Bulk Analysis

Many analytical methods are available to determine the chemical composition of powders. Most of these procedures are geared toward the analytical chemist, and a brief description of methods used in powder analysis are summarized in Table 6 and described in this section.

Table 6 Comparison of bulk analysis method for powders

Method

Samples

General uses

Limitations

XRPD(a)

Powder samples (1 mg usually adequate) of crystalline solids

• Identification of crystalline phases contained in unknown samples

• Quantitative determination of the weight fraction of crystalline phases in multiphase materials

• Characterization of solid-

Must be crystalline for phase identification which requires existence of standard patterns

state phase transformations

Inert gas Solids, chips, or powders of fusion usually 2 g or less, depending on material type and the expected amount of gases present

Quantitative determination of oxygen, nitrogen, and hydrogen in ferrous and nonferrous materials

Special precautions for metals with low boiling points Materials with stabile nitrides or oxides require addition of fluxes Method is destructive to the material

High- Solids, chips, or powders of 1 g Determination of carbon and sulfur in metals temperature or less, depending on type of and organics combustion material

Specimen must be homogenous Graphite-bearing specimens require special handling Method is destructive

Simultaneous multielement

• Detection limits

analysis

parts per billion to

Quantitative and qualitative

parts per million

analysis for over 70

• Cannot analyze for

elements with detection

noble gases

limits in the parts per billion

• Halogens and some

(ng/mL) to parts per million

nonmetals require

(/■'g/inL) range

vacuum

Determination of major,

spectrometer and

minor, and trace elemental

optics

components

• Sensitivity poor for

alkali elements,

especially

rubidium; cannot

determine cesium

ICP-AES

Liquids, gases, and solids; liquids are most common; sample size of 5 to 50 mL for solutions; 10 to 500 mg for solids

Solids, solutions, and gaseous (mercury) size depends on technique used--from a milligram (solids by graphite furnace atomic absorption spectrometry) to 10 mL of solution for conventional flame work

Quantitative analysis of approximately 70 elements

Detection limits range from subparts per billion to parts per million Cannot analyze directly for noble gases, halogens, sulfur, carbon, or nitrogen

Poorer sensitivity for refractory oxide or carbide-forming elements than plasma atomic emission spectrometry • Basically a singleelement technique

Characterization, Vol 10, ASM Handbook XRPD, X-ray powder diffraction.

ICP-AES, inductively coupled plasma atomic emission spectroscopy. AAS, atomic absorption spectroscopy.

If a pressed or sintered part is to be chemically analyzed, milling, drilling, or crushing can be used to obtain a representative sample. If the part contains oil, it should be removed by using a Soxhlet extractor with a solvent as detailed in ASTM B 328. Oil can also be removed by heating in a protective atmosphere at 705 to 815 °C (1300 to 1500 °F). However, this method cannot be used for materials such as sintered aluminum because they melt.

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