X-Ray Diffraction (XRD)

What X-Ray diffraction can tell

X-ray powder diffraction (XRD) is mainly used for phase identification of crystalline materials, crystal structure, preferred orientation, specific phases, and other properties such as average grain size, percent crystallinity and phase quantification ^[5].

Principle

In XRD, X-rays are generated in a sealed tube under vacuum. Electric current is applied to heat the filament in the tube, thereby emitting electrons. A high voltage is applied inside the tube to accelerate the electrons, and then the electrons hit the target and generate X-rays ^[3]. The detector detects the X-ray signal and then electronically processes it to convert the signal to a count rate. When the X-ray beam hits the sample and diffracts, we can measure the distance between the atomic planes that make up the sample by applying Bragg's law nλ = 2 d sinθ ^[3]. The conversion of diffraction peaks to the distance between adjacent planes of atoms d-spacing can identify substances, because each substance has a unique set of d-spacings. Usually, this is achieved by comparing the d-spacing with the standard reference pattern.

Discussion about XRD

XRD certainly is a good method to measure the crystallinity property of CDA material used as filters in the cigarette industry because of the significant differences in X-Ray diffractogram for different kinds of CA materials shown in figure 3 curve a) wood pulp; b) CTA; c) hydrolyzing CDA; d) CDA fibre ^[4].

XRD is very precise. The detection limit of XRD method for multiphase analysis is around 0.5% to 1%. The sampling depth is between ~20Å to ~30µm ^[5]. The uncertainty of measuring size of unit cell can be reduced to less than 0.1%. But XRD may be inaccurate because CDA is not a crystalline material. When use XRD testing CDA, the crystalline diffraction peaks can be partially superimposed due to the diffusion and both the edges of crystallized and non-crystallized peaks can coincide completely or mostly, the peak-fit process can be very hard and produces large errors ^[6].

This method could identify and analyze the crystallinity from both quantitative and qualitative prospects. XRD can qualitatively identify the property of crystallinity for CA by analyzing the different peaks in X-Ray diffractogram and quantitatively calculate the degree of crystallinity and grain size by the intensity of diffraction and the angle of incidence of the X-Ray beam. Here an example of data is listed as follows.

Example Data and Analysis

The curve d in figure 3 is the X-Ray Diffraction curve for CDA fibre ^[4]. It can be seen that there are only two peaks in this curve around 10° and 18°. This shows that the CDA is not a crystalline material, the amorphous structure predominates. By applying peak-fit processing to this curve, the related lattice parameters of CDA fibre can be got (here the detailed data is missing due to the authors did not list them in the easy for curve d but only for b) ^[4]. The amount of degree of crystallinity can be obtained and cell size in crystallinity area can be calculated out, which are shown in table 2. These data can verify that the crystallinity of this material is poor.

From figure3 curve b and d comparison, it can be seen that the degree of crystallinity of CDA fibre are lower than the other CA materials. Cellulose acetate samples of x - ray diffraction analysis in D/Max - 2550 PC 18 kw designed target determination on X-ray diffractometer. Test condition: Ni filter, Cu target Kα Ray, 40 kV tube voltage, tube flow 40 mA, scanning speed is 2degree per min, and 2θ Angle scanning range is from 5 to 50.

Figure 3.png

Figure 3. X-Ray diffractogram of CA materials ^[4]
X-Ray CDA table.png

Table 2. Crystalline properties of CDA fibre ^[4]

Cellulose Acetate

Cellulose Acetate

Charaterization Data

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