General method for dispersing and disaggregating particulate samples for quantitative SEM optical microscopic studies

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Powder Technology-Elsevier Sequoia SA., Lausanne-Printed in the Netherlands

Short Communication General method for dispersing and disaggregati g particulate samples for quantitative SENT and optical microscopic studies N_ THAULOW' and E_ W_ WHITE Materials Research Laboratory . Pennsylvania State Unirersit} •, .) .A .S University Park, Pa. (U (Received September 3, 1971)

The eutectic composition of 60 wt . % camphor and 40 wt- % naphthalene (C-N) is found to be an ideal medium for preparation of uniform dispersions of a variety ofinorganic materials . Particular advantages to this medium include : (1) A melting point of 32 °C allowing one to store and handle the C-N as a solid and to take advantage of its properties either as a solid or liquid . (2) The solid rapidly sublimes completely in vacuum leaving no residue to interfere with microscopic observations . (3) The C-N is unreactive with most inorganic powders. (4) In its solid form the C-N is a gently abrasive medium for mechanically breaking down agglomerates and light aggregates . (5) The viscosity of the C-N liquid near the melting point is high enough to prevent most materials from fractionating by sedimentation during normal handling times. So far this technique has been found quite satisfactory for dispersing powder samples of alumina, Portland cement, tungsten, coal mine dust, barium titanate, and rutile and shows considerable promise for dispersing magnetically agglomerated materials such as ferrite powders . As part of a program on the computer evaluation ofscanningelectronmicroscopeimages(CESEMI) t,2 for quantitative characterization of particulate samples, it has been necessary to devise a general method for dispersing a wide variety of materials . To be generally applicable, the dispersing medium must have a controllable viscosity, be generally un-

reactive with inorganic materials and be readily removed after the dispersion has been presented on the substrate . A dispersion suitable for CESEMI or automatic quantitative optical microscopy should consist of a representative single layer of particles, closely spaced but without particle contacts or agglomeration. The dispersion material which has been decided upon is a eutectic composition of naphthalene and camphor. These materials can readily be obtained in pure form. The eutectic composition has the dual advantages of transforming from a solid to a -viscous liquid at low temperature (32° C) and also completely sublimes in vacuum with no residue left to interfere with SEM or optical studies_

Properties of the camphor-naphthalene system In order to establish the most suitable composition of a mixture of camphor and naphthalene as a medium for dispersing particulate materials, the binary phase diagram was plotted as shown in Fig-1 .

NAPHTHALENE -CAMPHOR CC

°CI

150 N: CIO H8 Mur =12818

100

C: %f-h M z.r =1524

so

N +C 50

On leave from the Concrete Research Laboratory, Karistrup, Denmark-

T
Fig. 1 . Binary phase diagram of camphor and naphthalene .

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378 The melting point for pure naphthalene is 80 .2°C and for camphor 178 .8° C. The eutectic melting point is 32° C at a composition of 60 wt .'/,' camphor and 40 -.v-t-% naphthalene. The crystallization of melts with 50,60 and 70 wt . °o camphor was followed on glass slides using an optical microscope . In the 50 wt. % camphor liquid, large needle-shaped crystals of naphthalene were formed_ while in the 70 wt. °l camphor liquid, large

dendritic camphor crystals developed . In the 60 wt . % camphor liquid (eutectic mixture), individual crystals cannot be distinguished (Fig . 2)_ It was observed that the large crystals of eith, r camphor or naphthalene tended to push away part :_ies dispersed in the liquid, while the eutectic-formed crystals grew around the particles if the system was cooled rapidly. Therefore, a mixture of 60 wt_ % camphor and 40 wt . % naphthalene (henceforth called C-N) was found most suitable for further experiments on particle dispersion . As stated, this mixture freezes at 32° C to a white crystalline solid having a high vapor pressure and sublimes readily in vacuum leaving no residue. This transformation from solid to vapor phase without any liquid formed is considered very important for this application, because when a liquid particle suspension is evaporated the surface tension of the liquid tends to pull the particle together-The viscosity of the melt of C-N is high enough to prevent rapid sedimentation and fractionation of fine grained, high density materials (for example : tungsten powder, density= 19.4 g/cm3).

