U. K. Chaturvedi

A study of blistering in Mylar due to H+ ion implantation

Abstract A comprehensive study of blistering of Mylar under 100 and 250 keV H + -ion implantation has been made. The conversion of blisters to “Karnavalaya” and vice-versa due to electron beam excitation during scanning electron microscopy of the implanted sample is found to be an analog of similar events taking place during ion implantatio. This results in the development of complex blisters. The critical dose for blistering is found to be dependent on the residual surface stresses. It is higher when the stresses are compressive as when tensile. The gases emitted during ion implantation were analysed by a residual gas analyser. CO and CO 2 are found to be the main components resulting from radiolysis of the Mylar due to H + ion implantation. However, the emission of H 2 in this case remains inconclusive due to various experimental limitations. Various observed features on the Mylar surface after ion implantation were successfully explained on the basis of a proposed model of blistering based on the micro-structure of semicrystalline polymers.

Blistering observed in Mylar due to H+-ion bombardment☆☆☆

Abstract So far the blistering phenomenon due to ion implantation has been observed in metals and alloys, but no blistering has been reported in polymers like Mylar etc. A severe blistering in Mylar (polyethylene teraphthalate) has been observed by us, due to 250 keV H+-ion bombardment up to a dose level of 7×1015 ions/cm2, at ambient temperature. In metals and alloys the blistering is due to the pressure built up by the coalescence of the implanted gaseous species. However, in Mylar the cause is entirely different. In fact, it is attributed to the hydrogen gas released on account of the breakage of aliphatic C-H bonds of monomer, due to ion implantation. Very interesting features, like uniform unexfoliated circular blisters, circular rings (Karnavalayas) and black dots of different sizes along with their development sequence, have been observed. The observed features are explained on the basis of the aliphatic CH bond breakage theory due to ion bombardment.

Experimental CO2 evolution curve obtained during 250 keV D+ implantation on Mylar and its theoretical model

The partial pressure variation curve of the evolved CO2 during implantation of 250 keV D+ ion beam in Mylar has been experimentally measured and theoretically explained on the basis of a diffusion based model. Unlike the earlier percolation model, our model explains the rising portion of the curve (i.e. the pre-maximum portion) very satisfactorily. This pre-maximum portion is very important as most of the radiolytic processes take place during this short span, due to the high rate of energy deposition by the ion beam. The reasons for the failure of the percolation model in the pre-maximum portion are also explained.

Effect of Neutron and Proton Irradiation on Some Properties of Kapton

Photoacoustic, dielectric and surface-morphological properties of Kapton* were studied after exposing sheets (25 μm thick) separately to fast reactor neutrons (E>0.1 MeV) and to 250 keV protons up to doses 1.2×1018 (maximum)and 7.0×1015 cm−2, respectively. The photoacoustic spectrum (PAS) of the maximum neutron dose irradiated sample shows the red-shift of the absorption peak from 450 nm to 470 nm, whereas in the proton irradiated (7×1015 cm−2) sample, a new flattened absorption peak appears at λ ≃ 515 nm. There is a net enhancement in the D.C. dielectric constant of about 13% after the maximum neutron dose. It is attributed to the enhanced water absorptivity of the material due to neutron induced radiation damage.

A simple set-up for in-situ observation of the critical dose of blistering during ion implantation in polymers

A simple set-up was designed and fabricated for in-situ studies of the critical dose of blistering and changes on the surface morphology of the polymers during ion-implantation. Mylar, a very important polymer, stable at high temperatures, which shows severe blistering at dose level ∼- 7 × 1015, 250 keV H+ ions cm−2, was chosen for study with this set-up. The design of t some initial results are discussed here. Some major limitations are also described.

A modified diffusion-based model for radiolytically evolved gases during ion implantation in polymers

Abstract A refined theoretical model, based on 3-dimensional diffusion equation, has been proposed to explain our earlier experimental results of dynamic partial pressure variation of CO 2 evolved during 250 keV D + ion implantation in Mylar. This model gives a better fit with the experimental points than the percolation model in the pre-maximum as well as in the postmaximum region of the curve. The experimental H 2 evolution curve obtained by Davenas et al. in 500 keV Ar + implantation of PMMA, has also been explained by our model in a much better way than by the percolation model.

