Vertical Scanning Research Articles

Fiber optic white light interferometer for areal surface measurement

White light interferometers (WLI) are extremely powerful tools in the context of areal surface topography instrumentation. With lateral scanning mechanisms, larger objects with trans-scale structures are also measurable. However, the existing commercial WLIs are usually only applicable for off-line measurements in the laboratory. This paper presents a flexible WLI, in which the light source and the vertical scanning mechanism are separated from the interference probe using fiber-optic components. As a result, the interference probe is more compact, and portable for on-machine measurement. Meanwhile, a comprehensive algorithm that combines the improved gravity center method and phase-shifting technique is proposed for signal processing, which can balance computation time and noise robustness. The simulation results show the effectiveness by comparing with the traditional white light scanning interferometry methods. A 1000 nm step height standard was used as a test specimen to evaluate the proposed system and algorithm. The experimental data show that the measurement results are basically in good agreement with the calibrated value.

A rapid measurement method for structured surface in white light interferometry.

White light interferometry (WLI) is an effective and widely-used technique for structured surface measurement. However, it requires multiframe interferograms with vertical scanning to realise large-scale measurement, which is time consuming and computationally intensive. This paper proposes a rapid surface measurement method to realise surface recovery with a single interferogram by white light interferometry. First, the feasibility to solve the wrapped phase of a single white-light interferogram by Hilbert transform is certified. Then, unwrapped phases against zero optical path difference position (OPD) are achieved by a zero optical path difference detection algorithm applied to unwrapping process, which provides efficient surface recovery. To ensure the accuracy of phase solution in the proposed method, the necessary number and width of the interference fringes in the interferogram are analysed and determined based on Hilbert transform and sampling analysis. Finally, measurement results of a standard step sample and a standard reticle template are presented, which prove the accuracy and efficiency of the proposed method. LAY DESCRIPTION: As an effective and widely-used technique for structured surface measurement, white light interferometry (WLI) has the major advantage to measure noncontinuous surfaces using the short coherence length of a wide bandwidth source. However, frequently vertical scanning is required to get series of white light interferograms at different axial positions for surface recovery by recovered algorithms. The vertical scanning process is complicated and time consuming. This paper proposes a fast and efficient method to realise rapid surface measurement using only a single-frame interferogram based on WLI. First, the feasibility of using only one single white light interferogram to solve wrapped phases by Hilbert transform (HT) is discussed. Next, unwrapping process and zero optical path difference（OPD） detection algorithms are combined to unwrap phases against zero OPD position, which makes the structured surface recovery much easier. After that, the feasible number and width of interference fringes are determined based on sampling analysis and HT to guarantee the reliability and accuracy of phase solution in the proposed method. Finally, the accuracy and efficiency of this method is verified by measurement experiments of a standard step sample and a standard reticle template.

Two-Dimensional Laser Scanning Utilizing Inclined Rolling of an Ellipsoidal Reflector

Compact light detection and ranging (LIDAR) with wide scanning angle is required for the environment monitoring of automobile. We have developed a two-dimensional (2D) laser scanner for the LIDAR system with the horizontal scanning angle of 360 degrees. The scanner is composed of an inclined rolling ellipsoidal reflector equipped with a permanent magnet at the center, a ferromagnetic steel pillar, and six electromagnets. The reflector is attached on top of the pillar by magnetic force; the reflector is translationally-fixed but free in rotational motion. The electromagnets are arranged in a circular pattern under the reflector. Applying current pulses to the electromagnets subsequently, the reflector is rotated by the electromagnetic force. The maximum rotational speed of 85 rps (revolutions per second) was achieved with an elliptical reflector (major axis: 22 mm, minor axis: 18.2 mm, thickness: 0.2 mm) and a permanent magnet (diameter: 4 mm, thickness: 2 mm) at an applied current 28 mA to the electromagnets. We successfully demonstrated 2D laser scan with the horizontal scanning angles of 360 degrees, the vertical scanning angle of 18 degrees, and the rotational speed of 70 rps.

