K. Liddell

Yttrium oxynitride glasses: Properties and potential for crystallisation to glass-ceramics

Abstract Silicon nitride-based ceramics contain oxynitride glass phases at the grain boundaries which can impair subsequent high temperature properties. Studies of bulk glasses in the Y-Si-Al-O-N system have been carried out and it has been shown that up to 10 atomic % N can be incorporated into these oxynitride glasses. Nitrogen increases the viscosity, hardness and glass transition temperature of the glasses. Heat treatments of Y-Si-Al-O-N glasses have been carried out and the crystalline phases formed are reported. Further improvements are possible if glass-ceramic processes using two-stage heat treatments are introduced. This paper reviews the development of oxynitride glasses, the effects of nitrogen on properties and reports on the glassceramic heat treatments.

Phase transformation in the Al2O3–ZrO2 system

Co-precipitation methods have been used to produce 20 mol% Al2O3–80 mol% ZrO2 mixed oxides, from aqueous solutions of zirconium oxychloride and aluminium chloride, followed by precipitation with ammonia. The resulting gel was calcined at increasing temperatures, and X-ray diffraction confirmed that the structure remained amorphous up to 750°C and then crystallized as a single-phase cubic zirconia solid solution, but with a reduced unit-cell dimension. At higher temperatures, the unit-cell dimension increased and, above 950°C, this phase started to transform to a tetragonal zirconia (t-ZrO2) phase, again of reduced cell dimensions compared with t-ZrO2, with simultaneous appearance of small amounts of θ-Al2O3. Above 1100°C, the tetragonal phase transformed to monoclinic zirconia on cooling, and the amount of θ-Al2O3 increased. Above 1200°C, the θ-Al2O3 transformed to the stable α-Al2O3. These results confirm that aluminium acts as a stabilizing cation for zirconia up to temperatures of about 1100°C.

Subsolidus phase relationships in the systems Ln2O3-Si3N4-AlN-Al2O3 (Ln = Nd, Sm)

Abstract Subsolidus phase relationships in the systems Ln-Si-Al-O-N where Ln = Nd and Sm have been determined. Forty-four compatibility tetrahedra were established in the region Ln 2 O 3 -Si 3 N 4 -AlN-Al 2 O 3 . Within this region, LnAlO 3 and M′-phase (Ln 2 Si 3 . x Al x O 3 + x N 4 − x ) are the only two important compounds which have tie lines joined to β-sialon and AlN polytypoid phases. α-Sialon coexists with the M′ phase.

X-ray data for new Y-Si-Al-O-N glass ceramics

Abstract Since the 1970s an increasing number of crystalline oxynitrides have been observed as grain boundary phases in sialon ceramics. In particular, the well-known four- and five-component phases in the Y-Si-Al-O-N system have been accepted as the total picture in this system and the potential for new phases has not been considered. However, with further development of sialon glasses and glass ceramics, post-preparative heat-treatment has revealed a number of previously uncharacterised crystalline phases occurring particularly at temperatures below 1200 °C, Three such phases are discussed, IW, Q and D, all of which have been observed previously by other researchers but without X-ray diffraction data; Q-phase occurs in some rare earth as well as yttrium sialon systems. All these phases can be produced only within a limited temperature range and are critically dependent on starting composition and heat-treatment temperature, so the present data will complement those already existing for devitrified sialon glass products, with potentially more phases yet to be identified.

Heat treatment of wollastonite-type Y–Si–Al–O–N glasses

Cumulative work over the last twenty years has defined the glass-forming regions in several M–Si–Al–O–N systems (M = Mg, Ca, Y, Ln) with the resulting crystalline products identified after heat treatment. Glass-forming regions in nitrogen-rich sialon glasses have been recently reported and heat treatment of some of these glasses in the Y–Si–Al–O–N system has been performed. The crystallization of yttrium-containing glasses is particularly sensitive to small variations in composition and heat treatment temperature and in the current work the results of three series are discussed: (1) a single composition, Y15.2Si14.6Al8.7O54.6N6.9 (16 e/oN), treated at 30 °C intervals between 875–1410 °C; (2) compositions of a constant Y: Si:Al ratio of 3:3:2 and up to 32 e/oN and (3) selected compositions lying on the 28 e/o N plane. Two different sets of crystalline products are found to form above and below 1200 °C.

