Chapter 3 Experimental
3.3 Analysis and Characterizations
3.3.1 X-ray Diffraction
Phase identification was predominantly performed by X-ray diffraction (XRD) analysis. Typically, information like phase purity, crystallinity, and size and shape of unit cell can be obtained from a XRD pattern. The x-ray beam generated from the X-ray tube encounters the sample, and the diffracted X-ray beam must obey Bragg’s law:
T O 2d sin
n
(3-3)where n is the order of diffraction, positive integer; λ=wavelength of incident beam;
d=distance between corresponding crystal lattice plane; and θ=the incident angle between x-ray beam and atomic layers in the crystal, also called Bragg’s angle. Figure 3.6 simply describes the relationship between the incident beam, diffracted beam, d-spacing, and Bragg’s angle in Bragg’s law [65].
In this study, XRD analysis was carried out with the X-ray diffractometers (MAC Science/MXP and Philips/X’Pert) which use Cu Kα radiation (O = 1.5418 Å) as the source of X-ray. The X-ray generator equipped with a graphite monochromator and the applying voltage and current was 40 kV and 30 mA, respectively. The reflection data of powder samples was collected under a continuous-scanned T-2T mode at a scan-rate of 10 °/min. The scan-range was operated at the range of 10 to 80°. Figure 3.7 depicts the typical features of XRD experiment [72].
In addition, average crystallite size can be determined based on the Debye-Scherrer equation, adopting the full width at half maximum (FWHM) of the specified reflection in diffraction patterns, described as:
63 b B
B
d
T
O
cos 9 . 02
2 (3-4)
where d is the average crystallite size, λ is the wavelength of incoming X-ray, B is the revealed FWHM of the specific reflection, b is the line broadening width of instrument, and T is the Bragg angle. The XRD information of silicon and graphite based on Cu Kαradiation is listed in Table 3.4.
Figure 3.6 2-dimensional schematic representation of the Bragg’s law [73].
Figure 3.7 Basic features of XRD experiment [72].
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Table 3.4 The XRD information of materials involved in this study (λ= 1.5184 Å)
Si (27-1402)
Plane (111) (220) (311) (400) (331) (422) (511)
2θ (°) 28.466 47.344 56.171 69.196 76.452 88.118 95.052
I/Io 100 55 30 6 11 12 6
ZrO2(c) (27-0997)
Plane (111) (200) (220) (311) (222) (400)
2θ (°) 30.168 35.023 50.375 60.026 62.728 73.997
I/Io 100 24 80 60 10 12
ZrO2(m) (37-1484)
Plane (331) (422) (511) (640) (822) (771)
2θ (°) 13.097 14.767 15.669 21.736 25.733 30.202
I/Io 20 45 55 20 100 20
ZrO2(t) (42-1164)
Plane (101) (002) (110) (102) (112) (200) (103)
2θ (°) 29.832 34.024 34.855 42.355 49.511 50.112 58.328
I/Io 100 39 34 2 34 38 12
C(graphite) (41-1487)
Plane (002) (100) (101) (004) (103) (110)
2θ (°) 26.40 42.26 44.43 54.59 59.75 77.32
I/Io 100 2 4 6 1 3
3.3.2 Scanning Electron Microscopy
Morphology observation on a material can provide some important information in the initial stage before going further investigation, such as approximate particle size distribution, the shape and microstructure of particles, topography, and porosity, etc.
The scanning electron microscope (SEM) is a microscope that uses electrons instead of light to form an image. The SEM has advantages over traditional optical microscopes (OM) and has become one of the most heavily used instruments in
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research areas today. For instance, the SEM has a large depth of field, which allows more of a specimen to be in focus at one time. The SEM also produces images of high resolution, which means that closely spaced features can be examined at a high magnification. Moreover, preparation of the samples is relatively easy since most SEM only require the sample to be conductive.
The particle morphology was examined by SEM (FEI/Nova230). Before examination, the samples were first pasted on a carbon-tape glued holder, and then coated with platinum or gold by ion sputtering to ensure the conductivity. Energy dispersive x-ray spectroscopy (EDS, Oxford Instrument/model: 6587) were used to analyze the surface compositions of electrodes.
3.3.3 Pore Volume and Pore Size Distribution Analysis
The BET (Brunauer, Emmett, and Teller) surface area and pore size distribution, determined by nitrogen adsorption, of materials was conducted with a surface area analyzer (Micrometrics/ASAP 2010). BET method involves multiple-payer adsorption, of which equation is described as:
0 0
where V is the volume of adsorbed nitrogen (cm3/g); P, the pressure of adsorbed gas;
P0, the saturated vapor pressure ; Vm, the volume of mono-layer adsorbed nitrogen;
and C is constant. A linear relation of P/(V(P0-P)) and P/P0 can be obtained, which gives a slope of (C-1)/VmC and intercept of 1/VmC; and thus the specific surface area can be calculated based on the volume of adsorbed nitrogen. BJH (Brunauer, Joyner, and Halendar) scheme for determination of mesopore distribution is based on Kelvin equation and thickness equation, showing the relation between relative pressure and
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pore size. In this study, 5-points measurement was utilized for determining BET surface area, in which the relative pressure ranging from 0.06 to 0.20; in the meanwhile, 55-points detection was measured for mesoporous characteristics. The equilibrium interval time was set as 20 s.
3.3.4 Determination of Carbon Content
It is an important issue to determine the carbon content of the synthesized samples. In this research, ZrO2-Si is always coated with carbon to improve its intrinsically low electronic conductivity and cushion volume change of Si during Li+ insertion/extraction. Besides, different microstructures of carbon give various electrochemical properties. Thus, carbon content should be detected to calculate the actual specific charge and discharge capacity of the composite and to estimate its influence not only on cycle stability but also reversibility.
Elemental Analysis (EA) is conducted to measure the carbon content. It is a combustion analysis technique: a carefully-weighted amount is completely burned under oxygen atmosphere. Take carbon and hydrogen analysis of CxHy as an example, it can be described in the following equation:
2 2 2
x y 2
C H zO o yH OxCO
(3-6) The measurement is processed in presence of excess oxygen atmosphere. As the amounts of H2O and CO2in exhaust gas are measured, the original composition of carbon and hydrogen contained in the compound can be obtained. In this way, the carbon content of carbon can be known by measuring the amount of CO2. The machine used here is Heraeus VarioEL-III (for CHN).
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3.3.5 Thermo Gravimetric Analysis
Thermo Gravimetric analysis (TGA) was also performed with TGD 7000 (ULVAC Sinku-Riko, Inc.) thermal analyzer. Samples were heated to 800 oC in nitrogen at a heating rate of 10oC/min. The weight of testing samples was 30-35 mg and the precision of the method was ± 0.1 mg. In this research, TGA is used to observe the mass variation of gel or pitch with increasing temperature. Therefore, a proper calcination temperature could be determined for making ZrO2-Si-C composite.
3.3.6 Particle Size Distribution Analysis
The particle size distribution (PSD) was performed by Coulter Counter LS230 (Coulter Corp., USA). Samples were dispersed in appropriate solvent to avoid severe aggregation. The detective range ranges from 0.4 Pm to 2000Pm. The detecting method is “Static Light Scattering”. The theorem based on Raleigh Single Slit equation :
sin d
T O (3-7)
The scattering angle decrease with increasing detected particle size.
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