The ability of Listeria monocytogenes to form biofilm on surfaces relevant to the mushroom production environment

Due to its ubiquitous nature, Listeria monocytogenes is a threat to all fresh fruits and vegetables, including mushrooms, which are Ireland's largest horticultural crop. Although fresh cultivated mushrooms (Agaricus bisporus) have not been previously linked with listeriosis outbreaks, the pathogen still poses a threat to the industry, particularly due to its ability to form biofilms. This threat is highlighted by the multiple recalls of mushroom products caused by L. monocytogenes contamination and by previous studies demonstrating that L. monocytogenes is present in the mushroom production environment. In this study, the biofilm formation potential of L. monocytogenes strains isolated from the mushroom production environment was investigated on materials and at temperatures relevant to mushroom production. A preliminary assessment of biofilm formation of 73 mushroom industry isolates was undertaken using a crystal violet assay on polystyrene microtitre plates. The biofilm formation of a subset (n = 7) of these strains was then assessed on twelve different materials, including materials that are representative of the materials commonly found in the mushroom production environments, using the CDC biofilm reactor. Vertical scanning interferometry was used to determine the surface roughness of the chosen materials. All the strains tested using the CDC biofilm reactor were able to form biofilms on the different surfaces tested but material type was found to be a key determining factor on the levels of biofilm formed. Stainless steel, aluminium, rubber, polypropylene and polycarbonate were all able to support biofilm levels in the range of 4–4.9 log10 CFU/cm2, for seven different L. monocytogenes strains. Mushroom industry-specific materials, including growing nets and tarpaulins, were found to support biofilms levels between 4.7 and 6.7 log10 CFU/cm2. Concrete was found to be of concern as it supported 7.7 log10 CFU/cm2 of biofilm for the same strains; however, sealing the concrete resulted in an approximately 2-log reduction in biofilm levels. The surface roughness of the materials varied greatly between the materials (0.7–3.5 log10 Ra) and was found to have a positive correlation with biofilm formation (rs = 0.573) although marginally significant (P = 0.051). The results of this study indicate that L. monocytogenes can readily form biofilms on mushroom industry relevant surfaces, and additionally identifies surfaces of specific concern, where rigorous cleaning and disinfection is required.

Fast surface profilometry utilizing structured illumination microscopy based on the time-domain phase-shift technique.

Structured illumination microscopy (SIM) has attracted much research interest due to its high accuracy, strong adaptability, and high efficiency. Existing SIM is mainly based on the phase-shift technique, Hilbert transform technique, and global Fourier transform technique. The phase-shift technique is most widely applied for its higher accuracy, and both the phase-shift technique and Hilbert transform technique suffer from lower speed because multiple images are needed to obtain modulation information for each scanning step. The global Fourier transform technique has a higher speed, but the high-frequency information of the sample will inevitably be lost because a filter window is used. As a result, the global Fourier transform technique is limited to smooth surfaces. In this paper, a fast surface profilometry using SIM is proposed. It is based on the time-domain phase-shift technique (SIM-TPT), which combines one-fringe projection and phase shift. In this proposed measurement system, vertical scanning of the object is synchronized with the switching of the phase-shifted fringe pattern. As a result, only one fringe pattern must be projected, which enables a point-to-point processing defined as the local Fourier transform method in this paper to be utilized to extract the modulation information that will preserve the high-frequency information of the image so it can be applied to both smooth and rough surfaces. Compared to conventional SIM, SIM-TPT has a higher speed because it is a simpler system and can be applied to complex structures such as high roughness surfaces and steep edges.

Characterization of grain geometrical features for monolayer brazed grinding wheels based on grain cross-sections

The grain geometrical features of grinding wheels are critically important to evaluate their quality and grinding performance. Therefore, grain geometrical features should be comprehensively and systematically characterized. Such characterization parameters were defined from the geometric features of individual grains (for each grain) to the distribution features of grains (for a group of grains on a given wheel surface) on the basis of grain cross-sections. For each grain, characterization parameters included cross-section area, equivalent diameter, volume, and sharpness ratio of the grain. For a group of grains, the characterization parameters were distribution density, cross-section area, cross-section area-supporting rate, and volume proportion of the grains on a sectioning depth. The cross-sections of grains were extracted from the section-scanning images captured by an optical measurement system. A system based on optical vertical scanning using a narrow depth of field microscope was adopted to obtain the optical sectioning images, and the grain cross-section boundaries on each image were extracted through an image-processing program. The defined characterization parameters, vertical sectioning step, and sectioning numbers were calculated efficiently. A monolayer brazed grinding wheel with a grain size of 425–500 μm was selected for an experiment. The individual and group parameters, such as sharpness or strength, active volume, or area-supporting ratio, of grains were used for performance analysis. Experimental results indicated that the proposed parameters based on grain cross-sections are not only efficient to calculate but also effective to characterize the geometric features of grains for monolayer brazed grinding wheels.

