My Topics

CATHODE RAYS – THE DISCOVERY OF ELECTRON

The knowledge about the electron was derived as a result of the study of the electric discharge in the discharge tube by J.J. Thomson, 1896.

Discharge tube:

The discharge tube consists of a glass tube with metal electrodes fused in the walls as shown in the figure.

How Cathode Rays Are Produced:

Through a glass side-arm, air can be drawn with a pump. The electrodes are connected to a source of high voltage (10,000 Volts) and the air partially evacuated. The electric discharge passes between the electrodes and the residual gas in the tube begins to glow. If virtually all the gas is evacuated from within the tube, the glow is replaced by faintly luminous ‘rays’ which produce fluorescence on the glass at the end far from the cathode. The rays which proceed from the cathode and move away from it at right angles in straight lines are called Cathode Rays

Charge And Mass (E/M) Of The Cathode Particle:

By counterbalancing the effect of magnetic and electric field on cathode rays. Thomson was able to work out the ratio of the charge and mass (e/m) of the cathode particle. In SI units the value of e/m of cathode particles is – 1.76 × 188 coulombs per gram. As a result of several experiments, Thomson showed that the value of e/m of the cathode particle was the same regardless of both the gas and the metal of which the cathode was made. This proved that the particles making up the cathode rays were all identical and were constituent parts of the various atoms.

Electrons Are Named:

Dutch Physicist H.A.Lorentz named them Electrons. Electrons are also obtained by the action of X-rays or ultraviolet light on metals and from heated filaments. These are also emitted as β-particles by radioactive substances. Thus it is concluded that electrons are a universal constituent of all atoms.

PROPERTIES OF CATHODE RAYS

·         They travel in straight lines away from the cathode and cast shadows of metallic objects placed in their path.

·         Cathode rays cause mechanical motion of a small pin-wheel placed in their path. Thus they possess kinetic energy and must be material particles.

·         They produce fluorescence (a glow) when they strike the glass wall of the discharge tube.

·         They heat up a metal foil to incandescence which they impinge upon.

·         Cathode rays produce X-rays when they strike a metallic target.

·         Cathode rays are deflected by the electric as well as the magnetic field in a way indicating that they are streams of minute particles carrying negative charge.

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Reference:                                                  Essentials of Physical Chemistry by B.S. Bahl

Proper sample preparation is one of the most important requirements in the analysis of powder samples by X-ray diffraction. This statement is especially true for soils and clays that contain finely divided colloids, which are poor reflectors of x-rays, as well as other types of materials such as iron oxide coatings and organic materials that make characterization by XRD more difficult. Sample preparation includes not only the right sample treatments to remove undesirable substances, but also appropriate techniques to obtain desirable particle size, orientation, thickness, etc.

Analysis of powders by XRD requires that they be extremely fine grained to achieve good signal-to noise ratio (and avoid fluctuation in intensity), avoid spottiness and minimize preferred orientation. Reduction of powders to fine particles also ensures enough particle participation in the diffraction process. The recommended size range is around 1-5µm, especially if quantification of various phases is desired. For routine qualitative evaluation of mineral components, the samples are usually ground to pass through a 325 mesh sieve (45 µm). Grinding is accomplished either through hand grinding or in a mechanical grinder. The effects of excessive grinding include lattice distortion and possible formation of an amorphous layer (Beilby layer) outside the grains.

There are two types of mounts normally employed depending on the nature of crystallite orientation required. Random mounts are preferred when identification of phases in a specimen is required. In this type of mount, particles ground to 1-5µm are packed to a flat surface onto a sample holder to assume different orientations and ensure reflections from various hkl planes. Oriented mounts are used when analyzing clay minerals which rarely show strong diffraction effects from Bragg planes other than the (00l). In general, these are prepared by making slurry of the sample with distilled water. The water is then allowed to evaporate until the slurry is smeared into a sample holder (could be a glass slide or ceramic tile).

1.      Sample properties also influence the quality of a powder pattern by either reducing intensities or distorting intensities.

2.      Preferred orientation or texture: By texture we mean that the powder particles do not have an arbitrary shape but a strongly regular anisotropic shape, typically platelets or needles. On preparation these are then preferably oriented along the sample surface massively changing the peak intensities. Several techniques may be employed to minimize this effect.

·         The most efficient way is to form a slurry in a highly viscous liquid such as nail varnish. In such a liquid, the random orientation is retained on drying.

·         Alternatively, the anisotropic particle shape can be reduced by grinding in a ball   mill. This should be done with great care as excessive grinding can easily break down the particle size to the nanometer size and lead to amorphisation. It is recommended to try the effect of subsequent 5 minute grinding intervals to optimize the process on respective samples. In the case of coatings or thin films preferred orientation is often a desired effect. In this case Rietveld refinement can be used to determine the degree of texture.

3.      Crystallite size and strain: The broadness of a diffraction peak corresponds to the mean crystallite size in a reciprocal manner. The smaller the average crystallite size, the broader the reflections and the lower the absolute intensities. This effect becomes visible below and average crystallite size of less than 200 nm. Related to crystallite size broadening is strain broadening. Strain broadening occurs due to the presence of defects in crystals. Such strain can be introduced via substitution of constituting atoms but also via special thermal treatment. It is possible to distinguish strain broadening from size broadening as the angle dependence is much larger than in the latter case.

4.      Sample preparation height: Rotating sample holders improve the measurement statistics and thus provide the best results. However, they are not available for all machines. The most severe error during sample preparation is to fill the sample holder too high or too low. Both result in a significant shift of peak positions which can make the interpretation difficult.

Glass industry: