same spectrum and peaks at about 3361, 2939, 1710, 1567, 1447, 1335, 1096, 850, 657 [cm.sup.(-1)] that the effect of PVA is dominant over all the spectra. These peaks are in good accordance with literature (21), (22). Moreover, previous studies are shown that after calcinations at 500[degress]C, the intensity of those peaks due to PVA are decreased or almost disappeared, indicating the removal of PVA molecules from the fibers (23-25).

The large bands observed between 3500 and 3200 [cm.sup.(-1)] I are linked to the stretching 0--H. The vibrational hand observed between 2800 and 3000 [cm.sup.(-1)] refers to the stretching C--H from alkyl groups and the peak at 1710 [cm.sup.(-1)] is due to the stretching C=0 vibration. Peaks at about 1567, 1447, 1096 [cm.sup.(-1)], correspond to vC=C, vC--[H.sub.2]. vC--0--C, respectively. Moreover, intensity of the FTIR vibrations for boron doped sample is probably low because of the variation in nanofiber sample thickness.

The DSC thermograms of electrospun boron doped and undoped PVA/Zr-Y acetate nanofibers are shown in Fig. 2. The endothermic melting peak ([T.sub.m]) of PVA generally appears around 257[degrees]C (21). In this study, it can be seen from the thermogram of undoped PVA/ Zr-Y acetate nanofibers that the [T.sub.m] appears around 217[degrees]C and degradation temperature about 260[degrees]C. The [T.sub.m] for boron doped PVA/Zr-Y acetate nanolibers disappeared probably due to the crosslinking of PVA with boron and forming an amorphous structure. This shows that crosslinking decreases the degree of crystallinity which is in good agreement with literature (22).

It is believed that, as the boric acid used as a crosslinking agent, it can form a complex with PVA. In an aqueous solution of boric acid, the following dissociation equilibrium is established:

B[(OH).sub.3] + [H.sub.2]0 [left and right arrow] B[(OH).sub.4][(OH).sup.-] + [H.sup.2] (1)

Monoborate ion B[(OH).sub.4][(OH).sup.-] can form a didiol complex with PVA by hydrogen bonding (24). The many of studies have been carried out on the aqueous PVA/borate systems to show the mechanism of crosslinking of PVA caused by addition of boric acid (25), (26). PVA composite fibers undoped with boron have a clear degradation peak at 466[degrees]C, but boron doped fibers do not have such a clear degradation peak. This can be explained with the conformation of unperfected crystal by the addition of boric acid. The regularity of PVA crystal decreased and the peak related to the degradation of boron doped PVA/Zr-Y acetate nanofibers broadened and intensity of the peak decreased due to the intercrosslinking of boric acid (27).

Figure 3 shows the XPS survey spectrum and the high resolution detailed spectra of the boron doped sample calcined at 800[degrees]C. According to the result, the sample contains only Zr, Y, 0 as well as a small amount of carbon. The existence of C Is peak is mainly caused by C[O.sub.2]. which is absorbed by the surface of the sample. The XPS spectrum of the sample reveals a binding energy of the Zr 3[d.sub.5/2] peak of 182.6 eV, Y 3[d.sub.5/2] peak of 156.6 eV and that of the B Is peak of 193.8 eV. These values are characteristic of yttria stabilized Zr02 (27-31).

XRD patterns of the undoped YSZ samples calcined at different temperatures are given in Fig. 4. The six reflection peaks observed in the XRD pattern of samples calcined at 250 and 500[degrees]C correspond to (101), (110), (200), (211), (202), and (220), indicating tetragonal Zr02 (ICDD Card No:81-1544). Broad and the weak peaks imply poor crystallinity and fine grains for the YSZ powders. When the calcination temperature increases from 250 to 800C, the reflection peaks become sharper and stronger. This result is due to the improved crystallinity of the YSZ powders with increasing calcination temperature. Moreover, the average crystallite size increases from 9.28 to 22.79 nm. The average crystallite size (D) is determined by the Scherrer's equation:

D = k[lambda]/[beta]cos[theta] (2)

where K is the Scherrer constant (K = 0.94), 2 is the wavelength of Cu K[alpha] radiation (2 = 1.54 [Angstrom]), [beta] is the full width at half maximum of the peak (101) and [theta] is the Bragg angle of the peak. As observed from Fig. 4c, tetragonal patterns starts to transform into monoclinic pattern. There exist major peaks of monoclinic phase at 2[theta] = 28.09[degrees] (11-1), 31.41[degrees] (111), together with tetragonal patterns with a major peak appearing at 2[theta] = 30.02[degrees] (101). The amount of tetragonal and monoclinic phases present in undoped sample calcined at 800[degrees]C was estimated by comparing the peak heights under the characteristic peaks of tetragonal and monoclinic phases according to the Eq. 3. The amount of tetragonal phase was calculated as 71.4% and monoclinic as 28.6%.