Introduction of charged domain walls into BiFeO3 thin films using a pit-patterned SrTiO3 (001) substrate

This work demonstrated the artificial introduction of charged domain walls (CDWs) into a ferroelectric BiFeO3 (BFO) thin film by domain structure control using a pit-patterned SrTiO3 (STO) (001) surface. The pattern consisted of 1 × 1 μm square holes with sloped sides, fabricated on the STO (001) surface by electron beam lithography and Ar+ ion etching. Scanning electron and atomic force microscopy analyses demonstrated that the pit slopes had angles of 6.1°–7.6°, which were sufficient to limit the in-plane growth direction of the BFO at step edges on the STO surface, and thus control the domain structure. Lateral and vertical piezoresponse scanning force and transmission electron microscopy confirmed the artificial introduction of CDWs in the pit and showed that the sign of the CDWs could be reversed via ferroelectric polarization switching. This domain control technique based on a pit pattern provides a simple approach to the integration of ferroelectric DWs into functional devices.

Full-azimuthal imaging-DOAS observations of NO<sub>2</sub> and O<sub>4</sub> during CINDI-2

Abstract. A novel imaging-DOAS (differential optical absorption spectroscopy) instrument IMPACT (Imaging MaPper for AtmospheriC observaTions) is presented combining full-azimuthal pointing (360∘) with a large vertical coverage (∼41∘). Complete panoramic scans are acquired at a temporal resolution of ∼15 min, enabling the retrieval of NO2 vertical profiles over the entire panorama around the measurement site. IMPACT showed excellent agreement (correlation >99 %) with coincident multiaxis DOAS (MAX-DOAS) measurements during the Second Cabauw Intercomparison of Nitrogen Dioxide measuring Instruments (CINDI-2) campaign. The temporal variability of NO2 slant columns within a typical MAX-DOAS vertical scanning sequence could be resolved and was as large as 20 % in a case study under good viewing conditions. The variation of corresponding profiles and surface concentrations was even larger (40 %). This variability is missed when retrieving trace gas profiles based on standard MAX-DOAS measurements. The azimuthal distribution of NO2 around the measurement site showed inhomogeneities (relative differences) up to 120 % (on average 35 %) on short timescales (individual panoramic scans). This is more than expected for the semirural location. We explain this behavior by the transport of pollution. Exploiting the instrument's advantages, the plume's trajectory during a prominent transport event could be reconstructed. Finally, the potential for retrieving information about the aerosol phase function from O4 slant columns along multiple almucantar scans of IMPACT is demonstrated, with promising results for future studies.

A Statistical Approach for Analysis of Dissolution Rates Including Surface Morphology

Understanding mineral dissolution is relevant for natural and industrial processes that involve the interaction of crystalline solids and fluids. The dissolution of slow dissolving minerals is typically surface controlled as opposed to diffusion/transport controlled. At these conditions, the dissolution rate is no longer constant in time or space, an outcome observed in rate maps and correspondent rate spectra. The contribution and statistical prevalence of different dissolution mechanisms is not known. Aiming to contribute to close this gap, we present a statistical analysis of the variability of calcite dissolution rates at the nano- to micrometer scale. A calcite-cemented sandstone was used to perform flow experiments. Dissolution of the calcite-filled rock pores was measured using vertical scanning interferometry. The resultant types of surface morphologies influenced the outcome of dissolution. We provide a statistical description of these morphologies and show their temporal evolution as an alternative to the lack of rate spatial variability in rate constants. Crystal size impacts dissolution rates most probably due to the contribution of the crystal edges. We propose a new methodology to analyze the highest rates (tales of rate spectra) that represent the formation of deeper etch pits. These results have application to the parametrization and upscaling of geochemical kinetic models, the characterization of industrial solid materials and the fundamental understanding of crystal dissolution.

Fast structured illumination microscopy with reflectance disturbance resistibility and improved accuracy.