Improvements in the Vacuum System of a VDG Accelerator Used for Clean Ion Implantation

In order to obtain clean implanted surfaces free from beam-induced polymerization, and to avoid the limitations of diffusion pumps, two Varian Vaclon (triode) pumps of 30 lps and 110 lps capacity were incorporated into our AN-400 Van de Graaff accelerator, replacing the diffusion pumps. The 30 Ips pump is installed near the accelerating column before the switching magnet, while the larger one is fitted near the scattering chamber after the switching magnet. Although ion pumps have less throughput than cryo or diffusion pumps, the choice of Vaclon pumps was made after considering all the advantages and disadvantages of the various available pumps. The average vacuum of the beam line, with the existing neoprene 0-rings throughout, and without use of any cryogenic material, is found to be in the 10-6 torr range without the beam, which changes to the 10-7 torr range when the machine is loaded with the beam. Very satisfactory results are obtained with proton and deuteron beams, as these pumps have a very high pumping speed for H2 and D2. However, there may be some problems when one uses He+ and other inert gaseous ions, due to the low pumping speed of ion pumps for these gases, which may be overcome by interchanging the pump positions or by using additional pumps. The beam-induced polymerization has also been reduced considerably.

Effect of HCl vapours due to PVC, on PVDF samples during 250 keV H+ ion implantation

Abstract The role of a corrosive environment on polymeric targets during ion bombardment, has been observed. Presence of elastomeric seals or PVC etc. inside the vacuum chamber creates a partial pressure of HCl vapours. Observation of the decrease in the partial pressure of the HCl content as H+ ions impinge on a PVDF sample, indicates the possibility of attachments of HCl or Cl ions with the irradiated polymeric surface. This has been confirmed by residual gas analysis and by EDAX measurements of irradiated and unirradiated surfaces of PVDF samples.

Oxidising Role of Water Vapour in the 250 Kev D+ Induced Radiolysis of Polyimide Kapton-H

Residual water vapour present in the vacuum system has been observed to play a dominant oxidising role in the 250 keV D + induced radiolysis of polyimide (Kapton-H). The partial pressure (pp) of water in the vacuum system decreases sharply as the D+ beam impinges the polymeric surface, but soon after, it recovers to its initial value as the accumulated dose increases. Emission of CO 2 is observed which has its maximum at a time when the H 2 O partial pressure is at a minimum. The CO 2 level also returns to its original level with time. This complementary variation of CO 2 and H 2 O confirms that absorbed and adsorbed water molecules are radiolysed by the ion beam and initiate oxidation of the radiolytically evolved CO to yield CO 2 on and within the ion implanted surface of the polyimide. Further, the small enhancement in the 28 amu peak (N 2 + CO), which exhibits no maximum/minimum over the entire implantation time, can be understood in terms of the evolution of N 2 from the imide ring as a result of radiolysis of this nitrogen containing polymers.

Suitability of ion pump for evacuation of a closed He cycled Mössbauer cryostat

For Mössbauer studies at very low temperatures, the target holder kept at 11°K with the help of a closed He cycle Mössbauer shroud, has to be insulated by evacuating the outside enclosure surrounding the shroud. The target is kept in thermal contact with the cryostat cold finger through He gas at atmospheric pressure. This provides a vibrationless thermal contact between the two. The evacuation of the outside chamber has to be vibration free which can be accomplished by means of a suitable ion pump which is totally vibration free. However, use of Vac-ion pump here could not be made successful because the pump did not work at all after little pumping. To find out the cause of pumps failure, RGA was employed which showed the presence of He gas in the outside chamber at the time of purging of He gas in the internal chamber. The pump, however, worked efficiently again when the dry N2 gas purged in place of Helium. This points out that totally vibration free Vac-ion pumps cannot be used for evacuation of such cryostats. Here, diffusion pump was used subsequantly taking extra precautions to minimise the vibrations.