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D5187-21 – Standard Test Method Technical Guide
📐 Scope and Significance
This standard, designated ASTM D5187-21, establishes the procedure for determining the mean crystallite thickness (Lc) of calcined petroleum coke using X-ray diffraction. The crystallite size parameter Lc is a critical indicator of the coke’s heat treatment history and overall quality, reflecting the extent of calcination and affecting suitability for end use.
The standard explicitly covers the determination of the average crystallite thickness and does not determine the crystallite diameter (La). The methodology relies on empirical interpretation of the X-ray diffraction pattern and is unaffected by coke microporosity or the presence of non-crystalline additives such as dedust oil. Values are stated in SI (metric) units.
⚙️ Summary of the Test Method
A representative, pulverized sample of calcined petroleum coke (less than 75 µm) is packed into a sample holder and subjected to a monochromatic X-ray beam. Under carefully controlled conditions, the sample is rotated to produce a complete diffraction pattern.
The resulting pattern is analyzed specifically for the peak corresponding to the Miller indices hkl(002), which occurs at a characteristic lattice spacing (d) of 3.35 Å. The mean crystallite thickness (Lc) is then calculated from the location and shape of this peak, either by manual interpretation or through computer simulation of the peak profile.
💡 Technical Note: The precision of the Lc measurement is highly dependent on sample quality. Adherence to Practices D6969 (Preparation) and D6970 (Collection) is strongly recommended to ensure representative, reproducible results.
📊 Key Measured Properties and Apparatus
The core instrument is an X-Ray Powder Diffractometer, which must be equipped with an X-ray source, a monochromator or filter for restricting the wavelength range, a sample holder, a radiation detector, a signal processor, and a readout system (chart recorder or computer memory).
| 🟦 Term | 📏 Definition / Role |
|---|---|
| Crystallites | Stacks of graphitic carbon platelets located parallel to one another within the coke structure. |
| Lc | The mean or average thickness of crystallites in the sample, expressed in Angstrom units (Å, 10⁻¹ nm). |
| hkl(002) | The specific Miller indices identifying the crystalline planes of graphite with a lattice spacing (d) of 3.35 Å. |
| Theta angle (Θ) | The glancing angle measured by the X-ray goniometer, typically reported in degrees TwoTheta (°2Θ). |
⚠️ Critical Distinction: This test method is strictly designed for the determination of crystallite thickness (Lc). The crystallite diameter (La) is not determined by this test method and must be evaluated using separate analytical procedures.
| 🎯 Parameter | ⚡ Specification |
|---|---|
| Sample Particle Size | Less than 75 µm |
| Targeted Reflection | hkl (002) at d = 3.35 Å |
| Standard Units | SI (metric). Lc is reported in Angstroms (Å). |
| Measured Value | Mean Crystallite Thickness (Lc) |
This test method quantifies the extent of heat treatment applied to the coke. Lc serves as a direct indicator of crystalline ordering, which correlates with the material’s suitability for critical industrial applications.
❓ Frequently Asked Questions
🔍 What is the primary purpose of ASTM D5187-21?
The standard provides a reliable, empirical method for determining the mean crystallite thickness (Lc) of calcined petroleum coke, indicating the material’s thermal history and structural quality.
💡 Why is the Lc value important for industrial applications?
The Lc value directly reflects the degree of graphitization and calcination. It is a general measure of quality that affects the coke’s suitability for further use, such as in anode production for aluminum smelting.
⚡ What specific diffraction peak is analyzed in this test method?
The analysis focuses on the peak with Miller indices hkl(002), which corresponds to a lattice spacing (d) of 3.35 Å. The peak’s position and shape are used to calculate the Lc value.
📌 Can this method be used to measure crystallite diameter (La) as well?
No. This test method explicitly covers the determination of the average crystallite thickness (Lc) only. The crystallite diameter (La) is not determined by this method.