The conventional fast structured illumination microscopy (FSIM) without vertical scanning has been proposed by Lee for fast surface profiling. Nevertheless, this method is vulnerable to nonuniform surface reflectivity which is generally caused by the different reflection characteristic of hybrid materials and the environmental fluctuations. Consequently, traditional IFSIM cannot be used in the profile measurement for the samples with surface heterogeneity. In this paper, an improved FSIM configuration (IFSIM) is explored for improving the axial accuracy and suppressing the disturbance of the reflectivity. With this technique, a sinusoidal pattern produced by the digital micro-mirror device (DMD) is illuminated onto the sample. The reflected patterns are captured by two charge-coupled devices (CCDs) that are separately placed in and behind or before the focal plane. The subtraction and sum values of the two axial modulation response curves (AMRs) are divided as the new response function, which effectively suppresses the influence of the inhomogeneous reflectivity. Both wide dynamic range and enhanced axial accuracy can be selectively obtained by controlling the defocusing amount of the two differential detectors. The theoretical analysis and experiments are conducted to verify the feasibility of this method.

Full-field microsurface profilometry using image correlation without vertical scanning.

A full-field microprofilometry involving innovative image correlation was developed for profile measurement without vertical scanning. High-speed optical inspection has become critical for confirming precise dimensions in semiconductor fabrication such as microbumping in 3D stacked ICs and precision manufacturing. A digital micromirror device (DMD) is designed to serve as a point-light-source array in a quasiconfocal optical configuration and perform lateral scanning to minimize signal crosstalk between neighboring testing points. More importantly, multiple diffractive images are detected and measured with a prebuilt depth-correlated database to extract the height information of a tested surface. A 100-nm repeatability can be realized in the absence of a detector pinhole and without vertical scanning, thus achieving high-speed submicrometer-scale surface profilometry.

Extension of longitudinal measuring range of a confocal surface profilometer using a gradient-intensity probe beam

A confocal surface profilometer with a long vertical range was developed by using a gradient-intensity probe beam. In conventional confocal microscopy, vertical scanning of a tested surface by either stage height movement or the shifting of objective focus is time-consuming, thus making unacceptable measurement efficiency for in situ inspection. To overcome this, in this research, a quasi-Bessel beams (QBB) with the properties of narrow lateral spot and gradient-intensity axial distribution is adopted as a probe beam to generate an accurate intensity-to-height conversion for 3-D profile measurement. The influences of the incident Gaussian beam and the real axicon lens with an oblate-tip shape on the axial probe beam are calculated and analyzed. By switching the axicon lenses with different base angles, the measurable height of the surface profilometer can be ranged from a few millimeters to several hundred millimeters. According to the measured results, it was found that by using the axicon lens with a base angle of 20°, the measurable height reached at least 2 mm. By integrating a high-speed acquisition unit, the proposed profilometer can further increase the measurement efficiency, especially fitting in situ automatic optical inspection (AOI).

Remanufacturing of silicon powder waste cut by a diamond-wire saw through high temperature non-transfer arc assisted vacuum smelting.

Non-transfer arc assisted vacuum smelting technology is presented for the first time to recycle harmful silicon powder waste derived from diamond-wire cutting. The final yield and filling rate of silicon could reach 93.9% and 73%, respectively. The oxidation characteristics of silicon powder waste, formation mechanism of a hollow silicon block and removal efficiency of impurities were analyzed in detail to offer profound insights into a non-transfer arc granulation method. The critical conditions for continuous and efficient preparation of a solid silicon block were further confirmed through orthogonal experiments with 7.5 kW welder output power, 3 mm vertical scanning height, and 5 mm/s scanning speed. The yield of the silicon block was more than 96%. Meanwhile, the granulating process demonstrated a favorable effect on removing most impurities, such as C, Al, and Na, without introducing any other impurities. The residual impurities could be further removed by vacuum directional smelting. Given the extremely low equipment cost and huge room for improvement, this work is crucial for large-scale recovery of silicon powder waste.

Extended depth-of-field microscopic imaging using a polynomial transmissive phase plate

In this paper, a reflective light microscope for extending the depth-of-field with a polynomial phase plate is presented. The microscope with and without the polynomial phase plate is tested on imaging a USAF resolution test target and an in-vitro pig’s retina. Experimental results show that the polynomial phase plate can recover volumetric images identical to in-focus images within an acceptable range. We claim that the polynomial phase plate can be used in vertical scanning white-light interference microscopes and medical devices to improve the measurements of thick samples in real-time.