A combined 14N/27Al nuclear magnetic resonance and powder X-ray diffraction study of impurity phases in β-sialon ceramics

Beta-sialons are ceramic phases occurring in the SiO(2)-Si(3)N(4)-AlN-Al(2)O(3) system. A series of samples with differing compositions has been investigated by magic-angle spinning nuclear magnetic resonance (NMR) spectroscopy and powder X-ray diffraction (XRD). Although the constituent nitrogen nuclei occupy positions of low symmetry in the beta-sialon structure, 14N NMR spectra could be recorded for the samples examined. The origin of the 14N signal could be traced to the presence of an aluminium nitride (AlN) impurity phase with the help of 27Al NMR and XRD results. Similarly, the existence of Al(2)O(3) grains could be readily detected for a number of samples. Thus, the combination of 14N and 27Al NMR is shown to be an especially effective tool in identifying and characterizing impurity phases in sialon ceramics, complementing the results obtained from standard XRD analysis.

J-Phase solid solution series in the Dy-Si-Al-O-N system

Abstract Detailed studies on J-phase compositions of the type Dy 4 Si 2 − x Al x O 7 + x N 2 − x have demonstrated the existence of five different types of structure, all based on the monoclinic cuspidine arrangement characteristic of the end-members Dy 4 Si 2 O 7 N 2 and Dy 4 Al 2 O 9 . Series 1 and 3 extend into the system from the endmembers, in the ranges 0 ≤ x ≤ 0.4 and 1.6 ≤ x ≤ 2.0, respectively. Series 2 is characterised by a lower value of the b lattice parameter and occurs in the range 0.5 ≤ x ≤ 1.5, while Series 4 is observed in the range 0.8 ≤ x ≤ 1.1 alongside Series 2. Series 5 occurs in the range 1.4 ≤ x ≤ 1.5, at which point X-ray patterns are much simpler, indexing on an orthorhombic unit cell with a halved c -axis repeat. These observations are consistent with structural differences observed in naturally-occurring cuspidine minerals, and also with recent observations on phase transformations in mixed rare earth Ln 4 Al 2 O 9 compounds.

Preparation and crystal structure of U-phase Ln3(Si3 –xAl3 +x)O12 +xN2 –x(x≈ 0.5, Ln = La, Nd)

U-phase Ln3(Si3 –xAl3 +x)O12 +xN2 –x(Ln = La, Nd) occurs as a crystalline phase in rare-earth sialon ceramics formed by devitrification of grain-boundary glasses at 1000–1400 °C. The crystal structure of Nd U-phase has been determined from Cu-Kα X-ray powder diffractometer data and refined by the Rietveld full-profile technique to RF= 0.028. The space group is P321 and the cell dimensions are a= 7.974(1)A, c= 4.873(1)A and V= 268.29 A3. The structure is isomorphic with the La3Ga5GeO14 structure, and exhibits corner-shared layers of (Si,Al)(O,N)4 tetrahedra interconnected by AlO6 octahedra. The rare-earth cations occupy sites between the tetrahedral layers. Transmission electron microscopy and lattice imaging studies support the X-ray structural findings. The structural relationship of the U-phase to other nitrogen-containing ceramic phases is discussed.

J-phase structures in the Y-Si-Al-O-N system

Preparative studies carried out on compositions lying between Y4Si2O7N2 (J-phase) and Y4Al2O9 (YAM) show similar variations in structure to those already reported for related compositions in the analogous Dy-Si-Al-O-N system. Careful studies of X-ray diffraction patterns confirm that whereas all compositions have a cuspidene-like structure, there is a marked change in the a and b unit cell dimensions for x values in excess of approximately 0.8 in the general formula Y4Si2-xAlxO7+xN2-x, with much smaller changes in the c unit cell dimension and in the β angle. The results are compared with similar data previously presented for J-phase structures in the DySi-Al-O-N system.