%Tetragonal = Tetragonal/Tetragona/ + [SIGMA] Monoclinic x 100 (3)

The effect of boron doping on the YSZ samples calcined at 500 and 800[degrees]C are given in Fig. 4d and e. The spectra exhibits intense diffraction peaks and crystallinity is enhanced. The crystallite size was calculated as 11.34 and 24.74 nm for 500 and 800[degrees]C, respectively. It has to be noted that transformation of tetragonal zirconium to monoclinic form is fully decreased in the case of boron doped YSZ. This result is consistent with the literature (17), (32). Smith (17) also used boron as a sintering aid for calcia stabilized zirconia and he concluded that [B.sub.2][0.sub.3] is an effective sintering aid to form low-melting borate phases.

SEM micrographs of the neat electrospun PVA mat are presented in Fig. 5a and b. Fiber diameters were quantitatively measured using ImageJ software. ImageJ is a Java-based public domain program that contains basic digital image processing tools, and includes numerous tools that facilitate quantitative measurements which was originally developed at the National Institutes of Health, Bethesda, MA. Fiber diameters may vary depending on the applied electric field. Therefore, the constant electric field was applied to each sample in electrospinning process. The average fiber diameter for electrospun boron doped PVA/ Zr-Y acetate nanofibers was 531 nm and boron undoped nanabers was 402 nm.

The diameter distribution of electrospinning products is given in two histograms (Fig. 6a and b). All these electro-spun mats consisted of uniform, smooth fibers free of the defects forming a porous, highly interconnected architecture. From Fig. 6, it can be seen fiber diameters electrospun from boron doped PVA/Zr-Y acetate solution ranging 350-650 nm; while fibers electrospun from boron undoped PVA/Zr-Y acetate solution range 250-550 nm and diameter distribution is slightly smaller than that in boron doped PVA/Zr-Y acetate solution. Both boron doped PVA/Zr-Y and boron undoped PVA/Zr-Y fibers have a range of morphologies, including round fibers, some flattened oval shaped fibers. From these findings, it is understood that fiber diameter and diameter distribution increase with boron doping to the solution. These findings are in accordance with our previous researches (20), (33), (34).

In Fig. 7a and b, micrographs of boron doped samples at 250 and 500[degrees]C are presented. The average fiber diameters at 250 and 50[degrees]C were calculated as 360 and 305 nm, respectively. Moreover, in the case of calcined samples, some bundles and junctions were also evidenced.

In Fig. 7c, SEM micrograph of boron undoped calcined sample at 800[degrees]C is presented. The average fiber diameter of this sample was calculated as 256 nm. After calcination, the surface of the fibers became rough due to the removal of organic components. Because of burning-off of PVA, the fibers show a decrease in the diameter. SEM micrograph of boron doped calcined sample at 800[degrees]C is given in Fig. 7d. The disappearance of fibrous net like structure can easily be seen. It revealed that boron doping influenced the system and resulted needle crystalline structure. It should be noted that boron has a sticking property as boron is an acceptor atom with three valence hand electrons and hence to form covalent bond with other atoms in the crystalline network, the lack of one electron make the potential for bond formation much stronger. It is reported that due to this sticking property the growth rate increases linearly with boron doping. XRD results revealed that the boron doped sample calcined at 800[degrees]C has the highest crystallite size (35).

CONCLUSION

In this study, boron doped and undoped YSZ nanofibers and oxide nanostructured crystalline ceramics have been successfully prepared using an electrospinning method. Then, morphological changes and thermal properties of obtained fibers and ceramics were investigated comparatively.

It was found that the addition of boric acid into YSZ resulted in (i) the increase in diameters of electrospun PVA/Zr-Y nanofibers: (ii) the rise of electric conductivity, viscosity, pH and surface tension; (iii) the decrease of the degree of crystallinity of electrospun nanofibers due to the crosslinking of PVA with boron; (iv) needle crystalline structure in final composite material. Also, The XRD results indicated that the improved crystallinity of boron doped and undoped YSZ powders with increasing calcination temperature and resulted in the enlargement of YSZ grains. Moreover, boron doped and undoped YSZ powders crystallized to tetragonal and monoclinic phases. But, transformation of tetragonal zirconium to monoclinic form is fully decreased in the case of boron doped YSZ. These results indicated that PVA polymerized synthesis method for YSZ oxide composite materials was an effective route for the preparation of nanocrystalline materials.

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Tuncay Tunc, (1) ibrahim Uslu (2)

(1) Department of Science, Faculty of Education, Aksaray University, Aksaray 68100, Turkey

(2) Department of Chemistry, Chemistry Education Department, Gazi University, Ankara 06900, Turkey

Correspondence to: Tuncay Tunc; e-mail: tctunc@gmail.com

Published online in Wiley Online Library (wileyonlinelibrary.com).

[c] 2012 Society of Plastics Engineers

DOI 10.1002/pen.23345
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Author:Tunc, Tuncay; Uslu, Ibrahim
Publication:Polymer Engineering and Science
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Date:May 1, 2013
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