Thermal behavior and grain evolution of 24CrNiMoY alloy steel prepared by pre-laid laser cladding technology

The unique thermal behavior during the multi-layer laser rapid manufacturing process brings about unique microstructure for the as-produced parts. In this paper, a comprehensive three-dimensional self-consistent transient temperature field model of 24CrNiMoY alloy steel by pre-laid laser cladding technology (PLLC) was established, and the reliability of model was verified by the experiment. The simulation results showed that as scanning speed increased (5, 7, 9 mm/s), the peak temperature, dimensions and liquid lifetime of the molten pool decreased, while the G × R increased and G/R decreased at the same position. With increasing layer, the peak temperature of molten pool and the temperature of the whole domain increased gradually until forming a dynamic balance. The contrast experimental results showed that with scanning speed increased, the average size of equiaxed grain increased (4.682, 5.012, 5.462 μm) and the average aspect ratio of columnar grain decreased (11.890, 11.411, 10.593). During the multi-layer process, grain growth modes were different along the deposition sections. The structure in parallel scanning direction mainly consisted of equiaxed grains, while in the overlapping direction were equiaxed and columnar grains. There was a clear columnar-to-equiaxed transition (CET) boundary that controls the change in grain types. The grain growth behavior of a multi-layered in vertical scanning direction section was different from that of a single track. All the above results illustrated that temperature field had a significant effect on the thermal behavior during PLLC process, which affected the grain evolution of 24CrNiMoY alloy steel.

Ex vivo confocal laser scanning microscopy: An innovative method for direct immunofluorescence of cutaneous vasculitis.

Ex vivo confocal laser scanning microscopy (ex vivo CLSM) offers an innovative diagnostic approach through vertical scanning of skin samples with a resolution close to conventional histology. In addition, it enables fluorescence detection in tissues. We aimed to assess the applicability of ex vivo CLSM in the detection of vascular immune complexes in cutaneous vasculitis and to compare its diagnostic accuracy with direct immunofluorescence (DIF) microscopy. Eighty-two sections of 49 vasculitis patients with relevant DIF microscopy findings were examined using ex vivo CLSM following staining with fluorescent-labeled IgG, IgM, IgA, C3 and fibrinogen antibodies. DIF microscopy showed immunoreactivity of vessels with IgG, IgM, IgA, C3 and Fibrinogen in 2.0%, 49.9%, 12.2%, 59.2% and 44.9% of the patients, respectively. Ex vivo CLSM detected positive vessels with the same antibodies in 2.0%, 38.8%, 8.2%, 42.9% and 36.7% of the patients, respectively. The detection rate of positive superficial dermal vessels was significantly higher in DIF microscopy as compared to ex vivo CLSM (P < .05). Whereas, ex vivo CLSM identified positive deep dermal vessels more frequently compared to DIF microscopy. In conclusion, ex vivo CLSM could identify specific binding of the antibodies in vessels and showed a comparable performance to conventional DIF microscopy in diagnosing vasculitis.

Corrosion of carbon steel and the passivating properties of corrosion films formed under high-PT geothermal conditions

Corrosion is a major obstacle to a safe implementation of geotechnical applications. Using a novel approach that includes vertical scanning interferometry (VSI) and electrochemical impedance spectroscopy (EIS) we discuss time-dependent carbon steel corrosion and film formation at geothermally relevant temperatures (80–160 °C) in CO2-saturated mildly acidic NaCl brine. Iron dissolution kinetics follows a logarithmic rate at 80 and 160 °C and a linear rate at 120 °C. At 80 °C, high initial corrosion rates (first 24 h) generate H2 at a minimum rate of 12 μmol h−1 cm−2 and lead to the formation of a continuous ~100 μm thick porous corrosion film. It exhibits a duplex structure with a crystalline outer FeCO3 layer and an inner layer composed of a skeletal network of Fe3C impregnated with FeCO3. Being an electrical conductor we hypothesize the Fe3C to strongly enhance corrosion rates by providing additional cathodic sites. Pseudo-passivity due to an anodic film-forming reaction (presumably Fe-oxide) was observed at 120 and 160 °C, soon followed by the initiation of pitting at 120 °C. Steady-state corrosion rates at 160 °C are at least one order of magnitude lower than for 120 °C. Our experimental approach demonstrated potential for general applicability in studying corrosion-related phenomena.