SHORT COMMUNICATIONS

Specimen preparation The equipment used in this general method for dispersing particulate samples is rather simple and most of it is standard laboratory equipment : glass slides, cover glasses, hot and cold plates, spatula, vacuum jar, vacuum pump, small polyethylene bags and heat sealer . During preparation it is very important to avoid contamination such as dust from the air or grease from finger prints . All material which comes in direct contact with C-N, including the plastic b ag. is rinsed with alcohol and dried with a jet of pure freon gas from an aerosol . The stock mixture of C-N with 60 wt . % camphor and 40 wt- % naphthalene (eutectic composition) is made by weighing the necessary proportions and melting the mixture in a bottle on a waterbath . When the solution is homogeneous it is checked for solid contamination under an optical microscope and then quenched . The specimen preparation can be divided into four steps : (1) About 2 g C-N is cut from the stock mixture and transferred to a cleaned plastic bag (5 x 5 cm) . 20 mg of the powder sample is put into the bag . The bag is then sealed by a heat sealer . The optimum proportion between powder and C-N is strongly dependent on the particle size, size distribution and density of the powder, but can normally be found in a few trials. The powder and C-N is mixed by kneading the plastic bag with the fingers . By adjusting the amount of heat supplied to the bag, the viscosity of the melt and the amount of crystalline C-N can be controlled . The crystalline C-N helps to breakdown agglomerates . The kneading time is controlled to give the desired dispersion and disaggregation . About five minutes of mixing time is normal . Then the plastic bag is cooled under water to quench the melt_ (2) The plastic bag is cut open and a small sample of the mixture (dispersion) is transferred to a glass slide on the warm plate, heated with water. The mixture melts and is spread by means of a spatulaA cover glass, pressed over the melt, has the effect of forming a layer of constant thickness . A rapid crystallization of the C-N is very important for best results . (3) The cover glass is prized off by use of a razor blade, and the sample on the glass slide is transferred to a mechanically pumped vacuum chamber . (It is desirable to have a cold trap in the vacuum line .) After a few minutes in vacuum the C-N has sublimed Powder Teehnoh

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SHORT COMMUNICATIONS leaving the dispersed powder on the glass plate. (4) If the powder is to be studied by the SEM, the sample is vacuum coated with a few hundred A of gold or aluminum . If the X-rays generated in the particles under the electron bombardment in the SEM are to be analyzed it is important that the substrate does not contain the same elements as the particles . Polished and

Fig . 3_ Light micrograph of Portland cement aocader dispersed using the eutectic camphor-naphthalene mixture . (x 225)

conductively coated (boron or carbon for example) LiF crystals can be used instead of glass . Polished beryllium is also an ideal substrate for this application-The described method has been used to disperse a

range of inorganic materials including powder samples of aluminas, Portland cements (Fig . 3). tungsten, coal mine dusts, barium titanates, rutile, and ferrites. Most of the work was done on Portland cement (four different types with specific surface ranging from 3000 to 6000 cm- ;g measured by air permeability) and alumina (10 types) with varying (-0.1-75 Ian) average grain size . By X-ray diffraction it was shown that the C-N did not react with the cement There was also no evidence for reaction with anv of the other materials . However_ the dispersion method was tried on latex spheres without success becausthlxdiovetsmd_reh which changed the morphology of the spheres . Magnetic ferrite powders were dispersed with some degree of success In this case a very rapid cooling is important so that the individual particles do not have time to agglomerate again before the C-N mixture freezes. This work has been supported by the Office of Naval Research, Metallurgy Program under Contract No. N00014-67-A-0385-0007 and the United States Department of the Interior. REFERENCES I IL E McMillan, G. G_ Johnson, Jr-and E \V- White, Computer processing of binary maps of SEM images, Proe SEM S}znp_ IITRl, Chicago, Ill- 1969, pp . 439-44. 2 E W. White, H. Gorz G. G. Johnson, Jr. and R E McMillan, Particle size distributions of particulate alumiras from computer-processed SEM images. Proc . SE9I S}rmp_ IITRI. Chicago. Ill- 1970. pp- 57-64-

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