**2. Experimental procedure**

exchange process has been widely used to change the reflective index in a selected area of glass for producing waveguide devices [4, 5]. Starting about 50 years ago, ion exchange has been

Surface defects are responsible for the limited glass resistance and its large scatter [6]. The creation of a compressive stress layer in the surface of the material can limit the formation or propagation of flaws and improve the mechanical properties; thermal and chemical tempering of glass are two main methods for producing a compressive stress in the glass surface. The thermal method is widely used to make windows and other transparent flat structural

The ion exchange or chemical strengthening of glass was almost abandoned for many years because of the high processing cost and long-duration process. In recent years, this method has been reconsidered because of the possibility of mechanical treatment after strengthening, the applicability to complicated shape and limited thickness components, and the absence of

The ion exchange process is typically carried out by immersing the components made of a glass containing lithium or sodium in molten potassium nitride salt. The process can be carried out at a temperature between the melting point of the salt and the transforming temperature of the glass and takes times in excess to 4 h, depending on the required depth for the com‐ pressive stress layer. After finishing the process, the samples are removed from the bath and

The ion exchange process can be considered as an inter-diffusion reaction between the mobile ions in glass and the cations in the molten salt while the other glass components are considered as an immobile matrix of negative groups [10-13]. An external electric field can be the source of an extra driving force for the inter-diffusion of the mobile ions. This process is known as electric field-assisted ion exchange (EF-IOX) and it has been used especially for manufacturing waveguides [4, 5, 14, 15]. Three different procedures have been proposed. In the first one, a thin metal film as a source for ions is applied on the glass surface and the application of an electric field induces the oxidation/reduction at the interface with glass that generates cations that move into the glass on the anode side, creating the chemical concentration profile [5, 16, 17]. Alternatively, a molten salt can be used as the ion source at only the anode side of the sample, the cations at the anode side penetrates into the glass under the field [5]. In the last approach, the sample acts as a wall, separates two molten salt baths and each bath is connected to an electrode; by applying the electric field the cations in the salt start moving in the direction

By considering the ions flow and the electrical charge neutrality balance, different mathemat‐ ical modelling have been proposed for the ion diffusion during the exchange process [18]. For a large enough field, the concentration of exchanged ions, at distance x from the surface and

= ç ÷

æ ö m

è ø (1)

( ) <sup>0</sup> , <sup>2</sup> <sup>2</sup> *<sup>C</sup> x Et C x t erfc Dt*

employed also to improve the mechanical properties of silicate glass [1].

the salt on the surface is simply washed out by water [2, 9].

components [2, 7].

140 Ion Exchange - Studies and Applications

optical distortion [8].

of the electric field [4, 5, 14].

at time t, can be defined as:

Borosilicate test tubes from commercial sources, Fiolax clear, Schott, were used in this study. The glass transition temperature was measured by the differential scanning calorimeter (DSC) (DSC2010, TA Instruments, USA) method [21]. The chemical composition and the physical properties of the glass are shown in Table 1 [22]. The tubes having nominal thickness and outer diameter of 0.5 mm and 11.8 mm, respectively, were cut in 100 mm long samples. The limited thickness of the tubes allows thermal equilibrium between the sample and the salt bath during the process.


**Table 1.** The chemical composition and the physical properties glass tubes used for ion exchange

The samples were ultrasonically cleaned in distilled water for 5 min, washed with acetone and air dried. They were treated in a modified lab furnace schematically reported in Figure 1. One sample at a time was treated but current and applied voltage were constantly monitored and controlled during the process to guarantee a similar procedure for all samples. The applied electric field varied between 100 to 3000 V cm-1 and the current density was limited to 4, 8, and 16 mA cm-2. The samples were kept over the bath for 20 min before the process as a preheating step; then, they were filled with molten salt and immersed in the bath kept constant at 400±2°C. After applying the electric field. The tube was immediately emptied from the molten salt and held for 20 min over the bath. At the end of each cycle the samples were ultrasonically washed with distilled water. Ten samples were also treated by conventional ion exchange in a com‐ mercial lab furnace, Lema TC 20 A, Italy, for comparison, by holding 4 h at the identical temperature. Potassium nitrate salt (>99.5% pure) from commercial source was used.

**Figure 1.** Schematic experimental setup used for field-assisted ion exchange

A four-point bending test was carried out to determine the mechanical strength, using inner (L1) and outer (L2) span equal to 10.3 and 40 mm, respectively. The test was carried out in lab air (temperature≈20°C, relative humidity ≈40%) with a constant loading rate of 1.1 MPa/s. The failure stress was calculated as:

$$S\_b = \frac{4F\_{\text{max}}r\_2a}{\pi \left(r\_2^4 - r\_1^4\right)}\tag{2}$$

where a is (L2-L1)/2, r1 and r2 the internal and the external radius of the tube, Fmax the maximum load.

The presence of residual stresses on the surface layers was estimated by Vickers indentation method. The residual stress, σr, approximated as constant on the surface layers can be calcu‐ lated as: [23]

$$
\sigma\_r = \frac{K\_c}{\sqrt{\pi \Omega c}} \left( 1 - \chi\_r \frac{P^\*}{K\_c c^{\frac{3\xi}{2}}} \right) \tag{3}
$$

P\* being the indentation load, c the crack length, Kc the fracture toughness, χr and Ω two geometrical constants depending on the indenter and the crack shape, respectively.

The potassium penetration profile was measured on the fracture surface of test tube fragments. The fragments were fixed on an aluminum disk with an adhesive conducting tape and coated with Au-Pd alloy. The microscopic observation was carried out by a scanning electron microscope (SEM) (JSM 5500, Jeol, Japan). A clean path was chosen for analyzing the potassium Kα on a certain length, around 70 μm long, by using energy dispersion x-ray spectroscopy (EDXS) (EDS2000, IXRF System, USA) probe. The surface chemical composition of the glass before and after the treatment was also determined by EDXS.