Large-Displacement Vertical Electrostatic Microactuator Dynamics using Duty-Cycled Softening/Stiffening Parametric Resonance.

Electrostatic microactuators with large vertical scanning range (several hundred microns) at high frequency (hundreds to thousands of hertz) and chips sizes compatible with endoscopic microscopy have recently been demonstrated based on parametric resonance. This paper examines the use and modeling of mixed softening/hardening dynamics to help produce large ranges of motion in this class of mirrors. Origin of spring stiffening behavior in actuator design is described, followed by non-dimensional analysis of actuator motion trends. Experimental results are presented for a sample actuator design with up to 480 μm displacement at 1225 Hz and 60 V. Comparison to predicted trends and comments on benefits and limitations of modeling are provided.

A numerical-experimental investigation of heat distribution, stress field and crack susceptibility in Ni60A coatings

3D finite element model (FEM) was built for the simulation of temperature and stress field induced by laser cladding. The proposed model was validated by experimental results. The numerical width and depth of coating matched well with the experimental cross-section of macrograph. The viscosity of molten pool versus the temperature field was also discussed, and the maximum difference of viscosity in different zones was approximately 32.8%. High temperature and sharp thermal gradient was generated in the vicinity of heat source, which was prone to produce residual stress. The heat distribution obtained from temperature field was in consistent with the thermocouple measurements. Due to large plastic tensile deformation of materials, the longitudinal residual stress was susceptible to crack propagation in the laser cladding. The measured maximum value of longitudinal residual stress reached 492 MPa, which was lower than predicts. The coatings were apt to crack along the vertical scanning direction. In addition, the crack susceptibility of coatings was not only depended on residual stress, but also the microsegregation of Cr, Ni and Fe elements. The calculation results are helpful for systematically understanding the melt pool, heat cycles, and stress, which related to laser cladding Ni-based materials.

Comparing the reactivity of glasses with their crystalline equivalents: The case study of plagioclase feldspar

Abstract To evaluate the impact of atomic short- and long-range orders on silicate dissolution kinetics, the dissolution of amorphous and crystalline oligoclase was investigated at pH 1.5 and 10 at 90 °C. Experiments in solution saturated with respect to SiO2 am were additionally performed to constrain the effect of Si-rich surface layer formation on dissolution rates. The face-specific dissolution rates of the crystalline oligoclase and of the oligoclase glass were determined from element budget in solution and surface retreat measured by vertical scanning interferometry. The results show that atomic ordering primarily impacts solid reactivity, irrespective to the pH of the solution. A strong relation between the crystal surface orientation, the evolution of its topography and its dissolution rate was observed. The ( 0 0 1 ), ( 0 1 0 ) and ( 1 0 1 ¯ ) flat faces containing the strongest bonds dissolved the most slowly and their dissolution rates remained constant throughout the experiments. In contrast, the stepped ( 1 1 ¯ 1 ) face was characterized by the highest initial dissolution rate, but progressively decreased, suggesting that the preferential dissolution of stepped sites expose afterwards more stable planes. The differences in terms of etch pit density from one surface to another also explained the difference in dissolution rates for the ( 0 0 1 ) and ( 0 1 0 ) faces. The fluid chemistry suggested the formation of very thin (100–200 nm) Si-rich surface layers in acidic conditions, which weakly affected the dissolution rate of the pristine crystal. At pH 1.5, oligoclase glass dissolves at a rate similar to that of the fastest studied faces of the crystal, suggesting the absence of structural effect on oligoclase dissolution. Whereas Si-rich surface layers likely formed by interfacial dissolution-reprecipitation for oligoclase crystal, molecular dynamic calculations suggest that the slightly more open structure of the glass could also allow ion-exchange following water diffusion into the solid. This mechanism could explain why the surface layer of the glass is characterized by a different chemical composition. Results at pH 10 are strikingly different, as the oligoclase glass dissolves up to 20 times faster than its crystalline equivalent. This non-linear response of the material upon pH was linked to the density of critical bonds in oligoclase that is indeed pH-dependent. In acidic pH, the preferential dissolution of Al leaves a highly polymerized and relaxed Si-rich surface, whereas in basic pH the preferential dissolution of Si leads to a complete destructuration of the network because of the lack of Si O Al bonds.